Multimessenger astronomy with pulsar timing and X-ray observations of massive black hole binaries

Abstract

We demonstrate that very massive (>10^8\msun), cosmologically nearby (z<1) black hole binaries (MBHBs), which are primary targets for ongoing and upcoming pulsar timing arrays (PTAs), are particularly appealing multimessenger carriers. According to current models for massive black hole formation and evolution, the planned Square Kilometer Array (SKA) will collect gravitational wave signals from thousands of such massive systems, being able to individually resolve and locate in the sky several of them (maybe up to a hundred). By employing a standard model for the evolution of MBHBs in circumbinary discs, with the aid of dedicated numerical simulations, we characterize the gas-binary interplay, identifying possible electromagnetic signatures of the PTA sources. We concentrate our investigation on two particularly promising scenarios in the high energy domain, namely, the detection of X-ray periodic variability and of double broad K\alpha iron lines. Up to several hundreds of periodic X-ray sources with a flux >10^-13 erg s^-1 cm^-2 will be in the reach of upcoming X-ray observatories. Double relativistic K\alpha lines may be observable in a handful of low redshift (z<0.3) sources by proposed deep X-ray probes, such as Athena. (Abridged

Astrophysics of super-massive black hole mergers

“…Also, unlike LISA sources, PTA sources would be at an earlier stage in their inspiral and thus be much longer lived, allowing for even more extensive study. A sufficiently large sample of such sources would even allow us to test whether they are evolving due to GW emission or gas-driven migration [131,250,230] (a test that might also be done with LISA with only a single source with sufficient signal-to-noise [267]). …”

Gravitational wave emission from binary supermassive black holes

“…Sky location within few tens to few deg 2 is possible (see also [92,94]), and even sub deg 2 determination, under some specific conditions [103]. Even though this is a large chunk of the sky, these systems are extremely massive and at relatively low redshift (z < 0.5), making any putative electromagnetic signature of their presence (e.g., emission periodicity related to the binary orbital period, peculiar emission spectra, peculiar Kα line profiles, etc) detectable [104,105].…”

Resolving multiple supermassive black hole binaries with pulsar timing arrays. II. Genetic algorithm implementation

“…Can we locate them in the sky, and to what level of accuracy? All these questions are of great interest for the astrophysical community; precise sky localization of individual sources will allow the efficient search for electromagnetic counterparts [27,28], opening the new horizon of multimessenger astronomy. This is a second in a series of paper devoted to the exploration of the PTA potential of resolving multiple GW sources.…”