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SWIFT XRT Observations of the ULX M81 X-9

ATEL # 1505; S. B. Moore, J. M. Miller (University of Michigan)
on 2 May 2008; 18:44 UT
Password Certification: Jon Miller (

Subjects: X-ray, Binaries, Black Holes

We report on the well-known ultra-luminous X-ray (ULX) source M81 X-9 using several SWIFT XRT observations. These observations occurred on 2006 July 7, July 8, October 14, October 18, October 19, and November 22, and 2007 March 25. Using XSPEC v11 and fitting the spectra with a power-law model in the 0.3-10.0 keV band (and freezing the absorbing column density along the long of sight at 2.1 E+21 cm^-2), we obtained power-law indices ranging from 1.6 +/- 0.2 to 1.9 +/- 0.1 (90 percent confidence errors). The flux values ranged from 1.01 +/- 0.06 E+11 cgs to 1.71 +/- 0.04 E+11 cgs. Thus, while evidence for spectral variability is marginal, the evidence for flux variability is significant. There is no significant relation between the flux and the power-law indices. The calculated luminosity (assuming a distance of 3.4 Mpc) ranged from 1.2 +/- 0.1 E +40 erg/s to 2.5 +/- 0.2 E+40 erg/s. These luminosity values are consistent with prior observations of M81 X-9 (La Parola et al. 2001, Miller, Fabian, & Miller 2004), and demonstrate that even short observations with Swift can provide useful long-term flux monitoring of ULXs.

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