GCN Circular 39285
Subject
LIGO/Virgo/KAGRA S250206dm: Kinder confirmation of a possible artefact in AT 2025bbo
Date
2025-02-11T14:36:40Z (2 days ago)
From
Janet Chen at National Central University <janetstars@gmail.com>
Via
Web form
T.-W. Chen, A. Aryan (both NCU), S. Yang, Z. N. Wang (both HNAS), A. K. H. Kong (NTHU),
S. J. Smartt (Oxford/QUB), J. Gillanders (Oxford), A. Sankar. K, Y.-C. Pan, C.-C. Ngeow, M.-H. Lee, Y.-H. Lee, W.-J. Hou, H.-Y. Hsiao, Y. J. Yang, C.-H. Lai, C.-S. Lin, H.-C. Lin, J.-K. Guo (all NCU), L. L. Fan, G. H. Sun (both HNAS), H.-W. Lin (UMich), H. F. Stevance, S. Srivastav, L. Rhodes (all Oxford), M. Nicholl, M. Fulton, T. Moore, K. W. Smith, C. Angus, A. Aamer (all QUB), A. Schultz and M. Huber (both IfA, Hawaii) report:
We observed the field of the optical transient AT 2025bbo (Smartt et al., GCN 39244) within the skymap of the NSBH merger event S250206dm (The LIGO-Virgo-KAGRA Collaboration, GCN 39175) as part of the Kinder collaboration (Chen & Yang et al. 2024, arXiv:2406.09270). Observations were conducted using the 1m LOT and 40cm SLT telescopes at Lulin Observatory in Taiwan.
In our previous report (Lee et al., GCN 39252), we detected AT 2025bbo using template subtraction based on Pan-STARRS1 reference images (Chambers et al. 2016, arXiv:1612.05560). However, Huber et al. (GCN 39272) noted that the publicly available Pan-STARRS 3Pi reference frame is relatively shallow at the transient’s location. To reassess the detection, we performed additional analyses using different reference images.
To verify the flux excess, we employed the Python-based package AutoPhOT (Brennan & Fraser, 2022, A&A, 667, A62) to perform template subtraction using the "sfft" (Hu, 2022, ApJ, 936, 157) and "hotpants" (Becker A., 2015, ascl.soft. ascl:1504.004) alogorithms. Instead of Pan-STARRS1, we used SDSS template images for subtraction. The resulting difference image showed no residual flux at the transient’s position. Additionally, we applied the Kinder pipeline (Yang et al., A&A 646, A22) to subtract stacked images using SDSS templates. The results were consistent with our AutoPhOT analysis, showing no detection of the transient.
The details of the observations and measured 3-sigma upper limit (in the AB system) are as follows:
Telescope | Filter | MJD | Exposure | Magnitude | avg. Seeing | med. Airmass
LOT | r | 60715.463 | 300sec * 6 | > 20.33 | 1".61 | 1.39
LOT | r | 60716.453 | 300sec * 12 | > 21.24 | 1".58 | 1.41
SLT | i | 60716.455 | 300sec * 30 | > 20.33 | 2".53 | 1.68
The presented magnitudes are calibrated using the field stars from the SDSS catalog and are not corrected for the expected Galactic foreground extinction corresponding to a reddening of A_r = 0.24 mag in the direction of the burst (Schlafly & Finkbeiner 2011).
We conclude that AT 2025bbo is likely an artefact resulting from the use of a shallow reference image in our previous report. No significant flux excess was found when using SDSS as a reference.