Optical observations of the rapidly varying newly discovered transient Swift J1858.6-0814

ATel #12180; Maria Cristina Baglio, David M. Russell, Saarah Pirbhoy (NYU Abu Dhabi), Arash Bahramian (Curtin Uni.), Craig O. Heinke (University of Alberta), Paul Roche (Faulkes Telescope Project), Fraser Lewis (Faulkes Telescope Project & Astrophysics Research Institute, LJMU)
on 7 Nov 2018; 07:24 UT
Credential Certification: Maria Cristina Baglio (cristina.baglio@brera.inaf.it)

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We report on optical observations of the newly discovered transient source Swift J1858.6-0814 (first detected by the BAT instrument of the Swift satellite on October 25th, 2018; ATel#12151).
The source was observed in the r'-band using the the 2-metre Faulkes Telescope North on November 6th, 2018. We obtained a series of 50 30-seconds integration images. The target is well detected in all images at a position consistent with the Pan-STARRS position of a previous known object (ID 98112846453925483), as also reported in Vasilopoulos et al. 2018 (ATel#12164).
The source is found to be highly variable in our ~40 mins observation, with a fractional rms of 21 +/- 1 % on a ~45 seconds time resolution; in particular, we detect a maximum and minimum r'-band magnitude (which corresponds to a minimum and maximum flux) of 17.58 +/- 0.03 and 16.71 +/- 0.02, with a mean magnitude of 17.071 +/- 0.020 (from the fit of the light curve with a constant). This indicates that the target has slightly brightened in the optical with respect to previous observations (ATel#12164).

Finally, we considered the nH estimate given in ATel #12158, which is reported to vary in the range 1.5E21 - 3E21 cm^-2 (from NICER observations). Using the relation of Foight et al. 2016 (ApJ, 826, 66), from the nH we obtained an estimate of the absorption coefficient Av (~ 0.78 +/- 0.26) and of the E(B-V) (0.168-0.335). From Green et al. 2018 (MNRAS, 478, 651; see http://argonaut.skymaps.info/query?lon=113.14&lat=-57.61&coordsys=gal&mapname=bayestar2017 ), for an E(B-V) ~ 0.168 - 0.335 we could obtain an estimate of the distance of the target from the reddening-distance relation, which is found to be in the range ~ 0.6 - 6 kpc (but we caution that reddening is not constrained beyond 6 kpc).
With this estimate of the distance, we constructed the optical-X-ray correlation plot to identify the nature of the compact object of the system. For the X-rays luminosity, we considered a 2-10 keV unabsorbed flux in the range of 7.3(+/-2)e-11 - 1.47(+/-0.2)e-10 erg/s/cm2 (from observations obtained with the Swift satellite on 2018 November 4th and 2nd, respectively). For the optical luminosity, we considered the r'-band magnitudes reported in ATel#12164, which were taken on November 3rd, contemporaneous with the X-ray data. From the optical - X-ray correlation plot that we could build, considering trial values of distance (1, 3, 6, 8, 24 kpc), we conclude that the compact object of the system might be either a neutron star or a black hole, depending on the actual value of the distance. In particular, if the distance is low (~1 kpc), it it is more likely to be a neutron star; instead, if the distance turns out to be higher (>3 kpc), the compact object will more likely be a black hole.

We encourage multiwavelength observations, especially radio, and coordinated rapid timing data, e.g. simultaneous optical/infrared and X-ray observations.

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