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X-ray variability of V404 Cyg (GS 2023+33): prominent variable iron line in the Swift/XRT spectra

ATel #7694; S. E. Motta (University of Oxford), A. Beardmore (University of Leicester), E. Kuulkers (ESA/ESAC), D. Altamirano (University of Southampton)
on 23 Jun 2015; 10:11 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Sara Elisa Motta (sara.motta@physics.ox.ac.uk)

Subjects: X-ray, Binary, Black Hole, Transient

Referred to by ATel #: 7695, 7708, 7721, 7722, 7726, 7734, 7736, 7740, 7959

We report on the spectral variability of the black hole binary V404 Cyg (GS 2023+33) that recently entered the outburst phase and is currently showing exceptional variability at all wavelengths, in radio, millimetre and sub-millimetre (e.g. Atel #7658, #7661), infrared (e.g. Atel #7663), optical (Atel #7659, #7669), UV (e.g. Atel #7666), soft and hard X-rays (Atel #7647, #7665, #7648) and at gamma-rays (e.g. GCN #17934, #17948).

Since the first Swift/BAT trigger (June 15, 18:32 UT) the flaring activity of V404 Cyg has been increasing: the Swift/XRT light curve shows peaks at few tens up to >15000 c/s, while the baseline count rate has moved from the initial ~1c/s to >15 c/s. The source has shown significant spectral variability which can be partly ascribed to the variable column density already reported (e.g. Atel #7647 and #7665), which is consistent with the observed (variable) P Cygni profile observed in the Optical (Atel #7669), indicating the presence of outflows close to the source.

The XRT spectra taken after June, 18, 2015 are generally still hard. However, they are different from the XRT spectra taken between June 14 and 18, 2015 (Atels #7665, #7666): a disk component is now clearly detected in most of the spectra. The emission can be well-modelled with an absorbed thermal Comptonization component (nthcomp) with a variable photon index (between 1 and 2) plus a cool disk component, with temperature between 0.2 - 0.4 keV. In a few cases the XRT spectrum contains a strong iron emission line at ~6.4 KeV, with an equivalent width of ~0.27 keV and contributing up to 30% to the 0.3-10 keV flux. Such a prominent line was observed already in the Ginga X-ray spectra obtained during V404 Cyg 1989 outburst (see Oosterbroek et al. 1996, A&A, 309, 781O), and ascribed to reflection from the outer disk.

The iron line is strongest when the source count rate is lowest (< 100 c/s), the flaring not significant (less than a factor 4) and the disk hottest (Swift Observations taken on 2015-06-19 at 21:01, 2015-06-21 at 12:55, 2015-06-21 at 03:55, 2015-06-22 at 02:06). When the count rate is high (few thousands c/s) and the flaring strongest (variations of one or more order of magnitudes), the spectrum of the source is significantly harder, showing a curved tail above ~3 keV (consistent with the hard emission reported in ATel #7693), with faint or not detected disk component. In these cases the line is very faint or not detected at all. We did not find evidence of any clear correlations between the line properties and/or the overall spectral shape and the variations in column density.

Swift will continue monitoring the source in the coming weeks. We thank the Swift PI, Neil Gehrels, for approving the observations and the Swift schedulers for the prompt insertion into the observing schedule.