Renewed activity of V404 Cyg (GS 2023+338)

ATel #7647; E. Kuulkers (ESAC/ESA), S. Motta (U. Oxford), J. Kajava (ESAC/ESA), J. Homan (MIT), R. Fender (U. Oxford), P. Jonker (SRON & U. Nijmegen)
on 17 Jun 2015; 06:31 UT
Credential Certification: Erik Kuulkers (Erik.Kuulkers@sciops.esa.int)

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On June 15, between 18:32 and 19:09, hard X-ray flaring activity from V404 Cyg (GS 2023+338) triggered the Swift/BAT (GCN 17929) and Fermi/GBM (GCN 17932). A further GBM trigger occurred on June 16, 17:39 (GCN 17935). The strong X-ray flares have been confirmed by MAXI/GSC (ATel #7646). A quick-look analysis (GCN 17929) of the promptly downlinked XRT event data was reported to show a spectrum which could be fit with a power-law with a column density consistent with the Galactic value of 8.10 x 10^21 cm^-2 (see GCN 17929). Optical follow-up observations after the first Swift/BAT trigger showed the source fading from R = 12.65 to R = 15.43 in one and a half hours (GCN 17931).

We have re-analysed the prompt flare Swift/XRT data. We do not confirm the spectrum quoted above. The XRT spectrum from the prompt flare looks qualitatively very similar to that of highly absorbed binaries like IGR J16318-4848 (e.g., Walter et al. 2003, A&A 411, L427). This suggests that the source possibly exhibits a dense absorbing medium. The spectrum is difficult to characterise (partial covering absorption of a power-law with low index of about 0.3 fits). We find values of N_H around 5e23 cm-2. The total band average XRT count rate is about 0.9 ct/s.

We performed a Swift ToO on UT June 16 14:57-15:32. The total band average XRT intensity is ~1.1 cts/s. The spectrum can be modeled by a moderately absorbed power law with photon index of 1.22 (+/-0.14), with N_H = 4.5 (+/-0.5) x 10e22 cm-2, yielding an unabsorbed 0.5-10 keV flux of ~3e-10 erg/s/cm^2.

The X-ray flaring behaviour and similar heavy and strongly variable photoelectric absorption were present during the beginning of the previous outburst in 1989 (Terada et al. 1994, PASJ 46, 677; Oosterbroek et al. 1997, A&A 321, 776; Zycki et al. 1999, MNRAS 309, 561), and may indicate the source is getting into a new outburst.

Further Swift ToO observations have been approved; we encourage monitoring this source at all wavelengths.

We thank the Swift PI, Neil Gehrels, for approving the observations and the Swift schedulers for the prompt insertion into the observing schedule.