GCN Circular 39210

D. R. Young (QUB), J. H. Gillanders (Oxford), M. E. Huber, K. C. Chambers (IfA, Hawaii), S. J. Smartt, K. W. Smith (Oxford/QUB), S. Srivastav, F. Stoppa (Oxford), C. R. Angus, M. Nicholl, M. D. Fulton, M. McCollum, T. Moore, S. Sim, J. Weston, A. Aamer, 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):

We report observations of the Bilby.fits skymap of the NSBH merger event S250206dm (The LIGO-Virgo-Kagra Collaboration, GCN 39175
Loading...
 
 ) with the Pan-STARRS telescope system (Chambers et al., 2016, ArXiv e-prints, 1612.05560). The Pan-STARRS system comprises of two 1.8m telescope units located at the summit of Haleakala on the Hawaiian island of Maui, employing an SDSS-like filter system denoted as grizy, and a broad w-filter, which is a composite of the gri-filters. 

Tiling sequences of multiple 45-sec images were taken at each pointing position in the r-band with both Pan-STARRS1 and Pan-STARRS2. The images were processed with the Pan-STARRS pipeline. After astrometric and photometric calibration, reference images were subtracted from the target stacked images (Magnier et al., 2020a, ApJS, 251, 3; Magnier et al., 2020b, ApJS, 251, 6; Waters et al., 2020, ApJS, 251, 4). Transient candidates were identified and run through our standard filtering procedures, including rejection of artifacts with machine learning tools and cross-matching with galaxy, stellar and solar-system catalogs (e.g. Smith et al., 2020, PASP, 132, 1;  Smartt et al. 2024, MNRAS 528, 2299).

We covered 298 square degrees of the bilby.fits skymap 90% area, and covered a sky region totalling 32% of the event's full localisation. Data acquisition began at MJD 60713.23 or 2025-02-07 05:31:12 (UTC), 8.16 hrs after the merger event (which was on MJD 60712.89). The last image was taken at 60713.45. 

We found a number of plausible new extragalactic transient candidates which we have reported to the TNS. These were mostly found coincident with compact galaxies or ambiguous star/galaxy sources. We have no reason to favour any of them being a transient in a galaxy within the Bilby.fits distance of 348 +/- 114 Mpc and being a good candidate for the counterpart to S250206dm. Thus we do not highlight them here, but they can be found and matched on the TNS. 

The depths of these 45 sec images were typically (3.5 sigma) r < 20.0 +/- 0.4. Any transients we do find during further processing will be reported to the TNS, where they can be tracked, classified, searched, and commented upon. We encourage further information to be reported on the TNS object pages. We will re-process the 45 second exposures into deeper stacks and re-run them through the detection pipeline. 

Operation of the Pan-STARRS1 and Pan-STARRS2 telescopes is primarily supported by the National Aeronautics and Space Administration under Grant No. NNX12AR65G and NNX14AM74G, issued through the SSO Near-Earth Object Observations Program. Data processing is enabled by Queen's University Belfast and the University of Oxford, enabled through STFC grants ST/Y001605/1, ST/T000198/1 and ST/X001253/1, the Royal Society, and the Hintze Centre for Astrophysical Surveys.