Science and Reason: 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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