TITLE:   GCN CIRCULAR
NUMBER:  36307
SUBJECT: S240422ed: BlackGEM/MeerLICHT transient detection AT2024hgl/MLTJ080628.10-250116.4
DATE:    24/04/25 17:01:30 GMT
FROM:    Paul Groot at Radboud University Nijmegen <p.groot@astro.ru.nl>

P.J. Groot (Radboud, UCT, SAAO), H. Tranin (Barcelona), H. Wichern
(DTU), G. Leloudas (DTU), F. Stoppa (Radboud), I. Arcavi (Tel Aviv),
S. Bloemen (Radboud), P.M. Vreeswijk (Radboud), D. Pieterse (Radboud)
report on behalf of the BlackGEM/MeerLICHT Consortium:

Following the LVK Alert S240422ed the 0.6m BlackGEM/MeerLICHT
telescope array located at the ESO La Silla, Chile and SAAO
Sutherland, South Africa observatories observed the error box with the
ML1/BG2/BG4 telescopes using a predefined sky-grid tiling that covers
the accessible part of the error region using the ranked-tiling method
as outlined in Ghosh et al. (2016). Observations were obtained in the
q band, spanning 440-720nm, using a 2x10-tile strategy. Starting with
the highest probability tiles a set of 10-tiles was observed in the
q-band filter using 60s exposures each, which was repeated directly
after the first 10 tiles were observed. Subsequently tiles 11-20 were
observed in the same two-visit manner, etc.

Observations on the Updated error region of S240422ed (Biscoveanu et
al., GCN Circular 36240) were repeated on the nights of 2024-04-23 and
2024-04-24 following the same tiling pattern.

Transient candidates were extracted using the BlackBOX/ZOGY pipeline,
where astrophotometric calibrations are performed using Gaia DR3
photometry and astrometry. Possible asteroids are filtered using the
daily-updated lists of known asteroids from the Minor Planet
Center. All coordinates are given in the ICRS, epoch J2016.0
astrometric frame and magnitudes quoted are on the AB system.

Next to the MeerLICHT/BlackGEM detection of AT 2024hfx (Smith et al.,
GCN36303; Groot et al., GCN36304) we find one more candidate that
cannot be ruled out as the counterpart to S240422ed:

Name                   RA (ICRS) Dec(ICRS) Filter AB-Mag   Date-UT
MLTJ080628.10-250116.4 121.61711 -25.02119 q 19.92+/-0.16 2024-04-24
17:32:29 
""		       ""         ""       q 19.70+/-0.15 2024-04-24
17:45:43

The candidate was reported to the Transient Name Server as AT2024hgl.

The last non-detection at this position with MeerLICHT was obtained on
2022-01-28 and is therefore not constraining. Inspection of the public
ATLAS archive shows no recent brightening or flaring at the source
location.


The BlackGEM/MeerLICHT telescopes are designed, installed and operated
by a consortium of the following universities and institutions:
Radboud University, Netherlands Research School for Astronomy NOVA, KU
Leuven, Armagh Observatory and Planetarium, Hebrew University of
Jerusalem, South African Astronomical Observatory, Technical
University of Denmark, Tel Aviv University, Texas Tech University,
University of Amsterdam, University of Barcelona, University of
California at Davis, University of Cape Town, University of Durham,
University of Hamburg, University of Manchester, University of Oxford,
University of Potsdam, University of Valparaiso, University of
Warwick, Weizmann Institute

References:
Ghosh et al., 2016, A&A 592, 82