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Significant Decrease in Intensity and Variability of Millimeter Emission from V404 Cyg

ATel #7740; A. Tetarenko, G. R. Sivakoff (Alberta), M. Bremer (NOEMA), J. C. Miller-Jones (Curtin), K. Mooley, R. Fender, T. Staley, G. Anderson (Oxford) on behalf of a larger XRB collaboration
on 1 Jul 2015; 14:45 UT
Credential Certification: Gregory R Sivakoff (sivakoff@ualberta.ca)

Subjects: Radio, Millimeter, Request for Observations, Black Hole, Transient

Referred to by ATel #: 7763, 7959

We report follow up mm observations (ATel #7671, #7661, #7708) of the current outburst of the black hole X-ray binary, V404 Cyg, with the NOrthern Extended Millimeter Array (NOEMA; formerly the Plateau de Bure Interferometer). While V404 Cyg has shown strong variable multiwavelength emission (GCN #17929, #17931, #17929, #17931, #17933, #17938, #17940, #17944, #17945, #17946, #17948, #17949, #17956; ATel #7646, #7647, #7650, #7655, #7658, #7659, #7661, #7662, #7665, #7666, #7667, #7669, #7674, #7677, #7678, #7681, #7686, #7688, #7693, #7694, #7695, #7696, #7699, #7701, #7702, #7703, #7706, #7709, #7710, #7714, #7715, #7717, #7718, #7720, #7721, #7722, #7725, #7726, #7727, #7728), recently, dramatic decreases in flux at radio (ATel #7733), optical (ATel #7729, #7737), UV (ATel #7734), and X-ray (ATel #7731) frequencies have been reported.

Our NOEMA observations occurred on 2015 June 26/27, June 27/28, and June 29/30 (Epochs 1-3). Data were taken between June 26 22:24 - June 27 02:42 UTC (MJD=57199.9333-57200.1125), June 27 23:17 - June 28 03:05 UTC (MJD=57200.9701-57201.1285), and June 29 22:12 UTC - June 30 02:55 UTC (MJD=57202.9250-57203.1215). Data in Epochs 1 and 3 were taken in the 3mm (97.5 GHz for ~1.5 hours) and the 2mm (140.5 GHz for ~2.5 hours) bands in sequence, while only 2mm band data were taken in Epoch 2.

We observed multiple flares during Epochs 1 and 2 at 140.5 GHz, but not at 97.5 GHz, while Epoch 3 detected no flaring activity in either band. The average flux level declined from Epoch 1 to Epoch 3; all flux densities reported below are found from fitting a point source in the UV plane to the complete data sets. The errors reported do not include the ~15/10% systematic errors for absolute flux calibration in the 2mm/3mm bands.

 
Epoch    Frequency (GHz)    Flux Density (mJy) 
---------------------------------------------------------------- 
1         97.5              65.2 +/- 0.2 
1        140.5              46.9 +/- 0.3 
2        140.5              16.4 +/- 0.09 
3         97.5               5.3 +/- 0.1 
3        140.5               6.0 +/- 0.1 

Our NOEMA observations show much fainter flux densities, compared to the Jy level mm/sub-mm emission we observed with the SMA/JCMT on June 22 (ATel #7708). By combining the NOEMA data with simultaneous AMI-LA data at 16 GHz, we measure simultaneous spectral indices of: alpha_{16,97.5 GHz} = -0.65 +/- 0.06 and alpha_{16,140.5 GHz} = -0.61 +/- 0.08 in Epoch 1; alpha_{16,140.5 GHz} = -0.39 +/- 0.08 in Epoch 2; and alpha_{16,97.5 GHz} = -0.25 +/- 0.06 and alpha_{16,140.5 GHz} = -0.11 +/- 0.08 in Epoch 3, where flux density (f_nu) is proportional to nu^alpha. This suggests the radio through mm emission switched from being dominated by optically thin synchrotron jet ejecta to being dominated by an optically thick compact synchrotron jet.

This could represent the reestablishment of a compact jet associated with a transition from the soft to the hard accretion state. The latest X-ray spectra and flux reported (ATel #7731) are consistent with the source being in the hard state and at about 0.1% Eddington (given a 12 solar mass black hole; e.g., Shahbaz et al. 1994, MNRAS, 271, L10). It is unclear at this time whether V404 Cyg will continue to decay in the hard state (possibly following a similar decay pattern as the 1989 outburst; Han & Hjellming 1992, ApJ, 400, 304) or whether it will brighten and/or undergo other state transitions.

Observers with data between 2015 June 26 - 30 during the times listed above are encouraged to contact sivakoff@ualberta.ca to coordinate joint analyses.

Given the significant decline in intensity and variability activity reported here and at other wavelengths, further multi-wavelength observations are highly recommended.

We thank the NOEMA staff for rapidly scheduling our observations and the AMI scheduling team for their continued monitoring of the source.