Late‐Time X‐Ray, UV, and Optical Monitoring of Supernova 1979C

Abstract

We present results from observations of supernova (SN) 1979C with the Newton X-Ray Multi-Mirror (XMM-Newton) mission in X-rays and in UV, archival X-ray, and Hubble Space Telescope (HST) data, and follow-up ground-based optical imaging. The XMM-Newton MOS spectrum shows the best-fit two-temperature thermal plasma emission characteristics of both the forward (kT(high) = 4:1(-2.4)(+76) keV) and reverse shock (kT(low) = 0.79(-0.17)(+0.24) keV) with no intrinsic absorption. The long-term X-ray light curve, constructed from all X-ray data available, reveals that SN 1979C is still radiating at a flux level similar to that detected by ROSAT in 1995, showing no sign of a decline over the last 6 years, some 16-23 yr after its outburst. The high inferred X-ray luminosity (L(0.3-2) = 8 x 10(38) ergs s(-1)) is caused by the interaction of the SN shock with dense circumstellar matter, likely deposited by a strong stellar wind from the progenitor with a high mass- loss rate of. M approximate to 1.5 x 10(-4) M(circle dot) yr(-1) (v(w)/10 km s(-1)). The X-ray data support a strongly decelerated shock and show a mass-loss rate history that is consistent with a constant progenitor mass-loss rate and wind velocity over the past greater than or similar to 16,000 yr in the stellar evolution of the progenitor. We find a best-fit circumstellar medium (CSM) density profile of rho(CSM) proportional to r(-s) with index s less than or similar to 1.7 and high CSM densities (greater than or similar to 10(4) cm(-3)) out to large radii from the site of the explosion (r greater than or similar to 4 x 10(17) cm). Using XMM-Newton Optical Monitor data, we further detect a pointlike optical/UV source consistent with the position of SN 1979C, with B-, U-, and UVW1-band luminosities of 5, 7, and 9 x 10(36) ergs s(-1), respectively. The young stellar cluster in the vicinity of the SN, as imaged by the HSTand follow- up ground- based optical imaging, can only provide a fraction of the total observed flux, so that a significant contribution to the output likely arises from the strong interaction of SN 1979C with dense CSM.