The Blue Supergiant Progenitor of the Supernova Imposter AT~2019krl

TL;DR

This paper investigates the progenitor star of the transient AT~2019krl, revealing it was likely a blue or yellow supergiant with characteristics similar to LBV eruptions, based on multi-wavelength archival imaging and spectral analysis.

Contribution

It provides the first detailed analysis of the progenitor of AT~2019krl, combining archival imaging and spectroscopy to characterize its nature and compare it to similar transient phenomena.

Findings

Progenitor was a 13-14 M_sun blue/yellow supergiant or a more massive LBV.

Infrared peak magnitude was -18.4 mag, optical peak missed but estimated to be -13.5 to -14.5 mag.

Spectroscopy shows strong Hα and [N II] emission with high-velocity wings.

Abstract

Extensive archival \textit{Hubble Space Telescope}, \textit{Spitzer Space Telescope}, and Large Binocular Telescope imaging of the recent intermediate-luminosity transient, AT~2019krl in M74, reveal a bright optical and mid-infrared progenitor star. While the optical peak of the event was missed, a peak was detected in the infrared with an absolute magnitude of $M_{4.5\,\mu {\rm m}} = -18.4$ mag, leading us to infer a visual-wavelength peak absolute magnitude of $-$13.5 to $-$14.5. The pre-discovery light curve indicated no outbursts over the previous 16\,yr. The colors, magnitudes, and inferred temperatures of the progenitor best match a 13--14 M$_{\sun}$ yellow or blue supergiant (BSG), if only foreground extinction is taken into account, or a hotter and more massive star, if any additional local extinction is included. A pre-eruption spectrum of the star reveals strong H$\alpha$ and [N~{\sc ii}] emission with wings extending to $\pm 2000$\,km\,s$^{-1}$. The post-eruption spectrum is fairly flat and featureless with only H$\alpha$, \ion{Na}{1}~D, [\ion{Ca}{2}], and the \ion{Ca}{2} triplet in emission. As in many previous intermediate-luminosity transients, AT~2019krl shows remarkable observational similarities to luminous blue variable (LBV) giant eruptions, SN~2008S-like events, and massive-star mergers. However, the information about the pre-eruption star favors either a relatively unobscured BSG or a more extinguished LBV with $M > 20$\,M$_{\sun}$ likely viewed pole-on.