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Sub-millimetre Detection of V404 Cygni During December-2015 Outburst

ATel #8499; A. Tetarenko, G. Sivakoff (Alberta), J. Miller-Jones (Curtin), I. Coulson (EAO)
on 3 Jan 2016; 03:44 UT
Credential Certification: Gregory R Sivakoff (sivakoff@ualberta.ca)

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

Referred to by ATel #: 8501, 8520

On 2015 December 23, the black hole X-ray binary (BHXB) V404 Cygni entered into a new outburst phase (ATel #8453, #8454, #8455, #8457, #8458, #8462, #8466, #8475, #8482, #8489, #8494; GCN #18716, #18717, #18718, #18721, #18767, #18769, #18777, #18778, #18783, #18785), only ~4 months after the end of its last outburst (ATel #7959). Here we report on sub-mm observations of V404 Cyg taken with the James Clerk Maxwell Telescope (JCMT).

We observed V404 Cyg with JCMT SCUBA-2 on January 1 between 19:48-20:20 UTC (MJD=57388.825-57388.847). V404 Cyg was detected significantly at 850um (350 GHz) with an average flux density of 41 +/- 7 mJy, but not detected at 450 um, with a 3 sigma upper limit of 140 mJy. When we split the (~30 min) JCMT scan into two maps, the first half of the observation shows an average flux density of 58 +/- 19 mJy and the second half shows an average flux density of 38 +/- 10 mJy. The source was not bright enough, and the noise was too high, to accurately measure the flux density on shorter timescales. As V404 Cyg transits during the daytime at this time of year, the JCMT was operating in an specialized extended observing mode at the time of our observations.

Sub-millimetre detections of BHXBs typically arise from the optically thick, steady, compact synchrotron jet, where the jet is characterized by a flat to inverted spectral index; for S_nu proportional to nu^alpha, where S_nu is the flux density and alpha is the spectral index, alpha>0. Given the recent radio detections (ATel #8454, #8482) on the order of tens of mJy, this sub-mm emission could be consistent with that of a steady compact jet. However, we caution that there was significant variability on many timescales in the June outburst, which makes it difficult to interpret non-simultaneous data.

Moreover, during the June 2015 outburst of V404 Cyg, we detected extremely bright, optically thin, mm/sub-mm flares from discrete jet ejecta (ATel #7708). During these jet ejecta flares, detections of tens of mJy over 30 minute timescales were detected in between the peaks (at Jy level) of flares. Our detection on January 1 could have caught the tail end of a brighter flare. We note that there have been bright flares recently measured at X-ray frequencies (ATel #8489, #8494).

Further mm/sub-mm observations will be scheduled as this new outburst progresses. Given the rapid multiwavlength variability seen previously during the June 2015 outburst, coordinated multi-wavelength observations are highly recommended.

We thank the JCMT for rapidly scheduling our observations and allowing us to observe in atypical conditions.