GTC spectroscopy of MAXI J1750-327

ATel #17990; Teo Munoz-Darias, Daniel Mata Sanchez, Montserrat Armas Padilla, Manuel A. P. Torres, Enzo Saavedra, Alessandra Ambrifi (IAC, ULL)
on 19 Aug 2026; 16:50 UT
Credential Certification: Teo Muñoz-Darias (teo.munoz-darias@iac.es)

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We report on optical spectroscopy of the recently discovered X-ray transient MAXI J1750-327 / EP J175003.0-324244 (ATels #17963, #17964), whose multi-wavelength properties are consistent with a black hole X-ray binary in outburst (ATels #17969, #17971, #17974, #17977, #17978, #17979, #17983, #17988). Observations were carried out on Aug. 18, 2026, starting at 21:05 UT, immediately after evening twilight at the Observatorio del Roque de los Muchachos (ORM), using the OSIRIS spectrograph mounted on the 10.4-m Gran Telescopio Canarias (GTC). The slit was centred on the optical counterpart reported by Russell et al. (ATel #17971). Owing to the low declination of the source, the observations were obtained at an airmass slightly above 2. We employed the R2500V and R2500R grisms and a slit width of 0.8 arcsec, effectively covering the spectral range 4500--7500 Angs at a velocity resolution of ~120 km/s. The combined spectrum is consistent with those commonly observed in X-ray binary transients in outburst, and is characterised by the presence of H (Balmer), He I and He II emission lines. In particular, we detect conspicuous Halpha emission, as well as weaker features associated with He I -5876, He I -6678 and He II -4686. The field is relatively crowded and contributions from fainter nearby stars cannot be excluded, although the spectrum is clearly dominated by the optical transient. We do not find clear disc wind signatures in the prominent Halpha line, although the emission is slightly skewed towards the red. A simple Gaussian fit indicates a modest FWHM of ~900 km/s and a centroid velocity of approximately +170 km/s. However, underlying broad absorption components also appear to be present, as observed in similar sources during the early and late phases of their outbursts (see Mata Sanchez et al. 2026, A&A 707, A364). He II -4686 is more symmetric, with a centroid velocity consistent with zero. New spectroscopic observations will be taken in the forthcoming nights. We are thankful to the GTC team for their fast and efficient response to our ToO trigger.