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T. E. Strohmayer, R. F. Mushotzky (NASA/GSFC)
We report the discovery with the EPIC CCD cameras onboard XMM-Newton of a 54 mHz quasiperiodic oscillation (QPO) in the > 2 keV X-ray flux from the ultra-luminous X-ray source (ULX) X41.4+60 in the starburst galaxy M82. This is the first detection of a QPO in the X-ray flux from an extra-Galactic ULX, and confirms that the source is a compact object. The QPO had a centroid frequency of 54.3 ±0.9 mHz, a coherence Q = 5, and an amplitude (rms) in the 2 - 10 keV band of 8.5 %. Below 0.2 Hz the power spectrum can be described by a power-law with index of 1, and integrated amplitude (rms) of 13.5 %. The X-ray spectrum requires a curving continuum, with a disk-blackbody (diskbb) at T = 3.1 keV providing an acceptable, but not unique, fit. A broad Fe line centered at 6.55 keV is required in all fits, but the equivalent width (EW) of the line is sensitive to the choice of continuum model. There is no evidence of a reflection component. The implied bolometric luminosity is 4 - 5 x 1040 ergs s-1. Several archival Rossi X-ray Timing Explorer (RXTE) pointings at M82 also show evidence for QPOs in the 50 - 100 mHz frequency range. Several Galactic black hole candidates (BHCs), including GRS 1915+105, GRO J1655--40, and XTE 1550--564, show QPOs in the same frequency range, which at first glance suggests a possible connection with such objects. However, strong, narrow QPOs, as well as the broad Fe line provide solid evidence for disk emission, and thus present enormous theoretical difficulties for models which rely on either geometrically or relativistically beamed emission to account for the high X-ray luminosities. We discuss the implications of our findings for models of the ULX sources.
The author(s) of this abstract have provided an email address for comments about the abstract: Tod.E.Strohmayer@nasa.gov
Bulletin of the American Astronomical Society,
35#2
© 2003. The American Astronomical Soceity.