GCN Circular 40958
Subject
EP250704a / GRB 250704B: Pan-STARRS iz-band imaging and photometry
Event
Date
2025-07-04T19:49:50Z (14 days ago)
From
James Gillanders at University of Oxford <jhgillanders.astro@gmail.com>
Via
Web form
J. H. Gillanders (Oxford), M. Huber, K. C. Chambers (IfA, Univ. Hawaii), S. J. Smartt, K. W. Smith (Oxford, QUB), S. Srivastav, F. Stoppa (Oxford), M. Nicholl, D. Young, M. Fulton (QUB), T.-W. Chen (NCU, Taiwan) A. S. B. Schultz, T. de Boer, J. Fairlamb, G. Paek, C. C. Lin, T. Lowe, E. Magnier, P. Minguez, I. A. Smith, R. J. Wainscoat (IfA, Univ. Hawaii).
We observed the optical counterpart of EP250704a (Li et al., GCN 40941) and GRB 250407B (Wang et al., GCN 40940), first reported by Schneider et al. (GCN 40942), using the Pan-STARRS telescope system (Chambers et al., 2016, arXiv e-prints, 1612.05560) on MJD 60860.60 (2025-07-04 14:24 UTC), approximately 6.1 hours after the SVOM/GRM detection (Wang et al., GCN 40940). The Pan-STARRS system consists of 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.
Our observation consisted of 10x90s exposures in both the i and z filters. 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).
From these difference images, we measure the following AB magnitudes:
m_i = 20.20 +/- 0.03,
m_z = 20.01 +/- 0.07.
Our i-band detection is ~0.26 mags brighter than that recorded ~5.8 hours earlier by Schneider et al. (GCN 40942), while our z-band detection closely matches that measured ~5.4 hours earlier by Malesani et al. (GCN 40945). Further multi-band observations with Pan-STARRS are planned.