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

Zhongxiang Wang·Deepto Chakrabarty·Paul Roche·Philip A. Charles·Erik Kuulkers·Tariq Shahbaz·Chris Simpson·Duncan A. Forbes·Stephen F. Helsdon
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Abstract

We present multiband optical/infrared 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 best-fit extinction Av=0.68 and inclination cos(i)=0.65, assuming a distance of 2.5 kpc. The allowed inclination range requires that the pulsar's stellar companion has extremely low mass, 0.05-0.10 Msun. Some of the infrared observations are not consistent with disk emission and are too bright to be from either the disk or the companion, even in the presence of X-ray heating.

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Zhongxiang Wang, Deepto Chakrabarty, Paul Roche, Philip A. Charles, Erik Kuulkers, Tariq Shahbaz, Chris Simpson, Duncan A. Forbes, Stephen F. Helsdon. 2001-10-30. The Optical Counterpart of the Accreting Millisecond Pulsar SAX J1808.4-3658 in Outburst: Constraints on the Binary Inclination. https://doi.org/10.1086/338357

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