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Optical spectroscopy of V404 Cyg: evolution of the P Cygni profiles

ATel #7669; T. Munoz-Darias, D. Mata Sanchez, J. Casares (IAC-ULL)
on 19 Jun 2015; 13:42 UT
Credential Certification: Teo Muñoz-Darias (tmd@brera.inaf.it)

Subjects: Optical, Black Hole

Referred to by ATel #: 7671, 7674, 7677, 7681, 7686, 7694, 7699, 7702, 7708, 7721, 7722, 7734, 7740, 7959

We are performing an intensive observing campaign of the black hole transient V404 Cyg with the 10.4m GTC telescope in La Palma (see ATEL #7659).
We observed the system again on Jun 19, 2015 (00:56 02:40 UT) with the OSIRIS spectrograph attached to the GTC. We took 75x60s spectra (effective time resolution ~ 100s) using the R1000B grism, which cover the spectral range 4000-7500 Angs. In agreement with previous reports, we also observe strong flaring (e.g. ATELs #7646, #7650, #7655, #7658, #7659, #7661, #7662, #7666, #7665).
During our 2 hours run we witness two flare events, with continuum fluxes varying by a factor of ~8-10. However, even though an accurate flux calibration is still not available, the system was always fainter than the brightest points of the precedent night. This can be inferred from a direct comparison with a reference star that we included in the slit.
We keep detecting P-Cyg profiles, but they are less intense and show lower velocities than those seen 1 day ago (i.e. ATEL #7659). They are now much more obvious in the He I lines (e.g. 5875 A and 6678 A) than in the Balmer lines. From a combined analysis of both nights we can infer that the strength and associated velocity of the P-Cyg profiles are modulated with the continuum flux. At low flux the P-Cyg features are weaker in He I and almost absent in the Balmer lines. We also detect high excitation HeII-4686 and Bowen blend lines, which are stronger than in the precedent night. Their strength (flux once continuum is subtracted) is dramatically modulated with the continuum flux. They vary from not being present to be comparable to H-beta in time scales of 20 min. H-alfa strength seems to be more stable and almost continuum independent.

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.