V404 Cyg: the 2015 June outburst observed by BAT

[see also ATel#7755 (Segreto, Del Santo, D'ai', La Parola, Cusumano, Mineo, Malzac)]

We have analysed about 13 days (from MJD 57189 until MJD 57201) of BAT survey data on the recent outburst of V404 Cyg (Atel #7646).
A very bright X-ray flare (Swift/XRT observation reported in ATel #7736) has been observed with Swift/BAT at MJD 57199.533 (2015/06/26 12:47 UT) with a peak intensity of about 50 Crab in 15-30 keV. This occurred about 14 hours after the giant radio flare reported in the ATel #7716.
In spite of the short exposure time (2 min), we obtained a very good statistics spectrum at the flare peak, which extends up to 200 keV. The 15-200 keV spectrum can be well described (Chi^2_red=1.19(72)) by a a cutoff power-law with Ec= 78 +12-10 keV, Γ=2.21±0.07 plus a reflection component with Ω/2π=1.2 +0.2/-0.3.
The estimated 15-200 keV flux is 6.8E-7 erg/cm2/s which translates into a luminosity of 4.6E+38 erg/s, assuming a distance of 2.4 kpc (Miller-Jones, J. et al., ApJL, 706, 230). Extrapolation of this model over the range 1-500 keV results in a luminosity of 1.6E+39 erg/s which is consistent with the Eddington luminosity of a 12 M_Sun black hole system (Shahbaz, T. et al., 1994, MNRAS, 271, L1).

Products obtained from the BAT data are available upon request to the following address: bat@ifc.inaf.it

BAT light curve on V404 Cyg in two energy bands
BAT light curve of V404 Cyg

BAT spectrum of V404 Cyg collected during the peak of the last flare (integration time is 2 min)
BAT spectrum of V404 Cyg

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