高能暂现源 · 每日文献综述

GRB 221009A is the brightest gamma-ray burst (GRB) observed to date, and its afterglow has been characterized with unprecedented detail at TeV energies by LHAASO. Quite puzzlingly, it is also the most energetic GRB known. Among the questions posed by this mysterious source, however, the sheer energetics are hardly the most intriguing; an unprecedented, narrow, luminous emission line at around 10 MeV has been uncovered by a detailed spectral analysis of Fermi/GBM data immediately following the brightest peak in the GRB prompt emission and the peak of the TeV afterglow. As noted in the discovery article, the temporal evolution of the line properties can be explained as being due to high-latitude emission from a geometrically thin, relativistically expanding shell where annihilation of a large number of electron─positron pairs took place. We show that this interpretation yields stringent constraints on the properties of such a shell, which point to a process that happens at radii typical of external shocks. We then demonstrate that the shell could have been the blastwave associated with the GRB precursor, with the line arising after pair loading of the blastwave as it was illuminated by the bright and hard radiation of the GRB main event. The scenario, which also explains the abrupt initial rise of the LHAASO afterglow, requires the progenitor of the GRB to have been surrounded by a circumstellar medium (CSM) extending out to a few 10<SUP>15</SUP> cm, with a density n<SUB>ext</SUB> ∼ 10<SUP>8</SUP> − 10<SUP>9</SUP> cm<SUP>−3</SUP> reminiscent of those found from studies of type IIn supernovae. The consequences of such a CSM on the dynamics and emission of the external shock are yet to be fully explored. If future, more detailed work confirms the compatibility of the GRB 221009A afterglow emission with our scenario, this will provide a precious clue to the nature of the progenitor of this peculiar GRB, which could also be present in other bursts that feature a long quiescence followed by a bright emission episode with a hard spectrum.

We present optical/UV photometric and spectroscopic observations, as well as X-ray and radio follow-up, of the extraordinary event AT2019cmw. With a peak bolometric luminosity of <inline-formula><tex-math>$\mathrm{\sim 10^{45.6}\, erg\, s^{-1}}$</tex-math></inline-formula>, it is one of the most luminous thermal transients ever discovered. Extensive spectroscopic follow-up post-peak showed only a featureless continuum throughout its evolution. This, combined with its nuclear location, blue colour at peak and lack of prior evidence of an AGN in its host lead us to interpret this event as a `featureless' tidal disruption event (TDE). It displays photometric evolution atypical of most TDEs, cooling from <inline-formula><tex-math>$\mathrm{\sim 30}$</tex-math></inline-formula> to <inline-formula><tex-math>$\mathrm{\sim 10\, kK}$</tex-math></inline-formula> in the first <inline-formula><tex-math>$\mathrm{\sim 300}$</tex-math></inline-formula> d post-peak, with potential implications for future photometric selection of candidate TDEs. No X-ray or radio emission is detected, placing constraints on the presence of on-axis jetted emission or a visible inner-accretion disc. Modelling the optical light curve with existing theoretical prescriptions, we find that AT2019cmw may be the result of the disruption of a star in the tens of solar masses by a supermassive black hole (SMBH). Combined with a lack of detectable star formation in its host galaxy, it could imply the existence of a localized region of star formation around the SMBH. This could provide a new window to probe nuclear star formation and the shape of the initial mass function (IMF) in close proximity to SMBHs out to relatively high redshifts.