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Swift XRT in PC Mode Observes Softening X-ray Spectra as V404 Cyg Declines Towards Quiescence

ATel #7763; G. Sivakoff, A. Bahramian (UAlberta), Diego Altamirano (Southampton), James Miller-Jones (Curtin), D. Russell (NYU) on behalf of a larger XRB collaboration
on 5 Jul 2015; 18:16 UT
Credential Certification: Gregory R Sivakoff (sivakoff@ualberta.ca)

Subjects: X-ray, Request for Observations, Black Hole, Transient

Referred to by ATel #: 7772, 7788, 7797, 7959, 8462

Since 2015 June 27 the outburst of V404 Cyg has been declining from its outburst peak (ATel #7729, #7731, #7733, #7740, #7752). We report here on the first seven Swift XRT Photon Counting mode observations taken both from public DDT and guest investigator ToO (PI: Miller-Jones) programs after the source began to decline. We exclude Swift Window Timing mode observations due to the analysis difficulties created by the dust rings (ATel #7736). We see that as the source has significantly declined towards quiescence (well below the luminosity at which outbursts are typically first detected) its spectrum is softening.

Swift PC mode observations were analyzed under standard procedures; there is no indication of pileup. A local background close to the source that should exclude contamination from the dust rings was used. We fit the spectra with an absorbed power-law model (TBABS * PEGPWRLW) where we assumed Wilms et al. (2000) abundances and Verner et al. (1996, ApJ, 465, 487) cross sections. We tied the hydrogen absorption column (N_H) together in all spectral fits. We first also tied the power-law photon index together and fit N_H=(2.05+0.30/-0.27)*1e22 cm^-2 and power-law photon index Gamma=1.63+0.16/-0.15 with a chi-squared of 208.37 for 144 degrees of freedom (dof), where all errors reported here indicate 90% confidence intervals. When we allowed the photon index to vary between observations the fit significantly improved to a chi-squared of 129.76 for 153 dof. An F-test suggests the probability of this improvement being from random chance is 2.4e-12. The unabsorbed 0.5-10 keV luminosities, assuming a 2.4 kpc distance (Miller-Jones et al. 2009, ApJL, 706, 230), and power-law indices are reported in the table below. We fit a globally tied N_H=(1.88+0.27/- 0.25)*1e22 cm^-2. We note that our N_H are intermediate between the ~7e22 cm^-2 reported by ATel #7752 and those typically reported (~1e22 cm^-2; e.g. Reynolds et al. 2014, MNRAS, 441, 3656; Bernardini & Cackett 2014, MNRAS, 439 277).

MJD          Luminosity (1e34)  Photon Index 
57205.453    3.51+0.28/-0.27    1.34+0.18/-0.17 
57205.795    3.97+0.41/-0.40    1.34+0.24/-0.23 
57206.661    3.25+0.44/-0.37    1.91+0.28/-0.26 
57207.377    1.20+0.22/-0.17    1.91+0.33/-0.31 
57207.522    1.78+0.38/-0.28    2.22+0.31/-0.28 
57208.374    2.12+0.38/-0.28    2.19+0.28/-0.16 

V404 Cyg is continuing its decline towards quiescence (the 0.5-10 keV quiescent luminosities range from ~5e32 to ~1.5e33 erg/s with an average of 8e32 erg/s; Bernardini & Cackett 2014). Like most black holes declining towards quiescence, V404 Cyg shows a softening spectrum as its luminosity decreases (Corbel et al., 2006, ApJ, 636, 971; Plotkin et al. 2013, ApJ, 773, 59). Indices after MJD~57206 are consistent with the indices measured in quiescence (2.10-2.31; Bernardini & Cackett 2011)

Assuming an initial time of June 27 (MJD=57200) for when the decline began (ATel #7731), neither an exponential decline [a*exp{-(t-t_0)/tau}] nor a power-law decline [a*(t-t_0)^-b] fit the data well (chi-squared values of 71.3 and 91.9, respectively, for 5 degrees of freedom). There appears to be more structure in the X-ray light curves than that represented by these simple monotonic decline models. With extreme caution, we report that the best fit power-law decline model (for comparison with Han & Hjellming 1992, ApJ, 400, 304) extrapolates to X-ray luminosities of ~8e32 erg/s (i.e., quiescence) by MJD=57295 (2015 September 30).

Given the proximity and known distance of V404 Cyg, we highly encourage continued multiwavelength coordination (see ATel# 7735 for details of a web interface for such coordination) as the source continues its decline towards quiescence.

We thank the Swift team for their continued efforts in monitoring V404 Cyg both through public DDT and guest investigator ToO programs.