TITLE: GCN CIRCULAR NUMBER: 39791 SUBJECT: GRB250309B: Proof-of-concept for Joint ZTF+EP Experiment DATE: 25/03/20 20:29:02 GMT FROM: Tomas Ahumada Mena at Caltech tahumada@caltech.edu
T. Ahumada (Caltech), E. C. Bellm (UW), L. Yan, T. du Laz, M. M. Kasliwal (Caltech), Q. Y. Wu, S. Q. Jiang (NAOC, CAS), J. H. Wu (GZHU), Y. Liu, C. Jin, W. Yuan (NAOC, CAS)
report on behalf of the Zwicky Transient Facility Partnership and Einstein Probe Team
Starting Feb 27, 2025, the Zwicky Transient Facility (ZTF) partnership is collaborating with the Einstein Probe (EP) team to map multiple EP pointings concurrently every night with ZTF. By cross-matching ZTF alerts with EP alerts, we report our first proof-of-concept cross-match.
On March 9, 2025, Fermi-GBM detected GRB 250309B at 07:38:30 UTC (Preis and Greiner, GCN 39629; Fermi GBM team, GCN 39635; McDermott et al., GCN 39642). As part of the regular schedule, EP was set to cover the region, with ZTF scheduled to shadow EP. Both EP and ZTF subsequently observed the afterglow, ZTF25aaitvjt/AT2025dws.
The Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission began observations from 2025-03-09T10:38:05(UTC) (~T0+3h) with an exposure time of 4.8 ks. The WXT position of the source is R.A. = 210.800 deg, DEC = -8.500 deg (J2000) with an uncertainty of 3.1 arcmin in radius (90% C.L. statistical and systematic), which is consistent with the optical counterpart AT2025dws (Stein et al., GCN 39639) and the X-ray counterpart (Page and Evans, GCN 39649). The average WXT 0.5-4 keV spectrum can be fitted with an absorbed power law with a Galactic hydrogen column density of 3.0 x 10^20 cm^-2 and a photon index of 1.8 (-1.2/+1.6). The derived average unabsorbed 0.5-4 keV flux is 9.2 (-5.3/+10.8) x 10^(-12) erg/s/cm^2. The uncertainties are at the 90% confidence level for the above parameters.
In this particular example, we clarify that ZTF was double-triggered to observe this field - once by the joint ZTF+EP experiment and once by the IceCube neutrino experiment. Both experiments recovered AT2025dws. We observed a fading rate of 2.5 mag / hour in the r-band. This source was promptly reported by ZTF in Stein et al. GCN 39639.
Launched on January 9, 2024, EP is a space X-ray observatory to monitor the soft X-ray sky with X-ray follow-up capability (Yuan et al. 2022, Handbook of X-ray and Gamma-ray Astrophysics).
Based on observations obtained with the Samuel Oschin Telescope 48-inch and the 60-inch Telescope at the Palomar Observatory as part of the Zwicky Transient Facility project. ZTF is supported by the National Science Foundation under Award #2407588 and a partnership including Caltech, USA; Caltech/IPAC, USA; University of Maryland, USA; University of California, Berkeley, USA; University of Wisconsin at Milwaukee, USA; Cornell University, USA; Drexel University, USA; University of North Carolina at Chapel Hill, USA; Institute of Science and Technology, Austria; National Central University, Taiwan; Operations are conducted by Caltech's Optical Observatory (COO) and Caltech/IPAC.
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