The broad-line type Ic SN 2020bvc: signatures of an off-axis gamma-ray burst afterglow

TL;DR

SN 2020bvc exhibits signatures of an off-axis gamma-ray burst afterglow, providing evidence for hidden jets in supernovae not associated with observed GRBs, thus supporting the off-axis GRB hypothesis.

Contribution

This paper presents the first evidence of an orphan GRB detected via its associated supernova emission, revealing off-axis jet signatures in SN 2020bvc.

Findings

Detection of early optical decline and high-velocity spectra consistent with jet-cocoon emission.

X-ray observations indicating an afterglow component from an off-axis jet.

Estimated jet off-axis angle of approximately 23 degrees.

Abstract

Long-duration gamma-ray bursts (GRBs) are almost unequivocally associated with very energetic, broad-lined supernovae (SNe) of Type Ic-BL. While the gamma-ray emission is emitted in narrow jets, the SN emits radiation isotropically. Therefore, some SN Ic-BL not associated with GRBs have been hypothesized to arise from events with inner engines such as off-axis GRBs or choked jets. Here we present observations of the nearby ($d = 120$ Mpc) SN 2020bvc (ASAS-SN 20bs) which support this scenario. \textit{Swift} UVOT observations reveal an early decline (up to two days after explosion) while optical spectra classify it as a SN Ic-BL with very high expansion velocities ($\approx$ 70,000 km/s), similar to that found for the jet-cocoon emission in SN 2017iuk associated with GRB 171205A. Moreover, \textit{Swift} X-Ray Telescope and \textit{CXO} X-ray Observatory detected X-ray emission only three days after the SN and decaying onwards, which can be ascribed to an afterglow component. Cocoon and X-ray emission are both signatures of jet-powered GRBs. In the case of SN 2020bvc, we find that the jet is off axis (by $\approx$ 23 degrees), as also indicated by the lack of early ($\approx 1$ day) X-ray emission which explains why no coincident GRB was detected promptly or in archival data. These observations suggest that SN 2020bvc is the first orphan GRB detected through its associated SN emission.