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We report on time-resolved optical imaging of the X-ray binary SAXJ1808.4-3658 during its quiescent state and 2008 outburst. The binary,containing an accretion-powered millisecond pulsar, has a large sinusoidal-likemodulation in its quiescent optical emission. We employ a Markov chain MonteCarlo technique to fit our multi-band light curve data in quiescence with anirradiated star model, and derive a tight constraint of $50^{+6}_{-5}$ deg onthe inclination angle $i$ of the binary system. The pulsar and its companionare constrained to have masses of $0.97^{+0.31}_{-0.22} M_{\sun}$ and$0.04^{+0.02}_{-0.01} M_{\sun}$ (both 1$\sigma$ ranges), respectively. Thedependence of these results on the measurements of the companion's projectedradial velocity is discussed. We also find that the accretion disk had nearlyconstant optical fluxes over a $\sim$500 day period in the quiescent state ourdata covered, but started brightening 1.5 months before the 2008 outburst.Variations in modulation during the outburst were detected in our fourobservations made 7-12 days after the start of the outburst, and asinusoidal-like modulation with 0.2 mag amplitude changed to have a smalleramplitude of 0.1 mag. The modulation variations are discussed. We estimate thealbedo of the companion during its quiescence and the outburst, which wasapproximately 0 and 0.8 (for isotropic emission), respectively. This largedifference probably provides additional evidence that the neutron star in thebinary turns on as a radio pulsar in quiescence.

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