Blue Oscillations and Rapid Red Flares in Swift J1858.6-0814 Observed with ULTRACAM/NTT

ATel #12197; J. A. Paice (Univ. Southampton, IUCAA), P. Gandhi (Southampton), V. S. Dhillon (Sheffield), T. R. Marsh, M. Green (Warwick), E. Breedt (Cambridge)
on 10 Nov 2018; 05:29 UT
Credential Certification: Poshak Gandhi (p.gandhi@soton.ac.uk)

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We report on simultaneous multi-colour optical observations of the new X-ray transient Swift J1858.6-0814, first discovered by Swift/BAT on 2018 October 31 22:53 UT (ATel #12151). This source has been found to display significant variability, with flaring on timescales of seconds in both X-rays and Optical (ATel #12158, #12163, #12164, #12186). The nature of the source remains unclear, with both neutron star and black hole compact object scenarios suggested (ATel #12158, #12180). Variable radio emission supports the presence of a jet (ATel #12184). The Gaia Data Release 2 (Gaia Collaboration, 2018, A&A 616, A1) does not show any source with a measured parallax within a search radius of 2 arcseconds around the coordinates reported in ATel #12160.

Our observations were carried out with the fast triple-beam camera ULTRACAM on the ESO/NTT (Dhillon et al. 2007, MNRAS, 378, 825) on 2018 November 09, from 00:30-01:18 UT, for a total duration of ~48 minutes. u', g' and i' bands were observed simultaneously, and we achieved a cycle time of 0.92 s in g' and and i'. Up to 5 co-adds were used in u' to improve the signal-to-noise ratio. The median signal-to-noise per frame was 15, 31 and 24 in u', g' and i', respectively. Target counts were corrected with a simultaneously observed bright comparison star.

The source exhibits stronger variability in the blue on timescales of minutes, with rises and dips in flux by factors of ~2-5 (see link at the bottom). The fractional variability amplitude (Vaughan et al. 2003, MNRAS 345, 1271) was found to be ~0.42, ~0.34 and ~0.27 for u', g' and i', respectively. Additionally, sporadic, short flares are seen down to timescales of seconds. Similar single-filter fast optical flares were also reported in ATel #12186 from data with a time resolution of 7 s. We find these flares to be faster (<~5 s) and their colour to be significantly red.

Although the duration of our observation is relatively short, these multicomponent timing characteristics are reminiscent of those seen in the black hole X-ray binary V404 Cyg, which showed slow bluer-when-brighter oscillations as well as fast red sporadic flaring during its 2015 outburst (e.g. Kimura et al. 2016, Nature 529, 54; Gandhi et al. 2016, MNRAS 459, 554). Similarities to V404 Cyg have already been suggested in X-rays (ATel #12158).

We highly encourage further observations to better sample and characterise both the fast flares and the slower oscillations. In V404 Cyg, the rapid red flaring was seen to strengthen near the peak of the outburst (Gandhi et al. 2016). We also encourage rapid, simultaneous multiwavelength observations to search for sub-second optical/infrared delays with respect to X-rays, which would test the origin of the variations seen here. Ground-based optical/infrared visibility of the source is worsening, so quick turnaround is important.

ULTRACAM light curve segment of Swift J1858.6-0814