Discovery and Rapid Follow-up Observations of the Unusual Type II SN 2018ivc in NGC 1068

TL;DR

SN 2018ivc, an unusual Type II supernova in NGC 1068, exhibits a rapidly evolving light curve influenced by circumstellar interaction, with spectral features suggesting an asymmetric explosion and a progenitor possibly less than 12 solar masses.

Contribution

This study provides detailed early observations of SN 2018ivc, revealing unique light curve behavior, spectral features, and progenitor constraints, highlighting the importance of rapid follow-up for understanding supernova physics.

Findings

Rapidly changing light curve with multiple slope changes

Detection of high-velocity blueshifted emission lines

Progenitor mass likely less than 12 solar masses

Abstract

We present the discovery and high-cadence follow-up observations of SN 2018ivc, an unusual Type II supernova that exploded in NGC 1068 (D=10.1 Mpc). The light curve of SN 2018ivc declines piecewise-linearly, changing slope frequently, with four clear slope changes in the first 30 days of evolution. This rapidly changing light curve indicates that interaction between the circumstellar material and ejecta plays a significant role in the evolution. Circumstellar interaction is further supported by a strong X-ray detection. The spectra are rapidly evolving and dominated by hydrogen, helium, and calcium emission lines. We identify a rare high-velocity emission-line feature blueshifted at ~7800 km/s (in Ha, Hb, Pb, Pg, HeI, CaII), which is visible from day 18 until at least day 78 and could be evidence of an asymmetric progenitor or explosion. From the overall similarity between SN 2018ivc and SN 1996al, the \Ha{} equivalent width of its parent HII region, and constraints from pre-explosion archival Hubble Space Telescope images, we find that the progenitor of SN 2018ivc could be as massive as 52 Msun but is more likely <12 Msun. SN 2018ivc demonstrates the importance of the early discovery and rapid follow-up observations of nearby supernovae to study the physics and progenitors of these cosmic explosions.