Discovery of an intermediate-luminosity red transient in M51 and its likely dust-obscured, infrared-variable progenitor
Creators: name: Jencson, Jacob E.
name: Adams, Scott M.
name: Bond, Howard E.
name: van Dyk, Schuyler D.
name: Kasliwal, Mansi M.
name: Bally, John
name: Blagorodnova, Nadejda
name: De, Kishalay
name: Fremling, Christoffer
name: Yao, Yuhan
name: Fruchter, Andrew
name: Rubin, David
name: Barbarino, Cristina
name: Sollerman, Jesper
name: Miller, Adam A.
name: Hicks, Erin K. S.
name: Malkan, Matthew A.
name: Andreoni, Igor
name: Bellm, Eric C.
name: Buchheim, Robert
name: Dekany, Richard
name: Feeney, Michael
name: Frederick, Sara
name: Gal-Yam, Avishay
name: Gehrz, Robert D.
name: Giomi, Matteo
name: Graham, Matthew J.
name: Green, Wayne
name: Hale, David
name: Hankins, Matthew J.
name: Hanson, Mark
name: Helou, George
name: Ho, Anna Y. Q.
name: Hung, T.
name: Jurić, Mario
name: Kendurkar, Malhar R.
name: Kulkarni, S. R.
name: Lau, Ryan M.
name: Masci, Frank J.
name: Neill, James D.
name: Quin, Kevin
name: Riddle, Reed L.
name: Rusholme, Ben
name: Sims, Forrest
name: Smith, Nathan
name: Smith, Roger M.
name: Soumagnac, Maayane T.
name: Tachibana, Yutaro
name: Tinyanont, Samaporn
name: Walters, Richard
name: Watson, Stanley
name: Williams, Robert E.
Publisher: name: arXiv
Year: 2019
Subjects: subject: Solar and Stellar Astrophysics (astro-ph.SR)
subject: FOS: Physical sciences
subject: FOS: Physical sciences

description: We present the discovery of an optical transient (OT) in Messier 51, designated M51 OT2019-1 (also ZTF19aadyppr, AT 2019abn, ATLAS19bzl), by the Zwicky Transient Facility (ZTF). The OT rose over 15 days to an observed luminosity of $M_r=-13$ ($νL_ν=9\times10^6~L_{\odot}$), in the luminosity gap between novae and typical supernovae (SNe). Spectra during the outburst show a red continuum, Balmer emission with a velocity width of $\approx400$ km s$^{-1}$, Ca II and [Ca II] emission, and absorption features characteristic of an F-type supergiant. The spectra and multiband light curves are similar to the so-called "SN impostors" and intermediate-luminosity red transients (ILRTs). We directly identify the likely progenitor in archival Spitzer Space Telescope imaging with a $4.5~μ$m luminosity of $M_{[4.5]}\approx-12.2$ and a $[3.6]-[4.5]$ color redder than 0.74 mag, similar to those of the prototype ILRTs SN 2008S and NGC 300 OT2008-1. Intensive monitoring of M51 with Spitzer further reveals evidence for variability of the progenitor candidate at [4.5] in the years before the OT. The progenitor is not detected in pre-outburst Hubble Space Telescope optical and near-IR images. The optical colors during outburst combined with spectroscopic temperature constraints imply a higher reddening of $E(B-V)\approx0.7$ mag and higher intrinsic luminosity of $M_r\approx-14.9$ ($νL_ν=5.3\times10^7~L_{\odot}$) near peak than seen in previous ILRT candidates. Moreover, the extinction estimate is higher on the rise than on the plateau, suggestive of an extended phase of circumstellar dust destruction. These results, enabled by the early discovery of M51 OT2019-1 and extensive pre-outburst archival coverage, offer new clues about the debated origins of ILRTs and may challenge the hypothesis that they arise from the electron-capture induced collapse of extreme asymptotic giant branch stars.
description: 21 pages, 5 figures, published in ApJL