Searching for <i>Fermi</i> GRB optical counterparts with the prototype Gravitational-wave Optical Transient Observer (GOTO)

Mong, Y-L and Ackley, K and Galloway, D K and Dyer, Martin J and Cutter, R. and Brown, M J I and Lyman, Joe and Ulaczyk, K and Steeghs, Danny and Dhillon, Vik and O’Brien, P and Ramsay, G and Noysena, K and Kotak, R and Breton, Rene P and Nuttall, L and Pallé, E and Pollacco, D and Thrane, E and Awiphan, Supachai and Burhanudin, U and Chote, P and Chrimes, Ashley and Daw, E and Duffy, Christopher and Eyles-Ferris, R and Gompertz, Benjamin and Heikkilä, T and Irawati, P and Kennedy, Mark R and Killestein, Thomas and Levan, A and Littlefair, S and Makrygianni, L and Marsh, T and Mata-Sanchez, D and Mattila, S and Maund, Justyn and McCormac, J and Mkrtichian, D and Mullaney, J and Rol, E and Sawangwit, U and Stanway, Elizabeth and Starling, Rhaana and Strøm, P and Tooke, S and Wiersema, K (2021) Searching for <i>Fermi</i> GRB optical counterparts with the prototype Gravitational-wave Optical Transient Observer (GOTO). Monthly Notices of the Royal Astronomical Society, 507 (4). pp. 5463-5476. ISSN 0035-8711

Abstract

The typical detection rate of ∼1 gamma-ray burst (GRB) per day by the Fermi Gamma-ray Burst Monitor (GBM) provides a valuable opportunity to further our understanding of GRB physics. However, the large uncertainty of the Fermi localization typically prevents rapid identification of multiwavelength counterparts. We report the follow-up of 93 Fermi GRBs with the Gravitational-wave Optical Transient Observer (GOTO) prototype on La Palma. We selected 53 events (based on favourable observing conditions) for detailed analysis, and to demonstrate our strategy of searching for optical counterparts. We apply a filtering process consisting of both automated and manual steps to 60 085 candidates initially, rejecting all but 29, arising from 15 events. With ~3 GRB afterglows expected to be detectable with GOTO from our sample, most of the candidates are unlikely to be related to the GRBs. Since we did not have multiple observations for those candidates, we cannot confidently confirm the association between the transients and the GRBs. Our results show that GOTO can effectively search for GRB optical counterparts thanks to its large field of view of ~40 deg2 and its depth of ~20 mag. We also detail several methods to improve our overall performance for future follow-up programmes of Fermi GRBs.