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INTEGRAL detects a dramatic decrease in X-ray flux from V404 Cyg

ATel #7731; C. Ferrigno, E. Bozzo, R. Boissay (ISDC, University of Geneva, Switzerland), E. Kuulkers, P. Kretschmar (ESAC/ESA, Spain)
on 29 Jun 2015; 21:13 UT
Credential Certification: Carlo Ferrigno (Carlo.Ferrigno@unige.ch)

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

Referred to by ATel #: 7733, 7734, 7737, 7738, 7740, 7758, 7763, 7959, 8475

The INTEGRAL observatory is performing an intense monitoring of the on-going outburst from the black-hole X-ray binary V404 Cyg/GS 2023+338 (Atel #7695). The latest available data from the satellite revolution 1558 (from 2015-06-28 14:16 to 2015-06-29 17:37 UTC) revealed a dramatic decay of the source flux in all energy bands covered by the INTEGRAL instruments. The 5-200 keV X-ray flux derived from the combined JEM-X and IBIS/ISGRI data is 2.5×10-10 erg/s/cm-2 (~10 mCrab). The corresponding spectrum can be reasonably well described (reduced χ2/d.o.f.=1.41/68) by using a single power law with photon index Γ=1.9±0.2. We estimated a 2σ lower limit on the high-energy cutoff of 95 keV (all other uncertainties are quoted at 90% c.l.).

For comparison, we note that V404 Cyg exhibited an extraordinary flaring activity in INTEGRAL during the observational period extending from 2015-06-26 at 12 to 21:30 UTC, displaying a soft colour similar to the one reported in Atel #7702 and achieving a maximum flux of about 49 Crab (25-60 keV energy band). The flaring activity became already less pronounced during the period extending from 2015-06-26 at 21:30 to 2015-06-27 at 04:48 UTC, in which the source displayed a colour as hard as the one detected during the initial stages of the outburst (Atel #7662). A prolonged state of low X-ray emission was observed afterward until 14:24. In this case, the estimated source flux was ~17 mCrab (25-60 keV energy band). A few more flares were observed for the following 5 hours (reaching 1.7 Crab), before the source started to fade to the flux level mentioned above. The decay in X-ray flux observed by INTEGRAL occurred nearly contemporary to the one recently reported in the optical band (Atel #7729).

The light curve of the data described above can be viewed here (we display as green lines the Crab count rates in the 25-60 and 60-200 keV energy bands, which are are 172 and 63 cts/s, respectively). Additional INTEGRAL observations in the direction of V404 Cyg are already planned for the coming days.