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NuSTAR Observation of V404 Cyg During/After Decline

ATel #7752; Dominic Walton (JPL/Caltech), Fiona Harrison, Karl Forster (Caltech), Peter Jonker (SRON), John Tomsick (SSL/Berkeley), Poshak Gandhi, Diego Altamirano (Southampton), on behalf of a larger collaboration
on 2 Jul 2015; 16:52 UT
Credential Certification: Dominic Walton (dwalton@srl.caltech.edu)

Subjects: X-ray, Black Hole

Referred to by ATel #: 7763, 7797, 7959

We report on a NuSTAR (Harrison et al. 2013) observation of V404 Cyg during (or after) the rapid decline recently exhibited by the source (e.g. Atel 7731, 7740). The observation was performed from 2015-07-01 01:06 to 2015-07-01 15:11, spanning roughly half a day in duration and returning ~25ks good exposure (per focal plane module).

During this period, the lightcurve from V404 Cyg showed no evidence of fading further, with the typical count rate rising gradually from ~1 to ~3 ct/s (per module). Mild flaring up to ~10-15 ct/s was also observed. This may indicate that the drop in activity from V404 Cyg has now stabilised; further observations are required to determine whether this is the case.

The 3-79 keV spectrum obtained is well described (chisq/DoF = 767/765) with an absorbed powerlaw continuum and a neutral Fe K emission line. The best-fit photon index and column density are Gamma = 1.76+/-0.02 and Nh = (7.1+/-0.7)e22 cm^-2 (all errors are 90% confidence for 1 parameter of interest). This column is significantly higher than that typically reported in the literature (~1e22 cm^-2; e.g. Reynolds et al. 2014, Rana et al. in prep.). The iron emission has an equivalent width of 130+/-30 eV, an energy of 6.43+/-0.06 keV and appears to be marginally resolved by NuSTAR (sigma = 0.2+/-0.1 keV). Additionally, we can limit any high-energy exponential cutoff to Efold > 160 keV.

The average 3-79 keV flux is (2.02+/-0.03)e-10 erg cm^-2 s^-1, very similar to the most recent INTEGRAL flux reported (extending up to 2015-06-29 17:37; Atel 7731) considering the different bandpasses.

This is the second in a series of NuSTAR observations; the first was performed during the bright flaring phase on 24th June. Further NuSTAR observations are planned with an approximate ~5 day cadence.