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.
We report our analysis of the data obtained with the Large Area Telescope (LAT) onboard the Fermi Gamma-ray Space Telescope (Fermi) for the SS 433/W50 region. The total data length is ten years. Different from the previous results reported for this region (for which an old version of database was used), we show that excess emission is detected around the w1 region in the western lobe of the jets from SS 433. The region is bright at X-rays due to the interaction between the jet and the ambient medium. This detection also matches well the recent results of the very-high-energy detection of SS 433/W50 with the High Altitude Water Cherenkov (HAWC). However, the eastern regions that are slightly brighter in HAWC's observation are not detected in the Fermi data. Constructing the broad-band spectral energy distribution (SED) for the western region, we compare with the HAWC results for the eastern regions and discuss the possible origin of the emission. A leptonic scenario can provide a fit to the Fermi GeV spectrum and HAWC TeV detection, where the former and latter are due to the synchrotron radiation and inverse-Compton process respectively. However, the model can not explain the X-ray and radio emission from the region simultaneously, which thus requires further observational and theoretical studies of the region in order to clarify the reasons.
Cassiopeia A, a well-observed young core-collapse supernova remnant (SNR), is considered as one of the best candidates for studying very high-energy particle acceleration up to PeV via the diffusive shock mechanism. Recently, MAGIC observations revealed a γ-ray spectral cutoff at ∼ 3.5 TeV, suggesting that if the TeV γ-rays have a hadronic origin, SNR Cas A can only accelerate particles to tens of TeV. Here, we propose a twozone emission model for regions associated with the forward (zone 1) and inward/reverse shocks (zone 2). Given the low density in zone 1, it dominates the high-frequency radio emission, soft X-ray rim via the synchrotron process and TeV γ-ray via the inverse Comptonization. With a relatively softer particle distribution and a higher cut-off energy for electrons, emissions from zone 2 dominate the low-frequency radio, hard X-ray via the synchrotron process and GeV γ-ray via hadronic processes. There is no evi-dence for high-energy cutoffs in the proton distributions implying that Cas A can still be a PeVatron. Hadronic processes from zone 1 dominate very high-energy gamma-ray emission. Future observations in hundreds of TeV range can test this model.
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