Gamma-ray bursts without the gamma rays?

We discussed supernovae a bit in this recent post on gamma-ray bursts. There is now interesting new information on the connection between supernovae and gamma-ray bursts from two recently-described supernovae with atypical properties.

Let's first review a little. Gamma-ray bursts (GRBs) are identified by detection of relatively brief (usually less than a few minutes) but highly energetic emissions of gamma rays. Although there's a great deal of diversity, most events fall into one of two categories: "short", emitting strongly for less than two seconds, and "long", having strong gamma-ray emissions for more than a few seconds and higher total energy.

Short GRBs are less well understood, but they are commonly thought to result from the merger of two stellar-weight black holes or neutron stars. Short GRBs are not relevant for the present discussion.

Long GRBs are thought, with fairly general consensus, to result from certain types of supernovae (the "core-collapse" kind). One reason for this consensus is that the total energy output of a long GRB appears to be in the same neighborhood as that of a core-collapse supernova: around 10⁵¹ ergs (10⁴⁴ joules, if you prefer). Also, if a GRB occurs sufficiently nearby, we can optically identify it with a bright supernova event, and there have been several instances of this.

Supernovae also come in several different kinds, which differ in their internal mechanisms. The main types are Type I, which have no evidence of hydrogen in their spectra, and Type II, which do have hydrogen lines in the spectrum.

There are further subdivisions of the Type I case: Type Ia supernovae are thought to be associated with progenitors that are white dwarfs. These are the end stage reached by most stars, which have less than about ten times the mass of the Sun (10 M_⊙). Such stars lose most of their mass when they pass through an earlier red giant stage. After they have burned (via fusion) all of their hydrogen they shrink down to become white dwarfs with a mass less than about 1 M_⊙.

Eventually such a star can no longer produce energy even by fusing heavier elements. The star produces energy only by gravitational contraction. Contraction stops only when degeneracy pressure (due to the Pauli exclusion principle) prevents further collapse. Degeneracy pressure, however, can support a stellar mass only if it's less than 1.38 M_⊙ – the Chandrasekhar limit. If a star reaches this stage with a mass of more than 1.38 M_⊙, it will collapse further into a neutron star or black hole. If this doesn't happen, yet the star later accretes mass from an external source – such as a companion in a multiple star system – the star may wind up with enough additional matter to sustain an explosive fusion reaction – and then you get a Type Ia supernova. There's some controversy now whether this is more likely to happen due to the merger of two white dwarfs or gradual accretion from a companion, but that's irrelevant for the present discussion, because Type Ia supernovae are not associated with GRBs.

The problem with Type Ia supernovae is that there's no conceivable mechanism to create one particular hallmark of a GRB – namely, jets of matter ejected from the explosion in opposite directions at speeds near the speed of light. It takes a particular mechanism called a "central engine" to accelerate matter that dramatically. This mechanism is thought to be a rapidly spinning neutron star or black hole that is surrounded by a gaseous accretion disk. The matter is sucked in by the central object, but because it has such a large amount of angular momentum it is expelled outwards in jets along the axis of rotation, instead of falling into or onto the central object.

A "core collapse" supernova is required in order to produce the right conditions for a central engine of this sort to form. (And even in this case, only a small percentage of events seem to have just the right conditions.)

To have a core collapse supernova, it's necessary for the progenitor star to have a mass more than about 10 M_⊙. In this case, which is pretty rare, after a certain point, even though fusion of heavy elements may still be going on, the energy available from fusion becomes insufficient to support the entire mass of the star, and the whole thing then collapses very rapidly to a black hole or neutron star. If there is still some hydrogen remaining in the outer shell of the star immediately before collapse, you get a Type II supernova. Otherwise you get a Type Ib supernova, if there's still some helium remaining, or else a Type Ic (no helium).

If the progenitor star had a mass between about 10 M_⊙ and 20 M_⊙, the end result is a neutron star. Above 20 M_⊙ the result is a black hole. However, as noted, it's not known what conditions are required in order to have a central engine capable of producing a GRB. From the relative frequency of observed GRBs compared to Type Ib/c or Type II supernovae, the right conditions seem to occur only 1 or 2% of the time.

The new wrinkle that two recently described supernova events exhibit is that it is possible to have relativistic jets of matter from a supernova without detectable gamma ray activity. Just how fast are we talking about in terms of the ejected matter? Well, in an "ordinary" supernova matter is ejected at speeds, at most, only 2 or 3% of the speed of light (which is still plenty fast). Yet in the recent examples, the matter is accelerated to more than 50% of the speed of light.

How is the speed actually measured? Well, the interesting thing is that it's easy to measure at radio frequencies, using radio telescopes. Let's consider the case of the supernova known as SN 2009bb, to be specific, which was first observed on March 21, 2009. This one showed up in a galaxy called NGC 3278, which is about 130 million light-years away.

Astronomers didn't even have to act especially quickly to do the measurement – in fact it's best to do it several weeks after the event. Using radio interferometry it's straightforward to observe the size of the expanding sphere of radio emissions. Divide the radius of the sphere by the time since the initial event and you have (roughly) the rate of expansion.

Actually, it's slightly more complicated than that, because relativistic speeds are involved. It's necessary to apply equations of special relativity. Suppose R is the apparent radius of the sphere, and the time interval is Δt. Then the apparent velocity of expansion is R/Δt. It is even possible for this apparent velocity to exceed c (the speed of light).

However, this apparent velocity is not actually the rate at which the ejected matter is moving. Suppose that rate is denoted by v. Let β=v/c, the ratio of the actual speed to the speed of light. Define the quantity (Lorentz factor) γ=1/√(1-β²). Then what's actually true is that γβc=R/Δt. In the case of SN 2009bb the apparent velocity that was measured was ~0.85c. Solving for β you get β~0.65. That is, ejected matter was moving at about 65% of the speed of light. This matter was just lightweight electrons, but it still takes one honking explosion to move even electrons that fast – perhaps 30 times as fast as what one gets in an "ordinary" supernova.

Here's the research abstract:

A relativistic type Ibc supernova without a detected γ-ray burst

Long duration γ-ray bursts (GRBs) mark the explosive death of some massive stars and are a rare sub-class of type Ibc supernovae. They are distinguished by the production of an energetic and collimated relativistic outflow powered by a central engine (an accreting black hole or neutron star). Observationally, this outflow is manifested in the pulse of γ-rays and a long-lived radio afterglow. Until now, central-engine-driven supernovae have been discovered exclusively through their γ-ray emission, yet it is expected that a larger population goes undetected because of limited satellite sensitivity or beaming of the collimated emission away from our line of sight. In this framework, the recovery of undetected GRBs may be possible through radio searches for type Ibc supernovae with relativistic outflows. Here we report the discovery of luminous radio emission from the seemingly ordinary type Ibc SN 2009bb, which requires a substantial relativistic outflow powered by a central engine. A comparison with our radio survey of type Ibc supernovae reveals that the fraction harbouring central engines is low, about one per cent, measured independently from, but consistent with, the inferred rate of nearby GRBs.

There's a practical application of this discovery. It means that astronomers can use radio telescopes to identify supernova events in which a central engine is involved, even if no gamma rays are detectable. Eventually this will help figure out more precisely what conditions are necessary in order to create the central engine.

There are several possible reasons why gamma rays may not be detectable in such events. It could be that sufficiently powerful gamma rays simply are not produced in some cases. Or else they are produced, but quickly absorbed in the neighborhood of the event so that we never see them. Finally, since the gamma rays are presumably directed in a narrow beam, like the ejected matter, the beam simply is not along our line of sight.

A very similar result has also just been reported for the supernova SN 2007gr. This one is in a somewhat closer galaxy, NGC 1058, about 34 million light-years away. In this case, matter was ejected with β~0.52, corresponding to an apparent rate of 60% of the speed of light. Again, no associate gamma ray emission was detected.

The research report:

A mildly relativistic radio jet from the otherwise normal type Ic supernova 2007gr

The class of type Ic supernovae have drawn increasing attention since 1998 owing to their sparse association (only four so far) with long duration γ-ray bursts (GRBs). Although both phenomena originate from the core collapse of a massive star, supernovae emit mostly at optical wavelengths, whereas GRBs emit mostly in soft γ-rays or hard X-rays. Though the GRB central engine generates ultra-relativistic jets, which beam the early emission into a narrow cone, no relativistic outflows have hitherto been found in type Ib/c supernovae explosions, despite theoretical expectations and searches. Here we report radio (interferometric) observations that reveal a mildly relativistic expansion in a nearby type Ic supernova, SN 2007gr. Using two observational epochs 60 days apart, we detect expansion of the source and establish a conservative lower limit for the average apparent expansion velocity of 0.6c.

Soderberg, A., Chakraborti, S., Pignata, G., Chevalier, R., Chandra, P., Ray, A., Wieringa, M., Copete, A., Chaplin, V., Connaughton, V., Barthelmy, S., Bietenholz, M., Chugai, N., Stritzinger, M., Hamuy, M., Fransson, C., Fox, O., Levesque, E., Grindlay, J., Challis, P., Foley, R., Kirshner, R., Milne, P., & Torres, M. (2010). A relativistic type Ibc supernova without a detected γ-ray burst Nature, 463 (7280), 513-515 DOI: 10.1038/nature08714

Paragi, Z., Taylor, G., Kouveliotou, C., Granot, J., Ramirez-Ruiz, E., Bietenholz, M., van der Horst, A., Pidopryhora, Y., van Langevelde, H., Garrett, M., Szomoru, A., Argo, M., Bourke, S., & Paczyński, B. (2010). A mildly relativistic radio jet from the otherwise normal type Ic supernova 2007gr Nature, 463 (7280), 516-518 DOI: 10.1038/nature08713

Further reading:

Astronomers Find Rare Beast by New Means (1/27/10)

Oddball Cosmic Explosion Holds Clues to Universe's Biggest Bangs (1/27/09)

Newborn Black Holes May Add Power to Many Exploding Stars (1/27/10)

Astronomers in the Netherlands catch supernova, observe relativistic expansion (1/27/10)

Star Shoots out Material at Close to the Speed of Light (1/28/10)

Some baby black holes give boost, but no burst (1/29/10)

Supernovae linked to gamma ray bursts (1/28/10)

The Birth Place of the Type Ic Supernova 2007gr

Doctor Who and the Silver Spiral (1/27/10)

Magnetic fields in gamma-ray burst jets

Gamma-ray bursts (GRBs) are the most dramatic short-lived violent events observed in the universe. They are often described as releasing a quantity of energy, in less than a minute, that is at least as much as a star like the Sun releases in its entire 10 billion year lifetime. Since the first detection of a gamma-ray burst in 1967, the central question has been to determine the nature of the process or processes that can release so much energy so quickly.

We've discussed gamma-ray burst several times before, such as here, here, and here.

The defining characteristic property of a GRB is a rapid, highly energetic burst of gamma rays, lasting only a few seconds. Most of the GRB energy is released in that event. But beyond that, GRBs exhibit a bewildering diversity of characteristics – reflecting a diversity in the conditions that can produce a GRB.

The most noticeable difference observed in GRBs is that some events are over very quickly – within 1 or 2 seconds – while others include an "afterglow" of radiation less energetic than gamma rays, lasting as long as minutes in some cases. In a few instances there are events that have some properties of both "short" and "long" types of GRB.

Gamma rays cannot penetrate the Earth's atmosphere, so the initial phase of any GRB is detectable only from a satellite-based instrument. This is rather limiting in terms of the types of observations that can be made. For example, satellites lacked instruments that could record a spectrum of a GRB event in lower-energy electromagnetic radiation. Without a spectrum, astronomers cannot measure redshift, and hence the distance of the event.

The development and deployment within the last 10 years of systems that could use satellite detection of GRBs to activate automated ground-based telescopes have dramatically improved this situation. It's now possible to collect much more detailed data, so that astronomers have been able to learn a lot more about GRBs – in many cases.

Even so, many short GRB events are over in just a few seconds, and therefore much less is yet known about short GRBs. The best current guess is that such events are caused by the merger of a pair of binary neutron stars.

The research to be described here, therefore, concerns long GRBs, where it is relatively easy to study the characteristics of the lower-energy electromagnetic radiation, which makes up the afterglow for several minutes or even tens of minutes after the initial burst.

A general consensus has emerged that gamma-ray burst progenitors are certain types of supernovae. Not just any type, either, because the progenitor must be capable of releasing the amount of energy actually observed in a GRB. This rules out Type Ia supernovae, which result from a thermonuclear explosion of matter that has accreted onto the surface of a white dwarf star.

Type Ia supernovae are important in cosmology, because they all have roughly the same intrinsic brightness. This makes it possible to determine the approximate distance of a Type Ia supernova event, just from the observed brightness. By comparing this distance with the redshift of the supernova it is possible to determine how rapidly the universe has expanded in the past. This, in turn, is what made it possible to conclude, in 1997, that the expansion of the universe is accelerating.

However, the energy released by a Type Ia supernova is far too small to account for a GRB. Instead, a different supernova mechanism, known as "core collapse" is needed. Classification of supernovae is a little confusing, since it was originally done on the basis of spectral characteristics. Type II supernovae have a particular line in their spectrum due to hydrogen, while Type I supernovae do not.

Type II supernovae result from the collapse of very massive stars at the end of their lives, when they can no longer support their own weight by the pressure of fusion occuring in their constituent matter. But it turns out that some supernovae lacking the hydrogen spectral line are too energetic to result from the same mechanism as that of Type Ia supernovae. These types are known as Type Ib and Type Ic, and they also result from core collapses of massive stars (that have already burned off all their hydrogen).

So there are significant differences even among core collapse supernovae, resulting from such factors as the total mass of the progenitor star and the original composition of the star, among other things. Such differences can account for some of the differences observed if such supernovae are responsible for GRBs.

But by no means all core collapse supernovae produce a GRB. Theoretical considerations dictate that several other factors must also be present. For one thing, the supernova must result in the formation of a black hole that is massive enough to support a large accretion disk of matter orbiting around it. A neutron star, which is the alternative remnant of a supernova, just isn't massive enough. To get a sufficiently massive black hole, the progenitor star must be at least 40 Solar masses.

High mass alone, however, is not enough. The progenitor star must also be rotating rapidly enough that the angular momentum of the system is large enough to cause most of the matter and energy from the supernova explosion to be focused into a jet of angular width at most about 20 degrees. This concentrates most of the energy of the explosion into a narrow beam, so that the energy emitted in our direction matches what we actually observe. If the beam were not so narrow, the energy would not appear to be the magnitude that we observe.

There are additional factors that affect the varying characteristics of GRBs that we observe. In particular, the distribution of matter in the interstellar medium surrounding the supernova is important. It is the collision between the jets and this matter that determines the intensity and duration of the afterglow we observe for some time after the original burst.

And there's more. The jets of matter and energy from a GRB event may well be powered by the energy of the original explosion. But that's not the only possibility. Suppose there are strong magnetic fields surrounding the progenitor star. Then there will also be a considerable amount of energy in the magnetic flux, and this can also supply power to the jets.

Strong magnetic fields would have another consequence as well. The jets consist partly of electrons moving at relativistic speeds (very close to the speed of light). These electrons will follow a spiral path around lines of magnetic flux. This creates a type of electromagnetic radiation known as synchrotron radiation. If present, this radiation would make up part of the afterglow we can observe.

How would we know if synchrotron radiation, and hence magnetic fields, are present? That's simple – the radiation would be partly polarized, provided that the magnetic fields are orderly and not all tangled up.

And this is precisely what recent research has observed in the case of one particular GRB event (GRB 090102), which was detected January 2, 2009. Specifically, a polarization of 10±1% was observed at optical wavelengths. This degree of polarization is quite rare in astrophysical events, and it strongly suggests the presence of large-scale magnetic fields associated with GRB 090102. These fields should contribute substantially to the observable energy of the GRB.

Abstract:

Ten per cent polarized optical emission from GRB 090102

The nature of the jets and the role of magnetic fields in gamma-ray bursts (GRBs) remains unclear. In a baryon-dominated jet only weak, tangled fields generated in situ through shocks would be present. In an alternative model, jets are threaded with large-scale magnetic fields that originate at the central engine and that accelerate and collimate the material. To distinguish between the models the degree of polarization in early-time emission must be measured; however, previous claims of gamma-ray polarization have been controversial. Here we report that the early optical emission from GRB 090102 was polarized at 10 ± 1 per cent, indicating the presence of large-scale fields originating in the expanding fireball. If the degree of polarization and its position angle were variable on timescales shorter than our 60-second exposure, then the peak polarization may have been larger than ten per cent.

Steele, I., Mundell, C., Smith, R., Kobayashi, S., & Guidorzi, C. (2009). Ten per cent polarized optical emission from GRB 090102 Nature, 462 (7274), 767-769 DOI: 10.1038/nature08590

Further reading:

Magnetic Power Revealed in Gamma-Ray Burst Jet (12/9/09)

Huge Cosmic Explosions Fueled by Magnetism (12/9/09)

Gamma-ray bursts: Magnetism in a cosmic blast (12/10/09)

In the News this month: the role of magnetic fields in GRBs (1/3/10)

Space is very fine-grained

It would take you a lot longer to hike a significant distance over very hilly terrain than it would over a completely flat plain. For much the same reason, it would take light longer to cover the same distance depending whether the space through which it moves does or doesn't have large "hills".

But what does it mean for space to contain "hills"? And how large do "hills" need to be to make a difference?

Consider the second question first. There's no natural place on Earth that is perfectly flat, of course. So if you look closely enough, there are always "hills" of some size to cross when you're on a hike. But if the height of those hills is a lot less than the length of your stride, they will make little difference. In the same way, if the irregularities in the texture of space are much smaller than the wavelength of light, they won't make much difference either.

Physicists aren't even sure that space does contain "irregularities" at very short distances. No measurement yet made has given any evidence of irregularities. But all theories of quantum gravity – none of which is yet considered satisfactory – predict that irregularities must exist at a sufficiently small scale.

No attempted quantum theories of gravity are satisfactory yet, because physicists have not been able, with a mathematically consistent theory, to reconcile general relativity and quantum mechanics at very small scales. Nature, however, is somehow able to do the trick. So it seems quite plausible that space does have irregularities at sufficiently small scales. The only real question is "how small?"

Recent research now says, "smaller than can presently be detected."

And how would one go about detecting the smallest irregularities? With light whose wavelength is about the same size, of course. Which would mean photons with the highest energy one can find. So far, photons with the highest energy physicists know of are not indicating any quantum irregularities in space.

We're talking about photons with energies far higher than can be produced in laboratories. The most energetic photons correspond to the part of the electromagnetic spectrum known as gamma rays. By convention, physicists regard gamma rays as electromagnetic radiation with a wavelength of less than 10^-11 m (10 picometers). That corresponds to an energy of 10⁵ eV (eV = electron-volt). (Recall that photon energy is related to frequency by the relation E=ℎ&nu, where ℎ is Planck's constant and ν is the photon frequency. Since wavelength λ is inversely related to ν so is energy; i. e. E ∝ 1/λ.)

Gamma rays are produced naturally in some types of atomic decay, and such gamma rays have energies up to 10⁷ eV. That's not terribly energetic in the great scheme of things. The Large Hadron Collider will be able to accelerate protons up to energies around 10¹³ eV.

Since photons have no electric charge, unlike protons, they can't be accelerated the way protons can. Much more energetic gamma rays can be produced in particle-antiparticle annihilations, but such high-energy photons are rather rare, even in cosmic events like gamma-ray bursts, which may produce gamma-rays with energies of 3×10¹⁰ eV or more (3.34×10¹⁰ eV is the largest yet observed). That's 3×10⁵ times as energetic as the least energetic gamma rays and so corresponds to wavelengths ~.3×10^-16 m.

Fortunately, even with gamma rays in this energy range, much smaller irregularities in space can be detected if the gamma ray photons travel a very large distance. Suppose, for instance, that a gamma-ray photon were detectably slowed down only by 1 part in a million, a factor of 10^-6, over a distance of 100,000 light-years, about the diameter of a large spiral galaxy.

But really cosmic distances at which we can observe gamma-ray bursts are about 10¹⁰ light-years, or 10⁵ times the diameter of a galaxy. Since a light-year is about 10¹⁶ m, we're talking scales like 10²⁶ m. On that scale, the possible slow-down of a gamma-ray photon could be very noticeable – roughly 1 part in 10.

The result is that if we can detect how much a gamma-ray photon is slowed down over a distances around 10¹⁰ light-years, we could be able to probe energies much higher than those possessed by a gamma-ray photon itself, and therefore distance scales much smaller than a very energetic gamma-ray photon wavelength of about 10^-17 m.

Detecting small differences in the velocities of photons of different wavelengths is made much easier when both photons travel 10¹⁰ light-years over a time of (naturally) 10¹⁰ years. Gamma-ray bursts, fortunately, produce gamma rays over a range of energies that differ by several times 10⁵ from smallest to largest.

To be more concrete, suppose you have a gamma-ray burst at a redshift of z~.9. That corresponds to a distance of ~7×10⁹ light-years and a travel time of ~7×10⁹ years. The wavelength of photons traveling that distance is stretched by a factor of 1.9 (i. e. 1+z), but there's still a factor of several times 10⁵ between the wavelenghts of the most and least energetic gamma-ray photons, so they are readily distinguishable. If the difference in arrival time of gamma-ray photons is 1 second, that is only one part in 2×10¹⁷ (i. e. 7×10⁹ years times about 3×10⁷ seconds/year ≅ 2×10¹⁷ seconds).

It's reasonable to suppose that the slowdown of the least energetic gamma-ray photons due to irregularities in space is negligible. Suppose we could relate the slow-down of the most energetic photons to the actual size of spatial irregularities. For instance, suppose a spatial irregularity of 1 part in 10¹⁷ of the photon's wavelength caused a slow-down that was also 1 part in 10¹⁷ of the photon's velocity. That would cause a 2 second delay in photon arrival times – which is very readily detectable.

Recall that energetic gamma-ray photons from gamma-ray bursts have wavelengths of .3×10^-16 m or less. Thus we might expect to be able to detect spatial irregularities as small as ~.3×10^-33 m = 3×10^-34 m = 3×10^-32 cm.

In fact, it is now possible to measure arrival times of photons from gamma-ray bursts to within mere hundredths of a second. So we can probe length scales around 10^-33 cm. Interestingly enough, this is very close to the Planck length scale l_Planck ≈ 1.62×10^-33 cm.

That's just a back-of-the-envelope calculation based on hypothetical data. But we don't need to be hypothetical, because photons from the gamma-ray burst GRB 090510, which was observed on May 10, 2009, were measured very precisely. The most energetic photon observed had an energy of ~3.1×10¹⁰ eV (31 GeV), and it showed up precisely .829 seconds after the very first photons from the burst were detected. Further, spectroscopic observations of the afterglow from this burst showed a redshift of z very close to .9, as in our hypothetical example.

Since we don't have a reliable quantum gravity theory, we don't know exactly how much of a slowdown very high-energy, short-wavelength photons should experience. However, several theories predict that, to first order approximation, if v_ph is the effective average velocity of the photon, then its ratio to c, the speed of light, should satisfy |v_ph/c - 1| ≈ E_ph/(M_QGc²). One can think of M_QG as the "mass" that the photon would have in the quantum gravity theory so that the photon energy is c² times the mass.

Given that notation, then if you have two photons that differ in energy by ΔE, the difference in arrival times should be Δt ≈ (|ΔE|/(M_QGc²))D/c, where D is the distance traveled. In this relationship, all quantities except for M_QG are directly measured, which implies a value of M_QG.

The most sensible unit in which to measure M_QG is in terms of the Planck mass, M_Planck ≅ 2.17644×10^-5 g, which is quite a lot for small things, being about the mass of 20 million bacteria.

Most quantum gravity theories predict M_QG ≤ M_Planck, so that the ratio M_QG/M_Planck ≤ 1. Surprisingly, however, measurements made of GRB 090510 imply that this ratio is actually no smaller than 1.2, and could be quite a bit larger – as much as 100 or so.

There's a great deal of uncertainty in the estimate of Δt, the difference in arrival times between the 31 GeV photon and lower energy photons that were emitted at the same instant. That's because there is no way to know how long after the start of the GRB event the 31 GeV photon was emitted. Since it was observed .829 seconds after the very first photons, a large part of that delay could actually be due to emission of the 31 GeV photon at any point up to .829 seconds after the start. That would make the actual Δt much smaller, and M_QG much larger.

Only the most conservative assumption, with the 31 GeV photon emitted as early as possible, gives M_QG/M_Planck ≈ 1.2. More realistic assumptions would make the ratio 100 or more.

In any case, all of these estimates "strongly disfavor" the simplest theories of quantum gravity, in the words of the research paper describing the observations. Otherwise said, spacetime at the smallest scale must apparently be much less bumpy than most theories predict.

Here's the research paper and abstract:

A limit on the variation of the speed of light arising from quantum gravity effects (11/19/09)

A cornerstone of Einstein's special relativity is Lorentz invariance—the postulate that all observers measure exactly the same speed of light in vacuum, independent of photon-energy. While special relativity assumes that there is no fundamental length-scale associated with such invariance, there is a fundamental scale (the Planck scale, l_Planck ≈ 1.6×10^-33 cm or E_Planck = M_Planckc² ≈ 1.22×10¹⁹ GeV), at which quantum effects are expected to strongly affect the nature of space–time. There is great interest in the (not yet validated) idea that Lorentz invariance might break near the Planck scale. A key test of such violation of Lorentz invariance is a possible variation of photon speed with energy. Even a tiny variation in photon speed, when accumulated over cosmological light-travel times, may be revealed by observing sharp features in gamma-ray burst (GRB) light-curves. Here we report the detection of emission up to ~31 GeV from the distant and short GRB 090510. We find no evidence for the violation of Lorentz invariance, and place a lower limit of 1.2E_Planck on the scale of a linear energy dependence (or an inverse wavelength dependence), subject to reasonable assumptions about the emission (equivalently we have an upper limit of l_Planck/1.2 on the length scale of the effect). Our results disfavour quantum-gravity theories in which the quantum nature of space–time on a very small scale linearly alters the speed of light.

Abdo, A., Ackermann, M., Ajello, M., Asano, K., Atwood, W., Axelsson, M., Baldini, L., Ballet, J., Barbiellini, G., Baring, M., Bastieri, D., Bechtol, K., Bellazzini, R., Berenji, B., Bhat, P., Bissaldi, E., Bloom, E., Bonamente, E., Bonnell, J., Borgland, A., Bouvier, A., Bregeon, J., Brez, A., Briggs, M., Brigida, M., Bruel, P., Burgess, J., Burnett, T., Caliandro, G., Cameron, R., Caraveo, P., Casandjian, J., Cecchi, C., Çelik, �., Chaplin, V., Charles, E., Cheung, C., Chiang, J., Ciprini, S., Claus, R., Cohen-Tanugi, J., Cominsky, L., Connaughton, V., Conrad, J., Cutini, S., Dermer, C., de Angelis, A., de Palma, F., Digel, S., Dingus, B., do Couto e Silva, E., Drell, P., Dubois, R., Dumora, D., Farnier, C., Favuzzi, C., Fegan, S., Finke, J., Fishman, G., Focke, W., Foschini, L., Fukazawa, Y., Funk, S., Fusco, P., Gargano, F., Gasparrini, D., Gehrels, N., Germani, S., Gibby, L., Giebels, B., Giglietto, N., Giordano, F., Glanzman, T., Godfrey, G., Granot, J., Greiner, J., Grenier, I., Grondin, M., Grove, J., Grupe, D., Guillemot, L., Guiriec, S., Hanabata, Y., Harding, A., Hayashida, M., Hays, E., Hoversten, E., Hughes, R., Jóhannesson, G., Johnson, A., Johnson, R., Johnson, W., Kamae, T., Katagiri, H., Kataoka, J., Kawai, N., Kerr, M., Kippen, R., Knödlseder, J., Kocevski, D., Kouveliotou, C., Kuehn, F., Kuss, M., Lande, J., Latronico, L., Lemoine-Goumard, M., Longo, F., Loparco, F., Lott, B., Lovellette, M., Lubrano, P., Madejski, G., Makeev, A., Mazziotta, M., McBreen, S., McEnery, J., McGlynn, S., Mészáros, P., Meurer, C., Michelson, P., Mitthumsiri, W., Mizuno, T., Moiseev, A., Monte, C., Monzani, M., Moretti, E., Morselli, A., Moskalenko, I., Murgia, S., Nakamori, T., Nolan, P., Norris, J., Nuss, E., Ohno, M., Ohsugi, T., Omodei, N., Orlando, E., Ormes, J., Ozaki, M., Paciesas, W., Paneque, D., Panetta, J., Parent, D., Pelassa, V., Pepe, M., Pesce-Rollins, M., Petrosian, V., Piron, F., Porter, T., Preece, R., Rainò, S., Ramirez-Ruiz, E., Rando, R., Razzano, M., Razzaque, S., Reimer, A., Reimer, O., Reposeur, T., Ritz, S., Rochester, L., Rodriguez, A., Roth, M., Ryde, F., Sadrozinski, H., Sanchez, D., Sander, A., Saz Parkinson, P., Scargle, J., Schalk, T., Sgrò, C., Siskind, E., Smith, D., Smith, P., Spandre, G., Spinelli, P., Stamatikos, M., Stecker, F., Strickman, M., Suson, D., Tajima, H., Takahashi, H., Takahashi, T., Tanaka, T., Thayer, J., Thayer, J., Thompson, D., Tibaldo, L., Toma, K., Torres, D., Tosti, G., Troja, E., Uchiyama, Y., Uehara, T., Usher, T., van der Horst, A., Vasileiou, V., Vilchez, N., Vitale, V., von Kienlin, A., Waite, A., Wang, P., Wilson-Hodge, C., Winer, B., Wood, K., Wu, X., Yamazaki, R., Ylinen, T., & Ziegler, M. (2009). A limit on the variation of the speed of light arising from quantum gravity effects Nature, 462 (7271), 331-334 DOI: 10.1038/nature08574

Further reading:

Gamma-ray Observations Shrink Known Grain Size Of Spacetime (10/28/09) – Science News

Fermi Telescope Caps First Year With Glimpse of Space-Time (10/28/09) – NASA/JPL

7.3 Billion Years Later, Einstein’s Theory Prevails (10/28/09) – New York Times

Gamma-ray burst restricts ways to beat Einstein’s relativity (10/28/09) – Symmetry Magazine

Special relativity passes key test (10/28/09) – Physics World

Gamma-ray photon race ends in dead heat; Einstein wins this round (10/28/09) – Physorg.com

An intergalactic race in space and time (10/28/09) – Nature

Astrophysics: Burst of support for relativity (11/19/09) – Nature

Space-Time Observations Find Einstein Still Rules (10/28/09) – Space.com

Quantum gravity theories wiped out by a gamma ray burst (10/28/09) – Ars Technica

A Gamma Ray Race Through the Fabric of Space-Time Proves Einstein Right (10/29/09) – Discover

Nature, NYT report the demise of Lorentz-violating theories (10/29/09) – The Reference Frame

Einstein Still Rules, Says Fermi Telescope Team (10/28/09) – Universe Today

Most Distant Known Object In The Universe

On April 23 of this year the Swift Gamma-Ray Burst Telescope detected, just as it was designed to do, a gamma-ray burst (GRB). Within less than a day two of the most powerful Earth-based telescopes had begun studying the quickly fading light of the object, known as GRB 090423. (There were other observatories investigating it as well.)

Because the light from GRBs fades so rapidly, most such objects are detected from space-based instruments that are especially designed for the purpose, like Swift. On average about 1 GRB is detected every 2 or 3 days. But GRB 090423 turned out to be, perhaps, the most interesting one yet observed.

I've written about GRBs a couple of times before, such as here and here.

You can refer to those articles for more details, but the most widely accepted hypothesis concerning the nature of GRBs is that they result from either the core collapse supernova explosion of a massive star or the merger of neutron stars in a binary system. Both processes most likely occur. In the former case, the emission of light lasts somewhat longer than in the latter, so the corresponding types of GRBs are called either "long" or "short".

The long type lasts for more than 2 seconds and tends to be brighter than the short type (whose emissions last less than 2 seconds). Since short GRBs typically appear in locations where new star formation is not occurring, they must result from some cause other than the explosion of a very massive star, which is necessarily quite young. In both cases, there is a prolonged, but much dimmer "afterglow", which is the only part of the event that can be observed except from satellites such as Swift.

Not just any supernova or neutron star collision will result in a detectable GRB. Most of the energy from the event must also be directed in two very narrow and oppositely directed beams, and the Earth must be in the path of one of those. It is because the energy is so narrowly concentrated that a GRB can appear to be far brighter than an entire galaxy or quasar. For such narrowly focused beams to be produced, the progenitor star (or binary system) must have a great deal of angular momentum, due to rapid rotation.

The afterglow is thought to result from the interaction between the matter and energy contained in the beam and the interstellar medium at the location of the GRB. Although it's a secondary effect, there is a wealth of information that can be deduced from this afterglow.

However, the information concerns more than just details about the nature of GRBs – events that occur much closer to us are much more useful for that. There are two other things of far greater interest about which we can learn from very distant GRBs, such as 090423 and subsequent examples we are likely to observe.

Since this event had a redshift of z ≈ 8.2 it actually occurred very long ago, a comparatively short time after the big bang. About 630 million years after, to be more exact. GRB 090423 is farther and earlier than any other object or event we've actually observed, except for the cosmic microwave background (CMB). It is more distant in time and space than even any galaxy or quasar we've ever seen. (See here if you want to review how redshift works.)

GRBs, directly or indirectly, tell us about the nature of the largest stars at that time, which is one thing that's presently very difficult to study with other observations. There's simply no way to observe individual stars 13 billion light-years away. The other thing astronomers are very curious about regarding that time period is the nature of the intergalactic medium (IGM) then.

Let's start with the second of these things. 630 million years after the big bang is somewhere in the middle of what astronomers call the "cosmic dark ages". Precisely where it falls is what we don't know. I wrote about that subject here, last March.

We do know pretty closely when the "dark ages" began – about 380 thousand years after the big bang. That's when the CMB appeared. The time is equivalent to a redshift of z ≈ 1100. The CMB was, and still is, "blackbody" radiation, so wavelengths are distributed around a peak. At the peak of the distribution now, CMB photons have a wavelength of 1.9 mm, in the short microwave part of the spectrum. (That's a lot shorter, so more energetic, than in your microwave oven, where microwaves are about 12 cm.) But when the CMB actually appeared, its photons had a wavelength 1100 times shorter, around 2200 nm – in the infrared part of the spectrum. That's still much cooler and "darker" than human eyes can see.

Effectively, then, there wasn't much light in the universe, certainly not human-visible light, until the first stars showed up. So the time after the CMB appeared but before the first stars is known as the "dark ages". However, we don't know very closely when those first stars appeared – they're far too faint to observe, and even the first galaxies and quasars are too dim to be detected by our present instruments. GRBs seem to be our best observational hope, and GRB 090423 is the earliest we've seen yet.

Since existing GRB models presuppose an origin that involves massive stars, at least indirectly, we now know there were such stars 630 million years after the big bang. There is evidence in the GRB 090423 observations that the progenitor of this object was not one of the first population of stars to form. (Astronomers call such stars "Population III".) Also, GRB 090423 doesn't seem to have been one of the most powerful GRBs ever, so astronomers now figure that with current technology we may be able to find GRBs back to z ≈ 20. That would correspond to 250 million years after the big bang.

Theoretical models suggest that the first stars may have appeared even earlier than that, at perhaps 150 million years after the big bang. When they formed, these first stars did not contain elements heavier than helium, as such elements were first created (in other than trace quantities) only in the earliest generation of stars. (See here for a discussion of the earliest stars.)

The observations of GRB 090423 give some evidence that the galaxy in which it occurred did have small amounts of elements heavier than helium. Further, the afterglow that occurs following the peak brightness of any GRB is generally similar between both GRB 090423 and much closer GRBs. These facts are hints that the progenitor of GRB wasn't one of the Population III stars.

How does a GRB give us information about the intergalactic medium at z ≈ 8.2? That turns out to be closely related to the way that the redshift was estimated in the first place. Normally this is done by identifying known emission or absorption lines in a spectrum and simply calculating the amount of shift directly. However, GRB 090423 is so faint, and declined in brightness so quickly, that it wasn't possible in the time available to obtain a detailed spectrum.

Instead, an important feature of the IGM at that time comes to the rescue. The feature is that there is at that time a substantial, though not precisely known, quantity of un-ionized hydrogen atoms in the IGM. This hydrogen is left over from the period of "recombination" in which the CMB appeared. CMB photons are far too weak to "scatter" from hydrogen atoms, because quantum mechanics requires that a photon must have a certain minimal amount of energy to raise an electron bound in a hydrogen atom out of its "ground state" of lowest energy.

The minimum amount of energy required is that possessed by a 10.2 eV photon of wavelength 121.6 nm, which is in the ultraviolet part of the spectrum. This spectral point is known as "Lyman-α". (We covered this concept in detail here.) Ordinary stars like the Sun in the universe at present emit little light at this wavelength, but much larger, hotter, and brighter stars emit quite a bit of this ultraviolet light.

Consequently, as soon as such hot stars began to shine, the atomic hydrogen in the IGM began to reionize. There was a price to be paid for this, of course: all the photons with enough energy lost some of their energy in the process of reionizing the hydrogen. And so, most of the light whose photons had enough energy was absorbed or "scattered" in this reionization period.

Eventually most of the hydrogen in the IGM of the early universe would become fully ionized, so that very energetic photons could again pass unimpeded. That stage marks the end of the reionization period. The whole period may have lasted from 150 million years to a 900 million years after the big bang, but we are quite unsure of the actual endpoints.

GRBs may be able to tell us. To begin with, it's easy to derive the redshift of a GRB that flared during the reionization period. If the redshift is z, then in the "rest frame" of the GRB, the light will start to be absorbed strongly at 121.6 nm. But we will observe this drop-off to occur at a wavelength of (z+1)×121.6 nm. For example, if z = 8.2, we see the drop-off at 1120 nm, which is in the infrared. Reasoning backwards, we can compute z from the observed drop off.

If we have enough GRB observations with sufficiently good spectra available, we can infer the amount of reionization that has occurred from the rate of drop-off. Further, if we find GRBs where there is no drop-off, we can infer that reionization is complete. Unfortunately, GRB 090423 tells us little by itself, and only two other GRBs have been observed with z > 6, corresponding to somewhat less than 950 million years after the big bang.

Why do we care about the rate of reionization? Because it tells us indirectly about the rate of formation of very large, hot stars in the early universe. So it turns out that the two things that astronomers want to know about the early universe – the nature of the IGM and the rates at which stars were forming – are pretty closely related. On top of that, the distribution in time of early GRBs gives us an independent estimate of the rate of star formation, since the progenitors of GRBs are exactly the sort of large, hot stars that cause reionization.

Tanvir, N., Fox, D., Levan, A., Berger, E., Wiersema, K., Fynbo, J., Cucchiara, A., Krühler, T., Gehrels, N., Bloom, J., Greiner, J., Evans, P., Rol, E., Olivares, F., Hjorth, J., Jakobsson, P., Farihi, J., Willingale, R., Starling, R., Cenko, S., Perley, D., Maund, J., Duke, J., Wijers, R., Adamson, A., Allan, A., Bremer, M., Burrows, D., Castro-Tirado, A., Cavanagh, B., de Ugarte Postigo, A., Dopita, M., Fatkhullin, T., Fruchter, A., Foley, R., Gorosabel, J., Kennea, J., Kerr, T., Klose, S., Krimm, H., Komarova, V., Kulkarni, S., Moskvitin, A., Mundell, C., Naylor, T., Page, K., Penprase, B., Perri, M., Podsiadlowski, P., Roth, K., Rutledge, R., Sakamoto, T., Schady, P., Schmidt, B., Soderberg, A., Sollerman, J., Stephens, A., Stratta, G., Ukwatta, T., Watson, D., Westra, E., Wold, T., & Wolf, C. (2009). A γ-ray burst at a redshift of z ≈ 8.2 Nature, 461 (7268), 1254-1257 DOI: 10.1038/nature08459

Salvaterra, R., Valle, M., Campana, S., Chincarini, G., Covino, S., D’Avanzo, P., Fernández-Soto, A., Guidorzi, C., Mannucci, F., Margutti, R., Thöne, C., Antonelli, L., Barthelmy, S., De Pasquale, M., D’Elia, V., Fiore, F., Fugazza, D., Hunt, L., Maiorano, E., Marinoni, S., Marshall, F., Molinari, E., Nousek, J., Pian, E., Racusin, J., Stella, L., Amati, L., Andreuzzi, G., Cusumano, G., Fenimore, E., Ferrero, P., Giommi, P., Guetta, D., Holland, S., Hurley, K., Israel, G., Mao, J., Markwardt, C., Masetti, N., Pagani, C., Palazzi, E., Palmer, D., Piranomonte, S., Tagliaferri, G., & Testa, V. (2009). GRB 090423 at a redshift of z ≈ 8.1 Nature, 461 (7268), 1258-1260 DOI: 10.1038/nature08445

Other blog posts

Beyond the Farthest Star (12/14/09)

Reports on the newly-published research on GRB 090423

Most distant gamma-ray burst spotted (10/28/09)

A Blast From the Deep, Dark Past (10/28/09)

Astronomers explore 'last blank space' on map of the Universe (10/28/09)

Blast from the Past Gives Clues About Early Universe (10/28/09)

Blast from the Very Far Past (10/28/09)

Astrophysics: Most distant cosmic blast seen (10/29/09)

A big gamma-ray burst at a redshift of z ≈ 8.2 (10/29/09)

GRB 090423 at a redshift of z ≈ 8.1 (10/29/09)

Discovery of Radio Afterglow from the Most Distant Cosmic Explosion


Reports on the initial observation of GRB 090423

Scrambling to Read the Meaning Of the Sky's Most Ancient Flare (9/18/09)

Earliest astrophysical object yet seen (7/2/09)

Most Distant Known Object In The Universe / Science News (4/28/09)

The Farthest Thing Ever Seen (4/28/09)

Most distant object in the universe spotted (4/27/09)

The Most Distant Object Yet Discovered in the Universe (4/28/09)

Farthest Known Object: New Gamma-Ray Burst Smashes Cosmic Distance Record (4/28/09)

New Gamma-Ray Burst Smashes Cosmic Distance Record (4/28/09)

Ancient gamma-ray burst is most distant object ever seen (4/29/09)

Space Explosion Is Farthest Thing Ever Seen (4/28/09)

Telescope snaps most distant object (4/28/09)

Exploding star is oldest object seen in universe (4/29/09)

Cosmic blast sets distance mark (4/28/09)

Astronomers see oldest object in universe yet (4/28/09)

A glimpse of the end of the dark ages: the gamma-ray burst of 23 April 2009 at redshift 8.3

Tags: gamma-ray bursts, early universe

GLAST and gamma-ray astronomy

According to the present schedule, the GLAST gamma-ray telescope mission will be launched next week, on June 5, around noon EDT. If you're reading this before then, you can expect to find quite a bit of news coverage around that date. It's actually kind of a big deal, and I'll summarize some of the reasons for that here.

To begin with, you can find background information on the mission from NASA here and here.

In addition, there have been some good summaries already in science-oriented publications:

• Question and Answer with GLAST Scientists – good Q&A session available at the Sky and Telescope site
  
• GLAST goes for blast-off – very good 5/1/08 feature article provided by Physics World
  
• GLAST Mission Prepares to Explore the Extremes of Cosmic Violence – another very good article from the 5/23/08 issue of Science (sub. rqd.)
  

You can find good explanations of exactly what GLAST is in most of the above references.

What I'll summarize here are just some of the main objects and phenomena that GLAST is expected to help observe and study.

Gamma-ray bursts

Gamma-ray bursts (GRBs) have been discussed here several times, such as here and here. It is generally agreed that there are several different events that can cause a GRB, and a fair amount is known about the phenomenon already. For instance, the subtype known as a "long" GRB is thought to result from supernova explosions in which a high-energy jet of particles and radiation is emitted in a narrow beam that happens to point in our direction. But as yet we haven't measured the complete spectrum of energy from a GRB of any type. This spectrum can range from a few KeV to hundreds of GeV, and knowing it in detail would help determine the nature of the associated event much more accurately.

Dark matter

The visible universe that consists of luminous objects like stars and galaxies is composed of baryonic matter (mainly protons and helium nuclei). There may be at least as much baryonic matter in the form of diffuse gas that we cannot see. (Recent observations here and here.)

Yet it is essentially certain that the universe actually contains about four times as much matter that we can't detect at all (except by its gravitational effects) as all that baryonic matter put together. This is the dark matter. There are many theories about what this dark matter consists of, but in one of the main classes of theories, the matter consists of "weakly-interacting massive particles" (WIMPs). In most such theories, WIMPs can annihilate each other in pairs, giving off copious quantities of gamma-rays (among other things).

If some such theory accounts for a portion of the dark matter, GLAST will make it possible to estimate properties of WIMPs (e. g. their mass) by observing gamma-rays from locations where dark matter is expected to be concentrated, such as in the center of the Milky Way. This kind of information will complement and help corroborate observations made at the Large Hadron Collider, in which some kinds of WIMPs (if they exist at all) are expected to be created.

Solar gamma-rays

Although our Sun is a relatively weak source of gamma-rays compared to almost everything else mentioned here (even weaker than the Moon, where gamma-rays are produced when cosmic rays strike the surface), several solar events, such as flares and coronal mass ejections do produce gamma-rays. So GLAST will help us better understand solar events of this kind.

Supernova remnants

A large class of gamma-ray bursts are associated with the initial blast of a supernova event, and the gamma-rays from such bursts subside in a matter of minutes. But other gamma-rays may originate by other mechanisms from the supernova remnant long after the original event. Gamma-rays are thought to be produced in such remnants due to particles being accelerated to high energies in the blast and subsequently generating shock waves in the interstellar medium. The shock waves themselves are reasonably well understood, but how the particles are actually accelerated by the supernova blast needs much more elucidation, which GLAST can provide.

Pulsars

Another part of the remnant left over after a supernova is either a stellar-mass black hole, or else a rapidly spinning neutron star. Such a neutron star will produce jets of electromagnetic energy, usually at radio frequencies, and when the Earth is lined up with the jet the object is called a pulsar. These also emit gamma-rays. Since neutron stars are extremely small and dense, they have intense magnetic fields near their surface, and the fields reveal a lot about the nature of matter in the neutron star. The strong fields also convert gamma-rays into electron-positron pairs, so the overall gamma-ray spectrum can give us information about the object's magnetic fields, and about surface features that cause gamma-ray emission.

Supermassive black holes, active galactic nuclei, quasars, blazars

Supermassive black holes are thought to exist at the centers of most or all galaxies. We can estimate that they have masses ranging from 10⁵ to 10¹⁰ solar masses, yet there is a great deal more we would like to know about them, such as the process by which they form. (See here.) Most supermassive black holes are thought to be circled by an accretion disk of matter which has been attracted by the object's extreme gravity.

Depending on the amount of matter in the disk, large amounts of energy may be released as the matter falls into the black hole. Our own Milky Way has a smallish object of this sort, with a correspondingly small accretion disk. But if there is much more mass in the disk, one has a bright object called an active galactic nucleus (AGN). AGNs were more plentiful in the early days of the universe, before most of the available nearby matter had been consumed by the black hole, and especially active objects of this kind, usually at great distances, are called quasars.

Like supernovae (from which stellar-mass black holes or neutron stars are formed), supermassive black holes may emit powerful relativistic jets of particles and energy. If such a jet is pointed in our direction, we see an especially bright source, called a blazar. Most of the emitted electromagnetic energy from all these objects is in the gamma-ray part of the spectrum.

So one of the main objectives of GLAST is to measure how this spectrum varies over time, in order to get a better understanding of what is actually going on. For instance, there could be additional confirmation of a model of the relativistic jets as described in this recent research, and perhaps evidence as to whether the particles in the jets are protons or electrons.

Cosmic ray origins

As discussed in detail here, we are finally beginning to clear away some of the mystery surrounding the most energetic ultra-high-energy cosmic rays (UHECRs). But there's a lot more we'd like to know, such as whether these rays are mostly made up of relativistic protons, and what sort of process creates them in the first place. Gamma-rays are produced when UHECRs interact with interstellar gas and dust, so GLAST may be able to give us more information about UHECRs.

Primordial black holes

It is generally suspected that many small black holes (with masses covering a wide range, but much less than the mass of a star) could have been produced in the big bang. These are called primordial black holes. Their existence hasn't yet been confirmed. But Stephen Hawking made a strong case that any black hole will slowly emit weak electromagnetic radiation, due to quantum effects and called Hawking radiation. This radiation should be too weak to be directly observable. However, small primordial black holes should eventually evaporate completely by this process, and at the end disintegrate in a burst of gamma-rays. This is all rather conjectural, but if it happens, it may contribute to a continuous gamma-ray background that can be detected.

Cosmic gamma-ray background

In addition to discrete gamma-ray sources such as GRBs and supernova remnants, there is a diffuse background of gamma-ray photons, much like the cosmic microwave background (CMB), only vastly more energetic. Some of this background may be due to UHECRs, very distant and very powerful (TeV range) gamma-ray sources, or primordial black holes. But who knows what other kinds of sources might be out there? There will probably be some surprises, as well as a lot of useful information to be deduced, just as happened with the CMB.

Possible breakdowns of special relativity

Heading even further into speculative territory, various theorists of quantum gravity have proposed that even special relativity (as well as general relativity too) may break down under various conditions. For example, the speed of light might not be an exact constant, but might instead vary by a slight amount according to the energy carried by individual photons.

Thus not all gamma-ray photons from a GRB would need arrive at precisely the same time, and so any pattern in this radiation would be shifted very slightly depending on what part of the gamma-ray spectrum is observed. Even if the shift is as little as 1/1000 of a second (for photons that may have been travelling for billions of years), GLAST should be sensitive enough to detect the shift. That would certainly be quite a surprise if found.

Further reading:

GLAST Science Writer's Guide – An extremely informative 47-page document (PDF), with detailed descriptions of the relevant science, a glossary, and additional links

Simona Murgia: Dark Matter searches with GLAST – Blog posting that discusses the relevance of GLAST for dark matter searches

Tags: GLAST, Gamma-ray Large Area Space Telescope, gamma-ray bursts, supermassive black holes, quasars, blazars, pulsars, primordial black holes

Gamma-ray burst surprises in 2006

Gamma-ray bursts (GRBs) have been puzzling astrophysicists for a long time. A little over a year ago, the picture seemed to be getting clearer. But during the past year, it seems to have become rather more complicated. And the year saved the best surprise for last. This spurt of activity is largely due to a flood of data, obtained with the help of the Swift satellite, launched in November 2004 specifically to study GRBs. (See also here.)

GRBs first attracted attention because they were unlike any other highly energetic phenomenon known in the universe. A GRB includes a flux of gamma-rays, which are, by definition, the highest energy photons. But unlike other gamma-ray sources, such as occur when matter falls into a black hole, the gamma-ray flux persists only a short time – only a few minutes at most. Gamma-ray emissions associated with black holes persist indefinitely.

At first, it was not even known whether GRBs normally came from something in our own galaxy or something perhaps much farther away. After it was determined around 1992 that GRBs were distributed evenly across the whole sky, it was almost certain that most GRBs were not associated with our galaxy, since in that case they would be mostly located in the direction of the visible Milky Way.

After many more examples had been observed, it was apparent that GRBs were of at least two different types: short duration events ("short GRBs") lasting less than 2 seconds, and longer events ("long GRBs"), lasting from 2 seconds to a few minutes. When it became possible to obtain spectra from the galaxies associated with GRBs, and hence possible to determine approximate distances by means of the redshift, astrophysicists were surprised to find that long GRBs were usually extremely distant – more than 8 billion light-years. This implied that whatever caused the GRB had to be almost incredibly energetic, so that they were visible across so many billion light-years and their photons remained in the gamma-ray energy range, despite being red-shifted by a factor of 2 or more. (That is, the photon wavelength as observed was more than twice its length when it was emitted.) Short-duration GRBs, on the other hand, were found to be somewhat closer, and therefore their source must be 10 or 100 times less energetic.

It was easier to understand long GRBs, since almost the only explanation that could work was for the GRB to be produced in a supernova explosion in which most of the energy was concentrated into jets parallel to the axis around which the supernova's progenitor star rotated. It occurs because material ejected from the dying star just before its collapse forms an accretion disk around the newborn black hole. This material is subsequently sucked back into the black hole and produces particle jets and the blast of gamma-rays. The emitted photons have such high energy since most of the supernova energy is concentrated so narrowly. Eventually, accumulating observations confirmed that the characteristics of the light output in a long GRB was what would be expected in a very energetic supernova event. Such a supernova must result from the death of a very massive stars (more than 40 solar masses), and is sometimes called a "hypernova". This model is sometimes known as the "collapsar" model.

Short GRBs were harder to figure out, but eventually it appeared that they could be explained as the result of a collision between two neutron stars. The amount of energy emitted and the duration of the event appeared to be just about right.

I wrote here about these conclusions a little over a year ago. But the ink was hardly dry (so to speak) on that post before new findings emerged that suggested some short GRBs weren't fully with the program.

Two papers were published in December 2005, suggesting that not all short GRBs had the same origin:

Breakthrough in puzzle of giant explosions in space

The Hertfordshire team’s new result adds a further, unexpected twist to the tale: a significant proportion of short bursts seem to originate from galaxies much more local to us than those previously observed. These nearby short bursts, could, like their more distant brethren, result from the catastrophic collision of neutron stars, though if so then their outbursts must be much weaker. Alternatively they could be a fundamentally different kind of explosion. A prime candidate could be an exotic object called a magnetar — a lone neutron star with a magnetic field a hundred thousand billion times that of the Earth - tearing itself apart due to enormous magnetic stresses.

A second paper published at the same time was more specific about what might cause short GRBs that are more nearby, and hence less energetic. It leaned towards an explanation involving the merger of a neutron star and a black hole, rather than a magnetar disintegrating from magnetic stresses:

Witnessing The Flash From A Black Hole's Cannibal Act

An international team of astronomers reports the discovery of a third short gamma-ray burst, associated with a nearby elliptical galaxy. The low level of star formation in such galaxies and the detection of a second long-lasting flare indicate that this gamma-ray burst is most likely the final scream of a neutron star as it is being devoured by a black hole.

This paper was based on measurements of a GRB observed on July 24, 2005 (hence designated GRB 050724), as well as an earlier one (GRB 050509B). The first of these was located in a galaxy "only" about 3 billion light-years away. This showed that short GRBs might result from the release of 100 to 1000 times less energy than a typical long GRB. GRB 050724 had a longer "afterglow" than would be expected from a merger of neutron stars (which would collapse almost instantly to a black hole). But the afterglow would be consistent with the merger of a neurton star and a black hole, where the process begins with the neutron star being rent asunder, followed by the pieces falling into the black hole over a longer period of time.

Other accounts of these results can be found here, here, here, and here.

To summarize, as of December 2005, the most common type of short GRB was figured to be the result of a merger between two neutron stars, while atypical short GRBs could be either magnetars or the merger of a neurton star with a black hole. But new examples kept showing up.

In February 2006 a computer study showed that about 1% of short GRBs due to neutron star mergers should occur in globular clusters, which are tightly packed with stars, and so the chances of encounter are high. More normally, neutron star mergers should occur between stars that a part of a single binary system. But in fact from 10 to 30% of observed short GRBs occur in globular clusters, far more than would be expected. It was hypothesized that in the latter case, energy output would be less tightly beamed, and hence more likely to be observed. More details are here.

Just a little later, on February 18, a very unusual GRB was observed as part of a supernova event. Named GRB 060218, it was much longer than typical long GRBs – 33 minutes in duration. It was also relatively quite close (440 million light-years) and so much less energetic (by a factor between 10 and 100) than typical long GRBs. Remember this one – its importance will be described later. Details: here, here, here, here, here, here, and here.

In March three papers in Nature announced that observations from a number of ground-based and space-based instruments had confirmed that GRB 050904, which was first seen in September 2005, was the most distant GRB ever seen. Its redshift was measured to be 6.3, making it about 12.8 billion light-years away, and occurring when the universe was only about 900 million years old. The earliest previous GRB to be observed was dated to about 1.4 billion years after the big bang. The characteristics of GRB 050904 were typical of long GRBs, so the results show that such an event was possible at that early date. Details: here, here, here, here.

Starting later in March, additional doubts were expressed that short GRBs had a single, simple explanation in terms of merging neutron stars. First, analysis of short GRBs occurring on July 9 and July 24, 2005 showed X-ray flares minutes after the initial burst. Then a short burst on December 21, 2005 appeared to have the total energy of a typical long GRB – 10 times as much as the most energetic short GRBs known. Several models have been suggested for these and other anomalous bursts. And this is in addition to models involving magnetars, applicable to perhaps 10% of short GRBs. Here's a good summary of the situation: Cosmic Explosion Mystery Deepens.

In May, further analysis of the July 24 event showed that it radiated its energy in all directions. However, the December 21 event appeared to radiate its energy in narrow jets with opening angles between 4° and 8°. Because the energy was narrowly focused, the GRB appeard to be very bright, but the total energy was not as high as if it had radiated in all directions. Thus this event actually had an energy in the normal range for short GRBs. However evidence of jets from other short GRBs is sketchy. And it is difficult to explain jets in a neutron star merger model. The short GRB situation is looking rather messy. Reference: High-energy jets spew from short gamma-ray bursts.

As if all that were not enough, later in May an analysis of a short GRB that occurred on January 21 showed that the event was from 10.1 billion light years to 12.7 billions light years away – where it would be as distant as the long GRB of September 4, 2005. All previously measured short GRBs were no farther than 6.5 billion light years. Consequently, GRB 060121 might be as energetic as the most powerful long GRBs. This might mean the energy of the burst was concentrated in very narrow jets, so it was not as energetic as it appeared – as with GRB 051221. Alternatively, the characteristics of neutron stars in the early universe may have been different than more recent ones, though this seems like a stretch. Or perhaps some entirely different model, involving neither neutron stars nor supernovae is needed. Reference: Distant gamma-ray burst may be in class of its own.

Late in August, 4 papers appeared in Nature that gave detailed analysis of GRB 060218 – the one of very long duration (33 minutes) but low energy. The event has been put in a new class called an X-ray flash. It appears to have a jet structure and result from a Type Ic supernova, which involves the least heavy type of star (about 20 solar masses) that can go supernova when its hydrogen and helium supply is used up. Instead of leaving behind a black hole, its remnant may be a magnetar. (Unlike short GRBs possibly resulting from magnetars that self-destruct from their own magnetic fields, this GRB was produced in the supernova event.) References: here, here, here, here, here, here.

The news flow then went quiet for a few months. And then yet another surprising twist showed up. This involved observations of two more long but nearby and low-energy GRBs. Because they were of the long type, they (presumabley) did not involve neutron stars or magnetars. But these two seemingly did not involve supernovae either, unlike GRB 060218. The events occurred on May 5 and June 14 of this year. The findings were published in the December 21 issue of Nature.

GRB 060614 lasted 102 seconds and occurred at a distance of 1.6 billion light-years. (Nowadays that's considered relatively nearby.) In a long GRB due to a supernova, there is a rebrightening that lasts for days after the initial flare. This is the primary source of light that makes the supernova visible. It comes from the gravitational energy of collapse and the energy of fusion reactions which occur. If anything like that happened in these two cases, it must have been at least 100 times fainter than normal. GRB 060505 lasted only 4 seconds, which is still longer than a short GRB (under 1 second), and was somewhat more than 1 billion light-years distant.

Not only was there no evidence in GRB 060614 of the light normally seen following a supernova explosion, but it occurred in a galaxy with few young stars – the only kind that can go supernova, because they must be massive and short-lived. The problem is that it's very hard to understand the sustained emission of gamma-rays for 102 seconds except in a supernova event. It was also more energetic than the normal short GRB involving neutron stars. There are various speculations about what may have happened in these peculiar GRBs, but as yet no tenable models. It could have been a type of supernova collapse which produced little or no light. Or a merger involving neutron stars that continued to produce gamma-rays for an extended time, perhaps in a system of more than two neutron stars and black holes (a more complex version of GRB 050724). Or perhaps something else entirely. Apart from something exotic, one suggestion is that the distance estimate for GRB 060614 is off, and there was a visible supernova, but it was too far away to be visible.

This mystery will probably spawn various hypothetical models in the next year or two, and may lead to a better understanding of supernovae.

References: here, here, here, here, here, here, here, here, here.

If you have a subscription to Science here's a pretty good summary of the situation: Burst-Hunter's Rich Data Harvest Yields a Cosmic Enigma

Tags: gamma ray burst, supernova, neutron star, magnetar, astrophysics