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X-ray re-brightening of V404 Cyg observed by Swift.

ATel #8462; S. E. Motta (Uni. Oxford), C. Sanchez, J. Kajava, E. Kuulkers (ESA/ESAC), A. Beardmore, K. Page (Uni. Leicester), D. Altamirano, C. Knigge (Uni. Southampton), G. Sivakoff (Uni. Alberta).
on 24 Dec 2015; 16:29 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 #: 8466, 8475, 8482, 8489, 8499, 8501, 8507, 8512, 8520, 8531

The dynamically confirmed black-hole binary V404 Cyg has very recently and unexpectedly shown renewed activity (ATel #8453, #8454, #8455, #8457) only 4 months after the end of its previous outburst (ATel #7959).

Following the Swift/BAT trigger on V404 Cyg on 2015-Dec-23 at 05:19:52 UT (ATel #8455 and GCN Circ. 18716), we requested a Swift ToO in an attempt to determine whether the source is entering another active phase.

Swift/XRT observation took place on 2015-12-23 at 14:58:58UT, with a total exposure of ~1.5ks in photon counting (PC) mode. We measured an average count rate of ~0.2 counts/s in the 0.3-10 keV energy band, slightly lower than the count rate reported in ATel #8455. While this is still more than a factor of ten higher than the average quiescent count rate measured in 2012 (~0.014 counts/s), we note that Swift measured a count rate of ~0.15 counts/s in 2009 (Bernardini and Cackett 2014 MNRAS 439 2771).

The spectrum of the source is well fitted by an absorbed (Nh = (1.8 +0.8 -0.6) x 10^22 hard powerlaw (photon index 1.4+/- 0.4), leading to an observed flux of ~1.6 x 10^-11 erg/s/cm^2 in the 0.3-10.0 keV energy band, consistent with the spectra found during the decay of V404 Cyg into quiescence earlier this year (ATel #7763). The photon index is harder than the spectrum found after a softening event towards the end of that decay of V404 Cyg into quiescence. The measured absorption is higher than the Galactic value in the direction of the source, indicating possible mild absorption intrinsic to the source, either cause by residual material expelled during the June outburst, or ejected at the beginning of the re-brightening reported here.

No obvious dust scattering rings (previously detected from V404 Cyg during its last outburst, ATel #7736) can be seen in the PC image, which suggests that the flare detected by Fermi/GBM on Dec. 21st, 2015 (ATel #8457) was probably hard and/or its fluence was not particularly high. UVOT data do not show any significantly detected source. The 3 sigma limit on the magnitude is > 20.62 with an exposure of 1.4ks.

Further Swift observations have been requested and approved. INTEGRAL will stare at the source for 5 days starting from the 26th of December (see schedule at http://integral.esac.esa.int/isocweb/schedule.html?action=schedule&startRevno=1626&endRevno=1635 ). NuStar will observe the source as well, starting on December 24th. Multi-Wavelength observations are strongly encouraged.

We thank the Swift team for the prompt scheduling of the requested ToO. Merry X-mas everyone.