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 E \sim 6.7 keV Fe He \alpha emission ( from highly ionized iron ) is present in the central |r| < 200 pc , |z| < 100 pc of M82 in all datasets at high statistical significance , in addition to a possibly non-thermal X-ray continuum and marginally significant E = 6.4 keV Fe K \alpha line emission ( from weakly ionized iron ) .
No statistically significant Fe emission is found in the summed X-ray spectra of the point-like X-ray sources or the ULX 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 E=6.9 keV iron lines are detected .
We attribute the excess iron line emission to the Ultra-Luminous X-ray source 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 ( corrected for emission by unresolved low luminosity compact objects ) is L _ { X, 2 - 8 keV } \sim 4.4 \times 10 ^ { 39 } \hbox { $ { \thinspace erg } { \thinspace s% } ^ { -1 } $ } in the E = 2 – 8 keV energy band , and the 6.7 keV iron line luminosity is L _ { X, 6.7 keV } \sim ( 1.1 – 1.7 ) \times 10 ^ { 38 } \hbox { $ { \thinspace erg } { \thinspace s } ^ { -1 } $ } .
The diffuse hard X-ray continuum is best fit with power law models of photon index \Gamma \sim 2.5 – 3.0 , although thermal bremsstrahlung model with kT \sim 3 – 4 keV are also acceptable .
A simple interpretation of the hard diffuse continuum is that it is the bremsstrahlung associated with the 6.7 keV iron line emission .
This interpretation is problematic as it would imply an unrealistically low gas-phase iron abundance Z _ { Fe } \sim 0.4 Z _ { Fe, \odot } , much lower than the super-Solar abundance expected for supernova ejecta .
We explore the possibility of non-equilibrium ionization , but find that this can not reconcile the continuum and iron line emission .
Nor can Inverse Compton X-ray emission be the dominant source of the diffuse continuum as theory predicts flatter power law slopes and an order of magnitude less continuum flux than is observed .
The 6.7 keV iron line luminosity is consistent with that expected from the previously unobserved metal-enriched merged supernova 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 \sim 2 \times 10 ^ { 7 } { K } \hbox { $ { \thinspace cm } ^ { -3 } $ } which is consistent with independent observational estimates of the starburst region pressure .