GCN Circular 33979

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


We report on optical observations with the Gravitational-wave Optical Transient Observer (GOTO; Steeghs et al. 2021) in response to the LIGO/Virgo candidate BNS event S230615az. Targeted observations covering 686 sqr. degrees or 24.1% of the localisation probability of the BAYESTAR skymap were performed between 21:16 UT June 15 and 00:35 UT June 16 2023 (3.43 to 6.75 hours after trigger).


Exposures were taken in L-band (400-700 nm passband) and the median 5 sigma depth was 20.3 AB magnitudes.


Images were processed immediately after acquisition using the GOTO pipeline. Difference imaging was performed using recent survey observations of the same pointings. Source candidates were initially filtered using a classifier (Killestein et al. 2021) and cross-matched against a variety of contextual and minor planet catalogues. Human vetting was carried out in real time on any candidates that passed the above checks.


Within the 90% localisation region of the LVK skymap, we identify two priority candidate counterparts. Both are associated with catalogued galaxies that have photometric redshifts consistent with the GW distance posterior. We find no evidence of these sources prior to the GW alert in previous GOTO observations or the ZTF observations provided by the Lasair broker (Smith et al. 2019). We note possible low significance pre-GW flux in the ATLAS forced photometry server (Shingles et al. 2021) for GOTO23hn.


Name     | RA(J2000)   | Dec(J2000)   | Filter  | Mag(AB)        | D_L(Mpc) | Catalog

GOTO23hn | 12:40:35.55 | +17:02:15.27 | L       | 19.49 +/- 0.10 | 278.0 | WISExSuperCOSMOS

GOTO23hu | 13:22:55.23 | +08:09:49.57 | L       | 19.43 +/- 0.08 | 245.6 | GLADE+


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


Observations are ongoing.

GOTO (https://goto-observatory.org) is a network of telescopes 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), University of Turku, University of Portsmouth, University of Manchester and the Instituto de Astrofisica de Canarias (IAC)