The emergence of diffused gamma-ray burst afterglows from the discs of active galactic nuclei

Abstract

The disks of Active Galactic Nuclei (AGNs) have emerged as rich environments for the production and capture of stars and the compact objects that they leave behind. These stars produce long Gamma-Ray Bursts (LGRBs) at their deaths, while frequent interactions among compact objects form binary neutron stars and neutron star-black hole binaries, leading to short GRBs (SGRBs) upon their merger. Predicting the properties of these transients as they emerge from the dense environments of AGN disks is key to their proper identification and to better constrain the star and compact object population in AGN disks. Some of these transients would appear unusual because they take place in much higher densities than the interstellar medium. Others, which are the subject of this paper, would additionally be modified by radiation diffusion, since they are generated within optically thick regions of the accretion disks. Here we compute the GRB afterglow light curves for diffused GRB sources for a representative variety of central black hole masses and disk locations. We find that the radiation from radio to UV and soft X-rays can be strongly suppressed by synchrotron self-absorption in the dense medium of the AGN disk. In addition, photon diffusion can significantly delay the emergence of the emission peak, turning a beamed, fast transient into a slow, isotropic, and dimmer one. These would appear as broadband-correlated AGN variability with a dominance at the higher frequencies. Their properties can constrain both the stellar populations within AGN disks as well as the disk structure.

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

“…Even though a scattered photon does not disappear, its path to the observer is increased by ∼ 𝐻, causing diffusion of the prompt emission over a timescale Δ𝑡 ∼ 3𝑅 16 days. Such a temporally stretched burst would be undetectable in most circumstances (see also Wang et al 2022 for a more refined treatment of radiative transfer under these conditions).…”

“…Golant & Sironi 2022), it is currently understood that afterglows for both short and long GRBs are synchrotron radiation resulting from the collision of a relativistic shell with an external medium (Sari et al 1998;Panaitescu & Kumar 2000). To compute our afterglows, we used a code that has been used in various previous papers (Lazzati et al 2018;Perna et al 2022;Wang et al 2022) and performs the afterglow computation numerically. As input parameters to the afterglow computation we use 𝜖 𝑒 = 0.3 and 𝜖 𝐵 = 0.1, where 𝜖 𝑒 is the fraction of the shock energy that is given to electrons and 𝜖 𝐵 is the fraction of the shock energy given to tangled magnetic fields.…”

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

“…First, hard-to-soft bursts should be predominantly FREDs, possibly showing evidence of variability overlaid on the overall pulse profile. Second, if an afterglow is observed from such bursts, it should show the characteristics of high external density, such as a fast evolution and a spectrum characterized by high-frequency self-absorption (Wang et al 2022). Finally, should a precise localization be available, the burst should originate from the very center of the host galaxy.…”

“…Additionally, they can help constrain the neutrino background associated with relativistic sources (Fasano et al 2021;Zhu et al 2021a). As the prompt γ-ray emission is generally followed by longer-wavelength radiation, it would contribute to the AGN variability in the optical and infrared (Wang et al 2022). With the upcoming Vera Rubin observatory, some of these transients will be observed; hence, it becomes especially important to recognize them in association with their prompt γray emission.…”

Interacting Kilonovae: Long-lasting Electromagnetic Counterparts to Binary Mergers in the Accretion Disks of Active Galactic Nuclei

“…The emission of jets has been studied under the precondition of whether the progenitor has opened a cavity or not. For the non-cavity scenario, Perna et al (2021a) looked at the dynamics and radiation of the relativistic jet in the disk and studied the typical radii and observability of both the prompt and afterglow radiation; Wang et al (2022) simulated the diffused GRB afterglows from the disk of an AGN. Upon reprocessing by disk, the GRB afterglows became dimmer but lasted longer.…”