GCN Circular 39669
Subject
EP250304a: Chandra detection
Event
Date
2025-03-11T18:08:57Z (7 months ago)
From
Raffaella Margutti <rmargutti@berkeley.edu>
Via
Web form
R. Margutti (UC Berkeley), S. Pedrami (UC Berkeley), R. Chornock (UC Berkeley), Y. Yao (UC Berkeley), T. Laskar (U of Utah) report:
We observed EP250304a (Chen et al., GCN 39580) with the Chandra X-ray Observatory starting on 2025-03-10, 03:13:38 UT (i.e., T0+6.2d, observer frame; 20ks exposure) under a DDT proposal (PI Margutti). An X-ray source is detected with high confidence (>8 sigma Gaussian equivalent) at a location consistent with the EP transient (Zhang et al., GCN 39591). The spectrum is well fitted by a simple power law with photon index Gamma = 2.1 +/- 0.5, similar to previous Swift-XRT observations of this source (Page et al., GCN 39584). For a Gamma=2 index, the inferred 0.3-10 keV flux is ~1.6e-14 erg/s/cm2. These Chandra observations imply that the source has entered a phase of shallower X-ray flux decay compared with the earlier Swift-XRT observations, in close similarity with low-luminosity GRBs. In the soft X-rays, the evolution of EP250304a closely resembles that of the prototypical event GRB060218 (e.g., Campana et al., 2006).
We thank the entire CXO team for scheduling these very fast observations, which enabled unique constraints on the later time evolution of the EP source.