Prompt Emission of γ-Ray Bursts in the High-density Environment of Active Galactic Nucleus Accretion Disks

Abstract

Long and short γ-ray bursts (GRBs) are traditionally associated with galactic environments, where circumburst densities are small or moderate (few to hundreds of protons per cubic centimeter). However, both are also expected to occur in the disks of active galactic nuclei, where the ambient medium density can be much larger. In this work we study, via semianalytical methods, the propagation of the GRB outflow, its interaction with the external material, and the ensuing prompt radiation. In particular, we focus on the case in which the external shock develops early in the evolution at a radius that is smaller than the internal shock one. We find that bursts in such high-density environments are likely characterized by a single, long emission episode that is due to the superposition of individual pulses, with a characteristic hard-to-soft evolution irrespective of the light-curve luminosity. While multipulse light curves are not impossible, they would require the central engine to go dormant for a long time before reigniting. In addition, short GRB engines would produce bursts with prompt duration that would exceed the canonical 2 s separation threshold and likely be incorrectly classified as long events, even though they would not be accompanied by a simultaneous supernova. Finally, these events have a large dynamical efficiency, which would produce a bright prompt emission followed by a somewhat dim afterglow.

The effects of Time-Variable Absorption due to Gamma-Ray Bursts In Active Galactic Nuclei Accretion Disks

“…Perna & Loeb 1998;Böttcher et al 1999;Lazzati et al 2001;Frontera et al 2004;Robinson et al 2009;Campana et al 2021), bursts in the environment of Active Galactic Nuclei (AGN) accretion disks is a relatively new area of research with few dedicated studies thus far (for studies that have already been performed, see e.g. Perna et al 2021a;Yuan et al 2021;Zhu et al 2021b,a;Lazzati et al 2022).…”

A long-duration gamma-ray burst of dynamical origin from the nucleus of an ancient galaxy

“…In particular, the high densities result in the formation of an external shock very close to the progenitor, and the dissipation of energy within this shock causes bursts which are intrinsically short and hard to be smeared out and softened [11]. Although not conclusive, the similarity of GRB 191019A to these models is remarkable.…”

“…The burst properties could naturally be explained via compact object mergers involving white dwarfs (WD), neutron stars (NS) or black holes (BH). These may form dynamically in dense stellar clusters, or originate in a gaseous disc around the supermassive black hole [10,11]. Future electromagnetic and gravitational-wave observations in tandem thus offer a route to probe the dynamical fraction and the details of dynamical interactions in galactic nuclei and other high density stellar systems.…”

“…A very recent and explicit prediction of compact object mergers within discs is that intrinsically short-hard GRBs should become longer and softer [11], with a notable hard-soft evolution. This is exactly what is seen in GRB 191019A.…”

Observable signatures of stellar-mass black holes in active galactic nuclei

“…Recently, several studies have investigated emission from transients emerging from AGN disks. Zhu et al (2021c), Zhu et al (2021a), Perna et al (2021a), Yuan et al (2021), Wang et al (2022), andLazzati et al (2022) estimated the emission from gamma-ray bursts, and Perna et al (2021b) and Zhu et al (2021b) discussed the electromagnetic signatures expected from accretion induced collapse of neutron stars and white dwarfs. Yang et al (2021) studied the properties of tidal disruption of stars by stellar-mass BHs, while Grishin et al (2021) investigated supernova explosions, and Bartos et al (2017) and Stone et al (2017) estimated the electromagnetic emission produced by thermal radiation and/or outflows from circum-BH disks in AGN disks.…”