Multiband Studies of the Optical Periodic Modulation in the X-Ray Binary SAX~J1808.4-3658 during Its Quiescence and 2008 Outburst

Details Multiband Studies of the Optical Periodic Modulation in the X-Ray Binary SAX~J1808.4-3658 during Its Quiescence and 2008 Outburst Multiband Studies of the Optical Periodic Modulation in the X-Ray Binary SAX~J1808.4-3658 during Its Quiescence and 2008 Outburst

2011-08-24

arxiv.org/abs/1108.4745v1

Astronomy and Astrophysics

Astrophysics

High Energy Astrophysical Phenomena

10 pages, 7 figures, submitted to ApJ

Scientific paper

We report on time-resolved optical imaging of the X-ray binary SAX J1808.4-3658 during its quiescent state and 2008 outburst. The binary, containing an accretion-powered millisecond pulsar, has a large sinusoidal-like modulation in its quiescent optical emission. We employ a Markov chain Monte Carlo technique to fit our multi-band light curve data in quiescence with an irradiated star model, and derive a tight constraint of 43+/-3 deg on the inclination angle of the binary system. The pulsar and its companion are constrained to have masses of 1.6 (+0.31, -0.25) Msun and 0.07+/-0.01 Msun, respectively. We find that the accretion disk had nearly constant fluxes over a ~500 day period in the quiescent state our data covered, but was caught to have started brightening 1.5 months before the 2008 outburst. A change of the disk to a steeper spectrum was also seen at the onset of the brightening. Variations in modulation during the outburst were detected in our four observations made during 7-12 days after the start of the outburst, and a sinusoidal-like modulation with 0.2 mag amplitude changed to be flat with a small amplitude of 0.1 mag. The modulation variations are discussed. We estimate the albedo of the companion during its quiescence and the outburst, which was approximately 0 and 0.9 in the former and latter states, respectively. This large difference probably provides additional evidence for that the neutron star in the binary turns on as a radio pulsar in quiescence.

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