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Extreme Millimeter/Sub-millimeter and Radio Flares from V404 Cyg (GS 2023+338)

ATel #7708; A. Tetarenko, G. R. Sivakoff (Alberta), Ken Young (CfA), J. G. A. Wouterloot (EAO), J. C. Miller-Jones (Curtin) on behalf of a larger XRB collaboration
on 24 Jun 2015; 21:37 UT
Credential Certification: Gregory R Sivakoff (sivakoff@ualberta.ca)

Subjects: Radio, Millimeter, Sub-Millimeter, Black Hole, Transient

Referred to by ATel #: 7710, 7716, 7718, 7720, 7721, 7722, 7726, 7734, 7740, 7959, 8499

We report follow up radio and mm/sub-mm observations (ATel #7671) of the current outburst of the black hole X-ray binary, V404 Cyg, with the VLA, Submillimeter Array (SMA), and James Clerk Maxwell Telescope SCUBA-2 (JCMT). V404 Cyg has recently 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).

Our nearly simultaneous observations occurred on 2015 June 22. The VLA had scans on source from 10:37:24-14:38:39 UTC (MJD=57195.442-57195.610); the SMA had scans on source from 10:16:17-18:20:47 UTC (MJD=57195.428-57195.764); and the JCMT had scans on source from 10:49:33-14:41:09 (MJD=57195.451-57195.612). VLA data were taken in C (5.25 & 7.45 GHz) and K (20.8 & 25.9 GHz) bands, with the array in the A configuration, split into 2 sub-arrays of 14 and 13 antennas. The SMA data were taken with a tuning frequency of 230 GHz, in the subcompact array configuration, with 7 antennas. JCMT SCUBA-2 data were taken at 850 micron (350 GHz) and 450 micron (666 GHz).

We observed rapid variability in the emission from V404 Cyg. Multiple flares reach Jy levels with longer rise times at lower frequencies (see http://astro.physics.ualberta.ca/sivakoff/V404Cyg/20150622_V404Cyg.jpg). During the SMA track the largest flare occurred at 13:08:53 UTC (MJD=54195.548), where the flux rose from ~75 mJy up to ~6.0 Jy on a timescale of ~25 min. Upon splitting the brightest JCMT 850 micron scan that coincides with the time of the peak flux in the SMA data (12:26- 13:09 UTC) into smaller timescale maps (5 sec time bins), the source flux appeared to rise from ~400 mJy to ~5.3 Jy on a timescale of ~18 min. This is the largest mm/sub-mm flare ever observed from a black hole X-ray binary, far surpassing even the brightest ejections in GRS 1915+105 (Fender & Pooley, 2000). In the VLA data, flares in K band reach a peak of ~1.5 Jy with a rise time of ~25 min, and flares in C band reach a peak of 780 mJy with a rise time of ~40min. The data appear consistent with optical depth effects playing a major role; the lower frequency data are a smoothed, delayed version of the higher-frequency emission, in contrast to the GRS1915 oscillation events, whose amplitudes, rise, and decay times were similar from radio to IR (e.g., Mirabel et al. 1998).

Given the peak flux and rise time of the largest flare (in the SMA data), and assuming equipartition, the standard calculations for a synchrotron emitting plasma (see Fender, 2006) suggest that the minimum energy required to produce this flare is ~8.5e40 erg, corresponding to a mean power into the ejection event of ~5.6e37 erg/s, and that the equipartition magnetic field strength at minimum energy is 2.5 G. As we do not have constraints on the bulk motion of the ejecta at this time, these measurements are not corrected for doppler (de-)boosting. However, the power calculated for this event is consistent with the ejection events of other LMXBs (1e36-1e39 erg/s; e.g., Fender et al., 1999; Brocksopp et al., 2007, 2013; Curran et al., 2014) and comparable to the power measured for the repeated oscillation events seen in GRS 1915+105 (Fender & Pooley, 2000).

Observers with data between 2015 June 22 10:16:17-18:20:47 UTC (MJD=57195.428 57195.764) are encouraged to contact sivakoff@ualberta.ca to coordinate joint analyses.

Further mm/sub-mm observations with the SMA and JCMT will be scheduled as the outburst progresses. Given the nature of the extreme flaring activity reported here, coordinated multi-wavelength observations are highly recommended.

We thank the SMA, JCMT, and VLA for rapidly scheduling our observations.