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 ∼ 10 6 M to years for SMBHs of ∼ 10 8 M . Finally, a large fraction of events, especially in AGNs with SMBHs 10 7 M , display time-variable absorption in the X-ray band.

Neutron Star Mergers in AGN Accretion Disks: Cocoon and Ejecta Shock Breakouts

“…After the completion of this work, we learned that Perna et al (2020) have independently investigated the general properties of GRBs in an AGN disk. Different from our approach to focus on the characteristics of EM signals of BNS/NSBH mergers located at migration traps in AGN disks, they discussed the various possible outcomes of a GRB originating from different distances from the supermassive BH and different BH masses.…”

Outflow Bubbles from Compact Binary Mergers Embedded in Active Galactic Nuclei: Cavity Formation and the Impact on Electromagnetic Counterparts

“…• Short GRB jets from neutron star mergers are more likely to be seen without being choked, but gamma-ray spectra may be modified. The mergers of BNS and NS-BH binaries are also expected in AGN disks, which can induce unique EM signals (e.g., Zhu et al 2020;Perna et al 2020). BNS and NS-BH mergers are expected to occur at the migration trap located in R < 10 3 (Bellovary et al 2016) or 10 −2 pc (corresponding to R ∼ 10 4 for M • = 10 7 M ⊙ ) suggested by dedicated numerical calculations (Tagawa et al 2020b).…”

“…Recently, Zhu et al (2020) discussed the cases for binary neutron star and neutron star-black hole mergers and jet breakout emission from kilonova ejecta. Perna et al (2020) focused more generally on explosions in AGN disks and the ensuing breakout emission.…”