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INTEGRAL observes a bright flare and spectral changes in V404 Cyg: possible hard to soft transition ahead?

ATel #8500; S. E. Motta (Uni. Oxford), C. Sanchez-Fernandez, E. Kuulkers, J. Kajava (ESA/ESAC), E. Bozzo (Uni. Geneva)
on 3 Jan 2016; 11:39 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 #: 8501, 8507, 8510, 8512, 8516, 8520

We report the results of ongoing INTEGRAL TOO observations of V404 Cyg in spacecraft revolutions 1627 (from 2015-12-29 01:43 to 2015-12-31 03:04), and 1628 (from 2015-12-31 15:43 to 2016-01-02 19:41). The total observation time per revolution is ~170 ks.

During revolution 1627, V404 Cyg was observed by ISGRI and JEM-X instruments with following fluxes and significances:

IBIS/ISGRI 20-40 keV 2.67 +/- 0.09 cts/s ( 16.4 +/- 0.5 mCrab ) signif. = 31

IBIS/ISGRI 40-80 keV 1.83 +/- 0.06 cts/s ( 23.8 +/- 0.8 mCrab ) signif. = 31

JEM-X 1 3-10 keV 0.77 +/- 0.04 cts/s ( 3.3 +/- 0.2 mCrab ) signif. = 22

JEM-X 1 10-20 keV 0.33 +/- 0.03 cts/s ( 5.3 +/- 0.5 mCrab ) signif. = 11

JEM-X 2 3-10 keV 0.72 +/- 0.03 cts/s ( 3.1 +/- 0.2 mCrab ) signif. = 23

JEM-X 2 10-20 keV 0.32 +/- 0.03 cts/s ( 5.2 +/- 0.4 mCrab ) signif. = 13

In Rev. 1627 the source has globally brightened by a factor of about 3 compared to revolution 1626 (from 2015-12-26 07:59 to 2015-12-28 12:00, ATel #8475). The JEM-X + ISGRI spectrum can be fitted with a simple power law model (photon index 1.71 +/- 0.03).

During revolution 1628 the flux of V404 Cyg has increased further. The JEM-X and ISGRI light curves show significant flaring activity. Three main flaring periods are visible in different energy bands both in IBIS/ISGRI and JEM-X, with peaks at the times indicated below:

JMX1: 3-10 keV:

Time (UTC) Peak Flux (Crab)

2015-12-31 17:19:32.422 1.383 +/- 0.09

2015-12-31 18:38:22.351 2.95 +/- 0.13

2016-01-01 00:18:01.448 3.66 +/- 0.14

JMX1: 10-20 keV:

Time (UTC) Peak Flux (Crab)

2015-12-31 17:19:12.422 2.68 +/- 0.23

2015-12-31 18:38:05.951 6.12 +/- 0.35

2016-01-01 00:18:01.448 6.99 +/- 0.36

ISGRI, 20-40 keV

Time (UTC) Peak Flux (Crab)

2015-12-31 17:19:31.529 2.83 +/- 0.13

2015-12-31 18:38:06.427 7.9 +/- 0.3

2016-01-01 00:18:00.186 9.28 +/- 0.24

ISGRI, 40-80 keV

Time (UTC) Peak Flux (Crab)

2015-12-31 17:19:11.530 5.4 +/- 0.2

2015-12-31 18:38:06.427 15.27 +/- 0.46

2016-01-01 00:17:39.688 15.28 +/- 0.38

The first flaring period probably corresponds to the flare detected by Swift/BAT (Trigger 668949, CGN circular 18785) in the 15-150 keV energy band. The second flaring period most likely corresponds to the flares detected by MAXI on MJD 57387 at 18:38:04UT reported in ATel #8494. The last and strongest flaring period has been partially observed by Swift/XRT. In Obs. 00031403134 (started on 2016-01-01T17:30:58) a flare is visible in the Swift/XRT light curve (0.6-10keV energy band).

The JEM-X + ISGRI spectrum taken in rev. 1628 cannot be fitted with a simple power law and looks complex as in many spectra taken in June-July 2015. The hard tail now show a clear cutoff at ~150 keV, with a power law photon index of 1.3. Based on previous experience, this photon index probably indicates the presence of a fairly strong reflection component (Motta et al. in prep.) rather than an hardening of the source. The appearance of an high-energy cutoff and of a reflection component, might indicate that V404 Cyg is going through or will soon go through a canonical hard to soft transition.

The JEM-X and ISGRI light curves can be found here: ftp://ftp.sciops.esa.int/pub/ekuulker/JEMX_ISGRI_1628_lcr.pdf

Multi-Wavelength observations, especially in radio, are strongly encouraged in order to confirm a possible state transition.