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X-ray and optical/UV variability from V404 Cyg (GS 2023+338) observed by Swift (part 1 of 2)

ATel #7665; S. Motta (University of Oxford), A. Beardmore (University of Leicester), S. Oates ( IAA-CSIC/UCL-MSSL), N. P.M. Kuin (UCL/MSSL) A. Sanna (University of Cagliari), E. Kuulkers, J. Kajava, C. Sanchez-Fernanedz (ESAC)
on 19 Jun 2015; 08:52 UT
Credential Certification: Sara Elisa Motta (sara.motta@physics.ox.ac.uk)

Subjects: X-ray, Binary, Black Hole, Transient

Referred to by ATel #: 7669, 7671, 7674, 7677, 7681, 7686, 7694, 7695, 7702, 7708, 7721, 7722, 7734, 7736, 7740, 7959

X-ray and optical/UV variability from V404 Cyg (GS 2023+338) observed by Swift S. Motta (University of Oxford), A. Beardmore (University of Leicester), S. Oates ( IAA-CSIC/UCL-MSSL), N.P.M. Kuin (UCL/MSSL) A. Sanna (University of Cagliari), E. Kuulkers, J. Kajava, C. Sanchez-Fernanedz (ESAC)

On June 15 hard X-ray flaring activity from V404 Cyg (GS 2023+338) triggered the Swift/BAT (GCN 17929). Since then, the source repeatedly showed strong X-ray flares that caused the Swift/BAT trigger on the source (GCN 17944, 17945, 17946). Swift monitoring observations started on UT June 16, 2015. We here report on the XRT observations on June 18, and also give an account of all UVOT observations taken until June 18, 2015.

In summary, over the last four Swift observations the source showed low count-rates phases alternated to very strong flaring activity, with the Swift/XRT count rate peaking at > 1000 counts/s in the last Observation. The energy spectrum could be modelled in all cases with and absorbed powerlaw (index between 1.3 and 1.6) and variable column density. The observed fluxes varied between 1e-10 erg/cm^2/s and ~3e-08 ergs/cm^2/s. The strongly flaring X-ray activity now observed in V404 Cyg was also previously observed during the 1989 outburst closely resembling that of the BHC GRS 1915+105.

We report here the most relevant information for the four Swift pointings performed on June 18, 2015 (including both one planned observation and three BAT triggers).

1) Observation 00031403038
Date & time: June 18, 2015 00:19 UT
0.3-10keV flux: 1.46e-10 erg/cm^2/s
Count rate: ~13 counts/s
N_h: 1.30 +/- 0.13 10^22 cm^-2
Photon index: 1.56 +/- 0.10
The X-Ray light curve shows moderate flaring (peaks at ~30 counts/s). The power density spectrum shows flat-top noise and a QPO hint at ~1.8 Hz.

2) Observation 00644520000. Trigger 644596 (GCN 17944)
Date & time: June 18 2015, 00:44 UT
0.3-10keV flux: 3.26e-09 erg/cm2/s
Count rate: ~50 counts/s
N_h: 1.30 +/- 0.13 10^22 cm^-2
Photon index: 1.29 +/- 0.07
The X-Ray light curve shows strong flaring (peaks at ~100 counts/s). The power density spectrum shows a marginally significant QPO at 1.7 Hz (3.1 sigma).

3) Observation 00644596000 (no immediate follow-up). Trigger 644596 (GCN 17945)
Date & time: June 18 2015, 08:52 UT (Bat trigger), June 18, 09:55 UT (XRT follow-up)
0.3-10keV flux: 2.6e-10 ergs/cm^2/s
Count rate: ~1 counts/s
N_h: (1.)*10^22 cm^-2
Photon index: not constrained

Due to Observation constraints, Swift could not slew to point immediately to the source. Once the Swift/XRT observation resumed, the source was already back at ~1 count/s.

4) Observation 00644627000. Trigger 644627 (GCN 17946) Date & time: June 18 2015, 12:53 UT
0.3-10keV flux: 2.8e-08 ergs/cm^2/s
Count rate: ~255 counts/s
N_h: (0.702 +/- 0.017)*10^22 cm^-2
Photon index: 1.248 +/- 0.017
The X-ray light curve shows strong flaring on short time-scales (< 100s) and peaks at > 1100 count/s. The power density spectrum does not show any significant narrow feature between 0.01 and 100.0 Hz.