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INTEGRAL and Swift observations of V404 Cyg: going back to quiescence?

ATel #8510; S. E. Motta (Uni. Oxford), C. Sanchez-Fernandez, J. Kajava, E. Kuulkers (ESA/ESAC), J. Chenevez (DTU Space), V. Esposito, C. Ferrigno (Uni. Geneva), A. Beardmore, K. Page (Uni. Leicester)
on 6 Jan 2016; 13:40 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 #: 8512, 8516

We report the results of the INTEGRAL and Swift TOO observations of V404 Cyg in outburst since December 23, 2015 (ATel # 8453). Between INTEGRAL revolutions 1626 (December 26-28, 2015) and 1628 (December 31 2015-January 2, 2016) the flux from V404 Cyg increased by several orders of magnitude, going from a few mCrabs in Rev. 1626 (ATel #8475) up to 15Crab in the 40-80 keV energy band during the flares seen by INTEGRAL in revolution 1628 (ATel #8500).

At the end of INTEGRAL revolution 1628 we noticed a significant weakening of the source and during revolution 1629 (January 3-5, 2016) the source is not detected in the combined JEM-X1+2 mosaics of a total exposure time of 185.5ks. We derived the following 5-sigma upper limits:

2 mCrab in JEMX 3-10 keV energy band.

4 mCrab in JEMX 10-20 keV energy band.

3.5 mCrab in IBIS 20-40 keV energy band.

4.0 mCrab in IBIS 40-80 keV energy band.

Swift detected part of the flaring activity seen by INTEGRAL (on 2016-01-01, 17:30:58, ATel #8500) and then saw a significant decay of the flux, similarly to what can be seen in the INTEGRAL data. V404 Cyg was not detected during observation 00031403139 (2016-01-03, 00:02:20), but it become visible again (though faint) soon afterwards (0.09 cnt/s on 2016-01-03, 13:53:37).

During the last Swift pointings (taken on 2016-01-05, 23:20:58 and 2016-01-06, 00:55:58, respectively) V404 Cyg is detected with an average count rate of 0.13 (exposure 1.1 ks) and 0.05 cnt/s (exposure 0.96 ks), respectively (the count rate close to quiescence was 0.0197 cnt/s, ATel #7959). In the first case the spectrum of the source can be fitted by an absorbed (Nh = 1.7 * 10^22 g/cm^2) powerlaw with photon index equal to 2.0 +/- 0.6, which gives an absorbed flux of 0.4mCrab in the 0.3-10keV energy band, consistent with the INTEGRAL upper limits. In the second case the spectrum can be fitted by an absorbed (Nh = 2.0 * 10^22 g/cm^2) powerlaw with photon index equal to about 1.8, poorly constrained given the low S/N of the spectrum.

Given the erratic behaviour of the source it is still too early to say if this weakening is an actual return to the quiescence state, or we are just seeing an extended minimum in its light curve. Swift will continue monitoring the source during the upcoming days.