MAXI J0903-531: confirmation of Be star companion and possible binary period

ATel #14564; Monageng I. M. (UCT/SAAO), Coe M. J. (Southampton), Buckley D. A. H. (SAAO), Kennea J. A. (PSU), Strader J. (MSU), Townsend L. J. (SALT/SAAO)
on 20 Apr 2021; 16:16 UT
Credential Certification: Itumeleng Monageng (itu@saao.ac.za)

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On 18 April 2021 at 20:22:24 UTC we obtained a spectrum of the optical counterpart of the newly-discovered X-ray transient, MAXI J0903-531 (ATel #14555, #14557, #14559), with the Southern African Large Telescope (SALT) under the SALT transient follow-up programme (2018-2-LSP-001). The High Resolution Spectrograph (HRS) was used in Low Resolution (LR) mode (R~16 000)with an exposure time of 1800 secs covering a wavelength region 3750 - 8500 Angstroms. The spectrum shows a number of Balmer and helium absorption lines, with the H-alpha and H-beta lines in emission, and is consistent with an early spectral type star. The H-alpha emission line shows an asymmetric double-peaked profile that is dominated by the red peak and broad wings. The H-beta line shows a double-peaked morphology with a shell profile, where the central depression goes below the continuum level. The measured equivalent width (EW) and FWHM of the H-alpha line are -4.46±0.40 Angstroms and 9.39±0.02 Angstroms, respectively.

Using the criteria of Evans et al. (2004), we performed a spectral classification using the blue end of the spectrum (4000 - 5000 Angstroms). The absence of the HeII 4686, 4541 and 4200 lines indicate that the optical counterpart is of spectral type later than B0 while the presence of the OII 4415, CIII+OII 4650 and OII+NII 4631 blends make it earlier than B3. The absence of SiIV 4116 eliminates B1 and the ratio of the SiIII 4553 and MgII 4481 rules out a B3 spectral type (SiIII > MgII). The estimated distance from GAIA of 10 kpc (ATel #14557) and the average V-band magnitude from ASAS-SN of 13.3 mags constrain the luminosity class range to III-V. In summary, the spectral type of the optical counterpart is B1.5-2 III-Ve. We note that the distance estimate might vary, as the parallax measurements from GAIA are dominated by systematics which are expected to change in subsequent data releases. This can have an effect of altering the derived luminosity class. The spectral classification of the optical counterpart, together with the X-ray spectral properties (ATel #14557 and #14559) confirm that MAXI J0903-531 is a new Galactic Be X-ray binary system.

Data from the ASAS-SN project were used to investigate the optical behaviour of the proposed counterpart, and g-band data in particular were selected as they presented the most coverage. Globally the g-band flux has steadily increased from an average magnitude of 13.44 around JD 2458500 (16 January 2019) to 13.18 in the last few months. If these data are first detrended with a polynomial, then a Lomb-Scargle analysis reveals a strong modulation at a period of 3.84 days with an amplitude of 0.05 magnitudes. If the increase in g-band brightness is indicative of a growing circumstellar disk, then this modulation may indicate the binary period of the system. The measured H-alpha EW is consistent with the short orbital period based on the well-known correlation between the EW and orbital period in BeXBs (Reig et al. 2011), although there is a large scatter in the low-end of the relationship. Furthermore, the circumstellar disc could still be growing and so the measured EW might not be at maximum.