The near-infrared counterpart of the newly discovered transient Swift J151857.0-572147

ATel #16506; M. C. Baglio, P. D'Avanzo, M. Ferro, S. Campana, S. Covino, D. Fugazza, S. Motta, I. Mariani, M. M. Messa (INAF - OAB), D. M. Russell, P. Saikia, K. Alabarta, S. Rout (NYUAD)
on 5 Mar 2024; 20:58 UT
Credential Certification: Maria Cristina Baglio (cristina.baglio@brera.inaf.it)

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The newly discovered X-ray transient Swift J151857.0-572147 was first detected by Swift/XRT (ATel#16500). The source was at first thought to be a Gamma Ray Burst (GRB; GCN #35849, #35835) and was then classified as an X-ray transient (ATel #16500).
Radio observations performed with MeerKAT (ATel #16503) revealed a bright counterpart at the coordinates RA(J2000) = 15:18:57.5, Dec(J2000) = -57:21:48.7, consistent with the position obtained with Swift/XRT, originally reported in ATel #16500.

The 60cm Robotic Eye Mount (REM) telescope (La Silla, Chile) observed the field at optical (gri-bands) and near-infrared (NIR; H-band) frequencies as part of a monitoring program of the optical and NIR counterparts of GRBs. Observations took place on 2024 March 3rd (MJD 60372), soon after the Swift/XRT first detection, and on March 5th (MJD 60374).
The source is not detected at optical frequencies, likely due to the very high extinction (N_H = 5.6 +/- 0.1 x 10^22 cm^-2; ATel#16500).

The NIR counterpart is instead clearly detected at the coordinates: RA=15:18:57.138, DEC=-57:21:47.30, with an error of ~2''. The position is consistent with the coordinates of the radio counterpart (ATel #16503). The closest 2MASS catalogued source (H=14.9+/-0.1) is found ~10'' away from the suggested counterpart.

The average H-band magnitudes on the two dates are H=14.45+/-0.13 on MJD 60372.322682 (evolving from a minimum brightness of H=14.86+/-0.16 on MJD 60372.28188 to a maximum of H=14.09+/-0.08 on MJD 60372.3668) and H=12.33+/-0.06 on MJD 60374.195548. Data show that the transient is brightening in the NIR, consistently with the typical evolution of a low-mass X-ray binary outburst during the initial rise stage.

Monitoring in g, r, i, H-bands with REM will continue. We encourage multiwavelength observations to uncover the nature of the transient.

NIR finding chart