Possible optical counterpart of Swift J103441.7-571527

ATel #15884; David M. Russell, Kevin Alabarta, Maria Cristina Baglio, Payaswini Saikia (NYU Abu Dhabi), Fraser Lewis (Faulkes Telescope Project & Astrophysics Research Institute, LJMU)
on 2 Feb 2023; 13:30 UT
Credential Certification: David M. Russell (dave.russell5@gmail.com)

Tweet

We report on optical observations of the field of Swift J103441.7-571527, an X-ray transient discovered by the Neil Gehrels Swift Observatory on 27 January 2023 and reported on 30 January (ATel #15878). We observed the location of the transient with the Las Cumbres Observatory (LCO) 1-m robotic telescopes on 30-31 January (the LCO nodes at the South African Astronomical Observatory; SAAO, Sutherland, South Africa, and Cerro Tololo, Chile) and 2-m robotic Faulkes Telescope South on 31 January (at Siding Spring Observatory, Australia). Imaging observations were made using the SDSS i' and PanSTARRS Y filters. The exposure times were 300 sec in each filter for both the 1-m and 2-m observations.

A single, faint object is visible at the centre of the 2.6" Swift/XRT error region in the i'-band 2-m image, taken at 14:54 UT on 31 January (MJD 59975.62). The source is not quite significantly detected in all other images, likely due to a lack of sensitivity in Y-band and/or with the 1-m telescopes. No other source is detected within the XRT error circle; the closest star is 4.9" to the west of the centre of the error region. Aperture photometry was performed on the candidate counterpart and several isolated, unsaturated field stars in the APASS DR10 catalogue (Henden 2019) using PHOT in IRAF. The flux calibrated magnitude of the candidate counterpart is i' = 21.64 +- 0.17 (both statistical and systematic errors are included). A finding chart is linked below.

We propose that this source is likely to be the optical counterpart of Swift J103441.7-571527, but future observations, either showing variability, or classification from a spectrum, are required to confirm the association with the X-ray source. Given the high absorption along the line of sight (nH = 1.8 (+0.9, -0.7)e22 cm^-2; ATel #15878), our reported magnitude is consistent with the Swift UVOT magnitude limits measured at shorter wavelengths (u > 21.75, uvm2 > 22.20, uvw2 > 22.07; ATel #15878).

It was suggested that the source could be a cataclysmic variable (ATel #15878). It could also be a faint X-ray transient, i.e. a LMXB (low-mass X-ray binary) in the hard state, given the measured photon index of ~1.7 (+0.6, -0.5). The XRT X-ray count rate of 0.057 +/- 0.011 ct/sec (ATel #15878), taken in PC mode in the 0.3-10 keV energy range (P. Evans, private communication), corresponds to an unabsorbed X-ray flux of ~6.9E-12 erg/cm^2/s. The optical extinction is Av ~ 6.3 (+3.1, -2.4), adopting the nH/Av relation of Foight et al. (2016). Our optical magnitude results in a de-reddened (using A_i' = 0.666 Av) flux density of ~0.38 mJy. Using these values, we can place Swift J103441.7-571527 on the optical/X-ray luminosity correlation for X-ray binaries (e.g. Russell et al. 2006). The distance is unknown, but the source is in the Galactic plane, so we consider distances of 2 kpc and 8 kpc. At these distances, the X-ray luminosity is ~1.9E33 erg/s to ~3.1E34 erg/s (2-10 keV) and the source lies within the cluster of data points for black hole LMXBs (see plot linked below). Therefore, if the above source is indeed the optical counterpart, the optical/X-ray ratio is consistent with the source being a black hole LMXB, not a neutron star LMXB (unless the distance is < ~2 kpc) and not a HMXB. We cannot rule out a cataclysmic variable nature. Further observations are encouraged, including radio and X-ray follow-up observations, to confirm the nature of the source.

The Faulkes Telescope observations are part of an on-going monitoring campaign of ~ 50 low-mass X-ray binaries (Lewis et al. 2008). This work makes use of observations from the Las Cumbres Observatory (LCO).

Optical finding chart and optical/X-ray correlation for Swift J103441.7-571527