Broad-line type Ic SN 2020bvc. Signatures of an off-axis gamma-ray burst afterglow (Izzo et al.)

Vol. 639
1. Letters

Broad-line type Ic SN 2020bvc. Signatures of an off-axis gamma-ray burst afterglow

by L. Izzo, K. Auchettl, J. Hjorth, et al. 2020, A&A, 639, L11 alt

Gamma-ray bursts (GRBs) are the brightest explosions in the Universe. There are two types of GRBs, and the long-duration ones are the outcome of the death of massive stars. A narrow, collimated jet pointing toward the Earth marks the GRB. At the same time, the star explodes giving rise to a supernova. GRBs that explode in the local Universe are almost always associated with broad-line, type Ic supernovae (i.e., those that were stripped more during the evolutionary path which brought them to explode). The supernova emission is isotropic and the GRB emission is collimated: We do expect that most of the type Ic supernovae lack a GRB, being that the Earth was missed by the jet. However, if we are just outside the GRB narrow jet, it is possible to observe an "orphan" GRB, that is to say a relativistic explosion without a high-energy signature. In this letter, Izzo et al. provide strong evidence for an off-axis GRB or choked jet (i.e., a jet which fails to break out from the star). This evidence comes from delayed X-ray emission - GRBs have a decaying X-ray afterglow and supernovae rarely have X-ray emission - and from the very high expansion velocity which is 20% of the speed of light, which were revealed from an early optical spectrum that was taken 1.5 d after the explosion. The off-axis viewing angle is ~25 degrees. The GRB barely missed the Earth and the authors were able to detect the X-ray emission thanks to its closeness of just 120 Mpc.