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Highly Variable Millimeter/Sub-Millimeter Detections of V404 Cyg ( GS 2023+338) During its Renewed Activity

ATel #7661; A. Tetarenko, G. R. Sivakoff (Alberta), M. A. Gurwell, G. Petitpas (CfA), J. G. A. Wouterloot (EAO), J. C. Miller-Jones (Curtin) on behalf of a larger XRB collaboration
on 18 Jun 2015; 19:51 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Gregory R Sivakoff (sivakoff@ualberta.ca)

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

Referred to by ATel #: 7663, 7667, 7669, 7671, 7674, 7677, 7681, 7686, 7694, 7701, 7702, 7708, 7720, 7721, 7722, 7734, 7740, 7959

On 2015 June 15 18:32 UT (MJD~57188.772), the black hole low mass X-ray binary V404 Cyg (GS 2023+338) began to exhibit strong X-ray, optical, and radio flaring (GCN #17929, #17931, #17933; ATel #7646, #7647; GCN #17940, ATel #7650, 7658). Here we report on mm/sub-mm observations of V404 Cyg with the Submillimeter Array (SMA) and James Clerk Maxwell Telescope (JCMT) SCUBA-2.

Initial SMA observations on June 16 with scans on source from 17:36-18:16 UTC (MJD=57189.733-57189.761), were carried out with a tuning frequency of 230GHz, the array in its subcompact configuration, and four antennas. V 404 Cyg was detected significantly in the lower sideband (217.5 GHz; 123.6+/-7.5 [statistical errors only] mJy) and upper sideband (232.5 GHz; 125.8+/-7.5 [stat.] mJy); the combined sideband flux density at 230 GHz is 124.6+/-5.3 (stat.) mJy. Due to the limited data, these fluxes were measured directly in the uv-plane. For the SMA, we estimate a 7% systematic uncertainty in absolute flux calibration.

This marks the first time V404 Cyg has been detected at mm frequencies, and the second brightest mm flux density of a black hole X-ray binary ever detected, superseded only by GRS 1915+105 (Fender & Pooley, 2000, MNRAS, 318, L1). Millimeter detections of black hole X-ray binaries typically arise from the optically thick compact synchrotron jet, where the jet is characterized by a flat to steep spectral index; for S_nu proportional to nu^alpha where S_nu is flux density and alpha is the spectral index, alpha~0–0.7. If we assume a flat spectral index, then the implied radio luminosity (L_R=nu*L_nu) at 5 GHz is ~ 4e30 erg/s at the time of the June 16 SMA observation. This luminosity is approximately a factor of 40 higher than is typical for BH X-ray binaries at an X-ray luminosity (1-10 keV) of ~1e35 erg/s, and is instead more consistent with an X-ray luminosity of ~3e37 erg/s. A steeper spectral index (alpha = 0.7) would imply L_R=3e29 erg/s, more consistent with the radio luminosity implied by the X-ray luminosity (L_X~2e35 erg/s; ATel #7647).

A second set of SMA observations on June 17 with scans on source from 13:27-16:55 UTC (MJD= 57190.560-57190.705) were also carried out. Six antennas were used. The flux density dropped significantly from the previous day; we measured flux densities in the uv-plane of 19.4+/-1.2 (stat.) mJy in the lower sideband (219 GHz), 17.4+/-1.2 (stat.) mJy in the upper sideband (231 GHz), and 18.2 +/- 0.8 mJy in the combined sideband. We see tentative evidence for flux variability on the order of 80% within this observation, which could arise if the intrinsic flux density was variable, like the radio flares observed by ATel #7658, or if the polarization varied, as SMA measures only one polarization.

JCMT SCUBA-2 observations on June 17 with scans on source from 11:44-14:22 UTC (MJD=57190.489-57190.599) at 353 GHz tentatively detected V404 Cyg with a flux density of 7.0 +/- 1.6 mJy.

As part of the JACPOT XRB project, a VLA scan taken in A array at 6 GHz on June 17 from 10:49 - 10:52 UTC (MJD = 57190.451-57190.453) measured V404 Cyg to have a flux density of 87.3 +/- 0.3 (stat.) mJy/bm, with a probable 2% systematic error.

It is unclear if the differences in flux densities between VLA, JCMT, and SMA on June 17 arise due to flux variability or an inverted spectral. If the emission is described by a flat spectra, this would require extreme variability from 87 mJy down to ~3 mJy given our potential SMA variability. If we instead assume no temporal variability, we measure alpha_{6-350 GHz} = -0.45+/-0.02, consistent with the inverted index of optically thin jet ejecta.

Further mm/sub-mm observations with the SMA and JCMT will be scheduled. Given the nature of the rapid variability, coordinated multi-wavelength observations are highly recommended.

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