Electromagnetic Signatures of Relativistic Explosions in AGN Disks

Abstract: The disks of Active Galactic Nuclei (AGNs), traditionally studied as the feeders of the supermassive black holes (SMBHs) at their centers, have recently triggered a lot of interest also as hosts to massive stars and hence their neutron star (NS) and black hole (BH) remnants. Migration traps and gas torques in these disks favor binary formation and enhance the rate of compact object mergers. In these environments both long gamma-ray bursts (GRBs) from the death of massive stars and short GRBs from NS-NS and NS-BH mergers are expected. However, their properties in the unique environments of AGN disks have never been studied. Here we show that GRBs in AGNs can display unique features, owing to the unusual relative position of the shocks that characterize the burst evolution and the Thomson photosphere of the AGN disk. In dense environments, for example, the external shock develops before the internal shocks, leading to prompt emission powered by a relativistic reverse shock. The transient's time evolution is also compressed, yielding afterglow emission that is much brighter and peaks much earlier than for GRBs in the interstellar medium. Additionally, in regions of the disk that are sufficiently dense and extended, the light curves are dominated by diffusion, since the fireball is trapped inside the disc photosphere. These diffusion-dominated transients emerge on timescales of days in disks around SMBHs of $\sim 10^6 M_\odot$ to years for SMBHs of $\sim 10^8 M_\odot$. Finally, a large fraction of events, especially in AGNs with SMBHs $\lesssim 10^7 M_\odot$, display time-variable absorption in the X-ray band.