Results from Long-Term Optical Monitoring of the Soft X-Ray Transient SAX J1810.8-2609

TL;DR

This study presents long-term optical monitoring of the faint soft X-ray transient SAX J1810.8-2609, analyzing its 2007 outburst and correlating optical and X-ray data to understand emission mechanisms and disk behavior.

Contribution

It provides detailed multi-wavelength observations of the 2007 outburst, revealing the roles of X-ray reprocessing and viscous disk emission, and constrains the system's physical parameters.

Findings

Optical and X-ray light curves show multi-peak features.

X-ray lag of ~2 days during rebrightening suggests a small accretion disk.

Optical flux is influenced by both reprocessing and viscous heating.

Abstract

In this paper we report the long-term optical observation of the faint soft X-ray transient SAX J1810.8-2609 from OGLE and MOA. We have focused on the 2007 outburst, and also did the cross-correlate between its optical light curves and the quasi-simultaneous X-ray observations from swift. Both the optical and X-ray light curves of 2007 outburst show multi-peak features. Quasi-simultaneous optical/X-ray luminosity shows that both the X-ray reprocessing and viscously thermal emission can explain the observed optical flux. There is a slightly X-ray delay of 0.6+-0.3 days during the first peak, while the X-ray emission lags the optical emission by ~2 days during the rebrightening stage, which suggests that X-ray reprocessing emission contributes significantly to the optical flux in the first peak, but the viscously-heated-disk origin dominates the optical flux during rebrightening. It implies variation of the physical environment of the outer disk, even the source stayed in low-hard state during the whole outburst. The ~2 day X-ray lag indicates a small accretion disk of the system, and the optical counterpart was not detected by OGLE and MOA during quiescence, which constrained it to be fainter than MI = 7.5 mag. There is a suspected short-time optical flare detected at MJD = 52583.5 without X-ray counterpart detected, this single flux increase may imply a magnetic loop reconnection in the outer disk as proposed by Zurita et al (2003). The observations cover all stages of the outburst, however, due to the low sensitivity of RXTE/ASM, we cannot conclude whether it is an optical precursor at the initial rise of the outburst.