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)
and
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.

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

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

$\dot{\nu}=-K\nu^n$

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):

$n=\frac{\nu\ddot{\nu}}{\dot{\nu}^2}$.

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.

The Case of the Fickle Found Furby

Fast Radio Bursts

Early last week, Dusty discussed Fast Radio Bursts (FRBs, or as I prefer to call them, Furbies). To recap, they are short (~millisecond) bursts in the radio (GHz) frequencies, that seem to come from cosmological distances, as determined from their dispersion measures, related to the amount of time between arrivals of the pulse at different radio frequencies. The more free electrons between us and a radio burst, the longer this delay will be, with the time delay scaling as the inverse of the frequency squared. The column density of electrons indicate that these bursts come from either dense plasma environments, or from cosmological distances (with the electrons coming from the intergalactic medium).

The first detected FRB, known as the Lorimer burst. The $\nu^{-2}$ dispersion, large dispersion measure, and short duration define the class.

The other kind of Furbies, also radio emitters, but more easily localized, with varying optical colors.  

Since these bursts are so fast (it's in the name!), it is very difficult to figure out precisely where they are coming from. Until last week we didn't have any clear identification of a true distance or redshift. We know that they seem to be isotropically distributed in the sky (thus supporting the hypothesis that they are cosmological). Most have been discovered by the Parkes radio observatory in Australia, mostly due to survey design reasons, but several have also been seen by the Arecibo and Green Bank Radio Telescopes. One detected by GBT also showed signs of circular polarization, seeming to indicate that it was associated with a strongly magnetized/rotating object. So far about 17 have been detected, though when convolving this number with the beam size and observational strategies of the various radio surveys, this gives a rate of roughly thousands per sky per day!

A Furby Located?

Last Thursday, an extremely interesting Nature paper came out discussing the apparent localization of a Fast Radio Burst by an afterglow and the identification of a host galaxy. Keane et al. report that using their extremely impressive real time identification of FRB 150418. (most FRBs have been detected by an archival search of the data), a large number of followup telescopes were triggered with an afterglow in the radio (5 GHz and 7.5 GHz) detected by ATCA that faded over about 6 days.

From Keane et al. 2016, on the purported radio afterglow associated with FRB 150418. The detection are the two purple points and one green point on the left, with low level emission and upper limits after. The FRB was detected at time 0. 

If true this is a HUGE deal. It would be the first identification of an FRB host galaxy, providing a distance and redshift (z = 0.49). The combination of dispersion measure and redshift provide an estimate of the "missing" intergalactic baryon density, providing an estimate of the mass in the intergalactic medium (IGM) that agrees well with the cosmological results from WMAP.

The host environment, however is at odds with a magnetar being the source of FRBs. Having FRBs be associated with giant magnetar flares is, I feel, the leading candidate for their astrophysical source. However, magnetars exist in young stellar populations, while the galaxy in question is "red and dead", meaning it has an old stellar population with no star formation). Additionally the energetics of the afterglow (which contains much more energy than the initial FRB) suggest that it must come from something much more powerful than a magnetar flare. The authors even suggest that these factors indicate a NS merger as a source for the burst and afterglow. This, however, is problematic, as the rate of neutron star mergers is not nearly high enough to account for the inferred FRB rate, which is close to the supernova rate! If the NS-mergers accounted for a large fraction of the inferred rate of FRBs this would mean that 1) Short GRBs are extremely beamed 2) Initial LIGO should have probably seen a merger if they happen so often. The alternative many suggest is that there may be multiple progenitor classes (though the rates must still be comparable for it to have been picked up with so small a sample).

Keane et al. also examine the possibility of a false positive, by looking at the rate of radio transients, and conclude that there is a < 6% chance of a coincident transient detection. However, in the last few days it has emerged that only considering the radio transient rate may have been a crucial, but subtle mistake.

Not So Fast, Radio Burst!

Over the weekend Peter Williams and Edo Berger of Harvard CfA posted a preprint questioning the Keane et al. results. (They actually posted a link to their preprint on facebook on Friday morning, which is impressive turn around time indeed!) They argue that it is the rate of variable radio sources that should be considered, and show that there should be order unity of these expected in the beam of Parkes associated with the FRB. They argue that the "afterglow" then, is likely associated with AGN variability. Most compellingly (to me), they show how the evolving ratio of the 5.5 GHz to 7.5 GHz flux from the source is inconsistent with a relativistic fireball that one expects to be powering a true afterglow of a burst event. (This also jives with how it requires a lot more energy to power such an afterglow than most think would be available for a single compact object).

If the "afterglow" of FRB 150418 is indeed due to AGN variability, this would be easy enough to check with a campaign of follow-up observations, which of course Edo and Peter carried out. The result was this Astronomer's Telegram:

ATel 8752 from Williams and Berger reporting on the VLA follow up of the purported FRB host galaxy.

Using the VLA, they reported a 157 microJansky detection of the radio source, which suggests a re-brightening at the 3 sigma level. That's pretty damning of the afterglow interpretation, though there is no direct detection of variability in their two 1.5 hour observations. 

It is worth noting that radio transient/variability people that I know and respect think this closes the case, but some of my colleagues who study AGN for a living have told me that such variability from a radio quiescent AGN isn't actually expected (they said they expect such variability from a blazar source, though the optical observations make it clear it is not a blazar in this case). [EDIT - Edo writes to mention that since the source is only a few degrees from the Galactic plane, the variability seen is consistent with the level of ISM scintillation you'd expect from an AGN with no intrinsic variability. Interesting! Though pulsar people should not have missed this as they deal with scintillation all the time!] As an outsider to both fields I can't really provide a solid interpretation, but my general impression is that the radio sky (and particular quiescent radio galaxies) at these frequencies and timescales are not well studied. Regardless, the re-brightening, if held up, combined with the strange spectral evolution, does make it hard to believe the afterglow interpretation, and subsequently the association with a host galaxy and distance. 

A Furby, disassociated. 

Too bad!

Gearing Up to Find More FRBs

This post is inspired by a recent popular science article about the Molonglo radio telescope which has been upgraded and repurposed over the last few years to do, among other things, surveys of Fast Radio Bursts (FRBs). If you're not already familiar with FRBs, here's a brief primer.

From Lorimer et al. (2007). 

The first FRB detection was published in Lorimer et al. (2007). Using data from the Parkes radio telescope, they detected an extremely bright, non-repeating, millisecond duration radio burst. At radio frequencies, pulsed emission is dispersed by intervening diffuse cold plasma, meaning low-frequency emission arrives at Earth after high-frequency emission. Measurements of the amount of dispersion allow for inferences regarding the total electron content between the emission and observation locations. With FRBs, the dispersion is so great that it cannot be explained by the electron content of the Milky Way alone. It is possible that much of the dispersion is due to propagation through vast swaths of the very diffuse intergalactic medium and that the sources of FRBs are at cosmological distances. If that is the case, their brightness hints at truly extraordinary energetics and a very exciting new type of astrophysical transient.

Several more of these FRBs were detected by Thornton et al. (2013), all with Parkes. With very small number statistics, it began to appear that FRBs were not tracking the stellar density of the Milky Way and were instead distributed seemingly isotropically, consistent with a cosmological source population. It was worrisome that all of these were being detected with Parkes. Parkes might have just had this serendipitous combination of field-of-view and sensitivity to make it great for detecting these things, or maybe there was some devious source of radio frequency interference influencing the observatory. And indeed, Petroff et al. (2015) realized that a microwave at Parkes, when opened prematurely, caused so-called "perytons" which almost perfectly mimicked the dispersive sweep of an astrophysical radio burst. However, not all FRBs were perytons. Many of them appeared to be bonafide astrophysical events. FRB detections at Arecibo and The Green Bank Telescope helped to allay microwave fears and build the case for these things being astrophysical in nature.

Now, back to Molonglo. The Molonglo telescope is over 50 years old. It has a giant cylindrical collecting area with hundreds of dipole antennas. Over the last several years, it has been subjected to a massive overhaul of its digital systems and is now capable of processing vast quantities of high time resolution radio data. It can synthesize many beams and survey large areas of the sky at once. It is a fantastic example of a relatively new trend in radio instrumentation: digital processing and software developments are driving much of the new science at low radio frequencies (below 2 GHz). Similar examples are the remarkable European LOFAR telescope and the soon-to-be complete Canadian CHIME telescope. This recent paper describes new and improved digital data processing capabilities at Arecibo for FRB searches. Note that Arecibo's funding situation is currently very precarious even as the case for its science capabilities is growing.

In the Molonglo article, Matthew Bailes from Swinburne University of Technology describes how there are currently more theories for what causes FRBs than there are observed FRBs. This is certainly the case. There are only about 15 published FRBs and theories for what may cause them range from flare stars to supergiant pulses from pulsars to merging neutron stars to core collapse supernovae and more. I'd say you should expect this to change within about 18 months. New and existing but improved instruments are going to start finding these things by the dozen. We're on the verge of learning a lot about FRBs.

  

Random Topics: GW GRBs, mini-JWST, Exoplanets and lunch.

So after all the excitement last week, the subsequent week has been filled with discussions and other work getting caught up. For me, the last few days have been spent getting caught up on some review work that is going to be due soon, so this will be a fairly short post.

Fermi

On Friday, for our high-energy astrophysics meeting at UMD, we had a nice round-table discussion with one of the Fermi team regarding the Fermi-signal coincident with the LIGO detection GW150914. Their arXiv (not-yet peer-reviewed) paper is here, and discusses their detection.

Fermi-GBM detected a signal with a fluence of $10^{49}$ ergs/s  between 1 keV and 10 MeV, lasting about a second. The fluence, duration, and hardness was similar to a weak short-GRB. The reason it was so poorly localized was that the Fermi spacecraft was pointed in orthogonal to the optimal direction to localize the signal source. They characterize this as a roughly 2-3 sigma detection, with a 0.0022 false alarm probability, though there are some details of this FAP calculation that I don't quite follow (particularly, why this scales with time from the GW trigger event).

It is worth noting that Fermi expects to see events of this significance randomly every few thousand seconds or so, therefore there is a possibility this is just a coincidence. SWIFT-BAT did not detect anything, though it was also pointed in the wrong direction and doesn't have as wide field of a detector, so they had to make do with observations 2 days after the fact, during which there was no detection. More interestingly, the INTEGRAL-ACS which has a much larger collecting area than Fermi-GBM, (it's a coincidence shield that surrounds the spacecraft) but also much higher background, did NOT see anything at the same time as the Fermi-GBM detection. However, they are sensitive to a different energy band, and if the short-GRB like spectrum is taken as a given, INTEGRAL is expected to miss about 50% of the bursts Fermi catches at that fluence.

This of course did not stop many many so-called "creative" ideas from flooding the arXiv about how a BH-BH merger produced an electromagnetic GRB-like counterpart. I won't get into them here, and will leave the discussion for another day.

Mini-JWST

This isn't really all that research related but I just wanted to post a picture of our new JWST model we just got from MESAtech. The James Webb Space Telescope is of course the very expensive IR space telescope that carries the hopes, dreams, and opportunity cost of the entire astronomical community into the future. It will be revolutionary for extragalactic astronomy, mapping of the local group, and exoplanetary atmosphere studies, among other things.

This 1/40th scale model was 3D printed as a reward for kickstarting MESAtech's fully robotic model for high-school outreach. The pieces were all snap together and Vicky assembled it in about 45 minutes, with just some wire snips and a few pieces of tape to help tighten up some fits. We got it for $50 during the kickstarter, but they are now available for purchase from the MESAtech site for $75. It folds/unfolds, and at 1/40th scale is just about the right size compared to LEGO minifigs.

Exoplanets and Lunch

Lastly, this week I organized the inaugural Exoplanet lunch at UMD, in order to bring together Drake's group with some other exoplanet researchers, like myself. Personally, I'm hoping that a closer interaction with the exoplanet observers will lead to some interesting projects that will help me get a better feel for the details of exoplanetary data analysis and observations. Towards that end, I'm considering running a brief tutorial over the course of several lunches exploring the use of Dan Foreman-Mackey's emcee code for doing parameter estimation using Markov-Chain Monte-Carlo methods. Jake van der Plas has a very nice tutorial he ran at the last AAS, which would be a very good basis to start from.