Decoding the X-ray flare from MAXI J0709-159 using optical spectroscopy and multi-epoch photometry

TL;DR

This study investigates the X-ray flaring from MAXI J0709-159, linking it to optical spectral variability of HD 54786, and suggests it is a Be/X-ray binary with a neutron star companion based on multi-epoch observations.

Contribution

It provides the first optical spectroscopic and photometric analysis of MAXI J0709-159, identifying HD 54786 as a Be/X-ray binary and clarifying its evolutionary status.

Findings

HD 54786 shows variable He I emission lines.

The star has an effective temperature of about 20,000 K.

HD 54786 is likely an evolving off the main sequence Be/X-ray binary.

Abstract

We present a follow-up study on the recent detection of two X-ray flaring events by MAXI/GSC observations in soft and hard X-rays from MAXI J0709-159 in the direction of HD 54786 (LY CMa), on 2022 January 25. The X-ray luminosity during the flare was around 10^(37) erg/s (MAXI), which got reduced to 10^(32) erg/s (NuSTAR) after the flare. We took low-resolution spectra of HD 54786 from HCT and VBT facilities in India, on 2022 February 1 and 2. In addition to H-alpha emission, we found emission lines of He I in the optical spectrum of this star. By comparing our spectrum of the object with those from literature we found that He I lines show variability. Using photometric study we estimate that the star is having effective temperature of 20000 K. Although HD 54786 is reported as a supergiant in previous studies, our analysis favours it to be evolving off the main sequence in the Color-Magnitude Diagram. We could not detect any infrared excess, ruling out the possibility of IR emission from a dusty circumstellar disc. Our present study suggests that HD 54786 is a Be/X-ray binary system with a compact object companion, possibly a neutron star.