TITLE: GCN CIRCULAR NUMBER: 39195 SUBJECT: LIGO/Virgo/KAGRA S250206dm: Pan-STARRS observations of AT2025azm and AT2025azn DATE: 25/02/07 08:43:10 GMT FROM: James Gillanders at University of Oxford jhgillanders.astro@gmail.com
M. E. Huber (IfA, Hawaii), J. H. Gillanders (Oxford), K. C. Chambers (IfA, Hawaii) S. J. Smartt, K. W. Smith (Oxford/QUB), F. Stoppa, S. Srivastav (Oxford), D. R. Young, 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, A. Andersson (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), and C. Stubbs (Harvard)
We are currently observing the skymap of the LIGO/Virgo/KAGRA GW event S250206dm (LVK Collaboration, GCN 39175) with the twin Pan-STARRS telescope system (Chambers et al., 2016, arXiv:1612.05560) and will report detections of candidate optical transients to the TNS, and through GCNs. We also carried out targeted observations of AT2025azm and AT2025azn, two optical counterpart candidates reported by Hosseinzadeh et al., (GCN 39191). The Pan-STARRS system comprises two 1.8-m 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.
The last Pan-STARRS observations of these fields was 69 days ago for AT2025azm, and 87 days ago for AT2025azn. There are no reliable historical detections at these positions in our database (typical limits of i<21, w<22). There are a few excess flux detections, but these are likely subtraction residuals in the cores of the host galaxies.
Our targeted observations consisted of a sequence of 60 second exposures in griz filters for AT2025azm, and grizy filters for AT2025azn. Our observations of AT2025azm commenced at MJD 60713.27593 (2025-02-07 06:37:20.352 UTC), 9.20 hours after the merger event (LVK Collaboration, GCN 39175). Our observations of AT2025azn commenced at MJD 60713.28086 (2025-02-07 06:44:26.477 UTC), 9.31 hours after the merger event.
The resultant 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).
We do not detect either transient in any filter. The data were collected in poor seeing conditions (2-3 arcsec), and the 3.5 sigma limits for our 60 second exposures are:
AT2025azm: g<20.1, r<20.6, i<19.9, z<19.9 AT2025azn: z<20.2, r<20.7, i<20.3, g<20.7, y<20.4
As we detect no excess flux in any of the difference images, to limits similar to (or deeper than) those reported by Hosseinzadeh et al., (GCN 39191), it suggests to us that they are not real transients.
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.
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