[Paper Review] Observable Signatures of EMRI Black Hole Binaries Embedded in Thin Accretion Disks

Bence Kocsis, Nicolás Yunes+1 more|arXiv (Cornell University)|Apr 12, 2011

Astrophysical Phenomena and ObservationsPhysics and Astronomy189 references243 citations

TL;DR

This paper investigates electromagnetic (EM) and gravitational wave (GW) signatures of extreme mass-ratio inspirals (EMRIs)—stellar-mass compact objects spiraling into supermassive black holes—embedded in thin, radiation-pressure-dominated accretion disks. It shows that disk-induced migration causes a 10–1000 rad/year phase shift in GWs, detectable by LISA, and that gap refilling during late inspiral causes sudden EM brightening, enabling joint EM-GW constraints on disk physics.

ABSTRACT

We examine the electromagnetic (EM) and gravitational wave (GW) signatures of stellar-mass compact objects (COs) spiraling into a supermassive black hole (extreme mass-ratio inspirals or EMRIs), embedded in a thin, radiation-pressure dominated, accretion disk. At large separations, the tidal effect of the secondary CO clears a gap. We show that the gap refills during the late GW-driven phase of the inspiral, leading to a sudden EM brightening of the source. The accretion disk leaves an imprint on the GW through its angular momentum exchange with the binary, the mass increase of the binary members due to accretion, and its gravity. We compute the disk-modified GWs both in an analytical Newtonian approximation and in a numerical effective-one-body approach. We find that disk-induced migration provides the dominant perturbation to the inspiral, with weaker effects from the mass accretion onto the CO and hydrodynamic drag. Depending on whether a gap is present, the perturbation of the GW phase is between 10 and 1000 radians per year, detectable with the future Laser Interferometer Space Antenna (LISA) at high significance. The Fourier transform of the disk-modified GW in the stationary phase approximation is sensitive to disk parameters with a frequency trend different from post-Newtonian vacuum corrections. Our results suggest that observations of EMRIs may place new sensitive constraints on the physics of accretion disks.

Motivation & Objective

• To investigate how thin, radiation-pressure-dominated accretion disks modify EMRI gravitational wave and electromagnetic signals.
• To determine whether LISA can detect disk-induced perturbations in EMRI waveforms.
• To assess the detectability of disk parameters via GW Fourier transforms and phase shifts.
• To evaluate the role of disk gravity, mass accretion, and hydrodynamic drag in altering EMRI dynamics.
• To explore the potential for joint EM and GW observations to constrain accretion disk structure and physics.

Proposed method

• Used analytical Newtonian and numerical effective-one-body (EOB) approaches to model disk-modified EMRI waveforms.
• Computed disk-induced GW phase corrections due to migration, mass accretion, and disk gravity.
• Modeled gap formation via tidal forces from the secondary compact object and its refilling during late inspiral.
• Applied the stationary phase approximation to compute Fourier transforms of modified GW signals.
• Quantified detectability using dephasing and overlap measures relevant to LISA sensitivity.
• Incorporated disk potential and angular momentum exchange effects into the EOB Hamiltonian and radiation-reaction force.

Experimental results

Research questions

• RQ1 How does a thin, radiation-pressure-dominated accretion disk alter the gravitational wave phase of an EMRI?
• RQ2 What is the magnitude and detectability of disk-induced migration effects on EMRI inspirals?
• RQ3 Can EMRI gap refilling produce observable electromagnetic brightening?
• RQ4 How do disk gravity, mass accretion, and hydrodynamic drag compare in their influence on GW phasing?
• RQ5 Can the Fourier transform of disk-modified GWs distinguish disk parameters from post-Newtonian vacuum corrections?

Key findings

• Disk-induced migration causes a GW phase perturbation of 10–1000 radians per year, detectable with high significance by LISA.
• Gap refilling during the late inspiral phase leads to a sudden electromagnetic brightening of the EMRI source.
• The Fourier transform of disk-modified GWs exhibits a frequency trend distinct from post-Newtonian vacuum corrections, enabling disk parameter inference.
• Migration is the dominant perturbation to EMRI inspiral, with weaker contributions from mass accretion and hydrodynamic drag.
• The combined EM and GW signal provides a new, sensitive probe of accretion disk physics, including gap structure and mass supply rates.
• The study demonstrates that EMRI observations could constrain disk parameters such as surface density and viscosity through GW phasing.