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 50 ^ { +6 } _ { -5 } deg on the inclination angle i of the binary system .
The pulsar and its companion are 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 .
The dependence of these results on the measurements of the companion ’ s projected radial velocity is discussed .
We also find that the accretion disk had nearly constant optical fluxes over a \sim 500 day period in the quiescent state our data covered , but started brightening 1.5 months before the 2008 outburst .
Variations in modulation during the outburst were detected in our four observations made 7-12 days after the start of the outburst , and a sinusoidal-like modulation with 0.2 mag amplitude changed to have a smaller 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 0.8 ( for isotropic emission ) , respectively .
This large difference probably provides additional evidence that the neutron star in the binary turns on as a radio pulsar in quiescence .