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Some Background

General Relativity is currently the best theory physics has to explain gravity. Like any worthy theory, it makes predictions for how things ought to go if the theory is indeed the correct one; the one which describes the physical world that we observe. Many of these predictions have been tested and confirmed (e.g. tests of GR), which is partly why General Relativity (GR) is so celebrated.

There are predictions of GR however, which we have not yet been able to test, phenomena which become available to our senses only when generated in the strongest gravitational environments known to science, in the vicinity of black holes (BHs). One of these un-tested predictions is the existence of gravitational radiation (gravitons), an analogue to electromagnetic (EM) radiation (photons) but relaying information about the gravitational field rather than the EM field. Another difference is that gravitational radiation (and gravity in general) interacts very weakly with the normal matter that makes up any type of detector we might build to observe gravitational radiation. And so even though the merger of two, many-billion solar mass BHs at the center of a galaxy sends out more energy into gravitational radiation than 100 million supernovae, we have not yet been able to detect it (but soon: PTAs, eLISA, and for the merger of small BHs: LIGO).

My research focuses on predicting how we can observe the astrophysical events which create detectable gravitational radiation - *in the EM spectrum*. These astrophysical events include the merger of compact objects, black holes and neutron stars (NSs). Some examples from my work are detailed below.



Accretion onto Massive Black Hole Binaries:
Circumbinary Disks

Most galaxies have central BHs and and when galaxies collide, those black holes sink to the center of the newly formed galaxy. The two BHs could form a massive black hole binary (MBHB). The galaxy merger can also torque gas to its new nucleus, the MBHB will interact with this gas affecting the MBHB orbit and possibly creating very bright EM radiation via BH accretion.

In a 2013 paper Zoltan Haiman, Andrew MacFadyen, and I ran 2D hydrodynamical simulations of a thin circumbinary accretion disk and found that accretion onto the MBHB could be significant and also periodic. The periodicity occurs on specific timescales depending on the binary mass ratio (see here). This possible periodic signature can aid in identifing and interpreting MBHB systems.

In 2015 we used these predictions and new simulations, using the moving mesh hydrodynamical code DISCO, to interpet a MBHB candidate in the quasar PG 1302-102. Graham+2015 found PG 1302 to exhibit a steady, 5 year period variability in the optical. Single epoch mass measurements of the central BH(s), in addition to the 5 year period, cause the most likely explanation of the observed periodicity to be a MBHB with sub-parsec separation. In this paper we discuss the consequences for a MBHB in PG 1302 if the optical variability is caused by accretion onto the binary. Our simulations find that for binaries with a smaller BH with mass greater than a third of the larger BH, the dominate variability occurs not on the timescale of a binary orbit, but on the longer timescale of gaseous orbits in the circumbinary disk (CBD).

The movie below illustrates this. The movie shows density contours of a gas disk interacting with the binary over 20 binary orbits after the disc has evolved long enough to reach a quasi-steady state. A lopsided, central low-density cavity is cleared and the binary continually pulls gas streams from the outer CBD into the central cavity, these streams are flung back out in the CBD causing overdensities (black) to orbit at the inner edge of the CBD (I have a recent paper explaining the transition of CBD solutions to those with the lopsided cavity; characterization of the orbitting overdensity is future work). These overdensities in turn cause variability in the BH accretion rate as the streams which feed the BHs are fed by the orbiting overdensities.




Attributing the longer period of the orbiting overdensity with the observed periodicity in PG 1302 yields a smaller infered separation of the binary. This realization places the MBHB in PG 1302 well into the gravitatational radiation driven regime of inspiral, providing evidence that these types of gravitational wave emitting mergers do occur (despite the final parsec problem) and showing that EM signals of the inspiral can occur even at this late stage where the binary is running away to coalesence faster than it was previously thought that gas accretion could keep up with (see e.g. and references therein).

The closer inferred binary separation has other intruiging consequences which we explore in detail in the above linked paper (this one!).

Also, Zoltan Haiman, David Schiminovich and I have an exciting, new explanation for the MBHB in PG 1302 which came out September 17 in Nature. Here is press coverage of the article.


Neutron Star + Stellar Mass Black Hole Mergers

UNDER CONSTRUCTION

Movies from Big black hole, little neutron star: Magnetic dipole fields in the Rindler spacetime




Janna Levin, myself, and collaborators Norm Murray and Larry Price have a new paper titled: "Bright transients from strongly-magnetized neutron star-black hole mergers" on the observability of NS-BH mergers based on the BH-battery mechanism described in the above paper. Figure 1 from this paper was recently featured in PRD's Kaleidoscope.

Other Interesting Stuff

Prasenjit Saha and I found these analytic solutions for orbits around a Schwarzschild BH. Their discovery was motivated by work aiming to test general relativity using the S-stars orbitting our own Milky Way BH (see here). Let me know if you could use them too!



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