Detection of optical P-Cygni profiles in Swift J1858.6-0814

ATel #12881; T. Munoz-Darias, F. Jimenez-Ibarra, M. Armas Padilla, J. Casares, V. Cuneo, G. Panizo-Espinar, J. Sanchez-Sierras and M. A. P. Torres (IAC-ULL)
on 21 Jun 2019; 15:33 UT
Credential Certification: Teo Muñoz-Darias (teo.munoz-darias@iac.es)

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We report on time resolved optical spectroscopy of the X-ray transient and suspected X-ray binary Swift J1858.6-0814. The system was discovered in October 2018 (ATel #12151), and since then has displayed remarkable properties, such as strong flaring activity and significant intrinsic absorption (ATels: #12158, #12163, #12167, #12180, #12184, #12186, #12197, #12220, #12512). Observations were carried out with the instrument OSIRIS attached to the GTC-10.4m telescope in La Palma (Spain) over five different epochs (Mar 24, Apr 14, Apr 30, May 12 and Jun 6, 2019). We obtained 8-10 optical spectra (4000 - 7500 Angs) per epoch with a time resolution of 5 minutes. The spectra are typical of low-mass X-ray binaries in outburst, showing broad Balmer and He emission lines as well as the Bowen blend at ~4640 Angs.

The data obtained on Jun 6 show strong P-Cyg profiles (the blue-shifted absorption component reaches ~ 10 per cent below the continuum level) in the He I-5876 Angs, H_alpha and H_beta emission lines, implying the presence of an optical wind from this system. The outflow has a terminal velocity (blue edge) of ~2500 km/s, in the upper-end of the accretion disc cold wind velocities measured in the black hole transients V404 Cyg, V4641 Sgr and MAXI J1820+070 [Munoz-Darias et al. 2016 Nature 534; 2018 MNRAS 479; 2019 ApJ Lett. (arXiv:1906.04835)]. In particular, the strength and high-velocity of the wind features resemble those found in V404 Cyg and V4641 Sgr, systems that also share with Swift J1858.6-0814 the presence of strong flaring activity and intrinsic X-ray absorption (see e.g. ATel #12220). Conspicuous P-Cygni profiles in He I-5876 Ang were also observed on May 12, while weaker wind features (also in He I-5876 Ang) were witnessed on Apr 14 and Apr 30. They indicate outflow velocities in the range of 500-1500 km/s at the time of the observations.

We also obtained several photometric observations (g-band acquisition images) in each of the five epochs. These reveal (as reported in previous ATels) strong variability (17.5 -- 16.4 g-band mags; calibrated against Pan-STARRs field stars). In agreement with this, we also observe significant variability in the properties of the emission lines on the 5-min time-scale of our spectroscopy. Finally, we note that the first two epochs (aimed at covering simultaneous multi-wavelength campaigns) were obtained within small visibility windows under relatively poor conditions. In particular, this resulted in a lower signal-to-noise ratio for the Mar 24 observations, when no clear wind signatures are detected after a preliminary analysis (but significant H_alpha variability is observed).

More GTC spectroscopic observations are planned for the forthcoming weeks. We are thankful to the GTC team for their efficient response to our ToO triggers.