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

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⁺⁷⁶_-2.4 keV) and reverse shock (kT_low=0.79^+0.24_-0.17 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=8×10³⁸ 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˙~1.5×10^-4 M_solar 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 >~16,000 yr in the stellar evolution of the progenitor. We find a best-fit circumstellar medium (CSM) density profile of ρ_CSM~r^-s with index s<~1.7 and high CSM densities (>~10⁴ cm^-3) out to large radii from the site of the explosion (r>~4×10¹⁷ 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 9×10³⁶ ergs s^-1, respectively. The young stellar cluster in the vicinity of the SN, as imaged by the HST and 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.