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Multiple > 30 Crab hard X-ray flares seen with INTEGRAL during the current V404 Cyg outburst

ATel #7702; J. Rodriguez (SAp-AIM, CEA Paris-Saclay), C. Ferrigno (ISDC, Geneva), M. Cadolle Bel (Max Planck Computing and Data Facility, Garching), M. Clavel, A. Loh, S. Corbel (SAp_AIM, CEA), P. Laurent (SAp-APC, CEA), S. Markoff (U. Amsterdam, The Netherlands), J. Miller-Jones (Curtin Univ.), D. Russell (New York Univ., Abu Dhabi), J. Tomsick (UC Berkeley, USA), J. Wilms (Dr. Reimes Observatory, Germany)
on 24 Jun 2015; 09:10 UT
Credential Certification: Jerome Rodriguez (jrodriguez@cea.fr)

Subjects: X-ray, Gamma Ray, Black Hole, Transient

Referred to by ATel #: 7708, 7717, 7718, 7721, 7722, 7726, 7731, 7734, 7740, 7758, 7959

Following the renewed and highly variable activity of V404 Cyg (ATel #7646, #7647, #7650, #7655, #7658, #7659, #7661, #7662, #7663, #7665, #7666, #7667, #7669, #7671, #7674, #7677, #7678 ) we have triggered target of opportunity observations with INTEGRAL (PI Rodriguez).

The observations cover satellite revolution 1555 (2015 June 20-2015 June 22), and revolution 1556 (2015 June 23-2015 June 25). We report here on the data accumulated during revolution 1555, with the INTEGRAL/IBIS telescope. As was the case during the public ToO (ATel #7662) V404 Cyg is detected at very high significance in all the considered energy ranges: about 2000 (resp. 80) sigma in the 20-40 keV (resp. 150-300 keV) band. We extracted light curves with bins of 100 s in the 20-40 keV and 40-100 keV bands. We detect 11 flares with 20-40 keV flux greater than 1000 cts/s (about 6 times the flux of the Crab Nebula and pulsar in this range).

The strongest flares occurred around (peak time) MJD 57194.11, MJD 57194.31, and 57195.43 with 20-40 keV (resp. 40-100 keV) fluxes of respectively 5633 cts/s or 34 Crab (resp. 3300 cts/s or 31 Crab), 6950 cts/s or 42 Crab (resp. 3520 cts/s or 33 Crab), and 7226 cts/s or 44 Crab (resp. 3480 cts/s or 33 Crab). The former and the latter are single peaked, and they respectively lasted about 3000 s and 1000 s. The second has a doubled-peaked structure, and the whole event has an approximate duration of 9000 s. The hardness of these events slightly decreases with increasing flux.

We note that the instrumental response of IBIS/ISGRI at such a high flux is not precisely known and that these fluxes may be underestimated because of pile up in the detector. The nonflaring 20-40 keV (resp. 40-100 keV) mean flux is roughly 90 cts/s (40 cts/s) equivalent to about 0.55 Crab (0.4 Crab), implying a dynamic range up to at least 80 (resp. 87) for the brightest flare. We note that the mean nonflaring flux has also increased since the previous observation (ATel #7662) where the source had a 20-40 keV (resp. 40-100 keV) flux of roughly 30 cts/s (resp. 27 cts/s), while showing a significant softening. The hard X-ray behavior is globally consistent with the increased optical flaring activity and the amplitude of the optical flares (ATel #7681).

The incredibly high dynamic range of the variations seen at high energy prevents any meaningful global spectral analysis, and careful selection of the data is required. Following the detection of the source at very high energies with SPI (ATel #7693) we, however, extracted an IBIS "Compton-mode" spectrum. Very preliminary results show a detection at a rather high significance up to at least 1 MeV.

INTEGRAL will continue observing the source, over the next revolutions. In order to increase the scientific return we have decided to ease the access to the data once consolidated (in about a month from now) and make all the INTEGRAL consolidated data of our campaign publicly available (see also ATel #7695).