SN 2012cg: EVIDENCE FOR INTERACTION BETWEEN A NORMAL SN Ia AND A NON-DEGENERATE BINARY COMPANION

Abstract

We report evidence for excess blue light from the Type Ia supernova (Sn Ia) SN2012cg at 15 and 16 days before maximum B-band brightness. The emission is consistent with predictions for the impact of the supernova on a nondegenerate binary companion. This is the first evidence for emission from a companion to a normal SNIa. Sixteen days before maximum light, the B V -color of SN2012cg is 0.2 mag bluer than for other normal SNIa. At later times, this supernova has a typical SNIa light curve, with extinction-corrected M 19.62 0.02 B = - mag and m B 0.86 0.02 15 ( ) D =  . Our data set is extensive, with photometry in seven filters from five independent sources. Early spectra also show the effects of blue light, and high-velocity features are observed at early times. Near maximum, the spectra are normal with a silicon velocity v Si =−10,500 km s −1 . Comparing the early data with models by Kasen favors a main-sequence companion of about six solar masses. It is possible that many other SN Ia have main-sequence companions that have eluded detection because the emission from the impact is fleeting and faint.

Constraining the progenitor of the Type Ia Supernova SN 2012cg

“…In addition, it was found that the light curve of SN 2012cg was slightly narrower than that of a typical normal SN Ia, namely SN 2011fe . They also estimated the explosion date of SN 2012cg to be May 15.7 (UT), which is consistent with the explosion time of MJD=56062.5 (May 15.5 UT) estimated by Marion et al (2016). The green triangles represent the sub-Ch-mass scenario.…”

“…2 can only be shifted by about a factor of two due to the uncertainties of fitting parameters among different MS (or RG) companion stars. Marion et al (2016) presented early-time photometry of SN 2012cg, showing that blue excess light is seen at about 16 days before maximum B-band brightness. They reported that this blue early-time excess is consistent with predicted early-time emission of shocked gas that was proposed by Kasen (2010), suggesting SN 2012cg was generated from a binary system in the SD Ch-mass scenario.…”

“…At later epochs, this shock flux becomes weak and the light curve is dominated by the flux of the SN itself. Also, this early-time excess emission should be brightest in the UV band and become subordinate at longer optical wavelengths although it can still cause a blue colour evolution in the optical light curve (Kasen 2010;Marion et al 2016). Therefore, only predictions for the uvm2 filter (the uvw1 and uvw2 filters have an extended red tail) of the Swift/UVOT is shown as an example.…”

“…The analysis of observed early light-curves has been carried out for many nearby SNe Ia to look for evidence of these shock emissions (Hayden et al 2010;Brown et al 2012;Cao et al 2015;Marion et al 2016;Olling et al 2015), but only a sublumious SN Ia (iPTF14atg, Cao et al 2015) and one normal event (SN 2012cg, Marion et al 2016) have been reported as likely detections of the signatures of the interaction between SN ejecta and a stellar companion. Interestingly, Marion et al (2016) have shown that the analytical model of Kasen (2010) with a 6.0 M ⊙ MS companion star could provide an explanation for the excess blue luminosity detected in SN 2012cg at about three days after the explosion. They therefore reported that this was the first evidence for emission from the interaction of SN ejecta with a stellar companion star for normal SNe Ia, supporting the SD scenario.…”

“…It has been widely proposed that very early-time observations of SNe Ia can be used to place strong constraints on their progenitor systems (e.g., Kasen 2010;Nugent et al 2011;Rabinak, Livne & Waxman 2012, hereafter RLW12;Brown et al 2012;Cao et al 2015;Olling et al 2015;Marion et al 2016). In particular, Kasen (2010) suggested that the strong excess emission can arise from the collision of SN ejecta with its companion star in the SD progenitor system, and it should be detectable in the ultraviolet (UV) and blue optical bands in the first few hours to days after the SN explosion under favorable viewing angles.…”