TITLE: GCN CIRCULAR NUMBER: 39244 SUBJECT: LIGO/Virgo/KAGRA S250206dm: further Pan-STARRS observations and one candidate AT2025bbo DATE: 25/02/09 00:03:13 GMT FROM: Stephen Smartt at University of Oxford s.smartt@qub.ac.uk

S. J. Smartt (Oxford/QUB), M. E. Huber (IfA, Hawaii), K. W. Smith (Oxford/QUB), S. Srivastav(Oxford), D. R. Young (QUB), J. H. Gillanders (Oxford), K. C. Chambers (IfA, Hawaii), F. Stoppa (Oxford), M. Nicholl, M. D. Fulton, M. McCollum, T. Moore, S. Sim, J. Weston, A. Aamer, C. R. Angus, X. Sheng (QUB), P. Ramsden (QUB/Birmingham), L. Shingles (GSI/QUB), H. Stevance,(Oxford), L. Rhodes (TSI/McGill), A. S. B. Schultz, T. de Boer, J. Fairlamb, H. Gao, C. C. Lin, T. Lowe, E. Magnier, P. Minguez, G. Paek, A. Smith, R. J. Wainscoat (IfA, Univ. Hawaii), T.-W. Chen (NCU), A. Rest (STScI), C. Stubbs (Harvard)

Further to the Pan-STARRS coverage of the NSBH merger event S250206dm (The LIGO-Virgo-Kagra Collaboration, GCN 39175) described in Young et al. GCN 39210, we report updates from observations with the the Pan-STARRS telescope system (Chambers et al., 2016, ArXiv e-prints, 1612.05560).

We observed the skymap on a second night, again with individual r-band images of 45 seconds. The first and second night images were stacked and difference imaging was again run. We have covered 50% of the new, updated map of GCN 39231 (Bilby.offline1.multiorder.fits,0 ; LIGO-Virgo-Kagra Collaboration). We filter out candidates which have previous detections in Pan-STARRS, ZTF (Lasair Broker database) or ATLAS or are stars/compact galaxies with activity. In the new, much smaller skymap, we are left with only one possible candidate.

AT2025bbo : 01:37:17.27 +45:43:31.8. r = 20.45 +/- 0.2 The transient is 0.20" S, 0.26" W from the galaxy centre, SDSS J013717.29+454331.8. SDSS DR15 Photoz z = 0.079 +/- 0.0207 and photoz = 0.0657 from NED (source 2MASS phometric redshift survey). Non-detection 60713.3, and detected at r = 20.45 +/- 0.2 a day later on 60714.3. This implies M_r = -17.6 +/- 0.7 (accounting for foreground Milky Way extinction). The photometric redshift is compatible with the LVK estimate of the GW distance, but it is somewhat luminous for a kilonova.

We checked the ATLAS forced photometry server (Shingles et al. AstroNote 2021-7, Smith et al., 2020, PASP, 132, 1) and the Pan-STARRS all-sky database and neither show activity before the time of S250206dm. We caution that the excess could still be nuclear activity, or subtraction residuals and further data should elucidate. Other surveys should check for pre-GW flux excesses.

The coordinates and further details of AT2025bbo and other sources detected in our skymap are available on the IAU Transient Name server page for s250206dm : https://www.wis-tns.org/ligo/o4/S250206dm_20250206_212530

View this GCN Circular online at https://gcn.nasa.gov/circulars/39244. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...