A giant flare of V404 Cyg observed with Swift/BAT

ATel #7755; A. Segreto, M. Del Santo, A. D'Ai', V. La Parola, G. Cusumano, T. Mineo (INAF/IASF-Palermo, Italy), J. Malzac (IRAP, CNRS/Universite' de Toulouse)
on 3 Jul 2015; 08:27 UT
Credential Certification: Melania Del Santo (melania@ifc.inaf.it)

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We have analysed about 13 days (from MJD 57189 until MJD 57202) of BAT survey data on the recent outburst of V404 Cyg (GCN #17929, 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 cutoff power-law with E_c= 78 +12/-10 keV, Gamma=2.21+/-0.07 plus a reflection component with Omega/2Pi=1.2 +0.2/-0.3 (PEXRAV model in XSPEC). 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 energy 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 (BH mass estimation in Shahbaz, T. et al., 1994, MNRAS, 271, L1).

V404 Cyg: the 2015 June outburst observed by BAT