TITLE: GCN CIRCULAR NUMBER: 36258 SUBJECT: LIGO/Virgo/KAGRA S240422ed: no candidate counterparts from Pan-STARRS observations of the skymap DATE: 24/04/23 16:56:02 GMT FROM: Ken Smith at Queen's University Belfast k.w.smith@qub.ac.uk

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

We report observations of the Bilby.fits skymap of the NSBH merger event S240422ed (The LIGO-Virgo-Kagra Collaboration, GCN 36236) 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. In our primary NASA mission for Near-Earth Object discovery, we scan the visible night sky North of -50 degrees declination to magnitude depths m~22, weather and Moon permitting.

During focused science operations, we covered part of the accessible skymap of S240422ed. Tiling sequences of multiple 45-sec images were taken at each pointing position. Pan-STARRS1 was used with the i-band filter and Pan-STARRS2 with the z-band filter. We observed with a wedding cake strategy, with more exposures in the high probability regions and stacked the 45 second frames leading to a spatially varying sensitivity (the most sensitive regions being correlated with the highest probability density). 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).

We covered 167 square degrees of the bilby.fits skymap 90% area, and covered a sky region totalling 65% of the event's full localisation. We covered nearly all the 90% region above declination -30 degrees, due to lack of reference frames below that declination. Data acquisition began at MJD 60423.24267554 or 2024-04-23 05:49:27 (UTC), 8.2 hrs after the merger event (which was 60422.89946078). The last image was taken at 60423.32659085.

We found no new transient sources that had not been previously detected by Pan-STARRS before the merger event or reported to the IAU Transient name server by another survey. The depths of these stacked images were typically i < 19.9 +/- 0.4 , z < 19.5 +/- 0.3. 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.

The discoveries from this program are a byproduct of the Panoramic Survey Telescope and Rapid Response System (Pan-STARRS) NEO survey observations. Operation of the Pan-STARRS1 and Pan-STARRS2 telescopes is primarily supported by the National Aeronautics and Space Administration under Grant No. NNX12AR65G and Grant No. NNX14AM74G issued through the SSO Near-Earth Object Observations Program. Data are processed at Queen's University Belfast and are enabled through the STFC grants to Oxford and QUB : ST/Y001605/1, ST/T000198/1, ST/X001253/1.

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