Observable Signatures of Coalescing Compact Binaries

Brian Metzger

May 24, 2012

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Abstract: A fraction of neutron star (NS) and black hole (BH) binaries are formed sufficiently compact that they will inspiral and merge due to the emission of gravitational waves within a Hubble time. Such compact object mergers, in addition to representing some of the most extreme astrophysical events, are among the most promising sources for the direct detection of gravitational waves with ground based interferometers such as LIGO and Virgo. Maximizing the science return of such a detection will, however, require identifying a coincident electromagnetic (EM) counterpart. One possible source of EM emission is a short duration gamma ray burst (GRB), powered by the accretion of material that remains in a rotationally supported torus around the central black hole. Although observations of short GRBs are largely consistent with the merger model, the puzzling discovery has been made that many of them are followed by late ray flaring, which does not fit current theory and may require considering alternative progenitor models. Another source of EM emission from NS mergers is a supernova like optical transient, powered by the radioactive decay of heavy elements synthesized in neutron rich ejecta from the merger. I will present the first calculations of the radioactively powered transients from mergers that include realistic nuclear physics and radiative transport, and I will discuss the prospects for detecting and identifying such events following a gravitational wave trigger. In the second part of my talk I will describe a model for accretion following the tidal disruption of a white dwarf by a NS or BH. I will demonstrate that densities and temperatures in the disk are sufficiently high to burn the white dwarf material into increasingly heavier elements at sequentially smaller radii. Because the energy released by nuclear reactions is comparable to that released gravitationally, I introduce the concept of a “nuclear dominated accretion flow”. I will argue that outflows from the disk power a ~ week long transient, which may be related to recently-discovered classes of subluminous supernovae.