Research Note

Viscous timescale in high mass X-ray binaries

Physics DepartmentMiddle East Technical University, 06531 Ankara, Turkey
e-mail: altan@astroa.physics.metu.edu.tr

Received: 23 September 2010
Accepted: 5 February 2011

Abstract

Context. Low mass X-ray binaries are found to have very low frequency breaks in their power density spectra. The power density spectra below these breaks are nearly in white noise structure and at higher frequencies approximately follow the P_ν ∝ ν^-1.3 law.

Aims. To determine whether high mass X-ray binary power density spectra have similar properties and the findings for low mass X-ray binaries are also valid for high mass binaries, we analyzed the time series of high mass X-ray binary sources produced by All Sky Monitor of Rossi X-ray Timing Explorer.

Methods. We obtained the power density spectra of the high mass X-ray binaries using the cosine transform of autocorrelation function.

Results. We identified break frequencies for seven sources, namely OAO 1657-415, SS 433, Vela X-1, SMC X-1, 4U 1700-377, GX 301-2, and LMC X-1. The normalized break frequencies with respect to the orbital frequency (f_break/f_orbit) for sources OAO 1657-415, SS 433, SMC X-1 and LMC X-1 are consistent with those of Roche lobe overflow systems. The other high mass X-ray binary systems, Vela X-1, GX 301-2, and 4U 1700-377, however, have larger break frequency ratios, f_break/f_orb, which are indicative of short viscous times. These are all wind-accreting sources and the stellar winds in the systems allow the formation of only short radius discs. Consequently, we qualitatively distinguished the Roche lobe overflow binaries from the wind accreting system by comparing their normalized break frequencies.