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ABSTRACT 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, multiwavelength, time-dependent EM models are needed to identify the right host galaxy. We studied merging MBHBs embedded in a circumbinary disc (CBD) using high-resolution two-dimensional simulations, with a Γ-law equation of state, incorporating viscous heating, shock heating, and radiative cooling. We simulate the binary from large separation until after merger, allowing us to model the decoupling of the binary from the CBD. We compute the EM signatures and identify distinct features before, during, and after the merger. Our main result is a multiband EM signature: we find that the MBHB produces strong thermal X-ray emission until 1–2 d prior to the merger. However, as the binary decouples from the CBD, the X-ray-bright minidiscs 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 GW recoil or mass-loss. Looking for the 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.
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The dynamics and electromagnetic signatures of accretion in unequal-mass binary black hole inspirals
ABSTRACT We present a theoretical study of the gravitational wave (GW) driven inspirals of accreting black hole binaries with mass $$M = 10^7 M_\odot$$ and mass ratios between $$10^{-3}$$ and $$10^{-1}$$. Our results are based on analytic estimates, and grid-based hydrodynamics simulations run for many thousands of binary orbits before the merger. We show that the GW inspiral is evident in the light curves and colour evolution of a binary-hosting quasar over years to decades before a merger. The long-term electromagnetic (EM) signature is characterized by a gradual UV brightening and X-ray dimming, followed by an X-ray disappearance hours to days before the GW burst, and finally, a years-like re-brightening as the disc relaxes and refuels the remnant black hole. These time-scales are surprisingly insensitive to the normalization of the kinematic viscosity in the disc. The spectrum of quasi-thermal disc emission shows two peaks: one in the UV and another in the X-ray, associated with the outer and circum-secondary discs, respectively; emission from the inner disc is suppressed because the secondary consumes most of the inflowing gas. We discuss implications for real-time and archival EM follow-up of GW bursts detected by LISA.
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- Award ID(s):
- 2408034
- PAR ID:
- 10583738
- Publisher / Repository:
- Oxford University Press
- Date Published:
- Journal Name:
- Monthly Notices of the Royal Astronomical Society
- Volume:
- 539
- Issue:
- 2
- ISSN:
- 0035-8711
- Format(s):
- Medium: X Size: p. 1430-1447
- Size(s):
- p. 1430-1447
- Sponsoring Org:
- National Science Foundation
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