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Faint optical precursor to the 2015 outburst of the black hole binary V404 Cyg

ATel #7761; F. Bernardini (NYU Abu Dhabi), D. M. Russell (NYU Abu Dhabi), F. Lewis (Faulkes Telescope Project and Astrophysics Research Institute, LJMU)
on 4 Jul 2015; 16:42 UT
Credential Certification: Federico Bernardini (bernardini@nyu.edu)

Subjects: Optical, Binary, Black Hole, Transient

Referred to by ATel #: 7959, 8515, 16831

We've been monitoring V404 Cyg with Faulkes Telescope North (2-m, at Haleakala on Maui, Hawaii) since 2006, collecting data in the i', V, and R optical bands (200-sec exposures). For the photometry we adopt a circular aperture size of 3.6 arcsec (larger than we usually use) to encompass V404 Cyg and an unrelated field star located 1.4 arcsec to the north of the X-ray binary (e.g. Udalski & Kaluzny 1991 ), which is unresolved from V404 Cyg in some of our images. The nearby star has mag V = 18.90 +- 0.05 (Udalski & Kaluzny 1991), and an i'-band magnitude i' = 16.92 +- 0.01 (IPHAS-DR2, http://www.iphas.org/dr2/ ). The reported magnitudes below are therefore the combined measurements from V404 Cyg and this nearby star.

Our first clear detection of the new 2015 outburst is on 2015-June-19, four days after the initial detection of the new outburst (GCN #17929) by Swift/BAT in the hard X-rays. However, a faint but significant optical precursor is visible on 2015-June-8 and 2015-June-9. We folded the data on the orbital period (6.4714 days) using the ephemeris of Casares & Charles (1994). The quiescent optical flux varies with the orbital period, with some low amplitude flaring activity (see e.g. Shahbaz et al. 2003; Zurita et al. 2004). Figures showing the phase-resolved i', V, and R-band magnitudes of V404 Cyg are available at the link at the bottom of this ATel.

The phase-resolved V-band data up until 2015-June-2 are consistent with the long-term orbital modulation measured in 2014. On 2015-June-8, seven days before the outburst reported in GCN #17929, the magnitude was V = 17.74 +- 0.04, which is about 0.3 mag above the orbital modulation. On 2015-June-9, the magnitude is V = 17.69 +- 0.04, about 0.4 mag above the modulation, suggesting a faint precursor to the outburst. The phase-resolved R-band data up until 2015-April-8 are consistent with the long-term orbital modulation measured in 2014. The data between 2015-May-13 and 2015-June-2 are marginally above the modulation, but the exact modulation curve is uncertain and there is variability in the shape of the modulation from year to year (see also Zurita et al. 2004). On 2015-June-8 the magnitude is R = 16.78 +- 0.01, about 0.3 mag above the long-term modulation, and on 2015-June-9 it is R = 16.89 +- 0.01, still about 0.2 mag above, supporting the scenario of a faint precursor to the outburst. The phase-resolved i'-band data up until 2015-June-5 are consistent with the long-term orbital modulation measured in 2012 and 2014. While the source is showing some variability in the shape of the modulation from year to year, on 2015-June-9 the magnitude is i' = 15.51 +- 0.01, which is 0.1 mag above the modulation, confirming the presence of an optical precursor to the outburst.

We conclude that the outburst of V404 Cyg started on or before 2015-June-8, likely between 2015-June-2 and 2015-June-8, with a slow faint optical rise before the bright X-ray, optical, millimeter and radio flares. We note that the X-ray flux may have also been rising at this time if the source remained below the detection sensitivity limits of X-ray all-sky monitors.

The Faulkes Telescope observations are part of an on-going monitoring campaign of ~ 40 low-mass X-ray binaries (Lewis et al. 2008). This work makes use of observations from the Las Cumbres Observatory Global Telescope Network.

Optical light curves of V404 Cyg