An ultraluminous X-ray object with a 2 hour period in M51

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University of Chicago Press

Ultraluminous X-ray objects (ULXs) are off-nucleus point sources with L-X = 10(39)-10(41) ergs s(-1), but the nature 39 of such systems is largely unidentified. Here we report a 2.1 hr period observed in a Chandra ACIS observation for ULX M51 X-7, which is located on the edge of a young star cluster in the star-forming region in a spiral arm. In two ACIS observations separated by 1 yr, the ULX changed from a high-hard to a low-soft spectral state, in contrast to most Galactic low-mass X-ray binaries. On the basis of its period and spectral behaviors, we suggest that this ULX is a low-mass X-ray binary system, with a dwarf companion of 0.2-0.3 M. and a compact accretor, either a neutron star or a black hole, whose mass is not well constrained. Relativistic beaming effects are likely involved to produce the observed high X-ray luminosities, given its low accretion rate as inferred from a sustainable accretion scenario via Roche lobe overflow.

galaxies : individual (M51), X-rays : binaries, GALAXIES, EVOLUTION, CHANDRA, Astronomy & Astrophysics
Liu, J., Bregman, J., Irwin, J., Seitzer, P. (2002): An Ultraluminous X-Ray Object with a 2 Hour Period in M51. The Astrophysical Journal Letters, 581(2). DOI: 10.1086/346101