GCN Circular 36114
Subject
LXT 240402A/GRB 240402B: Swift follow-up observations
Date
2024-04-15T13:21:33Z (24 days ago)
From
Antonio Martin-Carrillo at UCD,Space Science Group <antonio.martin-carrillo@ucd.ie>
Via
Web form
A. Martin-Carrillo (UCD), S. Campana (INAF/Brera), D. B. Malesani (DAWN/NBI & Radboud Univ.), A. J. Levan (Radboud Univ.), G. Pugliese (Amsterdam), D. Xu (NAOC/CAS), N. R. Tanvir (Leicester), P. G. Jonker (Radboud Univ.), A. Rossi (INAF-OAS), P. O’Brien (Leicester) report on behalf of the Stargate collaboration:
Following up the LEIA trigger LXT 240402A (Xu et al., GCN 36016) associated with the Konus-Wind GRB 240402B (Ridnaia et al., GCN 3628) and the detection of its X-ray counterpart (Jia et al., GCN 36022) a series of Swift follow-up observations (PI Xu, ID: 203030 and PI Martin-Carrillo, ID: 20382) were performed between April 7 and April 14, with a total exposure time of 8 ks.
Combining all the data, a source is well detected with a count rate of 1.9 (+0.7, -0.6) x 10^-3 count s^-1 at the coordinates (J2000):
RA: 16:21:47.75 (245.4490);
Dec: +25:45:45.9’ (25.7627);
Error: 5.8” (90% confidence).
Using the spectral parameters of the EP-FXT detection (Jia et al., GCN 36022), this corresponds to an observed flux of 7.7*10^-14 erg cm^-2 s^-1. Assuming a mid-point for the Swift observations of 8.5 days after the LEIA trigger, and using a single power-law decay, the X-ray flux decays with an approximate index -1.2, typical for a GRB afterglow.
The source centroid lies within 1.5" from the optical counterpart reported by Levan et al. (GCN 36025) and Yang et al. (GCN 36027). The reduced localisation error strengthens its association with the proposed optical counterpart.
We thank the Neil Gehrels Swift Observatory team for making these ToO observations possible.