Recently, very high-energy photons above 100 GeV were reported to be detected from GRB 190114C and GRB 180720B at, respectively, 100-1000 s and 10 hours after the burst. We model the available broad-band data of both GRBs with the synchrotron plus synchrotron self-Compton (SSC) emission of the afterglow shocks. We find that the sub-TeV emission of GRB 180720B can be interpreted as the SSC emission from afterglow shocks expanding in a constant density circum-burst medium. The SSC emission of GRB 190114C dominates over the synchrotron component from GeV energies at ∼ 100 s, which can explain the possible hard spectrum of the GeV emission at this time. The extrapolated flux of this SSC component to sub-TeV energies can explain the high-significance detection of GRB 190114C by the MAGIC telescope. The parameter values (such as the circum-burst density and shock microphysical parameters) in the modeling are not unusual for both GRBs, implying that the detection of sub-TeV photons from these two bursts should be attributed to their large burst energies and low redshifts.

Some gamma-ray bursts (GRBs) have a tera–electron volt (TeV) afterglow, but the early onset of this has not been observed. We report observations with the Large High Altitude Air Shower Observatory of the bright GRB 221009A, which serendipitously occurred within the instrument field of view. More than 64,000 photons >0.2 TeV were detected within the first 3000 seconds. The TeV flux began several minutes after the GRB trigger, then rose to a peak about 10 seconds later. This was followed by a decay phase, which became more rapid ~650 seconds after the peak. We interpret the emission using a model of a relativistic jet with half-opening angle ~0.8°. This is consistent with the core of a structured jet and could explain the high isotropic energy of this GRB.

We report the discovery of an ultrahigh-energy (UHE) gamma-ray source, LHAASO J2108+5157, by analyzing the LHAASO-KM2A data of 308.33 live days. A significant excess of gamma ray–induced showers is observed in both energy bands of 25−100 and >100 TeV with 9.5σ and 8.5σ, respectively. This source is not significantly favored as an extended source with an angular extension smaller than the point-spread function of KM2A. The measured energy spectrum from 20 to 200 TeV can be approximately described by a power-law function with an index of −2.83 ± 0.18stat. A harder spectrum is demanded at lower energies considering the flux upper limit set by Fermi-LAT observations. The position of the gamma-ray emission is correlated with a giant molecular cloud, which favors a hadronic origin. No obvious counterparts have been found, and deeper multiwavelength observations will help to cast new light on this intriguing UHE source.

Soft gamma-ray repeaters (SGRs) are a mainly Galactic population and originate from neutron stars with intense (B ≃ 1015 G) magnetic fields (magnetars). Occasionally, a giant flare occurs with enormous intensity, displaying a short, hard spike followed by a weaker, oscillatory phase that exhibits the rotational period of the neutron star. If the magnetar giant flares occur in nearby galaxies, they would appear as cosmic, short-hard gamma-ray bursts (GRBs) without detecting the weak oscillatory phase. Recently, a short-hard GRB named GRB 200415A was detected, with a position coincident with the Sculptor Galaxy (NGC 253), raising the question of whether it is a classic short GRB or a magnetar giant flare. Here we show that magnetar giant flares follow a scaling relation between the spectral peak energy and the isotropic energy in 1 keV–10 MeV, i.e., , and locate in a distinct region of the E p–E iso plane from that of classic short GRBs. The relation can be well understood in the model that giant flares arise from the photosphere emissions of relativistically expanding fireballs. GRB 200415A, together with two other candidate giant flares (GRB 051103 and GRB 070201) follow this relation, which strongly favors the giant flare origin of these GRBs. The GeV emission detected by Fermi/LAT from GRB 200415A at 18–285 s can also be explained in the giant flare scenario. The total energy in the GeV emission implies a baryon load of ∼1023 g in the giant flare fireball of GRB 200415A.

The origin of the multi-band activities (outbursts/flares) of blazars is still a heavily debated topic. Shock and magnetic reconnection have long been considered as possible triggers for the multi-band activities. In this paper, we present an exploration of the origin of multi-band activities for a high-redshift (z =1.8385) FSRQ PKS 1502+106. Utilizing multi-band data from radio to γ-ray and optical polarization observations, we investigate two dramatic activities in detail: a γ-ray dominated outburst in 2015 and an optical dominated outburst in 2017. Our main results are as follows. (I) A fast γ-ray flare with a flux-doubling time-scale as short as 1-hr in 2015 is discovered. Based on the variability time-scale, the physical parameters of the flaring region (e.g, minimum Doppler factor, emission region size, etc.) are constrained. At the peak of the flare, the γ-ray spectrum hardens to Γ γ = 1.82 ± 0.04 and exhibits an obvious curvature/break characteristic that is caused by the typical "cooling break". Modelings of multi-band SEDs reveal a very hard electronic energy spectrum with the electronic spectral index of 1.07 ± 0.53. This result suggests that this fast γ-ray flare may be triggered by magnetic reconnection. (II) During the outburst in 2017, the optical polarization degree and optical fluxes show a very tight correlation. By analyzing Stokes parameters of polarization observations, our results show that this outburst could be triggered by a transverse shock with a compression ratio of η > 2.2, and the magnetic field intensity of the shock emission region is about 0.032 G.