TITLE:   GCN CIRCULAR
NUMBER:  39215
SUBJECT: LIGO/Virgo/KAGRA S250206dm: GOTO counterpart search update -  AT2025bar
DATE:    25/02/07 20:13:57 GMT
FROM:    kendall.ackley@warwick.ac.uk

D. Steeghs, K. Ackley, M.Kennedy, D. O’Neill, Y.Julakanti, S. Belkin, B. P. Gompertz, B. Godson, G. Ramsay, M. Dyer, J. Lyman, K. Ulaczyk, D. Galloway, V. Dhillon, P. O'Brien, K. Noysena, R. Kotak, R. Breton, L. Nuttall, E. Pallé, D. Pollacco, T. Killestein, and A. Kumar report on behalf of the GOTO collaboration:

We report on optical observations and candidate vetting with the Gravitational-wave Optical Transient Observer (GOTO; Steeghs et al. 2022) in response to the LIGO/Virgo/KAGRA candidate NSBH event S250206dm (The LIGO-Virgo-KAGRA Collaboration, GCN 39175). 

Targeted observations from both GOTO-N and GOTO-S are still ongoing. Here we report on analysis of data obtained beginning at Feb. 6 2025 21:31:11 UT, (+0.09h post trigger) and continuing through to Feb. 7 2025 18:07:58 UT (+20.71h post trigger). These cover 347 sqr. deg. within the 90% localisation contour, equivalent to 35.6% of the total 2D localisation probability.

Images were processed immediately after acquisition using the GOTO pipeline with an average 5-sigma depth of 20.1 mag (L band 400-700 nm). Difference imaging was performed using deeper template observations. Source candidates were initially filtered using a classifier (Killestein et al. 2021) and cross-matched against a variety of contextual and minor planet catalogs. Human vetting was carried out in real time on any candidates that passed the above checks. 

We report the detection of a variable transient, GOTO25aeb / AT2025bar (https://www.wis-tns.org/object/2025bar). Although individual detections are not very significant, the transient is detected multiple times and appears to be fading over the course of our observations (spanning 4 hours). It is likely associated with a host galaxy listed in the WISExSuperCOSMOS PhotoZ SVM catalogue as WISExSCOS J112243.56-453426.1 at a (photo) redshift of z ~ 0.17 (D_L = 840 Mpc). Although not necessarily matching the expectations for an EM counterpart, we believe further investigation of this transient is warranted.   


+--------------------------------------------------------------------------+
|  Internal name |  IAU name  |  RA(J2000) |  Dec(J2000)  |     Mag(AB)    |
+--------------------------------------------------------------------------+

|    GOTO25aeb | AT2025bar | 11:22:43.66, -45:34:28.58 | 20.32 +/- 0.15 |

+--------------------------------------------------------------------------+


Magnitudes were calibrated using ATLAS-REFCAT2 (Tonry et al. 2018) and not corrected for Galactic extinction.

GOTO (https://goto-observatory.org) is a network of telescopes that is
principally funded by the STFC and operated at the Roque de los Muchachos
Observatory on La Palma, Spain, and Siding Spring Observatory in NSW,
Australia, on behalf of a consortium including the University of Warwick,
Monash University, Armagh Observatory & Planetarium, the University of
Leicester, the University of Sheffield, the National Astronomical Research
Institute of Thailand (NARIT), the University of Turku, the University of
Portsmouth, the University of Manchester and the Instituto de Astrofisica
de Canarias (IAC).