Optical counterpart candidate to x-ray binary SLX 1746-331 during ongoing outburst.

ATel #15949; Vishal Joshi, Sandeep Rout, Santosh Vadavale, Shashikiran Ganesh, Neelam J. S.S. V. Prasad, Kevikumar Lad, Ashirbad Nayak, Abhijit Chakraborty (Physical Research Laboratory, Ahmedabad, India)
on 16 Mar 2023; 17:42 UT
Credential Certification: Vishal Joshi (vjoshi@prl.res.in)

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We report optical imaging observations of the field containing the black hole x-ray binary SLX 1746-331 during its ongoing outburst (ATEL #15937), using Faint Object Camera (FOC) on the 2.5m telescope at Mount Abu Observatory. Three images of the field were acquired in the SDSS-r' filter on UT 2023 March 23.61, under cloudy sky conditions. While one of these images was affected by clouds, the field was observed properly in the other two. We used a DECaPS dr2 image of the same field acquired in 2017 as a reference field.

We detect no transient at the position (R.A(J2000)=17:49:48.94; Dec(J2000)=-33:12:11.6; uncertainty +/- 0.1") previously reported as the NIR counterpart of the 2007 outburst of the source (ATEL #1252, #1633). while comparing with the DECaPS image, we could see a star (obj_id: 4882429761662852497; r mag ~21.1) at the same position.

However, we detect a transient star in the field, with R.A(J2000)=17:49:48.97; Dec(J2000)=-33:12:17.2; uncertainty +/- 0.3"). At this position, we find a star recorded in the DECaPS image (obj_id: 4882429761665101551, r mag ~21.8). Using the DECaPS field stars as a standard, we obtained an r mag of 18.8 +- 0.2 for the same star detected in our image. This suggests that the star has brightened by approximately 3 mag. The transient star is located 2.7" away from the Swift-XRT position RA(J2000) =17h 49m 48.84s; Dec(J2000) = -33d 12m 15.0s; uncertainty +- 2.6") (ATEL #15943) but is 5.6" away from the NIR counterpart detected in 2007. It is worth noting that during 2007 outburst, the separation between the positions of NIR counterpart and Swift-XRT detection was 3.4", but it was in the opposite direction to that of the counterpart reported here.

If the optical/NIR transient stars detected in the 2007 outburst and in our observations are the true counterpart of the x-ray outbursts of SLX 1746-331 in years 2007 and 2023, then the stars outbursted in these years are different stars rather than the same one.

The observed and reference images are found here.

The research work at the Physical Research Laboratory is funded by the Department of Space, Government of India.