The Optical Counterpart of the Accreting Millisecond Pulsar SAX J1808.4−3658 in Outburst: Constraints on the Binary Inclination

Abstract

We present multiband optical/IR photometry of V4580 Sgr, the optical counterpart of the accretion-powered millisecond pulsar SAX J1808.4-3658, taken during the 1998 X-ray outburst of the system. The optical flux is consistent with emission from an X-ray-heated accretion disk. Self-consistent modeling of the X-ray and optical emission during the outburst yields a best-fit extinction of A v = 0.68 +0.37 0.15 and an inclination of cos i = 0.65 +0.23 -033 (90% confidence), assuming a distance of 2.5 kpc. This inclination range requires that the stellar companion of the pulsar has extremely low mass, M c = 0.05-0.10 M⊙. Some of the IR observations are inconsistent with disk emission and are too bright to be from either the disk or the companion, even in the presence of X-ray heating.