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Rapid Bright X-ray Flares from V404 Cyg During December-2015 Outburst

ATel #8489; Sebastian Heinz (UW-Madison, USA), Peter Jonker (SRON, NL), Lia Corrales (MIT, USA), Niel Brandt (PSU, USA)
on 31 Dec 2015; 19:44 UT
Credential Certification: Sebastian Heinz (heinzs@astro.wisc.edu)

Subjects: X-ray, Black Hole, Transient

Referred to by ATel #: 8494, 8499, 8501, 8507

After an exceptionally bright outburst in June 2015 (e.g., ATEL #7647), V404 Cyg entered another outburst phase in late December 2015, described in ATEL #8453, #8454, #8455, #8457, #8462, #8475, #8482.

Ongoing Swift monitoring of the current outburst of V404 Cyg indicates that the source is exhibiting very rapid X-ray flares, reaching significantly higher fluxes than the baseline level of the outburst reported previously. The most recent Swift observation (ObsID 00031403133, 2015 Dec 31.4) contains a 320s segment (MJD 57387.4) at approximately 100 mCrab, with a 2-10 keV flux of 2e-9 ergs/s/cm^2 (+/- 14%), brightening by approximately two orders of magnitude from the baseline level within less than 4000 seconds (see Swift lightcurve of the outburst at the link below), suggesting that the source may currently be brightening significantly. Coincident brightening in the radio was also reported for MJD 57387.42 in ATEL #8482.

The spectrum of this flare appears very hard and strongly absorbed, however, spectral extraction of the photon-counting mode observations is limited by severe pile-up in the core of the PSF during the flare.

The highly variable lightcurve of the source and the gaps in coverage of the observations suggests that such flares may have occurred throughout the current outburst, with a series of rapid flux changes observed around MJD 57383.5, though the peak brightness observed during that episode was about an order of magnitude lower in flux, at about 10 mCrab.

The rapid variability and intervals of large flux, as well as the suggested current brightening of the source warrant ongoing multi-wavelength monitoring of the source.

Like other authors of ATels addressing the current outburst from this source, we would like to express our appreciation to the Swift team for their dedication and the prompt scheduling in response to this flare during the holiday season.

Dec 2015 Swift X-ray Lightcurve of V404