GTC optical spectroscopy reveals outflow-related features in Swift J1727.8-1613

ATel #16216; Daniel Mata Sanchez, Teo Munoz-Darias (IAC)
on 29 Aug 2023; 07:55 UT
Credential Certification: Teo Muñoz-Darias (teo.munoz-darias@iac.es)

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Swift J1727.8-1613 (originally GRB 230824A: GCN #34536, #34537) has been recently classified as a new low-mass X-ray binary (LMXB) undergoing a bright X-ray outburst (ATel #16205, #16206, #16207,#16210, #16215). Further follow-up of the source revealed bright emission across the electromagnetic spectrum (e.g., radio ATel #16211 and optical ATel #16208,#16209 wavelengths). We performed optical spectroscopy of Swift J1727.8-1613 on 26/08/23 (21:00:20) at the Gran Telescopio Canarias (GTC), equipped with the OSIRIS instrument. We used a slit width of 0.8 arcsec and grism R1000B, for a total exposure time of 4x150s. The data were reduced making use of semi-automatic routines based on IRAF and MOLLY tasks. Despite the unfavourable Moon conditions (17 deg separation and 78% illumination), the bright optical counterpart of the system (g~13; ATel #16208) allowed us to obtain a high quality combined spectrum (SNR~100). The combined spectrum shows strong, single-peaked emission lines throughout the spectral range covered by our observations (~3700 to 8000 Angs), confirming previous reports (ATel #16208). In addition to H, He I and He II emission lines, we also detect metallic transitions, including the Bowen blend and the Fe II 'forest' at 5150-5300 Angs. We report on the detection of sharp blue-shifted absorption troughs in a number of lines, including the hydrogen Balmer series (e.g. H-beta, H-gamma, H-delta), but also He II 4686. The blue-shifted absorptions have similar properties across the spectrum, with blue-edge velocities of ~1400 km/s and depths of ~3 per cent below the continuum level. Based on their similarity to previous studies of LMXBs in recent years (see e.g. Munoz-Darias 2016, 734, 75, Nature; Panizo-Espinar et al. 2022 A&A 664, A100), we associate these features with P-Cygni profiles, thus indicating the presence of an optical outflow in Swift J1727.8-1613. We will continue to monitor this source at optical wavelengths as long as the visibility of the object allows. We are thankful to the GTC team for their support and rapid response to our target-of-opportunity trigger.