Optical spectroscopy of Swift J1727.8-1613 confirms a new low-mass X-ray binary hosting a black hole candidate

ATel #16208; A. J. Castro-Tirado, R. Sanchez-Ramirez, M. D. Caballero-Garcia, I. Perez-Garcia, E. Fernandez-Garcia, S. Guziy and Y.-D. Hu (IAA-CSIC Granada, Spain), M. Blazek, I. Hermelo and V. Pinter (Calar Alto Observatory, Spain), P. J. Meintjes and H. J. van Heerden (UFS, South Africa), A. Martin-Carrillo and L. Hanlon (UCD, Ireland), D. Hiriart and W. H. Lee (UNAM, Mexico), I. M. Carrasco-Garcia (SMA, Malaga, Spain), I. H. Park (SKKU, Korea), M. Gritsevich (Univ. of Helsinki, Finlandia), A. Castellon, C. J. Perez del Pulgar and A. Reina (UMA, Malaga, Spain)
on 25 Aug 2023; 17:55 UT
Credential Certification: Alberto J. Castro-Tirado (ajct@iaa.es)

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We are following up the optical counterpart of the hard X-ray transient Swift J1727.8-1613 (GRB 230824A: GCN #34536, #34537) also detected by INTEGRAL (GCN #34540) and MAXI/GSC on 2023 August 24 (GCN #34544) and NICER (GCN #34549). According to the data gathered by BOOTES Global Network of robotic telescopes (starting on Aug 24, 19 UT at the BOOTES-6/DPRT plus clear filter), the reported optical counterpart (GCN #34543) is extremely blue (g = 13.41+/-0.07, r = 13.30+/-0.02, i = 13.38+/-0.08), as seen on the BOOTES-5/JGT 0.6m frames taken on Aug 25, 06 UT (ugriZY filters). In addition to the above mentioned photometric observations, optical spectroscopy (wavelength range 3,200-11,000 A, resolution 10 A) was gathered on Aug 24, 22 UT, at the 2.2m telescope at the Calar Alto Observatory in Almeria (Spain). A preliminary reduction of the low resolution CAFOS spectra shows a blue power-law continue plus several emission and absorption features. We identify, amongst others, H-alpha, H-beta, He I 5875, 6678 and He II 4686. Further analysis is currently undergoing. The high hard X-ray flux together with the optical spectrum characteristics support a black hole candidate in a low-mass X-ray binary system on its way to the maximum luminosity during this first recorded outburst, in agreement with the MAXI monitoring (ATEL #16206). Multiwavelength follow-up observations are highly encouraged.