Saturday, March 19, 2016

Black hole spins and gas dynamics

J1649+2635: 4th jet seen in a spiral galaxy. SDSS.

Yesterday I went to a talk by Massimo Dotti on linking the spin evolution of black holes to gas dynamics, and I thought it would be interesting to write about this.

How do BH spins evolve?
They evolve through

1.  BH mergers (new spin for the remnant black hole)
2.  gas accretion

Gas accretion may be a) coherent, when the angular momentum of the accreted gas is in the same plane as the spin of the black hole. It then fairly quickly results in a maximally spinning black hole. Or b) chaotic. Then the BH accretes small gas clouds of random orientation. This spins down the black hole. Or c) hybrid. The BH is most likely to suffer a combination of coherent and chaotic accretion. The spin is a vector that changes during the accretion process.

Why should we care about black hole spin?
1. Spin influences Black Hole (BH) growth - generally, high spin slows down accretion
2. Affects the occupation fraction of black holes in galaxies, e.g., a black hole may be kicked out of a galaxy - see Chandra observation of a black hole ejection.
3. encodes information about the fueling process of the black hole and the evolution of the host galaxy
4. May be linked to jet production. The jet could spin down the black hole. The process that produces the jet is not understood. It is believed that a fast spinning accretion disc produces a powerful magnetic field, which is in contact with the black hole. The spinning black hole drags the magnetic field, winding it in a cone. The jet extracts energy from the black hole spinning it down.

Matching observations of BH Spin (Sesana et al. 2014)

So far BH spin is measured in only spiral galaxies that are still accreeting today. The smaller spins have large errors. They are all measured via K-alpha line fitting. Overall, the picture is consistent with a hybrid gas dynamics as expected.

 What next?
 Observe more jets like J2345-0449 (Bagchi et al. 2014),  in spiral galaxies for fast spinning central black holes. Most jets are currently observed in elliptical galaxies. Measure the spin of more black holes. Launch LISA.

What about Pulsar Timing?
Pulsar timing arrays observe black holes that are, generally, too far from merger to constrain the spin of the final black hole. The spin of the individual black holes cannot be measured as far as I understand.

What about LIGO?
Stellar mass black holes also power relativistic jets, but the nature of the jets is likely different, where the jets are more likely to be powered by the disc and not by the spin of the black hole (e.g., Diaz Trigo et al. 2013). This remains to be tested as LIGO sees sources that can also be observed electromagnetically.

Monday, March 7, 2016

A Repeating FRB: Guest Post by Paul Scholz

This is a guest post by Paul Scholz, currently a graduate student working with Vicky Kaspi at McGill University. Paul specializes in radio and X-ray observations of pulsars. He was second author on this Nature Paper describing the discovery of a repeating(!!) fast radio burst, and was the first person to notice the repeated bursts it in the data from the follow-up observations. -dave

There's been a few posts already on this blog about FRBs  but a new result in a Nature paper last week, on which I'm co-author, adds a big twist to the story. As a reminder (or in case you haven't read Dave's and Dusty's posts) Fast Radio Bursts (FRBs) are short (~millisecond) bursts detected at radio frequencies, that originate from cosmological distances, or at least that's what's implied from their dispersion measures. The twist is that we've discovered repeating bursts from the same source as a previously discovered FRB. First I'll give a bit of background and then I'll get to why the result is exciting.

FRB 121102 was detected in the PALFA survey in 2013 by Laura Spitler and was the first FRB seen at a telescope other than Parkes. This was an important step in out understanding of this new phenomenon as it put to rest fears that FRBs were caused be some terrestrial or atmospheric phenomena local to Parkes. (since then the Green Bank Telescope has also found an FRB

The Parkes FRBs have so far never been seen to repeat despite quite a bit of telescope time dedicated to determining whether or not they do. This, along with their extreme distances, has led to many cataclysmic explanations for FRBs such as the collapse of a rapidly rotating neutron star into a black hole, and the merger of neutron stars or white dwarfs

For the PALFA FRB we set out to put a robust limit on the repeatability of bursts from the source of FRB121102. So, we performed a campaign of observations using Arecibo in May and June 2015. When we searched those observations for bursts we found some very bright signals. In total we found 10 new bursts that had the same dispersion measure as the original FRB 121102.

11 bursts from FRB 121102 including the original burst. They're corrected for the $\nu^2$ dispersion sweep and the shape of the instrumental bandpass.

It was immediately obvious that this was a big deal in the study of FRBs. Our discovery of repeating bursts from FRB 121102 shows that the source of these bursts cannot be cataclysmic, that is the source must survive the event. Also, six of the bursts were found within 10 minutes of each other, that says that it can't be caused by an event that occurs on longer timescales than that, such as giant flares from magnetars.

A model for FRBs that does fit with these bursts that can repeat on relatively short timescales is supergiant pulses from an extragalactic pulsar or magnetar. This also fits with the unusual and highly variable burst spectra that we see (see the right panels in the above plot), since similar spectral variability is seen in the Crab pulsar, the prime example of a young, energetic pulsar in a supernova remnant in our Galaxy. Another that might fit is radio counterparts to short (millisecond to second) X-ray bursts from a magnetar.

At any rate, this result shows that FRBs can repeat. But, it most definitely
does not imply that all FRBs repeat and that some of them cannot be of the 
cosmological, cataclysmic flavor. The Keane et al. Nature paper that came out 
a week before ours suggested the merging of two neutron stars into a black
hole (i.e. a cataclysmic event) as the origin of their bursts. But the host-galaxy
association that that interpretation was based on has since been questioned (see Dave's post). Still, keep in mind that the Parkes-discovered
FRBs have been followed up for many hours and no repeat bursts have
been found. Perhaps they are all repeating and Parkes is missing the
fainter bursts that we can see from FRB 121102 since we're using Arecibo. 
But this seems like especially bad luck, surely Parkes would have 
seen at least a second burst from one of the 15 FRBs discovered there by now.

This leads to the exciting prospect that there are two or more physical origins for FRBs. Time will tell: we'll keep characterizing FRB 121102, other FRBs will continue to be monitored for repeats, and in the next few years large field-of-view radio telescopes, such as the CHIME telescope in Canada, should give us a flood of new FRBs that will give us a much better picture of the FRB population.

EDIT - For posterity, here is a repeating Furby. -dave

Wednesday, March 2, 2016

Spherical photon orbits around Kerr (what I did this week)

Did you know that light can orbit a black hole?

These orbits are unstable: give your photon a tiny push in or out and it will either plunge into the horizon or shoot out to infinity. It's like trying to balance a pencil on its tip: you have to perfectly tune these orbits.

But still! Light can orbit a black hole. Ever wondered what these trajectories look like?

I spent a bit of time this past week making a simulator for spherical photon trajectories around rotating black holes (this is part of the reason why I'm behind on my posts here; sorry!). Now you can easily see all these trajectories in your browser, just by dragging around the slider controls.

Besides being unstable, these orbits generally fill the whole (truncated) sphere of allowed locations. That's because generically, the frequencies of azimuthal motion and longitudinal motion are incommensurate. But, rationals are dense in the reals, so there are actually resonant orbits all over parameter space. This means the orbits recur. They can be really pretty! Here's a picture of one of the 4:3 resonances:
Studying the frequency ratios and locations of resonances is also really cool. Those are encoded in this colorful contour plot. To understand what this plot means, I recommend that you head there now to play with the simulation, and read a lot more about the physics!

Pump the Brakes...

There has been a deluge of important papers coming out lately and I feel that it is my duty as a pulsar astronomer to not let this one slip by you. This little gem on the braking index of pulsar J1640-4631 is a bit of a paradigm breaker. The abstract alone is just chocked full of awesome nuggets. To explain why it's so important, I suppose I should first explain what a braking index is.

The rotational frequencies, $\nu$, of pulsars are observed to decrease with time. If the simplest toy model of a pulsar as a misaligned dipole corotating with a neutron star is true, this spin-down is anticipated from dipole radiation and can be quantified as


where $K$ is some constant that depends on the magnetic field strength/orientation and the moment of inertia of the neutron star, and $n$ is the braking index. For straightforward dipole radiation spin-down, $n=3$. If the second derivative of $\nu$ can be measured, the braking index can be measured (assuming the above parameterization is valid):


Now, it is exceedingly difficult to measure $\ddot{\nu}$ because it is typically tiny, like a few times $10^{-22}$ inverse cubic seconds (gotta love that unit). Before this paper, $\ddot{\nu}$ had only been measured for about 8 pulsars and the inferred braking index has always been less than 3. Well known phenomena like pulsar wind nebulae can easily reduce the braking index, so these measurements are not incredibly surprising.

As the above figure shows, these authors have measured a braking index greater than 3, and this requires new phenomenology such as evolving magnetic structure, or, tantalizingly, gravitational wave emission. Okay, it's probably not gravitational waves in this case, but who knows.

This is a big deal and it's a cool measurement of a really cool system. The pulsar is very young and is associated with a supernova remnant. The pulsar powers a wind nebula that is the brightest TeV gamma-ray source in the Galaxy.  As far as we can tell with radio telescopes in the southern hemisphere, this is a radio quiet pulsar. All of these inferences were done using X-rays from the pulsar collected with the space-based X-ray telescope NuSTAR. The timing model for this pulsar is incredibly simple and constrained by only around 20 measurement epochs spread out over just two years. Measuring a $\ddot{\nu}$ in just two years is remarkable and only possible because it seems to be such a large value. Anyways, this is a great and short little paper that I highly recommend you take a look at.

Update: The authors claim to have ruled out "timing noise" as the source of this $\ddot{\nu}$, but an eminent colleague of mine is skeptical. More time will tell, and this is still an exciting source to monitor.