A Two Hour Quasi-Period in an Ultra-luminous X-Ray source in NGC628

Details A Two Hour Quasi-Period in an Ultra-luminous X-Ray source in NGC628 A Two Hour Quasi-Period in an Ultra-luminous X-Ray source in NGC628

2005-01-14

arxiv.org/abs/astro-ph/0501306v1

Astrophys.J. 621 (2005) L17-L20

Astronomy and Astrophysics

Astrophysics

12 pages, 3 figures. accepted for publication in ApJ Letter

Scientific paper

10.1086/428902

Quasi-periodic oscillations and X-ray spectroscopy are powerful probes of black hole masses and accretion disks, and here we apply these diagnostics to an ultraluminous X-ray source (ULX) in the spiral galaxy NGC628 (M74). This object was observed four times over two years with the Chandra X-ray Observatory and XMM-Newton, with three long observations showing dramatic variability, distinguished by a series of outbursts with a quasi-period (QPO) of 4,000-7,000 seconds. This is unique behavior among both ULXs and Galactic X-ray binaries due to the combination of its burst-like peaks and deep troughs, its long quasi-periods, its high variation amplitudes of $>90$%, and its substantial variability between observations. The X-ray spectra is fitted by an absorbed accretion disk plus a power-law component, suggesting the ULX was in a spectral state analogous to the Low Hard state or the Very High state of Galactic black hole X-ray binaries. A black hole mass of $\sim2$--$20\times10^3 M_\odot$ is estimated from the $f_b$--$M_\bullet$ scaling relation found in the Galactic X-ray binaries and active galactic nuclei.

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