MAGIC Observations of the Nearby Short Gamma-Ray Burst GRB 160821B ^*

Abstract

The coincident detection of GW170817 in gravitational waves and electromagnetic radiation spanning the radio to MeV gamma-ray bands provided the first direct evidence that short gamma-ray bursts (GRBs) can originate from binary neutron star (BNS) mergers. On the other hand, the properties of short GRBs in high-energy gamma rays are still poorly constrained, with only ∼20 events detected in the GeV band, and none in the TeV band. GRB 160821B is one of the nearest short GRBs known at z = 0.162. Recent analyses of the multiwavelength observational data of its afterglow emission revealed an optical-infrared kilonova component, characteristic of heavy-element nucleosynthesis in a BNS merger. Aiming to better clarify the nature of short GRBs, this burst was automatically followed up with the MAGIC telescopes, starting from 24 seconds after the burst trigger. Evidence of a gammaray signal is found above ∼0.5 TeV at a significance of ∼ 3 σ during observations that lasted until 4 hours after the burst. Assuming that the observed excess events correspond to gamma-ray emission from GRB 160821B, in conjunction with data at other wavelengths, we investigate its origin in the framework of GRB afterglow models. The simplest interpretation with one-zone models of synchrotronself-Compton emission from the external forward shock has difficulty accounting for the putative TeV flux. Alternative scenarios are discussed where the TeV emission can be relatively enhanced. The role of future GeV-TeV observations of short GRBs in advancing our understanding of BNS mergers and related topics is briefly addressed.

High-Energy Alerts in the Multi-Messenger Era

“…If an alert occurs when the 'GRB mode' is enabled, the automatic alert system takes control over all the subsystems to point to the position of the alert and start data taking as quickly as possible. Sub-minute times have been achieved from receiving the alert to taking data [19,20].…”

Upper limits on the very high energy emission from GRBs observed by MAGIC

“…In particular for this study we consider those GRBs without any (hint of) detection in the VHE range. According to this criterion, GRB 160821B [10] and GRB 190114C [5] are excluded from this study. Also GRB 190829A [9], detected by H.E.S.S., is not included here in order to have a dedicated study of this GRB.…”

“…( [8,9]) of the detection of four GRBs at energies greater than one hundred GeV. They were followed, more recently, by the announcement of the observation of other GRBs at very high energies, albeit with different significance ( [10,11]). Having achieved the aim of revealing the first GRBs at TeV energies, the research now focuses on identifying whether there are common characteristics in GRBs showing emission components at the highest energies.…”

An Archival Search for Very-High-Energy Counterparts to Sub-Threshold Neutron-Star Merger Candidates

“…and MAGIC observations of GRBs show promise for the ability of IACTs to make a detection of a short GRB stemming from a BNS merger. Since 2016, four long GRBs have been detected by the two observatories [9][10][11][12], and a ∼ 3 hint of a short GRB (160821B) has been reported by MAGIC [13], which began observing ∼ 24 s after the onset of the burst. As BNS mergers are expected to be a source class of short GRBs [14], this result is particularly encouraging for the future of such work.…”