The Disk Veiling Effect of the Black Hole Low-Mass X-ray Binary A0620-00

TL;DR

This study investigates the veiling effect in the optical light curves of the black hole low-mass X-ray binary A0620-00, revealing the accretion disk's dominant role in non-ellipsoidal variability and providing key system parameters.

Contribution

It provides the first simultaneous spectroscopic and photometric analysis of A0620-00, quantifying the veiling effect and measuring the companion's spectral type, rotational velocity, and mass ratio.

Findings

The extra optical flux correlates with veiling emission, mainly from the accretion disk.

The companion star is classified as K2V.

Measured rotational velocity of the companion is 83.8 km/s.

Abstract

The optical light curves of quiescent black hole low-mass X-ray binaries often exhibit significant non-ellipsoidal variabilities, showing the photospheric radiation of the companion star is veiled by other source of optical emission. Assessing this "veiling" effect is critical to the black hole mass measurement. Here in this work, we carry out a strictly simultaneous spectroscopic and photometric campaign on the prototype of black hole low-mass X-ray binary A0620-00. We find that for each observation epoch, the extra optical flux beyond a pure ellipsoidal modulation is positively correlated with the fraction of veiling emission, indicating the accretion disk contributes most of the non-ellipsoidal variations. Meanwhile, we also obtain a K2V spectral classification of the companion, as well as the measurements of the companion's rotational velocity $v \sin i = 83.8\pm1.9$ km s$^{-1}$ and the mass ratio between the companion and the black hole $q=0.063\pm0.004$.