Where the action is in black hole jets

The object known simply as 3C 279 is rather distinctive for several reasons, in spite of the rather unassuming name. For one thing it's an active galaxy – that is, it has a supermassive black hole at its center, and that black hole is sucking in surrounding matter at a rate high enough to generate as much energy as all stars the in the galaxy where it resides combined. Only about 1% of visible galaxies are active galaxies like 3C 279.

But that's not all. 3C 279 is also a radio galaxy, a subset of only about 10% of active galaxies that also feature strong radio-frequency emissions. Such strong emissions are generally thought to be produced by a violent outflow of matter from the vicinity of the black hole in the form of narrow jets. The flow is so violent that matter in the jets reaches velocities close to the velocity of light.

And if that's not enough, one of the jets of 3C 279 is pointed almost straight at us. Only a few percent of active radio galaxies are oriented that way, by chance. Because we're looking essentially straight into the most active part of the object, with basically no dust or gas to obscure the view, 3C 279 appears especially luminous – the term for such an object is "blazar".

Although its jet is aimed right at us, there's nothing to be particularly concerned about, since 3C 279 has a redshift of z=0.536, which means it's actually about 6.5 billion light years away.



3C 279

I just wrote at some length about active galaxies, here, in some detail, so you might like to review that if you need to refresh your memory on many basics of the subject. There may be some aspects of the present discussion that will make more sense in light of that.

Even though 3C 279 came to the attention of astronomers over 40 years ago, because of its unusual apparent brightness and radio emissions, it is not an especially powerful active galaxy, as those things go. The central black hole is estimated to have a mass around 6×10⁸ M_⊙, somewhat short of 10⁹ M_⊙ that is typical of the largest quasars.

The peak velocity of matter in the jet of 3C 279 has been inferred to be about 99.8% of the speed of light, which is "relativistic" by anyone's definition. In other words, this velocity is v=0.998c. It's customary to express this velocity as β = v/c = 0.998. The inference is based on apparent (but not real) "superluminal" (faster than light) motion of jet-related material. This is a common phenomenon seen in active galaxy jets that are nearly parallel to our line of sight. A related quantity, the Lorentz factor, is defined as γ =  (1-β²)^-1/2, so in this case γ ≈ 16. That'll play a role in an important calculation later.

Since astronomers have been interested in 3C 279 for over 40 years, it's been studied a lot, although that's been difficult, because of its rather large distance. Radio galaxies like this produce electromagnetic emissions all the way from radio frequencies on up to gamma rays – spanning 11 or 12 orders of magnitude in photon energy, from under .001 eV to over 100 Mev. Many of those frequency ranges can be observed only from instruments in space, so until recently it hasn't been possible to observe a single object continuously for long periods of time in many bands. This has now been done for the blazar 3C 279 – and perhaps by chance something rather interesting showed up, which could only have been observed in an active galaxy whose jet is nearly parallel to our line of sight.

A change in the optical polarization associated with a γ-ray flare in the blazar 3C 279

It is widely accepted that strong and variable radiation detected over all accessible energy bands in a number of active galaxies arises from a relativistic, Doppler-boosted jet pointing close to our line of sight. The size of the emitting zone and the location of this region relative to the central supermassive black hole are, however, poorly known, with estimates ranging from light-hours to a light-year or more. Here we report the coincidence of a gamma (γ)-ray flare with a dramatic change of optical polarization angle. This provides evidence for co-spatiality of optical and γ-ray emission regions and indicates a highly ordered jet magnetic field. The results also require a non-axisymmetric structure of the emission zone, implying a curved trajectory for the emitting material within the jet, with the dissipation region located at a considerable distance from the black hole, at about 10⁵ gravitational radii.

The main thing that this paper reports is an "event", evidently some sort of disturbance affecting the jet, manifested in the spectrum of 3C 329. The event was most pronounced in the γ-ray part of the spectrum, which – in this object – is the dominant and also the most variable part. The γ-ray flux of 3C 279 can vary over an order of magnitude, and at one point – the beginning of the event – the flux increased rapidly from an already elevated level to its maximum value, then dropped a little more slowly, over a span of 20 days, to its minimum.

Other parts of the spectrum were also affected, but not so dramatically. Flux in ultraviolet, optical, and near infrared bands also decreased from somewhat elevated levels during the same 20 days, though there was no spike up at the start. There was, however, little change in the X-ray and radio bands during this period.

There was one additional dramatic change in the same period. The percentage of polarization in optical emissions (blue) dropped from 30-40% down to 10% before recovering at the end of the period. And at the same time, the direction of polarization changed smoothly over the 20 days by about 180°.

Since our line of sight is nearly parallel (to within about 2°) to the jet, it is difficult to distinguish where in 3C 279 different emissions originate. Strong γ-ray emissions are typically associated with jets (when present), but a key question that the paper examines concerns what part of the jet the dramatic changes in γ-ray flux could have been associated with. Was it relatively close to the black hole, or much farther out?

It's not too surprising that there was little change in X-ray flux, since that's normally associated in active galaxies with the "corona", which is symmetrically distributed in a region with a radius of a few hundred light years around the black hole. Radio emissions, however, do generally originate from the jets, but often from "lobes" at very large distances from the center. In this case it would seem that the source of radio emissions had little to do with the "event".

On the other hand, since distinct changes in ultraviolet, optical, and infrared flux – as well as the dramatic change in polarization – occurred at exactly the same time as the γ-ray "event", it's natural to suppose that whatever caused the disturbance affected a part of the jet where these emissions originated.

So what can be said about the size and (perhaps) location of the disturbance? The key fact is that the event was observed to last 20 days. However, since we're dealing with matter moving at a relativistic velocity, it doesn't at all follow that the disturbance affected only a portion of the jet about 20 light days in extent. It's not hard to calculate the "actual" size of the disturbance, but it does take a little work.

Suppose we let r₀ be the distance along the jet from the central black hole at which the disturbance began, and r₁ be the distance at which the state of the jet has returned to "normal". Then r₁ >r₀. The distance d = r₁ - r₀ is what we want to compute. If v is the average velocity of matter in the jet when the event occurred, then we have already noted v = 0.998c, so that β =v/c = 0.998.

Note that d is the distance in our reference frame. The time (in our frame) it takes light to travel that distance is d/c. If we assume that the matter within the jet that's subject to the disturbance is moving with velocity v, then the time it takes the leading edge of that matter to go the same distance is d/v. Since c > v, d/v > d/c, so by the time the leading edge reaches r₁ it is lagging behind the corresponding photons by a time interval Δt = d/v-d/c > 0. Since we're aiming only for an approximation, assume for simplicity that the spatial extent of the disturbance (from leading to trailing edge) is small compared to d. Then the photons from the time the disturbance began at r₀ will reach us by the same interval Δt ahead of the photons from the time the disturbance ended, when the affected matter was at r₁.

So the data we have to work with are just β and the observed elapsed time between the start and end of the event: Δt ≈ 20 days. We'll get an expression for d in terms of Δt.

We have Δt = d/v-d/c = (d/c)(c-v)/v = (d/c)(1-β)/β. Multiplying that by (1+β)/(1+β) gives Δt = (d/c)(1-β²)/[β(1+β)]. Using 1-β² = 1/γ² and solving for d gives d = γ²cΔtβ(1+β). But in this example, β≈1, so d ≈ 2γ²cΔt.

Plugging in actual numbers, Δt≈1.7×10⁶ seconds, c≈3×10⁸ m/sec, and γ²≈256 gives d≈2.6×10¹⁷ m. A light year is about 9.5×10¹⁵ m, so d is about 27 light years. That's quite an extensive part of 3C 279's jet that is affected by the disturbance.

Another way to appreciate the size of that number is to compare it to the Schwarzschild radius of the black hole, r_s=2GM_BH/c². M_BH is roughly 6×10⁸ M_⊙, so with G=6.67×10^-11 m³ kg^-1 sec^-2 and M_⊙=1.99×10³⁰ kg, we find r_s≈1.8×10¹² m. Thus the size of the disturbance is more than 100 thousand times the black hole's Schwarzschild radius – 5 orders of magnitude.

So, now that we have some idea of the impressive extent of this "disturbance", is it possible to draw any conclusions about what caused it?

To begin with, keep in mind that we have assumed the matter that is disturbed is propagating along the jet with velocity v=0.998c. That assumption is the reason the estimate of d is so large, because of the factor γ². If v≪c, then β≈0 and γ≈1, so that d ≈ cΔt ≈ 5×10¹⁴ m – about a factor of 500 smaller. In this latter case the disturbance affects the jet only in a small zone at a distance of r₀ from the black hole, and the matter "flows through" this zone without much long-lasting effect. This could happen, for example, if there is a narrow knot in the magnetic fields that keep the jet constricted.

The evidence that this latter possibility is not in fact what's happening is that the polarization of light turns around by 180° in almost perfect synchrony with the event in which γ-ray flux has a large bump. This implies that there's a large-scale bend in the jet at that point, so that the direction of the jet crosses over our line of sight. This apparent change of direction persists far longer than the event itself.

Of course, that hypothesis still doesn't explain either the γ-ray flare or the change of direction itself. It is possible, for instance, that the jet encounters, at an oblique angle, some large concentration of matter that deflects the jet. Perhaps the jet passes very close to another black hole. We simply don't know.

There was no guarantee that we could quickly learn how to explain all the behavior of black hole jets with ease – so the observational effort must continue, with increasingly sensitive equipment and larger data sets.

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Further reading:

Extreme Jets Take New Shape (2/17/10)

Fermi pins down a colossal accelerator (2/18/10)

Astrophysics: Cosmic jet engines (2/18/10)


Related articles:

Active galaxies and supermassive black hole jets (4/25/10)

Winds of Change: How Black Holes May Shape Galaxies (4/19/10)

Galactic black holes may be more massive than thought (6/8/09)

Black hole outflows from Centaurus A (2/6/09)

Evidence that quasars are powered by black holes (10/21/06)

The wind from a black hole (7/8/06)

Active galaxies and supermassive black hole jets

Most galaxies have a supermassive black hole in their center – sometimes even more than one. These black holes can have masses up to ten billion solar masses (10¹⁰ M_⊙) or more. One of the largest known examples is part of a binary system, and it weighs in at 1.8×10¹⁰ M_⊙ – see here, here, or here. (There are exceptions, such as the nearby M33, which apparently does not have a central black hole of mass more than 3000 M_⊙.)

All black holes gravitationally attract any nearby matter, because of their high mass, which is generally ≥ 1 M_⊙ even for comparatively tiny stellar mass black holes. Such matter does not necessarily fall directly into the black hole, but instead can go into orbit around the black hole. If there is enough matter close to the black hole, and if it is pulled in rapidly enough, the results can be a spectacular light show, such as one might see (if one could see simultaneously at all wavelengths from very high radio frequencies to X-rays) in the Centaurus A galaxy, about 13 million light years away:



This is a view of the whole galaxy – you can see that the central area, which contains the black hole, is unusually bright, and there are jets extending more than the radius of the galaxy itself in both directions perpendicular to the galactic plane. Centaurus A is an example of an "active galaxy", and it shows the impressive effects produced by the central black hole of such an object. (For more about Centaurus A and this image, see here, here. Another image: here.)

The innermost region of an active galaxy, which is the interesting part, is called an "active galactic nucleus" (AGN). This is a general term for a number of puzzling astronomical objects that were noticed at first on account of their unusually vigorous output of energy, but whose similarities were not immediately recognized. AGNs were eventually deduced to be (in almost all cases) just relatively ordinary galaxies with massive central black holes that appear to be responsible for liberating at least as much energy as all the stars in the remainder of the galaxy.

Although most galaxies seem to have a supermassive black hole in their center, behavior of AGNs is rather unusual, and AGNs are somewhat rare in the nearby universe, but more common at large distances – hence at an earlier time in the universe. AGN behavior requires not only a supermassive black hole, but also a substantial amount of surrounding interstellar gas that fuels their energy output. AGNs are rather profligate in using their fuel, so presumably most of the available interstellar gas close to a central black hole is consumed over a relatively short period of time (compared to the age of the universe). Consequently, in most nearby galaxies the fuel was used up long ago.

The Milky Way has a relatively small central black hole associated with the radio source named Sagittarius A* (Sgr A*). The black hole has a mass of ~4×10⁶ M_⊙. Sgr A* is not nearly luminous enough to be considered an AGN, so evidently either there is not now enough nearby interstellar gas, or perhaps the black hole is just too small to have ever attracted enough.

How luminous does a galactic nucleus need to be in order to qualify as an AGN? It's really a question of how bright the nucleus is compared to the rest of the galaxy. Messier 77, also known as NGC 1068, is the first galaxy now considered to have an AGN that came to special attention. In 1908 E. A. Fath obtained its spectrum and found it had unusually strong emission lines. (This was at a time when it was still assumed that nebulae were simply fuzzy objects inside our own galaxy.) V. N. Slipher later obtained a better spectrum and noted that the width of the lines implied high velocities – hundreds of kilometers per second. NGC 1068 is fairly nearby – 47 million light years away – and rather large, with a diameter of 170,000 light years (compared to the Milky Way's diameter of 100,000 light years). NGC 1068 is still under active study at this time – see here.

Finally in 1943 Carl Seyfert recognized that NGC 1068 was similar to a number of other galaxies that formed a distinct class, based on the nature of their spectra and because their innermost regions were as bright as the entire rest of the galaxy. This concentration of luminosity in the center was not only exceptional, but it was quite unlikely to be physically possible for a sufficient number of stars to be located in such a small volume of space. Naturally, galaxies of this sort became known as Seyfert galaxies.

Other peculiarities of Seyfert galaxies were eventually recognized as well. For example, their spectra contain broad, strong emission lines of hydrogen, helium, nitrogen, and oxygen. This in turn implied that the emitting material had to be in rapid motion in order to produce Doppler broadening of the emission lines. And this in turn implied that a large amount of mass needed to be concentrated in a small volume to account for such high velocities. The characteristics of high central luminosity and broad emission lines tended to occur together enough to justify recognizing Seyfert galaxies as a distinct class, which made up about 1% of nearby spiral galaxies.

About 15 years after Seyfert galaxies were discovered, another peculiar type of astrophysical object was noticed – quasars, or, as they were sometimes known, "quasi-stellar-objects". These came first to attention as strong sources of radio emission, in early radio telescope surveys, such as the original Third Cambridge Catalogue of Radio Sources. The strong radio signal was somewhat mysterious, since electromagnetic radiation at radio frequencies (up to 100 GHz at the high end) is normally emitted only by rather cold matter (under about 2 degrees above absolute zero).

Many of these sources ("radio galaxies") were eventually identified with optically visible objects, many of which had already been cataloged as unremarkable stars. But when they came under attention as radio sources and their optical spectra were obtained, their high redshift implied that they needed to have an astonishingly high intrinsic luminosity – too luminous to be explainable simply as very large galaxies containing even the brightest possible stars.

Phenomena that are seemingly impossible to explain in terms of familiar examples tend to draw a lot of attention – and this is true, in some respects, of quasars even today. A lot of the mystery is now explainable in terms of active galaxies that are powered by supermassive black holes, and we'll get more into that in a little bit. But there are also some AGN features, such as the jets that a few AGNs expel at relativistic velocities, which are still not well understood.

Although there was still controversy at the time whether redshift could be used as a reliable gauge of distance, if the conventional redshift interpretation (Hubble expansion) was assumed, then quasars would need to be (what was considered at the time, ca. 1960) extremely distant. One of the earliest-recognized quasars, 3C 273, which is the quasar with the largest apparent magnitude and is visible through amateur telescopes, has z=0.158, corresponding to an optical distance of ~2.4 billion light years. 3C 273 was therefore intrinsically far brighter than any star, about 100 times as bright as an entire spiral galaxy. 3C 273 is sometimes considered to be the nearest unambiguous quasar. So bright quasars are absent from the local universe, although there are ambiguous cases as close as ~800 million light years (z=0.06).

It's now pretty clear that all quasars are extremely luminous active galaxies, but when 3C 273 and similar objects were first discovered they were point-like objects without the visible appearance of a galaxy. After all, the rest of the galaxy of which they were a part was only 1/100 as bright as the nucleus. So the objects were referred to as "quasi-stellar objects" (QSOs), or quasars for short. (Sometimes the term QSO was reserved for the minority of such objects that did not have appreciable radio emissions, while "quasar" meant a QSO with a strong radio signal.)

It's also clear now that the distinction between Seyfert galaxies and quasars is rather arbitrary – the brightest Seyferts have characteristics much like the least bright quasars. Further, there are a small number of characteristics which may or may not be present in either class. Some of the "optional" features that may be present include X-ray emissions, narrow (optical or ultraviolet) emission lines in the spectrum, broad (optical or ultraviolet) emission lines, strong radio emissions, and evidence of relativistic jets of matter (such as seen in Centaurus A, above).

The last two of these features – radio emissions and jets – almost always are either present or absent together. Naturally, the jets are expected to account for the radio emissions, and there are very reasonable models that explain the connection. However, only about 10% of all AGNs, when Seyfert galaxies are included, have the jets and strong radio emissions, so whatever physical process is responsible for AGN behavior doesn't automatically produce relativistic jets as well.

Indeed, the actual physical process responsible for jets is not well understood, and it's one of the more mysterious phenomena in astrophysics. The research to be discussed here is a contribution towards elucidation of the mystery.

Physical models

But first, let's quickly review the physical model that now has consensus support to account for AGNs in general. The main characteristic, which was at first the most puzzling, is the enormous quantity of energy emitted in a very small volume of space. How small a volume? One indication is that the energy output can vary over periods of just a few days. Consequently, the diameter of the source must be only a few light days – about 10 times the diameter of Pluto's orbit. And yet the source may produce energy ranging from 1 to 100 times as much as an entire good-size galaxy containing hundreds of billions of stars. A little breath-taking, when you think about it.

So where does all this energy comes from? It's not thermonuclear energy that's produced by fusion the way that stars do. The material around even the largest black hole is not dense enough. That's basically because of what's known as the Eddington limit. Any time a sufficiently large mass of gas is collapsing under the force of gravity, the potential energy of the gas is converted to electromagnetic radiation. The energy released as radiation increases to the point where the outward radiation pressure equals the pressure due to gravity, stopping the collapse. This is what prevents very large stars – more than about 100 M_⊙ – forming out of a large mass of gas. The same process also limits the rate at which gas can collapse around a black hole.

However, in the case of a supermassive black hole it is exactly this conversion of potential energy into radiation that supplies the enormous output of energy found in AGNs. Let's look at a simple calculation to show just how effectively a very large compact gravitating object can release energy.

Consider a supermassive black hole whose mass M_BH is 10⁹ M_⊙. Since M_⊙=1.99×10³⁰ kg, we have M_BH=1.99×10³⁹ kg. The black hole is surrounded by an event horizon – the boundary from the inside of which neither matter nor radiation can escape. In the simple case of a non-rotating black hole the event horizon is a sphere whose radius is the Schwarzschild radius, which is r_s=2GM_BH/c², where G=6.67×10^-11 m³ kg^-1 sec^-2 is the gravitational constant and c=3×10⁸ m/sec is the speed of light. Plugging things into the formula gives r_s=2.96×10¹² m. That's very close to the radius of the orbit of Uranus.

Next let's ask how fast an object or particle in orbit around a supermassive black hole might be moving. There is a very simple formula for orbital velocity: v≅(GM/r)^½, where M is the mass of the central object, and r is the radius of the orbit. That's an approximation, since it makes some assumptions – the orbit is nearly circular, and the mass of the orbiting object is much less than M – reasonable for the sake of discussion. Squaring both sides and rearranging: r=GM/v². We could plug in various values and see what we get, but suppose we want to know r in some reasonable units, such as the black hole Schwarzschild radius r_s=2GM_BH/c². Then using plausible values for v, say v=c/10. That's "small" enough that relativistic effects are minor: the Lorentz factor γ=(1-(v/c)²)^-½≅1.005. The result is that r/r_s=100GM_BH/2GM_BH=50.

So an object or particle in orbit around a black hole at a distance of 50 r_s is moving at 1/10^th of the speed of light. Notice that this isn't dependent on the black hole's mass – it's true for any black hole. (It doesn't apply to objects that aren't black holes, since their Schwarzschild radius is very small, much smaller than the size of the object.) This calculation isn't necessarily realistic physically, since it neglects a number of other considerations, for example gas viscosity and turbulence. But it shows that black holes are nothing to be trifled with – they can have rather sizable physical effects.

In fact, although c/10 is not quite a relativistic velocity, it's still rather sprightly. For instance, at that rate one could get from the Sun to the Earth in an hour and 23 minutes – faster than the commute into a big city in bad traffic. It's also a velocity that gives even something as small as a proton quite a bit of kinetic energy. Let's compute it. The proton mass m_p≅1.67×10^-27 kg. Kinetic energy E=mv²/2 = (1.67×10^-27)(3×10⁸/10)²/2 ≅ 7.5×10^-13 kg m² sec^-2 = 7.5×10^-6 ergs. Since one erg is 6.2415×10¹¹ eV (electron volts), the kinetic energy of a proton moving at 1/10^th the speed of light is about 4.68×10⁶ eV = 4.68 MeV.

That's not chicken feed – it's well within the gamma ray range (100 keV to several 10s of GeV). What this means is that in any collision between protons moving this fast, it's no sweat at all to give off gamma-ray photons, or photons of any other form of electromagnetic energy. And this is how black holes of any size, from stellar mass up to the supermassive kind, can convert a substantial fraction of the mass-energy of matter that falls in sufficiently close to electromagnetic radiation.

Given all this, the questions that occupy astrophysicists interested in supermassive black holes, AGNs, quasars, and the like include: What's the exact physical configuration in which the energy is released? What processes bring about the energy release? How do these physical details explain observable effects, such as total energy output, emission lines, relativistic jets, and so forth?

Astrophysicists have been working on these questions for at least 50 years, since the first quasars were discovers, and a consensus has emerged about many of the physical details.

The main feature that all AGNs have (at least in the standard model) is a substantial accretion disk of matter orbiting around them. In many cases that have been studied in detail, there's a lot of evidence for such disks, besides the powerful emission of electromagnetic energy at frequencies from far infrared to far ultraviolet. As the name implies, the disks are flat and relatively thin. The inner and outer radii of the disks vary from case to case, but since there are minor fluctuations of output over periods of days, the inner radius must be on the order of at most a few light-days, around 10¹⁴ m, or 1000 times the size of the Earth's orbit.

Detailed analysis of the physics indicates that the innermost part of the disk should be the hottest, with the temperature gradually tapering off toward the outside. Since the emission is thermal ("black body"), the relation between temperature and wavelength is given by Wien's law: T=b/λ_max, where b≅2.9×10^-3 m-K. λ_max is the wavelength at which intensity per unit wavelength is maximized. Thus peak temperatures may range from 300,000 K when λ_max=10^-8 m (ultraviolet) down to 300K when λ_max=10^-5 m (infrared) – possibly even more at the high end. Higher energies and temperatures actually occur with stellar mass black holes instead of the much larger supermassive ones, because the maximum rate at which matter can accrete (the Eddington limit) is higher for smaller black holes.

As explained above, the energy to heat disk material to such temperatures comes ultimately from gravitational potential energy as matter falls inward and gains kinetic energy, which manifests as heat and ultimately electromagnetic radiation. The efficiency of this conversion of matter into EM energy can actually reach about 10%, which is a lot higher than nuclear fusion, for which the efficiency is only about 0.7%.

The net result is that a certain fraction of matter (mostly diffuse hydrogen and helium gas) in the vicinity of a black hole is converted to electromagnetic energy. This process can go on for a long time (perhaps hundreds of millions of years) until the matter is mostly used up or falls into the black hole itself. Calculations have verified that this process is entirely adequate to account for the observed luminosity of AGNs.

Eventually there is not enough matter sufficiently close to the black hole to be sucked in, and the process stops. This is why most quasars are observed only at great distances – more than a billion light years – because they no longer have the means to sustain the extremely high luminosity. It could be that all or most galaxies go through a quasar/Seyfert/AGN phase. One can even make a rough estimate of how long this phase lasts. Only about 1% of galaxies are Seyfert/AGN, so any given galaxy ought to be in that phase for only about 1% of the age of the universe, i. e. perhaps 130 million years.

As noted above, there are various prominent characteristics that may or may not accompany the high luminosity of an AGN, including broad and narrow emission lines in the spectrum, strong radio emissions, and relativistic jets.

The broad emission lines are thought to originate in clouds of colder gas (under ~100 K) orbiting outside the accretion disk. Although such clouds emit little EM energy, they consist of atomic hydrogen, helium, and traces of heavier elements. Intense radiation coming from the disk will put these atoms in an energetically excited state. But when electrons drop back from higher energy levels, spectral lines at frequencies characteristic of each atomic species are emitted. Because the clouds are in rapid motion, the emission lines are broadened due to varying amounts of Doppler shifting of the spectral lines.

One would expect that such clouds should be present around all supermassive black holes. However, there are many AGNs, both quasars and Seyfert galaxies, in which broad emission lines are not observed. Seyferts were originally placed into one of two classes, according as broad lines were either present or absent. But now intermediate cases are known with only weaker broad lines, so intermediate types are recognized according to the prominence of broad line features.

The thinking now is that there is no actual difference between AGNs with and without broad lines. Instead, the full or partial absence of broad emission lines is ascribed to the degree by which the broad-line clouds are hidden within a thicker torus-shaped ring of even colder gas and dust that surrounds both the accretion disk and the inner clouds. Because of the thick toroidal shape, if our line of sight to the object is mostly face-on, the inner disk and the clouds will be visible. But if we see the object mostly edge-on, those features will be partially or fully hidden.

Narrow emission lines are seen in the spectra of most AGNs. The fact that the lines are narrower indicates that the gas they come from is not moving as rapidly as the gas responsible for broad lines. The type of lines and other evidence indicates that the source of the narrow line emissions is a large but diffuse corona of very hot, ionized gas, which can extend for many light years, surrounding all other parts of the AGN. In AGN that are close enough, the corona is large enough that its actual size can be measured directly. Much of the emissions from the corona is at far ultraviolet and X-ray wavelengths, showing that temperatures in the corona must be quite high.

Approximately 10% of AGNs, both Seyfert galaxies and quasars, are "radio loud" – that is, a source of strong radio frequency emissions. In fact, it was strong radio emissions from the first quasars to be recognized that sharply distinguished them from the normal stars they appeared to be at visible wavelengths. Now that many quasars can be recognized by the high redshift of their spectra – indicating very distant and hence very luminous sources – it turns out that only about 10% of quasars are radio loud.

Long baseline radio interferometry makes it possible to "see" the source of the radio emissions in some detail. The source is not spherically symmetrical, but instead takes the form of very long, narrow "jets", as seen in Centaurus A. Such jets can be hundreds of thousands of light years long. The evidence is that these jets consist of plasmas in which electrons near the central black hole can have relativistic velocities – with Lorentz factors of 10⁴ or more. Electromagnetic emissions from jets often run all the way from radio up to X-rays.

The only plausible physical model for the jets requires very strong magnetic fields. These fields collimate the matter into narrow jets – which emanate in opposite directions from the central black hole – and accelerate the plasma's charged particles to extreme velocities. Relativistic electrons moving in a helical pattern around the jet axes are responsible for the radio emissions via synchrotron radiation.

Many of the details presented so far are based largely on theoretical models, even though astronomers have known of AGNs for over 50 years. Observational studies of active galaxies – quasars in particular – are difficult, since most of the objects are quite distant, and much of the action occurs in a volume of only ~100 cubic light years – impossible to resolve with existing technology. But observational evidence for some of the details is slowly accumulating. The research we're now ready to discuss is an example.


The Hard X-Ray View of Reflection, Absorption, and the Disk-Jet Connection in the Radio-Loud AGN 3C 33

We present results from Suzaku and Swift observations of the nearby radio galaxy 3C 33, and investigate the nature of absorption, reflection, and jet production in this source. We model the 0.5-100 keV nuclear continuum with a power law that is transmitted either through one or more layers of pc-scale neutral material, or through a modestly ionized pc-scale obscurer. The standard signatures of reflection from a neutral accretion disk are absent in 3C 33: there is no evidence of a relativistically blurred Fe Kα emission line, and no Compton reflection hump above 10 keV. We find the upper limit to the neutral reflection fraction is R < 0.41 for an e-folding energy of 1 GeV. We observe a narrow, neutral Fe Kα line, which is likely to originate at least 2000 R_s from the black hole. We show that the weakness of reflection features in 3C 33 is consistent with two interpretations: either the inner accretion flow is highly ionized, or the black-hole spin configuration is retrograde with respect to the accreting material.

3C 33 (which means it is object number 33 in the Third Cambridge Catalogue of Radio Sources) has a redshift z=0.0597, which equates to a distance of about 800 million light years. 3C 33 is one of the brightest narrow-line radio galaxies (NLRGs). An NLRG is a radio-loud AGN in which the spectrum contains narrow width emission lines but there is little or no evidence of broadened spectral lines.

At visible and infrared wavelengths 3C 33 is nothing special to look at, but radio images show structures typical of radio galaxies, with pronounced lobes on both sides of the central object. (See here and here for images at various wavelengths.)

For an explanation of why 3C 33 is an interesting object of study, we need to go into a little more detail about what makes up the "hard" X-ray part of the spectrum of an AGN. This involves photons having energies from 1 keV to 120 keV.

The temperature of the plasma that makes up the corona is much higher than the temperature of the gas in the accretion disk, which is mostly un-ionized. Even the hottest parts of the accretion disk have their intensity peaks in the ultraviolet, with wavelengths of at most 10 nm, which implies temperatures of about 300,000 K by Wien's law. Hard X-rays with 12 keV photons are two orders of magnitude smaller in wavelength, implying temperatures around 30 million K. Quite a difference. So it's not unreasonable to regard the gas in the accretion disk as "cold" – compared to the gas of the corona.

It's somewhat messy to describe what happens with very energetic radiation from the corona interacting with the less energetic radiation from the hottest (innermost) parts of the accretion disk. However, various simulation studies have investigated models where a hard X-ray spectrum is "reflected" from an opaque slab of relatively "cold" gas that mostly emits in the ultraviolet. High-energy photons can reflect off of lower energy electrons in the process of Compton scattering. The high-energy photons lose some of their energy in the process, while the lower-energy particles gain energy. The reverse can also happen: low-energy photons from the accretion disk can scatter off higher energy electrons and photons in the corona. In this process (inverse Compton scattering) the lower-energy photons gain energy.

The starting assumption is that the X-ray spectrum of the corona is approximated by a power law in which the distribution of number of photons of given energy is a power -α (with α>0) of the energy, i. e. N(E) ∝ E^-α. Models with typical assumptions about the configuration of the accretion disk suggest that there should be a slight enhancement of number of photons at energies above 10 keV in the corona X-ray spectrum. This enhancement is referred to as the "Compton reflection bump".

The observational evidence is that this Compton bump is usually found in the X-ray spectra of radio-quiet AGNs. But radio-loud AGNs tend not to have this feature in the X-ray spectra, or have it only weakly. This suggests that there may be something different about the accretion disks of radio-loud AGNs – that is, AGN that also have a jet structure responsible for their radio emissions.

There is one other common feature in AGN X-ray spectra – an emission line from fluorescing iron (Fe) atoms around 6.4 keV. This is called the Fe Kα line. It is normally observed to be relativistically broadened, indicating that it arises from accretion disk reflection. Again, the Fe Kα line is commonly found in radio-quiet AGNs and not in radio-loud AGNs. This is a further indication of something different about the accretion disks of radio-loud AGNs.

3C 33 is a radio-loud AGN, so it's a good candidate for closer investigation. However, there's an additional complication in all this in the radio-loud case. The jets radiate over the full EM spectrum, not just at radio frequencies. In particular, there's an X-ray component to the spectrum, and it's especially strong at the base of the jets. If this part of the jets adds its contribution to the X-ray spectrum the shape of the spectrum will be changed so that the Compton bump (if any) is harder to distinguish.

As it happens, most of the radio galaxies previously studied have been of the broad line sort. Recall that this means we are seeing the galaxy more or less along the axis of the jet, so that the base of the jet and the surrounding accretion disk are not obscured by the outer torus of cold gas and dust. 3C 33, however, is a narrow line radio galaxy (NLRG). That means that the common axis of the jets and the accretion disk is at a large angle (more than ~60°) to our line of sight. Consequently we can't see the accretion disk or the base of the jets directly, due to the obscuring dust, and so broad emission lines aren't visible.

That's actually good, because it means unobscured X-ray emissions from the jets are relatively minor and would not make it difficult to detect a Compton bump and Fe Kα lines – if they were present. If there were a Compton bump, it would be due to photons reflected from the accretion disk and scattered to higher energies in the corona. Since the corona may be hundreds of light years in radius, it is not obscured.

Nevertheless, what this research has shown is that a Compton bump and significant Fe Kα fluorescence are not present in 3C 33. Therefore there's probably something different from the norm of AGNs about the accretion disk of 3C 33. And the most natural assumption is that difference is related to the jets.

What could the difference be? Another research team that has considered the issue of lack of Compton bump in radio-loud AGNs hypothesized that the hottest inner part of the accretion disk could be partially ionized. Therefore it would be semitransparent and not reflect photons strongly. Calculations showed that this was a viable hypothesis.

The team responsible for the present research has a different hypothesis: the strong magnetic fields that create the jets also force the inner part of the accretion disk farther away from the black hole – provided that the black hole itself is spinning in the opposite direction ("retrograde") from the accretion disk. So the research team suggests that the lack of Compton bump is possible evidence for opposing spins of black hole and accretion disk.

Even without effects due to the magnetic field, a retrograde spin of the black hole would cause the radius of the smallest stable circular orbit outside the black hole to be larger than in the prograde case. In other words, the material that would otherwise orbit closer to the black hole isn't there since it has to fall into the black hole. Since magnetic field lines cannot be anchored in a black hole, they must be attached to the accretion disk, and thus assume a different shape than they would if disk and black hole were spinning in the same direction.

At this point, there is no direct evidence for retrograde spin. The competing hypothesis of a semitransparent inner accretion disk isn't ruled out. Further study will be required to distinguish between the two hypotheses.

Evans, D., Reeves, J., Hardcastle, M., Kraft, R., Lee, J., & Virani, S. (2010). THE HARD X-RAY VIEW OF REFLECTION, ABSORPTION, AND THE DISK-JET CONNECTION IN THE RADIO-LOUD AGN 3C 33 The Astrophysical Journal, 710 (1), 859-868 DOI: 10.1088/0004-637X/710/1/859

Further reading:

Black hole spin may create jets that control galaxy (2/11/10)

Backward Black Holes Control Fate of Galaxies (2/12/10)

The Hard X-Ray View of Reflection, Absorption, and the Disk-Jet Connection in the Radio-Loud AGN 3C 33 – arXiv copy of research paper


Related articles:

Winds of Change: How Black Holes May Shape Galaxies (4/19/10)

Galactic black holes may be more massive than thought (6/8/09)

Black hole outflows from Centaurus A (2/6/09)

Evidence that quasars are powered by black holes (10/21/06)

The wind from a black hole (7/8/06)


Other resources

Black Hole Models for Active Galactic Nuclei – excellent technical introduction by Martin Rees

3CRR Atlas Home Page

NASA/IPAC Extragalactic Databse: NED

Galaxies are slowly running out of gas

Galaxies are made of stars, and stars are made of... gas. So a large part of understanding how galaxies evolve and grow is understanding how much "gas" (literally, not "gasoline") is present in galaxies – but has not yet been incorporated in stars – at different periods in the history of the universe.

What periods of the universe are most interesting in this regard? The answer is: periods somewhat less than the first half of the universe's existence since the time of the big bang, roughly the first 5.5 billion years, 40% of the total. That's because astronomers have good reason to believe that is the time when star formation, and hence galaxy growth, occurred most vigorously.

Assuming the best current estimate, that is has been about 13.7 billion years since the big bang, this means we're interested in observing the universe as it was more than 8.2 billion years ago. That's quite a long time ago, and until fairly recently observation of objects that far back in time has been infeasible. Technology is only now becoming available to study the details of such a remote time.

Astronomers find it convenient to represent distance (in either space or time) in terms of redshift. Because it takes light a finite amount of time to travel, any observable object is seen not as it looks "today", 13.7 billion years after the big bang, but instead as it looked a some time T<13.7, and so we see the object as it looked 13.7-T billion years ago. The light from such an object has taken 13.7-T billion years to reach us.

Due to the expansion of the universe, the wavelength of any photon of light has been increased by a factor of (z+1), where z is the observed redshift – z=0 corresponding to nearby objects for which the shift is negligible. z increases as a complicated function of the distance of the object, but it increases in a regular way as the distance increases. For objects at an age T=5.5 billion years, corresponding to a distance of 8.2 billion light years, the redshift would be about 1.1.

Astronomers have now done surveys of galaxies around z≈1.1. It's not easy, but there is plenty of data, even though only very large, bright galaxies can be observed in detail at that distance. It's even more difficult, though still feasible, to survey galaxies that are even more remote, say at z≈2.3, which corresponds to T≈2.9 billion years.

For the research under discussion here, the investigators relied on existing surveys to sample from, because of the difficulty of doing new surveys from scratch. There was a trade-off to be made. In order to be able to study a selected sample of galaxies in sufficient detail, it's desirable to pick the largest, brightest galaxies. On the other hand, it's also important to study galaxies that are representative of "typical" mature galaxies today, such as our Milky Way. Unfortunately, the most luminous objects at large z tend to be atypical things like quasars and merging galaxies. Those are "freaks", quite unlike typical nearby galaxies, and whatever we might learn about them might not tell us much about the typical case.

So the investigators had to select galaxies for study that were as large and bright as possible, but still "normal". In this case, they included only galaxies of estimated stellar mass (excluding dark matter) of ≥ 3×10¹⁰ M_⊙. (1 M_⊙ is our Sun's mass.) Since we are inside the Milky Way and can't see all of it (because of thick dusty regions), it's hard to be sure of our galaxy's total stellar mass, but it's estimated to be about 5×10¹⁰ M_⊙. (Ref: here.)

An important objective of the research was to get a better understanding of galaxies in which new stars are actively being formed – unlike the Milky Way and other nearby large spirals, which are currently forming stars at the rate of about 5 M_⊙ per year. Star formation rate is something else that's easier to determine from outside the galaxy, by measuring light flux in various parts of the spectrum (especially infrared and ultraviolet). For the present research, only galaxies with a star formation rate ≥ 40 M_⊙ per year were selected.

For reasons we're coming to, the required observations are difficult and time-consuming, so for this kind of preliminary study it was necessary to work with small numbers. The net result is that the study was done with 11 galaxies selected from one survey, with z≈1.2, and 12 galaxies selected from another survey with z≈2.3.

Remember that the ultimate objective of the research is to determine how much gas is available for star formation in typical galaxies at the given values of z. That's what is so difficult that it had not been done before (for such distant galaxies).

It's relatively straightforward to determine how much of a galaxy's mass is in the form of stars. This "stellar mass" is proportional to the intrinsic luminosity of the galaxy (which is known since the galaxy distance is known), because most galaxies consist of stars with a predictable distribution of stars of given mass and luminosity ("initial mass function").

However, the total mass of a galaxy also includes non-baryonic dark matter, whose mass in the universe as a whole is known to be about 5 times as large as the mass of "ordinary" baryonic matter. The total mass of a galaxy can sometimes be inferred from measuring galaxy rotation curves. The baryonic matter of a galaxy consists of stars, gas, and (perhaps) massive nonluminous objects such as black holes. Even if one knew reliably the total mass of a galaxy, including dark matter, and one could neglect the contribution of nonluminous objects, one still could not estimate the mass of gas as the difference between the mass of a galaxy's stars and the roughly 17% of total mass that baryonic matter represents in the universe as a whole.

That's because there's no a priori reason to expect that a 1:5 ratio of baryonic matter to non-baryonic matter is present in any particular galaxy – it might well be either more or less. So there needs to be some way to measure fairly directly the amount of matter a galaxy contains in the form of gas that could form stars.

It is known that stars form only out of gas that's rather cold – with temperature less than 100 K. This is simply because hotter gas has a higher internal pressure that prevents the gravitational collapse that's necessary to form a star. Gas that cold is very hard to detect. Black body radiation at 100 K peaks at 29 μm, in the far infrared, and cooler gas emits at even longer wavelengths. Redshift stretches the wavelengths even more (by factors of 2 or 3 for z=1 or 2). Most of the radiation at such wavelengths is blocked by our atmosphere, so is observable only from space – and the necessary instruments don't exist yet.

Fortunately, black body radiation is not the only type of electromagnetic emission from cold gas. Vibrations and rotations of gas molecules also radiate at certain frequencies. Now, most of a galaxy's cold gas is in the form of atomic helium and molecular hydrogen. It would be convenient if hydrogen molecules, especially, had emissions at convenient wavelengths for ground-based observation, but no such luck. It turns out, however, that there is one molecule present in small amounts in interstellar gas which does have a convenient emission: CO (carbon monoxide). CO has a rotational emission at 870 μm (346 GHz). At the values of z of interest here (1.2 and 2.3) these fall into the 2 mm and 3 mm bands – which can be observed.

In a nutshell, then, what the research under discussion did was to measure the total flux from the selected galaxies at the appropriate wavelengths. This indicates that amount of cold CO gas present in the galaxies. Studies of nearby galaxies show that this accurately indicates the total amount of cold gas present. From this, and the estimated mass in the form of stars, one has fraction of total mass (stars + gas) represented by the cold gas available to form stars.

For the galaxies in the sample, this fraction was found to be 34% at z≈1.2 and 44% at z≈2.3. By contrast, contemporary large spiral galaxies have fractions in the 3% to 12% range – quite a difference.

For a summary of the results, here's the abstract:

High molecular gas fractions in normal massive star-forming galaxies in the young Universe

Stars form from cold molecular interstellar gas. As this is relatively rare in the local Universe, galaxies like the Milky Way form only a few new stars per year. Typical massive galaxies in the distant Universe formed stars an order of magnitude more rapidly. Unless star formation was significantly more efficient, this difference suggests that young galaxies were much more molecular-gas rich. Molecular gas observations in the distant Universe have so far largely been restricted to very luminous, rare objects, including mergers and quasars, and accordingly we do not yet have a clear idea about the gas content of more normal (albeit massive) galaxies. Here we report the results of a survey of molecular gas in samples of typical massive-star-forming galaxies at mean redshifts of about 1.2 and 2.3, when the Universe was respectively 40% and 24% of its current age. Our measurements reveal that distant star forming galaxies were indeed gas rich, and that the star formation efficiency is not strongly dependent on cosmic epoch. The average fraction of cold gas relative to total galaxy baryonic mass at z = 2.3 and z = 1.2 is respectively about 44% and 34%, three to ten times higher than in today’s massive spiral galaxies. The slow decrease between z ≈ 2 and z ≈ 1 probably requires a mechanism of semi-continuous replenishment of fresh gas to the young galaxies.

The results from this research about star formation rates (SFR) are especially interesting. From other research involving much larger samples, it's known that when SFR is plotted against galaxy stellar mass, the distribution can be fit by a power law:

SFR (M_⊙/year) = 150 (M_*/10¹¹M_⊙)^0.8×([1+z]/3.2)^2.7

In this equation, M_* is the galactic stellar mass. Thus SFR depends on total stellar mass of a galaxy, which makes sense, because the larger the galaxy, the more cold molecular gas is available to make stars. Further, because of the factor involving 1+z (where z is redshift), the SFR curve is shifted upwards at larger z – the rate of star formation is greater, in a regular way, at earlier times in the universe.

This equation is pretty close even in the nearby universe, where z=0. For a galaxy the size of the Milky Way (which is not among the largest of spirals), M_* is estimated as 5×10¹⁰ M_⊙, predicting SFR of about 3.7 M_⊙/year – which is surprisingly accurate. So SFR has continued to decline in a fairly regular way.

Interestingly enough, however, in the galaxies sampled in the present research, the percentage of cold gas in a galaxy does not appear to have any clear relationship to either the SFR or the total stellar mass of a galaxy. So almost all of the variation in SFR is related to the total stellar mass. This is what it means to say that the "efficiency" of star formation is not very dependent on percentage of cold gas or cosmic epoch. Instead, SFR is probably largely dependent on total available cold gas, which is proportional (at a given z) to a galaxy's stellar mass.

One additional interesting conclusion can be drawn from the research. Namely, given the SFR in sampled galaxies at z≈2.3, there ought to be much less cold gas in equivalent galaxies at the later time (about 2.3 billion years later) corresponding to z≈1.2 than is actually observed. Much of that cold gas should have been incorporated into stars. Yet the amount of cold gas actually observed at the later time is more than the original amount less what was converted to stars. And so there is apparently more cold gas added over time, even though, as a whole, galaxies really are "runnng out of gas".

Tacconi, L., Genzel, R., Neri, R., Cox, P., Cooper, M., Shapiro, K., Bolatto, A., Bouché, N., Bournaud, F., Burkert, A., Combes, F., Comerford, J., Davis, M., Schreiber, N., Garcia-Burillo, S., Gracia-Carpio, J., Lutz, D., Naab, T., Omont, A., Shapley, A., Sternberg, A., & Weiner, B. (2010). High molecular gas fractions in normal massive star-forming galaxies in the young Universe Nature, 463 (7282), 781-784 DOI: 10.1038/nature08773

Further reading:

Young galaxies gorge on gas (2/10/10)

Why Today's Galaxies Don't Make As Many Stars As They Once Did (2/11/10)

Early Galaxies Formed Stars Fast Because They Had More Gas (2/10/10)

Stellar Baby Boom of Early Universe Explained (2/11/10)

Ancient Galaxies Packed More Raw Material for Stellar Formation (2/10/10)

In the News this month: the molecular content of early galaxies (3/4/10)

Astrophysics: Less greedy galaxies gulp gas (2/11/10)

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)