When a Black Hole Stops Being Hungry

Figure 1: Artist's rendition of a tidal disruption event

It is a rare treat in astronomy when we can actually watch something occur in “real time” as opposed to trying to piece together objects at different stages in their evolution to form a coherent physical picture.  This is one reason astronomers were so excited about the unusual transient Swift 1644+57 (Sw 1644+57) , which went off in March of 2011.  This object was discussed in several astrobites at the time (see here and here) but as a brief review: Sw 1644+57 was first detected with the Swift Gamma-Ray telescope on March 28th 2011, and was quickly found to be unlike any other gamma-ray burst.  After combining observations from the Gamma-rays down to the radio the prevailing opinion in the community is that Sw 1644+57 is the result of a star being torn apart and swallowed by a supermassive black hole (these events are usually referred to as tidal disruption events, see Figure 1).  Not only that, but there was evidence that a jet of material moving at relativistic speeds had formed in the process.  Although relativistic jets are seen elsewhere in astrophysics (e.g. Active Galactic Nuclei, or AGN), this object gave us the unprecedented opportunity to observe the jet as it was forming.  Well, now, a year and a half later, new observations suggest that we may have just seen the jet turn off.

Background:

Before we get to the new observations, there are three pieces of background information which may be helpful:

Figure 2: A cartoon of the process which creates synchrotron emission

(1) Synchrotron Radiation:  Synchrotron radiation is formed when charged particles moving at relativistic speeds accelerate around magnetic field lines (See Figure 2).  In many explosive astronomical events, which generate shock waves, synchrotron emission is thought to occur in the outer regions where the shock slams into the interstellar material.  Unlike spectral lines, synchrotron emission produces a continuum of radiation.  The exact shape of this continuum is dependent on properties such as the distribution of energies possessed by the relativistic particles, the radius of the emitting region, etc.  Hence, as long as your observations cover certain important features, modeling a portion of a synchrotron spectrum as a function of time allows you to (i) decipher the evolution of those physical parameters and (ii) predict the amount of radiation which should exist at other wavelengths due to the synchrotron source.

(2) Accretion Timescales: When an object is tidally disrupted, it is not sucked into the black hole all at once.  Rather, it is ripped to shreds by the strong tidal forces and flung back out into space.  However, because the gravitational pull of a black hole is so high, most of the material will eventually be pulled back in, and there are several theoretical models which predict the rate at which this fallback should occur.

(3) The Eddington Mass Accretion Rate:  The Eddington luminosity is defined as the luminosity at which the radiation force outward equals the gravitational force inward.  For sources where the luminosity is due to accretion, one can also define that Eddington accretion rate as (you guessed it), the accretion rate which would produce a luminosity equal to the Eddington luminosity.  Now, although we really don’t understand the generation of relativistic jets in tidal disruption events (they weren’t even predicted before Sw 1644+57), there are models for jets in other contexts which suggest powerful jets can only be supported if the mass accretion rate is higher than the Eddington mass accretion rate.

Figure 3: X-ray light curve of Sw 1644+57. Note the rapid decline at recent times. The blue line indicates the X-ray flux expected from the region of the forward shock (derived from modeling the radio synchrotron emission)

Observations:

In Figure 3 we show the X-ray light curve for Sw 1644+57 for the past ~600 days (black points).  One can see that although there is quite a bit of variation, the X-ray luminosity has been casually declining for the past several hundred days. This decline is consistent with the predicted fallback rate described in (2) above – thus giving additional credence to the idea that Sw 1644+57 was a tidal disruption event. However, look at the last two data points.  Something interesting happened very recently which caused the X-rays to absolutely plummet over a very short time scale.  The final data point (black square) was taken with the Chandra X-ray Observatory, rather than Swift.

In order to explain this, today’s paper also examines the long-term radio behavior of the source (See Figure 4).  An earlier paper by the same group showed that by modeling the radio emission as synchrotron one could infer the presence of a large relativistic outflow associated with the event.  Interestingly, much of the observed X-ray emission in Figure 3 falls well above that predicted by extending the synchrotron spectrum to high energies (even accounting for Compton scattering) and, in addition, the X-ray and radio variability have not been seen to correlate with one another.  These are two of the reasons why it has generally been accepted that the radio and X-ray emission from Sw 1644+57 originate from different locations.  At least until now.  One of the major points in today’s paper is that, after the recent rapid decline, the remaining X-ray emission is fully consistent with the levels predicted by the radio emission (as shown by the blue line in Figure 3), and also inconsistent with one of the major other possible sources of X-ray emission in this context: hot blackbody emission from an accretion disk.

Figure 4: Radio Observations used to model the synchrotron emission from the relativistic jet

Interpretation:

So what does this mean? The interpretation outlined in the paper can be summarized as follows:  At all times, the majority of the radio emission we see is produced at large radii where the shock from a relativistic jet is interacting with the interstellar material.  This process also produces X-rays, but at early times there were dominated by another process – one which occurred much closer in to the black hole, and was closely tied to the fallback of material/accretion (as indicated by the decline timescale).  However, the recent rapid decline in X-ray emission and the fact that the remaining emission is consistent with that expected from the region of the forward shock indicates that this process has ceased. The authors interpret this as the turning off of the relativistic jet.

Interestingly, if one associates the turn-off of the jet with the time when the mass accretion rate falls below the Eddington rate (as implied by point 3 above) one can calculate the total mass accreted.  Assuming a black hole mass of 10^6 solar masses the authors calculate that all of the emission from Sw 1644+57 up until the relativistic jet turned off was due to the accretion of … wait for it… 0.15 solar masses of material.  Only 15% the mass of our sun caused all of this.

Now, clearly there are many things we still do not understand, but no one can argue that Sw 1644+57 is proving an incredibly useful testing ground for our theories of relativistic jets and tidal disruption events.

About Maria Drout

I am currently a Hubble, Carnegie-Dunlap Fellow at the Carnegie Observatories in Pasadena, CA and an associate of the Dunlap Institute in Toronto, ON. I recently received my PhD from the Harvard University Department of Astronomy, and was previously based both at the University of Cambridge (M.A.St.) and the University of Iowa (B.S.). My research focuses on understanding the evolution and death of massive stars, and the origin of unusual astronomical transients.

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