GTC resumes the spectroscopic monitoring of Swift J1727.8-1613
ATel #16490; Daniel Mata Sanchez, Teo Munoz Darias, Montserrat Armas-Padilla, Jorge Casares, Manuel A. P. Torres (IAC)
on 28 Feb 2024; 16:03 UT
Credential Certification: Daniel Mata Sanchez (matasanchez.astronomy@gmail.com)
Subjects: Optical, Binary, Black Hole, Transient
The black hole candidate Swift J1727.8-1613 (originally GRB 230824A: GCN #34536, #34537) is a low-mass X-ray binary (LMXB) undergoing a bright X-ray outburst (ATel #16205, #16206, #16207,#16210, #16215). Since its discovery, the source has been monitored across the electromagnetic spectrum as it evolved through the hardness-intensity diagram. Optical observations (including spectroscopy; see ATel #16208,#16216) ceased at the end of October due to Sun constraints.
Mata Sanchez et al. (2024; A&A Letters 682, L1) reported on the spectroscopic evolution of the source during the first two months of the outburst. This covered the transition from the hard to the soft state, with the last spectrum obtained when the source was in the bright soft state. The spectra revealed variable and asymmetric emission profiles for the Balmer series, HeI and HeII transitions, sometimes consistent with the presence of an accretion disc wind. A distance of ~2.7 kpc was also proposed.
We have obtained new optical spectroscopy of Swift J1727.8-1613 with OSIRIS at the Gran Telescopio Canarias (GTC) on the nights of 21/02/24 (06:30:21) and 26/02/24 (06:17:49). A total of 4x450s integrations were obtained on the first night and 4x400s on the second, using two spectral configurations that result in a wavelength coverage of 4000-7600 Angs. Aperture photometry on the acquisition images reveals that the target is ~3 magnitudes fainter compared to the outburst peak, with g= 15.996 +- 0.003 (21/02/24) and g= 16.144 +- 0.004 (26/02/24).
The spectra show broad (FWHM~1700 km/s) double peaked emission lines corresponding to the Balmer series (e.g. H-alpha, H-beta, H-gamma, H-delta), He I (5876 and 6678), and a prominent (EW~3 Angs) He II 4686 emission. These features are characteristic of a black hole LMXB in the soft state. The ubiquitous double-peaked profiles and the disappearance of the previously reported outflow signatures are consistent with the current picture for the spectral evolution in the optical regime (see e.g. Munoz-Darias et al. 2019, ApJ Letters, 879, L4). We will continue to monitor this source at optical wavelengths, as well as encourage multiwavelength follow-up . We are grateful to the GTC team for their support and rapid response to our target of opportunity trigger.
