Search for: All records

Abstract We present X-ray to radio frequency observations of the bright long gamma-ray burst GRB 210702A. Our Atacama Large Millimeter/submillimeter Array 97.5 GHz observations show a significant rebrightening by a factor of ≈2 beginning at 8.2 days post-burst and rising to peak brightness at 18.1 days before declining again. This is the first such rebrightening seen in a millimeter afterglow light curve. A standard forward shock model in a stellar wind circumburst medium can explain most of our X-ray, optical, and millimeter observations prior to the rebrightening, but significantly overpredicts the self-absorbed radio emission, and cannot explain the millimeter rebrightening. We investigate possible explanations for the millimeter rebrightening, and find that energy injection or a reverse shock from a late-time shell collision are plausible causes. Similar to other bursts, our radio data may require alternative scenarios such as a thermal electron population or a structured jet to explain the data. Our observations demonstrate that millimeter light curves can exhibit some of the rich features more commonly seen in optical and X-ray afterglow light curves, motivating further millimeter wavelength studies of GRB afterglows. 

Abstract Like most galaxies, the Milky Way harbors a supermassive black hole (SMBH) at its center, surrounded by a nuclear star cluster. In this dense star cluster, direct collisions can occur between stars before they evolve off the main sequence. Using a statistical approach, we characterize the outcomes of these stellar collisions within the inner parsec of the Galactic center (GC). Close to the SMBH, where the velocity dispersion is larger than the escape speed from a Sun-like star, collisions lead to mass loss. We find that the stellar population within 0.01 pc is halved within about a billion years because of destructive collisions. Additionally, we predict a diffuse population of peculiar low-mass stars in the GC. These stars have been divested of their outer layers in the inner 0.01 pc before migrating to larger distances from the SMBH. Between 0.01 and 0.1 pc from the SMBH, collisions can result in mergers. Our results suggest that repeated collisions between lower-mass stars can produce massive (≳10M_⊙) stars, and that there may be ∼100 of them residing in this region. We provide predictions on the number of so-called G objects, dust- and gas-enshrouded stellar objects, that may result from main-sequence stellar collisions. Lastly, we comment on uncertainties in our model and possible connections between stellar collisions and the missing red giants in the GC.