On the optical counterpart of Swift J1658.2-4242

ATel #11358; David M. Russell (NYU Abu Dhabi), Fraser Lewis (Faulkes Telescope Project & Astrophysics Research Institute, LJMU), Guobao Zhang (Yunnan Observatories, Chinese Academy of Sciences)
on 26 Feb 2018; 16:37 UT
Credential Certification: David M. Russell (dave.russell5@gmail.com)

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We report on optical observations of the field of Swift J1658.2-4242 with the Las Cumbres Observatory (LCO) 1-m robotic telescopes. The X-ray (ATel #11306, #11321) and radio (ATel #11322) properties of the source suggest this new transient could be a black hole X-ray binary (BHXB). The source was discovered by Swift BAT on 16 February 2018 (GCN #22416) and was also detected on 14 February 2018 by INTEGRAL (ATel #11306). Possible X-ray pulsations were reported at a frequency of ~0.115 Hz with Swift XRT (ATel #11311), however a follow-up NuSTAR observation detected a periodocity that increased in frequency from ~0.138 Hz to ~0.166 Hz, indicating that this is a QPO, commonly seen in BHXBs in the hard state. A flat radio spectrum observed with ATCA, and the position of the source on the radio - X-ray correlation, favour a BHXB at a distance of > 3 kpc (ATel #11322).

There is an optical and infrared source in several catalogues consistent with the position of the radio and X-ray counterpart (ATel #11334, #11342). An optical spectrum of this object taken with the SOAR telescope (ATel #11342) is consistent with a mid to late K star, with foreground extinction E(B-V)=1.4+/-0.1 mag and no accretion signatures. It was unclear from the SOAR observation if the source had brightened from catalogue magnitudes.

We observed the field of Swift J1658.2-4242 with the 1-m LCO network of telescopes (the nodes at Cerro Tololo, Chile and the South African Astronomical Observatory (SAAO), South Africa) on 22, 23, 24 and 25 February in the SDSS i' and r'-bands. We detect a single source at the radio position (see link to our finding chart below). The PSF of the object is consistent with being a point source (the seeing was 1.0" in our best image on 22 Feb), and we detect no other sources close by (the nearest star is 8 arcsec away). Using several stars from the VPHAS+ DR2 survey (Drew et al 2014, MNRAS, 440, 2036) to calibrate the field, we report the following AB magnitudes for this source:

22 Feb (MJD 58171.3): i_{AB}=19.05 +- 0.06
23 Feb (MJD 58172.1): i_{AB}=19.07 +- 0.06
24 Feb (MJD 58173.3): i_{AB}=19.11 +- 0.07; r_{AB}=20.31 +- 0.11
25 Feb (MJD 58174.3): i_{AB}=19.09 +- 0.05; r_{AB}=20.05 +- 0.06

The VPHAS catalogued AB magnitudes are i_{AB}=19.02 +- 0.04; r_{AB}=20.20 +- 0.09. Our LCO magnitudes agree with the VPHAS values within errors, with no evidence for variability, except possibly a slight brightening in the r'-band filter, although this would need follow-up observations to confirm. All X-ray binaries brighten at optical wavelengths during outburst, so we conclude that this source is very likely to be a foreground line-of-sight star unrelated to the X-ray binary, as the SOAR spectrum also suggested (ATel #11342). However, we note that this chance alignment is surprising given the density of stars in our image (see link to our finding chart below).

The total Galactic extinction in this direction is A_v~58.6 mag (ATel #11334), and the neutral hydrogen column measured from X-ray spectroscopy is NH ~ (1.3 - 1.9) x 10^{23} cm^{-2} (ATel #11307, #11310, #11321, #11336). Using the NH/A_v relation of Zhu et al. (2017, MNRAS, 471, 3494), this NH corresponds to a dust extinction of A_v ~ 62 - 91 mag. It is therefore not surprising that we do not detect the optical counterpart. Deep infrared observations are encouraged (K-band or longer wavelengths) to attempt to detect the counterpart.

The 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 (LCO).

Swift J1658.2-4242 LCO optical finding chart