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Detection of transient optical P-Cygni profiles in V404 Cyg

ATel #7659; T. Munoz-Darias, D. Mata Sanchez, J. Casares (IAC-ULL), A. W. Shaw, P. A. Charles (Southampton), A. Ferragamo, J. A. Rubino-Martin (IAC-ULL)
on 18 Jun 2015; 15:57 UT
Credential Certification: Teo Muñoz-Darias (tmd@brera.inaf.it)

Subjects: Optical, Black Hole

Referred to by ATel #: 7662, 7663, 7667, 7669, 7671, 7674, 7677, 7681, 7686, 7694, 7699, 7702, 7708, 7718, 7721, 7722, 7734, 7737, 7740, 7959

Following previous telegrams (ATELs #7646, #7647, #7650, #7655, #7658) reporting renewed activity in the black hole X-ray binary V404 Cyg, we observed the system on Jun 18, 2015 (00:59–01:50 UT) with the OSIRIS spectrograph attached to the GTC-10.4m telescope in La Palma (Spain). We took 35x60s spectra (effective time resolution ~ 100s) using the R1000B grism, which cover the spectral range 4000-7500 Angs. We observe the flux fading a factor of ~4 along our observing run (50 min). In the first 8 spectra we detect clear P-Cyg profiles in the strong He I-5875, H-beta and H-alfa emission lines. The blue absorptions are shifted by ~1500 km/s. P-Cyg profiles are not present in the remaining spectra taken after a sharp drop in flux (factor of 3).
Four hours later, an additional spectrum was obtained with ACAM attached to the WHT-4.2m telescope during twilight. A P-Cyg profile seems to be present in H-beta. We note that P-Cyg profiles were also observed during the 1989 outburst (Casares et al. 1991, MNRAS, 250, 712). V404 Cyg is so far the only black hole X-ray transient that has shown this phenomenology.

In addition, two spectra were fortuitously obtained with the WHT equipped with the ISIS double-arm spectrograph at 5.3 UT on June 15 (i.e.13h before the X-ray detection by Swift/BAT; GCN #17929). The WHT spectra show single-peaked Balmer and HeI emission lines together with weak HeII 4686 emission. The Balmer lines are slightly narrower than typically seen in quiescent profiles but an order of magnitude stronger, with EW(Ha)=273-299 Angs compared to ~19 Angs in quiescence (Casares 2015 arXiv:1506.00639). Furthermore, the Balmer lines are skewed and redshifted by ~100 km/s with respect to the binary systemic velocity, suggesting possible blueshifted absorptions. This indicates that outburst activity already started before the X-ray detection by Swift/BAT.

New spectroscopic observations will be taken in the forthcoming nights.
We are thankful to the GTC team for a fast and efficient response to our ToO trigger.