The upcoming Laser Interferometer Space Antenna (LISA) is expected to detect gravitational waves (GWs) from massive black hole binaries (MBHB). Finding the electromagnetic (EM) counterparts for these GW events will be crucial for understanding how and where MBHBs merge, measuring their redshifts, constraining the Hubble constant and the graviton mass, and for other novel science applications. However, due to poor GW sky localization, multi-wavelength, time-dependent electromagnetic (EM) models are needed to identify the right host galaxy. This dissertation investigates electromagnetic (EM) signatures to accompany compact black hole binaries, specifically those that occur prior to, during, and following the merger, as well as those originating via self-lensing flares (SLFs).
Chapter 2 considers equal-mass merging massive black hole binaries (MBHBs) embedded in a circumbinary disk (CBD), using high-resolution two-dimensional simulations, with a 𝚪-law equation of state, incorporating viscous heating, shock heating, and radiative cooling. Beginning from before the decoupling limit and transitioning through into post-merger, distinct EM features are identified before, during, and after the merger. The main result is that the MBHB produces strong thermal X-ray emission until 1-2 days prior to the merger. However, as the binary decouples from the CBD, the X-ray-bright minidisks rapidly shrink in size, become disrupted, and the accretion rate drops precipitously. As a result, the thermal X-ray luminosity drops by orders of magnitude, and the source remains X-ray dark for several days, regardless of any post-merger effects such as gravitational wave (GW) recoil or mass loss. Looking for this abrupt spectral change where the thermal X-ray disappears is a tell-tale EM signature of LISA mergers that does not require extensive pre-merger monitoring.
Chapter 3 follows up on and extends the results of Chapter~\ref{chap:ch2} by investigating the effects to the EM spectrum for unequal-mass MBHBs via comparable simulations. This work corroborates the findings of a several order of magnitude drop in the thermal X-ray luminosity near the time of merger, but with delayed timing than found in an equal-mass system, while the source still remains X-ray dark for hours post-merger. The main result, however, is a new signature, a sharp spike in the thermal X-ray emission just before the tell-tale steep drop occurs. This adds an additional EM signature that can be used to identify EM counterparts of LISA's unequal MBHBs before the merger and potentially measure the mass ratio of the system through EM means.
Finally, Chapter 4 addresses the EM signature of self-lensing flares (SLFs). SLFs are expected to be produced once or twice per orbit by an accreting MBHB, if the eclipsing MBHBs are observed close to edge-on. Again, using high-resolution two-dimensional viscous hydrodynamical simulations of a CBD embedding a MBHB, a very high-cadence output of these hydrodynamical simulation is used as inputs for a general-relativistic ray-tracing code to produce synthetic spectra and phase-folded light curves.
The main results show a significant periodic amplification of the flux with the characteristic shape of a sharp flare with a central dip, as the foreground black hole (BH) transits across the minidisk and shadow of the background BH, respectively. These corroborate previous conclusions based on the microlensing approximation and analytical toy models of the emission geometry. A realistic concern with incorporating a physical disk was that the CBD might obscure our view of the SLF, considering they only appreciably occur for a near edge-on line of sight. However, this work shows that the CBD is in fact more a friend than foe in the detection, because while the CBD does indeed block other sources of emission that constitute noise, the bent trajectories of the light from the lensed minidisks remain visible even for these edge-on configurations.
| Identifer | oai:union.ndltd.org:columbia.edu/oai:academiccommons.columbia.edu:10.7916/1g9r-1198 |
| Date | January 2024 |
| Creators | Krauth, Luke Major |
| Source Sets | Columbia University |
| Language | English |
| Detected Language | English |
| Type | Theses |
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