Multicolour Optical Photometry of V404 Cyg
ATel #8531: Vitaly Neustroev (University of Oulu) & George Sjoberg (AAVSO)
The black-hole X-ray binary V404 Cyg spent only few months in quiescence since the previous outburst in the summer of 2015 (ATel #8515). On 2015 December 23, the transient entered a new period of activity (GCN #18716; ATel #8453, #8454, #8455, #8457, #8458, #8462, #8501). In order to study the optical photometric behaviour of V404 Cyg during this outburst, we obtained five sets of multicolour photometric data. These observations were performed on 2015 December 29 and 31 and on 2016 January 4, 6 and 10 with the 0.35-m Celestron C14 robotic telescope, located at New Mexico Skies in Mayhill, New Mexico. We used an SBIG ST-10XME CCD camera with Johnson-Cousins BVRI Astrodon Photometric filters.
The images were taken for approximately 40-60 minutes after sunset, in a sequence B-I-R-V with exposure times of 120-240 s for each filter. In order to improve the confidence of our measurements, we aligned and summed all the nightly images for each filter and then measured the average magnitudes. Fluxes were calibrated using the secondary standards established by A. Henden (see the link to the AAVSO web-site below). The quoted uncertainties of these stars are less than 0.02 mag.
During our observations we were lucky to catch the system in both relatively high and low states. On 2015 December 29 (MJD=57385.104), the observations gave the following magnitudes:
B=17.73+-0.15, V=16.16+-0.05, R_c=14.65+-0.04, I_c=13.39+-0.03
On 2015 December 31 (MJD=57387.106), we found the following magnitudes:
B=18.77+-0.18, V=17.03+-0.07, R_c=15.64+-0.05, I_c=14.51+-0.04
The observations on the following nights gave the following magnitudes:
MJD=57391.091: V=18.04+-0.15, R_c=16.59+-0.06, I_c=15.36+-0.05
MJD=57393.064: V=17.68+-0.07, R_c=16.21+-0.05, I_c=15.15+-0.06
MJD=57397.078: V=18.12+-0.19, R_c=16.49+-0.04, I_c=15.36+-0.04
During the latter nights the source was too faint to be detected in the B band, although on 2016 January 10 (MJD=57397.078) there was a tentative detection with the B magnitude being 19.5+-0.3. V404 Cyg is still about a half magnitude brighter than during the previous quiescent period (ATel #8509).
We note, that despite a 2 mag variation of the source during our observations, we detected no significant changes in colours. The average colour indices are B-V=1.57+-0.17, V-R=1.49+-0.09, R-I=1.16+-0.08. However, these colours are bluer than they were in quiescence (Szkody & Margon, 1989, IAU Circ. No. 4794), and redder than they were during a brighter state of previous outbursts (Jones & Carter, 1989, IAU Circ. No. 4794; Marti et al., 2016, A&A, in press). We will continue our monitoring in the following nights, while the source is still visible.
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