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)

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