A Silver Anniversary Observation of the X-Ray-Luminous SN 1978K in NGC 1313

Abstract

We describe the results of a 2003 Chandra ACIS-I observation of SN 1978K. The spectrum shows little flux below 0.6 keV, in contrast to the 2002 Chandra ACIS-S observation, which showed flux to 0.4 keV. Fitting the ACIS-I spectrum alone leads to two solutions depending on the value of the column density. A joint fit using a dual thermal plasma model applied to the ACIS-I and a contemporaneous XMM-Newton spectrum, which if fit alone also leads to a two-column solution, yields a single column density fit. The fitted temperature of the joint fit for the soft component remains constant within the errors from previous Chandra, XMM-Newton, and ASCA data. The hard temperature recovers from its 2000-2002 decline and corresponds to an increase in the column density during that time. The hard (2-10 keV) light curve is confirmed to be declining. The derived number density represents a lower limit of ~10⁵ depending on the adopted filling factor of the emitting volume, leading to an estimated mass cooling rate of ~0.1-0.15 M_odot yr^-1.