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 (email@example.com)
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.