SALT spectroscopy of the candidate optical counterpart to SRGA J124404.1-632232/SRGE J124403.8-632231: a likely obscured Be X-ray binary

ATel #14364; D. A. H. Buckley (South African Astronomical Observatory), M. Gromadzki (U. Warsaw), L. Townsend (SAAO), I. Monageng (SAAO), A. Lutovinov (IKI, RAS), V. Doroshenko (U. Tuebingen), A. Rau (MPE)
on 1 Feb 2021; 16:04 UT
Credential Certification: David Buckley (dibnob@saao.ac.za)

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On 2021 January 29, commencing 23:34:05 UTC, we obtained spectra of the proposed optical counterpart to SRGA J124404.1-632232/SRGE J124403.8-632231 (ATel #14357, #14361), 2MASS J12440380-6322320, with the Southern African Large Telescope (SALT). The Robert Stobie Spectrograph was used in low resolution mode (R = 420), covering the region 3300-9000 Å. Two repeat spectra, with 1000 s exposures, were obtained and the combined spectrum shows a strong Hα emission line, a weaker Hβ emission line and a largely featureless continuum, rising to longer wavelengths, with several helium lines in absorption. The spectrum confirms the optical counterpart of SRGA J124404.1-632232/SRGE J124403.8-632231 as a highly reddened early-type Be star, consistent with the low Galactic latitude (-0.5 degrees) and the Gaia EDR3 distance estimate of 5.8 kpc (ATel #14357). This implies the system is a likely new Galactic Be/X-ray binary (BeXRB) system.

The Hα line is single peaked, but shows a slight asymmetry, with the peak flux on the red side of the line centre. The equivalent width (EW) and FWHM of the line are measured as -54.2 ± 2.7 Å and 17.55 ± 0.48 Å, respectively. The Hβ line EW is approximately -5A, though this is hard to measure precisely due to the lower S/N in the blue end of the spectrum. The H α EW is large compared to most other BeXRBs. Based on the proposed orbital period of 138 days (ATel #14361) and the well-known correlation between orbital period and Hα EW in BeXRBs (Reig 2011, Ap&SS, 332, 1), the expected EW for this system is around -35Å, so our measurement suggests that the disc has grown to be larger than average.

Additional higher resolution observations are being scheduled with SALT to reveal the structure seen in the Balmer emission lines, and to classify the spectral type.