RXTE View of the Starburst Galaxies M82 and NGC 253

Abstract

The two nearby starburst galaxies M82 and NGC 253 were observed for 100 ksec over a 10-month period in 1997. An increase of the M82 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 M82 includes ~38 of the emission from the Seyfert 1 galaxy M81. The best-fitting model for the earlier emission from M82 is thermal with kT = 6.7 +/- 0.1 keV. 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 M82. 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 M82; the corresponding limit for NGC 253, with index 2.0, is 48%.

18 pages, 4 figures, accepted for publication in A&A

References in corpus (3)

• X-Ray Spectral Components of Starburst Galaxies
• Stability of the Cyclotron Resonance Scattering Feature in Her X-1 with RXTE
• ROSAT X-Ray Observations of the Spiral Galaxy M81

Cited by in corpus (14)

• High Mass X-ray Binaries as a Star Formation Rate Indicator in Distant Galaxies
• The 2-10 keV luminosity as a Star Formation Rate indicator
• Discovery of X-ray QPOs from an Ultra-luminous X-ray Source in M82: Evidence Against Beaming
• Arp 299: a second merging system with two active nuclei?
• 2-10 keV luminosity of high-mass binaries as a gauge of ongoing star-formation rate
• A 62 Day X-Ray Periodicity and an X-Ray Flare from the Ultraluminous X-Ray Source in M82
• Stellar and gaseous abundances in M82
• Is M82 X-1 Really An Intermediate-Mass Black Hole? X-ray Spectral and Timing Evidence
• Suzaku Observations of M82 X-1 : Detection of a Curved Hard X-ray Spectrum
• A broadband X-ray spectral study of the intermediate-mass black hole candidate M82 X-1 with NuSTAR, Chandra and Swift
• Starburst Galaxies and the X-Ray Background
• A Major X-ray Outburst from an Ultraluminous X-Ray Source in M82
• Spectral evolution of the ultraluminous X-ray sources M82 X-1 and X-2
• Hard X-ray/Soft gamma-ray experiments and missions: overview and prospects