Paper on electromagnetic counterparts of binary MBH mergers

A new paper considers different mechanisms leading to an electromagnetic counterpart to a GW binary MBH merger event. The authors suggest that most LISA events of that type could have a detectable counterpart.

The various possible EM counterparts. M is the mass of the primary MBH, q is the mass ratio

The possibility of LISA GW sources to have a detectable GW counterpart is a very exciting but little explored avenue. A counterpart would palliate LISA's limited angular resolution and allow to identify the host galaxy, hence allowing (through redshift measurement) the determination of the rest-frame mass of the progenitor MBHs and the study of characteristics of the merged galaxy.

In a paper recently posted on astro-ph (http://www.arxiv.org/abs/astro-ph/0605624), Dotti at al. review the various processes that can lead to an electromagnetic counterpart for the GW signal produced by the merger of two massive BHs. In most cases, an accretion disk (either around the more massive MBH or the binary) is involved and the authors also present a useful summary of recent research on how such cold gas can solve the infamous "last parsec problem", i.e. allow the binary MBH to merge despite the relative inefficiency of extraction of orbital energy by stars (particularly in nuclei hosting  MBH more massive than  ~10⁷ Msun).

The authors consider "precursor" signals as well as EM counterpart following the GW event. A precursor EM signal would be large-amplitude luminosity variations followed by a sudden disappearence of a nuclear source as the disc around the primary MBH is perturbed and destroyed.  Another possibility is the tidal disruption of stars whose orbits around the primary are perturbed by the secondary, a scenario which would require further investigation. "Afterglows" could be produced by the resumption of accretion on the merged MBH once the central gap in the disk has filled.

The authors argue that (putting aside tidal disruptions), systems more massive than a few 10⁷ Msun could have an EM precursor and still show nuclear starburst activity when the MBHs merge. Less massive systems will not have a precursor but an afterglow that could be detected by future Xray missions such as XEUS a few years after the GW signal in most cases.