RXTE view of the starburst galaxies M 82 and NGC 253

The two nearby starburst galaxies M 82 and NGC 253 were observed for ∼100 ksec over a 10-month period in 1997. An increase of the M 82 flux by a factor ∼2 was measured during the period July-November, when compared with the flux measured earlier in 1997. The flux measured in the field centered on M 82 includes ∼38% of the emission from the Seyfert 1 galaxy M 81. The best-fitting model for the earlier emission from M 82 is thermal with kT ≃ 6.7 ± 0.1. In the high flux state, the emission additionally includes either an absorbed second thermal component or absorbed power-law component, with the former providing a much better fit. A likely origin for the temporal variability is a single source in M 82. The flux of NGC 253, which did not vary significantly during the period of observations, can be well fit by either a thermal spectrum with kT ≃ 3.8 ± 0.3 keV, or by a power law with photon index of 2.7 ± 0.10. We have also attempted fitting the measurements to more realistic composite models with thermal and power-law components, such as would be expected from a dominant contribution from binary systems, or Compton scattering of (far) IR radiation by radio emitting electrons. However, the addition of any amount of a power-law component, even with cutoff at 20 keV, only increases chi-square. The 90% confidence upper limit for power law emission with (photon) index 1.5 is only 2.4% of the 2-10 keV flux of M 82; the corresponding limit for NGC 253, with index 2.0, is 48%.