ABSTRACT The afterglow emission from gamma-ray bursts (GRBs) is believed to originate from a relativistic blast wave driven into the circumburst medium. Although the afterglow emission from radio up to X-ray frequencies is thought to originate from synchrotron radiation emitted by relativistic, non-thermal electrons accelerated by the blast wave, the origin of the emission at high energies (HE; ≳GeV) remains uncertain. The recent detection of sub-TeV emission from GRB 190114C by the Major Atmospheric Gamma Imaging Cherenkov Telescopes (MAGIC) raises further debate on what powers the very high energy (VHE; ≳300 GeV) emission. Here, we explore the inverse Compton scenario as a candidate for the HE and VHE emissions, considering two sources of seed photons for scattering: synchrotron photons from the blast wave (synchrotron self-Compton or SSC) and isotropic photon fields external to the blast wave (external Compton). For each case, we compute the multiwavelength afterglow spectra and light curves. We find that SSC will dominate particle cooling and the GeV emission, unless a dense ambient infrared photon field, typical of star-forming regions, is present. Additionally, considering the extragalactic background light attenuation, we discuss the detectability of VHE afterglows by existing and future gamma-ray instruments for a wide range of model parameters. Studying GRB 190114C, we find that its afterglow emission in the Fermi-Large Area Telescope (LAT) band is synchrotron dominated. The late-time Fermi-LAT measurement (i.e. t ∼ 104 s), and the MAGIC observation also set an upper limit on the energy density of a putative external infrared photon field (i.e. ${\lesssim} 3\times 10^{-9}\, {\rm erg\, cm^{-3}}$), making the inverse Compton dominant in the sub-TeV energies.
more »
« less
GeV Signatures of Short Gamma-Ray Bursts in Active Galactic Nuclei
Abstract The joint detection of gravitational waves and the gamma-ray counterpart of a binary neutron star merger event, GW170817, unambiguously validates the connection between short gamma-ray bursts and compact binary object (CBO) mergers. We focus on a special scenario where short gamma-ray bursts produced by CBO mergers are embedded in disks of active galactic nuclei (AGNs), and we investigate the γ -ray emission produced in the internal dissipation region via synchrotron, synchrotron self-Compton, and external inverse Compton (EIC) processes. In this scenario, isotropic thermal photons from the AGN disks contribute to the EIC component. We show that a low-density cavity can be formed in the migration traps, leading to the embedded mergers producing successful GRB jets. We find that the EIC component would dominate the GeV emission for typical CBO mergers with an isotropic-equivalent luminosity of L j ,iso = 10 48.5 erg s −1 that are located close to the central supermassive black hole. Considering a long-lasting jet of duration T dur ∼ 10 2 –10 3 s, we find that the future Cherenkov Telescope Array (CTA) will be able to detect its 25–100 GeV emission out to a redshift z = 1.0. In the optimistic case, it is possible to detect the on-axis extended emission simultaneously with GWs within one decade using MAGIC, H.E.S.S., VERITAS, CTA, and LHAASO-WCDA. Early diagnosis of prompt emissions with Fermi-GBM and HAWC can provide valuable directional information for the follow-up observations.
more »
« less
- Award ID(s):
- 2108467
- NSF-PAR ID:
- 10341192
- Date Published:
- Journal Name:
- The Astrophysical Journal
- Volume:
- 932
- Issue:
- 2
- ISSN:
- 0004-637X
- Page Range / eLocation ID:
- 80
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Abstract Many gamma-ray bursts (GRBs) have been observed from radio wavelengths, and a few at very high energies (VHEs, >100 GeV). The High Altitude Water Cherenkov (HAWC) gamma-ray observatory is well suited to study transient phenomena at VHEs owing to its large field of view and duty cycle. These features allow for searches of VHE emission and can probe different model assumptions of duration and spectra. In this paper, we use data collected by HAWC between 2014 December and 2020 May to search for emission in the energy range from 80 to 800 GeV coming from a sample of 47 short GRBs that triggered the Fermi, Swift, and Konus satellites during this period. This analysis is optimized to search for delayed and extended VHE emission within the first 20 s of each burst. We find no evidence of VHE emission, either simultaneous or delayed, with respect to the prompt emission. Upper limits (90% confidence level) derived on the GRB fluence are used to constrain the synchrotron self-Compton forward-shock model. Constraints for the interstellar density as low as 10 −2 cm −3 are obtained when assuming z = 0.3 for bursts with the highest keV fluences such as GRB 170206A and GRB 181222841. Such a low density makes observing VHE emission mainly from the fast-cooling regime challenging.more » « less
-
null (Ed.)ABSTRACT Recently, ground-based Imaging Atmospheric Cherenkov Telescopes have reported the detection of very-high-energy (VHE) gamma-rays from some gamma-ray bursts (GRBs). One of them, GRB 190829A, was triggered by the Swift satellite, and about 2 × 104 s after the burst onset the VHE gamma-ray emission was detected by H.E.S.S. with ∼5σ significance. This event had unusual features of having much smaller isotropic equivalent gamma-ray energy than typical long GRBs and achromatic peaks in X-ray and optical afterglow at about 1.4 × 103 s. Here, we propose an off-axis jet scenario that explains these observational results. In this model, the relativistic beaming effect is responsible for the apparently small isotropic gamma-ray energy and spectral peak energy. Using a jetted afterglow model, we find that the narrow jet, which has the initial Lorentz factor of 350 and the initial jet opening half-angle of 0.015 rad, viewed off-axis can describe the observed achromatic behaviour in the X-ray and optical afterglow. Another wide, baryon-loaded jet is necessary for the later-epoch X-ray and radio emissions. According to our model, the VHE gamma rays observed by H.E.S.S. at 2 × 104 s may come from the narrow jet through the synchrotron self-Compton process.more » « less
-
Abstract We investigate prospects for the detection of high-energy neutrinos produced in the prolonged jets of short gamma-ray bursts (sGRBs). The X-ray light curves of sGRBs show extended emission components lasting for 100–1000 s, which are considered to be produced by prolonged engine activity. Jets produced by such activity should interact with photons in the cocoon formed by the propagation of the jet inside the ejecta of neutron star mergers. We calculate neutrino emission from jets produced by prolonged engine activity, taking account of the interaction between photons provided from the cocoon and cosmic rays accelerated in the jets. We find that IceCube-Gen2, a future neutrino telescope, with second-generation gravitational-wave detectors will probably be able to observe neutrino signals associated with gravitational waves with around 10 years of operation, regardless of the assumed value of the Lorentz factor of the jets. Neutrino observations may enable us to constrain the dissipation region of the jets. We apply this model to GRB 211211A, a peculiar long GRB whose origin may be a binary neutron star merger. Our model predicts that IceCube is unlikely to detect any associated neutrinos, but a few similar events will be able to put a meaningful constraint on the physical quantities of the prolonged engine activities.more » « less
-
more » « less
ABSTRACT The recent discovery of TeV emission from gamma-ray bursts (GRBs) by the MAGIC and H.E.S.S. Cherenkov telescopes confirmed that emission from these transients can extend to very high energies. The TeV energy domain reaches the most sensitive band of the Cherenkov Telescope Array (CTA). This newly anticipated, improved sensitivity will enhance the prospects of gravitational-wave follow-up observations by CTA to probe particle acceleration and high-energy emission from binary black hole and neutron star mergers, and stellar core-collapse events. Here we discuss the implications of TeV emission on the most promising strategies of choice for the gravitational-wave follow-up effort for CTA and Cherenkov telescopes more broadly. We find that TeV emission (i) may allow more than an hour of delay between the gravitational-wave event and the start of CTA observations; (ii) enables the use of CTA’s small size telescopes that have the largest field of view. We characterize the number of pointings needed to find a counterpart. (iii) We compute the annual follow-up time requirements and find that prioritization will be needed. (iv) Even a few telescopes could detect sufficiently nearby counterparts, raising the possibility of adding a handful of small-sized or medium-sized telescopes to the network at diverse geographic locations. (v) The continued operation of VERITAS/H.E.S.S./MAGIC would be a useful compliment to CTA’s follow-up capabilities by increasing the sky area that can be rapidly covered, especially in the United States and Australia, in which the present network of gravitational-wave detectors is more sensitive.
- Free Publicly Accessible Full Text
-
Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.3847/1538-4357/ac6ddf
