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Preliminary Radio/UV/X-ray Fluxes from July 10 for V404 Cyg as it Fades Towards Quiescence

ATel #7788; G. R. Sivakoff, A. Bahramian, C. O. Heinke, A. Tetarenko (UAlberta), C. Knigge (Southampton) E. Bozzo, V. Esposito (ISDC, Switzerland), R. P. Fender, T. D. Staley, G. E. Anderson, K. P. Mooley (Oxford), E. Hodges-Kluck (Michigan), E. Kuulkers (ESA/ESAC), S. Markoff (Amsterdam), J. C. Miller-Jones (Curtin), S. Motta, R. P. Fender, T. D. Staley, G. E. Anderson, K. P. Mooley (Oxford), S. Oates (IAA-CSIC/UCL-MSSL) on behalf of a larger XRB collaboration
on 12 Jul 2015; 11:49 UT
Credential Certification: Gregory R Sivakoff (sivakoff@ualberta.ca)

Subjects: Black Hole, Transient

Referred to by ATel #: 7797, 7959

As part of the community's monitoring of the 2015 outburst of the black hole X-ray binary V404 Cyg (ATel #7773), and to help plan future observations, we report preliminary results from observations taken close to 2015 July 10 at radio, UV, and X-ray frequencies.

VLA radio observations were taken simultaneously on July 10 at 1.0-2.0 GHz (L), 4.7-5.6 GHz (C_l), 6.9-7.8 GHz (C_h), 13.0-13.9 GHz (Ku_l), and 16.9-17.8 GHz (Ku_h). AMI-LA observations were taken at 16 GHz on July 10/11. Ten Swift observations on July 10 took data in the Photon Counting mode at X-rays and in the U or UVM1 filters in the UV; we also include the U band data from the last observation of July 9. We fit the Swift X-ray spectra with an absorbed power-law model (TBABS*PEGPWRLW), assuming Wilms et al. (2000, ApJ 542, 914) abundances and Verner et al. (1996, ApJ, 465, 487) cross sections. We tied the power-law photon index (Gamma=1.8+/-0.3; 90% confidence interval) together of all PC mode observations of this outburst and allowed the hydrogen absorption column (N_H) to vary between observations. This differs slightly from ATel #7763, where we tied the column density together and allowed the photon index to vary. The former provides a statistically superior fit and was not considered during the preparation of ATel #7763. We summarize the fits as a plot that we will continue to update. Integral observations from July 9 to 11 place upper limits on the hard X-ray flux.

Below we summarize the radio flux densities, UV (AB) magnitudes, unabsorbed 0.5-10 keV X-ray luminosities, assuming a 2.4 kpc distance (Miller-Jones et al. 2009, ApJL, 706, 230), and Crab-scaled flux measured during these observations. The Swift X-ray luminosities indicate the median, minimum, and maximum extent of the individual observing intervals throughout the day; all other flux errors indicate 1-sigma confidence intervals. The exposures listed below indicate the approximate total on-source exposure in seconds.

MJD               Band           Flux Density/Mag/Lum/Flux         Exp.[s] 
57213.523+/-0.020 VLA:L              0.96+/-0.20 mJy                1548 
57213.523+/-0.020 VLA:C_l            0.90+/-0.03 mJy                1626 
57213.523+/-0.020 VLA:C_h            0.96+/-0.03 mJy                1626 
57213.522+/-0.021 VLA:Ku_l           1.16+/-0.04 mJy                1460 
57213.522+/-0.021 VLA:Ku_h           1.13+/-0.05 mJy                1460 
57213.168+/-0.063 AMI:16GHz          1.14+/-0.07 mJy                9252 
57214.035+/-0.063 AMI:16GHz          1.08+/-0.07 mJy                9252 
57212.950+/-0.362 UVOT:U             21.08+/-0.12 mag               6852 
57213.449+/-0.329 UVOT:UVW1          23.12+/-0.24 mag               1727 
57213.410+/-0.360 XRT:0.5-10keV      1.03(-0.20/+0.83)*1e34 erg/s   8000 
57213.205+/-1.076 INTEGRAL:20-40keV <2.0 mCrab (3-sigma limit)     32500  
57213.205+/-1.076 INTEGRAL:40-80keV <2.5 mCrab (3-sigma limit)     32500  

The simultaneous VLA data indicate an inverted spectral index (alpha= 0.21+/-0.04, for flux density S_nu proportional to nu^alpha) typical for compact synchrotron jets. We note that lower energy photons leak into the Swift UVOT UVW1 filter, and its reported magnitude does not correct for this leak. The optical and X-ray decay have been relatively flat recently, indicating quiescence is unlikely to be reached soon. The Swift X-ray luminosities are variable, with an excess variance of ~27%.

We encourage the continuing monitoring across all frequencies as the evolution of the 2015 outburst of V404 Cyg continues.

We thank the staff of AMI, INTEGRAL, Swift, & VLA for their rapidly scheduling these observations and continued efforts on behalf of the entire community.