An Accurate Determination of the Optical Periodic Modulation in the X-Ray Binary SAX J1808.4-3658

TL;DR

This study precisely measures the optical periodic modulation of the X-ray binary SAX J1808.4-3658, confirming its origin from the irradiated companion star and supporting the pulsar's rotational power in quiescence.

Contribution

We determined the optical period with high precision and confirmed its consistency with the X-ray orbital ephemeris, clarifying the modulation's origin and the presence of the accretion disk.

Findings

Optical period P=7251.9 s with 2.8 s uncertainty.

Optical modulation caused by irradiation of the companion star.

Accretion disk persists during quiescence, contrary to some models.

Abstract

We report on optical imaging of the X-ray binary SAX J1808.4-3658 with the 8-m Gemini South Telescope. The binary, containing an accretion-powered millisecond pulsar, appears to have a large periodic modulation in its quiescent optical emission. In order to clarify the origin of this modulation, we obtained three time-resolved $r'$-band light curves (LCs) of the source in five days. The LCs can be described by a sinusoid, and the long time-span between them allows us to determine optical period P=7251.9 s and phase 0.671 at MJD 54599.0 (TDB; phase 0.0 corresponds to the ascending node of the pulsar orbit), with uncertainties of 2.8 s and 0.008 (90 % confidence), respectively. This periodicity is highly consistent with the X-ray orbital ephemeris. By considering this consistency and the sinusoidal shape of the LCs, we rule out the possibility of the modulation arising from the accretion disk. Our study supports the previous suggestion that the X-ray pulsar becomes rotationally powered in quiescence, with its energy output irradiating the companion star, causing the optical modulation. While it has also been suggested that the accretion disk would be evaporated by the pulsar, we argue that the disk exists and gives rise to the persistent optical emission. The existence of the disk can be verified by long-term, multi-wavelength optical monitoring of the source in quiescence, as an increasing flux and spectral changes from the source would be expected based on the standard disk instability model.