Iron Line and Diffuse Hard X‐Ray Emission from the Starburst Galaxy M82

We examine the properties of the diffuse hard X-ray emission in the classic starburst galaxy M82. We use new Chandra ACIS-S observations in combination with reprocessed archival Chandra ACIS-I and XMM-Newton observations. We find that E ~ 6.7 keV Fe Heα emission is present in the central | r | < 200 pc, | z | < 100 pc of M82 in all data sets, in addition to a possibly nonthermal X-ray continuum and marginally significant E = 6.4 keV Fe Kα line emission. No statistically significant Fe emission is found in the summed X-ray spectra of the pointlike X-ray sources or the ULXs in the two epochs of Chandra observation. The total nuclear region iron line fluxes in the 2004 April 21 XMM-Newton observation are consistent with those of the Chandra -derived diffuse component, but in the 2001 May 6 XMM-Newton observation they are significantly higher and also both E = 6.4 and 6.9 keV iron lines are detected. We attribute the excess iron line emission to the ULX in its high state. In general, the iron K-shell luminosity of M82 is dominated by the diffuse component. The total X-ray luminosity of the diffuse hard X-ray emission is L X,2-8 keV ~ 4.4 × 10 39 ergs s -1 in the E = 2-8 keV energy band, and the 6.7 keV iron line luminosity is L X,6.7 keV ~ (1.1-1.7) × 10 38 ergs s -1 . The 6.7 keV iron line luminosity is consistent with that expected from the previously unobserved metal-enriched merged SN ejecta that is thought to drive the larger scale galactic superwind. The iron line luminosity implies a thermal pressure within the starburst region of P / k ~ 2 × 10 7 K cm -3 , consistent with independent observational estimates of the starburst region pressure.