TITLE: GCN CIRCULAR NUMBER: 35982 SUBJECT: X-ray transient EP 240315a: Second epoch Chandra observations DATE: 24/03/27 09:06:39 GMT FROM: Andrew Levan at Radboud University a.levan@astro.ru.nl

A.J. Levan (Radboud & Warwick), P.G. Jonker (Radboud), D.B. Malesani (DAWN/NBI & Radboud), N.R. Tanvir (Leicester), B.P. Gompertz (Birmingham), A. Saccardi (GEPI/Obs. de Paris), A. de Ugarte Postigo (CNRS/OCA & LAM), A. Martin-Carrillo (UCD), D. Perley (LJMU), D. Xu (NAOC), G. Tagliaferri (INAF, Brera), J. Palmerio (GEPI/Obs. de Paris)), K.E. Heintz (DAWN/NBI), K. Wiersema (Hertfordshire), M. Grazia Bernardini (INAF, Brera), M. Ferro (INAF, Brera), P. Jakobbson (U. Iceland), P. D’Avanzo (INAF, Brera), R. Salvaterra (INAF, Milan), S. Vergani (GEPI/Obs. de Paris), G. Pugliese (API, Amsterdam), Y. Julakanti (Leicester) report for the Stargate collaboration:

We obtained a second epoch of Chandra observations of EP240315a (Zhang et al., GCN 35931; Chen et al., GCN 35951) beginning at 04:34 on 26 March 2024, approximately 10.4 days after the transient detection and 6 days from the first epoch of observations (Levan et al., GCN 35963). A total of 18.5 ks of observations were obtained (compared to 10 ks at the first epoch). At the location of AT2024eju (Srivastav et al. GCN 35932) we do not detect any source. More formally, we identify a single photon at the source location, and set a 2-sigma upper limit of <2.4e-4 count s^-1 at this epoch.

The inferred decay rate from 3 days to 10.4 days is steeper than t^-2.1, and suggests a relatively rapid decay. This is consistent with the post-jet break decay rate for a GRB afterglow.

We again thank Pat Slane, Vinay Kashyap and the staff of the CXC for their excellent support and rapid scheduling of these observations.

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