DRAFT
2025-07-21 13:46:36
Type: Object/s-Discovery/Classification
Bibcode: 2025TNSAN.222....1G
BlackGEM/GOTO discovery of AT2025rtk in galaxy ESO073-027, also hosting AT2024afbw
Authors: P.J. Groot (Radboud/UCT/SAAO) , G. Ramsay (Armagh/Warwick), H. Tranin (Barcelona), J. van Roestel (Amsterdam), P.M. Vreeswijk (Radboud), D. O'Neill (Birmingham), A. Kumar (RHUL) , D.T.H. Steeghs (Warwick)
Source Group: BlackGEM
Keywords: Supernova, Surveys, Time-domain, Transient
Abstract:
The BlackGEM and GOTO collaborations announce the discovery of AT2025rtk in the emission-line galaxy ESO073-027 at z=0.02 at q=18.8/L=19.02. The host galaxy is also the host of AT2024afbw, discovered in late 2024, and located 5.5" away from AT2025rtk. Both sources are distinctly different, being located on either side of the nucleus of ESO073-027.

The BlackGEM and GOTO consortia report the detection of BGEM J201505.09-705933.8 / GOTO25fem / AT2025rtk at RA,DEC (ICRS) = 303.77125, -70.992747 = 20:15:05.100, -70:59:33.89 in the emission-line galaxy ESO073-027 at z=0.02. The transient is newly discovered at BlackGEM q-band magnitude of q=18.82 +/- 0.08 on MJD = 2025-07-20 03:04:32UT with an intranight 6-sigma non-detection limit (for transients) of q>20.51 at MJD 2025-07-20 01:42:46, i.e. 1.7 magnitudes deeper only 82 minutes earlier, although further analyses may reveal lower signifance detections before the first detection date. The GOTO experiment first detected AT2025rtk at 2025-07-19 15:04:15.744UT at L=19.04. The source has been detected eight times since with BlackGEM in both the q- and r-bands, and has brightened to q=18.16+/-0.09 at 2025-07-21 05:11:21UT.

AT2025rtk is located only 5.2" away from the recent transient AT2024afbw, which was detected by both the Gaia Transient Alert team and the GOTO consortium (TNS AT reports 237358, Hodgkin et al. 2024, and 251337, O'Neill et al., 2025). The combined GOTO/ATLAS light curves of AT2024afbw show that the transient had faded beyond
detection in the GOTO L-band at L > 19.4 by MJD 2025-05-06 15:24:00UT.  As the astrometric precision of all surveys involved is better than a few tenths of arcseconds, depending mostly on source brightness, AT2025rtk and AT2024afbw are two distinct transients in the same galaxy. Both events are also inconsistent with the nucleus of ESO073-027, which is known to display AGN activity. AT2025rtk and AT2024afbw are located on opposite sides of the galaxy's nucleus, as shown in attached view where the location of each transient is superposed onto the DESI Legacy Survey colour composite of ESO073-027.


BlackGEM is designed, built and operated at the ESO La Silla Observatory by a consortia of universities and institutes, including Radboud, NOVA, KU Leuven, Armagh, Barcelona, Tel Aviv, Weizmann, HUJI, Potsdam, Hamburg, DTU Space, Manchester, Warwick, Durham, UC Davis, TexasTech, LCO, Valparaiso.

The Gravitational-wave Optical Transient Observer (GOTO) project acknowledges the support of the Monash-Warwick Alliance; University of Warwick; Monash University; University of Sheffield; University of Leicester; Armagh Observatory & Planetarium; the National Astronomical Research Institute of Thailand (NARIT); Instituto de Astrofísica de Canarias (IAC); University of Portsmouth; University of Turku, and the UK Science and Technology Facilities Council

Show current TNS values
Catalog Name Reported RA Reported DEC Reported Obj-Type Reported Redshift Host Name Host Redshift Remarks TNS RA TNS DEC TNS Obj-Type TNS Redshift
TNS 2024afbw 20:15:05.850 -70:59:29.76 20:15:05.850 -70:59:29.76
TNS 2025rtk [BGEM J201505.09-705933.8] 20:15:05.100 -70:59:33.89 20:15:05.100 -70:59:33.89 SN Ia 0.0206

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