The Historical X-ray Transient KY TrA in quiescence

TL;DR

This study provides detailed optical observations of the X-ray transient KY TrA in quiescence, confirming its counterpart, analyzing its variability, and estimating its orbital parameters and mass function.

Contribution

It offers the first deep optical imaging and spectral analysis of KY TrA in quiescence, improving positional accuracy and estimating key system parameters.

Findings

Confirmed the optical counterpart of KY TrA.

Detected H-alpha emission characteristic of X-ray transients.

Estimated the orbital period and mass function of the system.

Abstract

We present deep optical images of the historical X-ray Transient KY TrA in quiescence from which we confirm the identification of the counterpart reported by Murdin (1977) and derive an improved position of alpha=15:28:16.97 and delta=-61:52:57.8. In 2007 June we obtained I, R and V images, where the counterpart seems to be double indicating the presence of an interloper at ~1.4 arcsec NW. After separating the contribution of KY TrA we calculate I=21.47+-0.09, R=22.3+-0.1 and V=23.6+-0.1. Similar brightness in the I band was measured in May 2004 and June 2010. Variability was analyzed from series of images taken in 2004, spanning 0.6 h, and in two blocks of 6 h during 2007. We find that the target is not variable in any dataset above the error levels ~0.07 mags. The presence of the interloper might explain the non-detection of the classic ellipsoidal modulation; our data indicates that it contributes around half of the total flux, which would make a variation <0.15 mags not detectable. A single spectrum obtained in 2004 May shows the H-alpha emission characteristic of X-ray transients in quiescence with a full-width-half-maximum FWHM=27000+-280 km s/s. If the system follows the FWHM -- K_2 correlation found by Casares (2015), this would correspond to a velocity semi-amplitude of the donor star of K_2=630+-74 km/s. Based on the outburst amplitude and colours of the optical counterpart in quiescence we derive a crude estimate of the orbital period of 8 h and an upper limit of 15 h which would lead to mass function estimates of ~9 M_solar and <16 M_solar respectively.