X-Ray Monitoring of Ultraluminous X-Ray Sources

Abstract

X-ray monitoring observations were performed with the Swift observatory of the ultraluminous X-ray sources Holmberg IX X-1, NGC 5408 X-1, and NGC 4395 X-2 and also of the nuclear X-ray source in NGC 4395. Holmberg IX X-1 remains in the hard X-ray spectral state as its flux varies by a factor of 7 up to an (isotropic) luminosity of 2.8E40 erg/s. This behavior may suggest an unusually massive compact object. We find excess power at periods near 60 days and 28 days in the X-ray emission from Holmberg IX X-1. Additional monitoring is required to test the significance of these signals. NGC 5408 X-1 and NGC 4395 X-2 appear to remain in the soft spectral state found by Chandra and XMM with little variation in spectral hardness even as the luminosity changes by a factor of 9. We found an outburst from the nuclear source in NGC 4395 reaching an X-ray luminosity of 9E40 erg/s, several times higher than any previously reported.

4 pages, to appear in ApJ

References in corpus (9)

• X-ray Properties of Black-Hole Binaries
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• The Orbital Period of the Ultraluminous X-ray Source in M82
• New flaring of an ultraluminous X-ray source in NGC 1365
• Multiwavelength Monitoring of the Dwarf Seyfert 1 Galaxy NGC 4395. II. X-ray and Ultraviolet Continuum Variability

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