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INTEGRAL observations of intense X-ray and optical flaring from V404 Cyg

ATel #7662; C. Ferrigno, S. Fotopoulou (ISDC, University of Geneva, CH), A. Domingo and J. Alfonso-Garzón (CSIC-INTA), Spain.), J. Rodriguez (Cea-Saclay, France), S. Motta (U. Oxford), E. Kuulkers, C. Sanchez-Fernandez, Jari Kajava (ESAC/ESA) M. Cadolle Bel (Max-Planck Computating and Data Facility, Garching Germany)
on 18 Jun 2015; 21:57 UT
Credential Certification: Carlo Ferrigno (Carlo.Ferrigno@unige.ch)

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

Referred to by ATel #: 7667, 7669, 7671, 7674, 7677, 7681, 7686, 7693, 7695, 7702, 7708, 7714, 7717, 7721, 7722, 7726, 7731, 7734, 7740, 7758, 7759, 7959

A public target of opportunity INTEGRAL observation of the black-hole candidate V 404 Cyg has started on 2015-06-17 at 21:29 and is currently ongoing. We report on the data accumulated until 2015-06-18 17:03 (all times are in UTC). V 404 Cyg is reported to be active from radio to X-rays since 2015 June 16 ( ATEL #7659, ATEL #7655, ATEL #7650, GCN #17929, GCN #17931, GCN #17933, GCN #17938, ATEL #7646, ATEL #7647, GCN #17940, ATEL #7650, and GCN #17944) with radio flaring activity (ATEL #7658).

V 404 Cyg is detected at very high significance and we extracted a light curve with bins of 100 s in the 20-40 keV and 40-100 keV bands. We detect an intense flaring activity with increasing intensity and corresponding shorter dimmer phases: the first active phase took place from 2015-06-17 23:15 to 2015-06-18 00:56, the second one from 06:42 to 09:49, and the third, most intense one from 10:47 to 15:36. During the latter period the source reached a flux of approximately 6 (6.5) Crabs in the 20-40(40-100) keV at 13:58. The IBIS/ISGRI light curve is available here, where horizontal green lines indicate the Crab count rates and the bottom panel is the hardness ratio. A similar flaring behaviour is observed at lower energy (3-20 keV) with JEM-X.

The optical monitor observations are divided in two groups (the gap is due to the INTEGRAL pointing strategy). The first part covers from 2015-06-18 04:27 to 05:17; in these first observations, V404 Cyg is barely detected, with a mean V-magnitude of V~15.6 mag. The second part spans from 10:07 to 11:06 and the source is clearly detected. Its brightness varies on time scales of minutes in tight correlation with the X-ray band: there is a first rapid brightening from V=~15.00 mag to V=~13.01 mag; then, a second brightening reaching V=~14.2 mag, and then a decay to values around V=~15.6 mag. The latest OMC V-band light curve is available here.

The average broad-band 5-250 keV spectrum is well described (χ2red=1.8 for 64 d.o.f.) by a power-law with high-energy cutoff (Γ=1.10±0.05, Ecut=105+11-9 keV). The 20-200 keV flux is 1.0×10-8 ergs/cm2/s during a dead-time corrected exposure of 45 ks.

A significant spectral variability can be inferred from the hardness ratio reported in the light curve. By analysing the spectra accumulated in the INTEGRAL science windows (single exposure lasting about 3.4 ks), we found that during the second flare (20-200 keV flux of 2.6 ×10-8 ergs/cm2/s) the spectral parameters are Γ=0.42±0.05 and Ecut=55±3 keV. While at the peak of the third flare (20-200 keV flux 1.03×10-7 ergs/cm2/s), we obtained Γ=1.06±0.05 and Ecut=63±3 keV.

Further observations of this surprising object are scheduled with INTEGRAL for the next days and a follow-up at all wavelengths is encouraged.