RXTE studies of the starburst galaxies M82 and NGC253

The two nearby starburst (SB) galaxies M82 and NGC253 were observed for ∼100 ksec over a 10-month period in 1997. While the NGC253 flux did not vary significantly, the M82 flux jumped by a factor ∼2 in July to a total luminosity of $4×1040 erg s−1.$ The earlier emission from M82 has a thermal shape with $kT≃6.63±0.08.$ In the high flux phase, the additional emission requires either an absorbed second thermal component, or absorbed power law component, with the former providing a better fit. But given the unphysically weak Fe K emission if thermal, nonthermal power law emission is more likely. M82 could therefore be a combined SB/LLAGN, with the dominant emission powered by the SB activity. The best-fit emission from NGC253 if thermal has $kT=4.2±0.4 keV$ or index $2.6±0.1$ if power law.