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INTEGRAL/Swift observations of V404 Cyg back in outburst

ATel #8475; S. E. Motta (Uni. Oxford), C. Sanchez-Fernandez, J. Kajava, E. Kuulkers (ESA/ESAC), K. Page (Uni. Leicester), E. Bozzo, D. Malyshev (uni. Geneva)
on 30 Dec 2015; 09:47 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 #: 8482, 8489, 8494, 8499, 8500, 8501, 8507, 8510, 8512, 8516

The black hole candidate V404 Cyg has recently entered a new outburst phase (ATel #8453, #8454, #8455, #8457, #8462).

INTEGRAL performed a ToO observation of V404 Cyg in spacecraft revolution 1626 (from 2015-12-26 07:59 to 2015-12-28 12:00). The total observation time was about 170 ks.

V404 Cyg was observed by ISGRI and JEM-X instruments with the following fluxes and significances:

isgri 20-40 keV 0.76 +/- 0.09 cts/s (4.7 +/- 0.6 mCrab) significance=8.9

isgri 40-80 keV 0.615 +/- 0.06 cts/s (8.0 +/- 0.8 mCrab) significance=10.5

jmx 3-10 keV 0.24 +/- 0.03 cts/s (1.0 +/- 0.1 mCrab) significance = 16.5

jmx 10-20 keV 0.066 +/- 0.026 cts/s ( 1.1 +/- 0.4 mCrab)

The IBIS/JEM-X light curve shows moderate flickering, comparable to the one seen in the Swift/XRT data. No significant flares are visible in the lightcurve.

The spectra of IBIS/ISGRI, JEMX-1 and Swift/XRT (Obs. taken on 2015-12-26 08:08:23, 2015-12-26T22:59:30, 2015-12-27 08:12:58, 2015-12-27 12:59:58 ) can be simultaneously fitted with and absorbed power law (Nh = 3.2 +/- 0.5 x 10^22 g/cm^2) with a photon index of 1.6 +/- 0.2, consistent with the photon index from the fits to the IBIS/ISGRI and JEM-X spectra alone (1.9 -0.3/+0.4). No soft excess nor a narrow line around 6.4 keV are visible in the joint spectrum and no high-energy cutoff is required. The addition of a reflection component does not improve the fit. The observed flux in the (0.6-200) keV energy band is 2.4x10^-10 ergs/cm^2/s

The multi-wavelength data obtained so far all show a much fainter outburst with respect to the one we witnessed during the summer of 2015. The current outburst is characterised by the same underlying moderate flaring activity that was observed, during the rise and decay of the June 2015 outburst (ATel #7959), although with a substantial lack of the strong flares (where the flux of the source increased of order of magnitudes in matters of minute, see e.g. ATel #7731) that characterised the previous outburst(s). It might be therefore possible that V404 Cyg is going through a standard X-ray outburst.

INTEGRAL will continue observing the source until the end of the year, while Swift will continue the monitoring until the 6th of January. Multi-Wavelength observations are strongly encouraged.

We would like to sincerely thank the INTEGRAL and Swift teams for the fast re-planning of the observing schedule over the Christmas break.