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A stall in the decline of V404 Cyg observed by NuSTAR

ATel #7797; Dominic Walton (JPL/Caltech), Fiona Harrison, Karl Forster, Brian Grefenstette, Vikram Rana (Caltech), Peter Jonker (SRON), John Tomsick (SSL/Berkeley), Diego Altamirano, Poshak Gandhi (Southampton), on behalf of a larger collaboration
on 14 Jul 2015; 08:46 UT
Credential Certification: Dominic Walton (dwalton@srl.caltech.edu)

Subjects: X-ray, Black Hole

Referred to by ATel #: 7959

We report on two recent NuSTAR (Harrison et al. 2013) observations of V404 Cyg obtained during its apparent decline from outburst towards quiescence. These observations were performed from 2015-07-06 06:41 to 2015-07-06 21:41, and from 2015-07-08 23:11 to 2015-07-11 12:46, returning good exposures of ~25 and ~100 ks (per focal plane module), respectively.

As expected from the overall decline exhibited by V404 Cyg recently (e.g. Atel #7763), the first of these two observations showed a strong drop in flux in comparison to the prior NuSTAR observation (performed on 2015-07-01; see Atel #7752). However, during the second observation (on 2015-07-08) on the source was observed at the same flux as the first, no further decay is apparent. This indicates the average flux of V404 Cyg has remained stable for at least 5 days. Short-term variability is seen at a level consistent with the Swift snapshots reported in Atel #7788, which also suggest the decline has slowed.

The spectra obtained from these two observations are also consistent. An analysis of the combined data finds that the 3-79 keV spectrum during these epochs is well described (chisq/DoF = 601/673) simply with an absorbed powerlaw continuum and a neutral Fe K emission line (see also Atel #7752). The best-fit photon index and column density are Gamma = 1.91+/-0.03 and Nh = (1.6 +0.7,-0.4)e22 cm^-2 (all errors are 90% confidence for 1 parameter of interest; the abundance set of Wilms et al. 2000 is assumed). Both have evolved from the NuSTAR observation on 2015-07-01 (Atel #7752) towards the values typically observed in quiescence (e.g. Reynolds et al. 2014, Bernardini & Cackett 2014, Rana et al. in prep.), as expected, but have not yet reached these values. In particular, the continuum has softened since the observation on 2015-07-01, broadly consistent with the trend reported in Atel #7763. The iron emission has an equivalent width of 50+/-20 eV, significantly weaker than the NuSTAR observation on 2015-07-01, an energy of 6.40+/-0.08 keV and is unresolved by NuSTAR (sigma < 0.2 keV). Additionally, we can limit any high-energy exponential cutoff at this epoch to Efold > 180 keV.

The average 3-79 keV flux during these epochs is (2.51+/-0.05)e-11 erg cm^-2 s^-1, roughly an order of magnitude fainter than the observation on 2015-07-01. The source is still comfortably above its quiescent level by at least an order of magnitude.

These are the third and fourth exposures in a series of NuSTAR observations performed during the current outburst of V404 Cyg. Further observations are planned.