Long-term remnant evolution of compact binary mergers

Abstract

We investigate the long-term evolution and observability of remnants originating from the merger of compact binary systems and discuss the differences to supernova remnants. Compact binary mergers expel much smaller amounts of mass at much higher velocities, as compared to supernovae and therefore the free expansion phase of the remnant will be short (~ 1 - 10 yr). In general the remnants will be observable for a considerable time (~ 10^6 - 10^7 yr). Events releasing large amounts of kinetic energy may be responsible for a subsample of observed giant HI holes of unknown origin as compact binaries merge far away from star forming regions. If the ejecta consist primarily of actinides, on long timescales the expelled material will contain mainly the few quasi-stable nuclei in the actinides range. Consequently the abundance of each isotope in the ejecta might be of the order of a few percent. During their decay some actinides will produce observational signatures in form of gamma ray lines. We particularly investigate the gamma ray emission of Am 243, Cm 247, Cm 248 and Bi 208 and estimate their observability in nearby remnants. Detections of the gamma ray lines with INTEGRAL will be possible only in very advantageous cases but these remnants are promising targets for future instruments using focusing optics for soft gamma rays. Due to the low mass expelled in mergers and due to the lack of free electrons in the ejecta, the merger remnants might be significantly fainter in bremsstrahlung and synchrotron radiation than comparable supernova remnants. Hence merger remnants might represent a candidate for very recently discovered 'dark accelerators' which are hard gamma ray sources with no apparent emission in other bands.