GCN Circular 36385
Subject
LXT 240402A / GRB 240402B: VLT X-shooter redshift of host
Date
2024-05-03T15:23:10Z (16 days ago)
From
nrt3@le.ac.uk
Via
Web form
N. R. Tanvir (U. Leicester), J. Palmerio (GEPI/Obs. de Paris), D. B. Malesani (DAWN/NBI & Radboud U.), A. J. Levan (Radboud U.), A. Saccardi (GEPI/Obs. de Paris), A. Martin-Carrillo (UCD), P. G. Jonker (Radboud U.), K. Wiersema (U. Hertfordshire), Y. Julakanti (U. Leicester) report on behalf of the Stargate collaboration:
We obtained observations of the location of the X-ray transient LXT 240402A (Xu et al., GCN 36016; Jia et al., GCN 36022; Levan et al., GCN 36025), coincident with GRB 240402B detected by GECAM-C (Xue et al., GCN 36017) and Konus-Wind (Ridnaia et al., GCN 36028), using the X-shooter spectrograph at the VLT. Observations began at 04:04 (UT) on 03 May 2024, approximately 31 days after the trigger. The spectrum covers a wavelength range roughly 0.31 to 2.1 mum, with a total integration time of 4800 s.
A weak continuum is seen throughout the spectrum, and we clearly detect emission lines of [O II], H-beta, [O III], H-alpha, and tentatively Ly-alpha, at a common redshift of z = 1.551. We propose that these lines are from the host of the transient. Based on the gamma-ray fluence seen by Konus-Wind in the 20 keV - 10 MeV band (Ridnaia et al., GCN 36028) we conclude an isotropic-equivalent energy release in gamma-rays of 4.7x10^52 erg. This places it very close to the mean Amati relationship (Amati et al. 2006 MNRAS 372 233), and thus is consistent with a long-GRB origin.
We thank ESO support staff Zahed Wahhaj and Francisco Caceres for undertaking these observations.