Supernovae and Gamma-Ray Bursts
Since the discovery of an association between long-duration gamma-ray bursts (GRBs) and core-collapse supernovae (SNe) via SN1998bw/GRB980425, it has become evident that long GRBs are related to a rare sub-class of SNe, which develop highly energetic and collimated relativistic outflows, likely powered by a central engine (an accreting black hole or neutron star). However, it is still a mystery what makes some core-collapse SNe explode producing an associated relativistic ejecta (GRBs).
The physics behind the GRB-SN connection is tightly linked to a question of broad interest in the field of astrophysics, namely, how do massive stars end their lives. Massive stars, as primary sources of radiative ionization, heating, and nucleosynthesis products, play a crucial role in the evolution of galaxies and the whole universe.
Observationally we know that all SNe associated with GRBs are of the type dubbed broad-lined (BL) with stripped envelopes (Ic), characterized by large photospheric velocities. What we do not know is exactly what fraction of those BL-Ic SNe also make GRBs (from some viewing angle). Determining this fraction accurately can shed light on whether the intrinsic rate of radio-luminous BL-Ic SNe is the same as that of GRBs, or else if there is a larger variety of explosions with properties in between BL-Ic and GRBs.
Motivated by these considerations and by the key fact that relativistic ejecta in SN explosions can be probed effectively using radio observations (see top Figure), I work to unravel the missing link between GRBs and SNe using the Karl G. Jansky Very Large Array and the Zwicky Transient Facility.
Surveys like the Palomar Transient Factory (PTF) and its successor, the Zwicky Transient Facility (ZTF), are now discovering BL-Ic SNe at a much higher rate than in the past (see bottom Figure), thus giving us the opportunity to build a statistical sample of BL-Ic SNe discovered independently of a gamma-ray trigger. Working in tandem with the VLA, ZTF is now opening the way to clarifying the physics behind the GRB-SN connection.
Right Figure: Cumulative distribution of the number of BL-Ic SNe (non triggered by GRBs) with radio follow-up observations (at t<300 d since explosion). Our PTF/iPTF/ZTF programs have contributed substantially to the sample available to the community. We include the projection for ZTF II.
Figure Above: Radio (GHz) light curves of relativistic SNe (dots), low-luminosty GRBs (stars), and recent upper-limits on 8 BL-Ic events discovered by ZTF (downward pointing orange triangles). Our ZTF+VLA programs are probing well below SN1998bw thanks to the increased rate of nearby BL-Ic discoveries. See Corsi et al. 2016, Corsi et al. 2017, and Ho et al. 2020.