GCN Circular 38637
Subject
GRB 241217A/EP241217b: tentative VLT spectroscopic redshift z=1.879
Date
2024-12-19T17:49:44Z (20 days ago)
From
Benjamin Schneider at MIT <bschn@mit.edu>
Via
Web form
B. Schneider (LAM), D. B. Malesani (DAWN/NBI and Radboud Univ.), J. T. Palmerio (CEA/Irfu), A. Saccardi (CEA/Irfu), S. D. Vergani (GEPI/Obs. de Paris), Z. P. Zhu (NAOC), D. Xu (NAOC), G. Pugliese (UvA), J. P. U. Fynbo (DAWN/NBI), L. Izzo (INAF-OACn and DARK/NBI), A. Martin-Carrillo (UCD), N. R. Tanvir (U. Leicester), A. J. Levan (Radboud Univ. and Warwick Univ.), A. Rossi (INAF-OAS), S. Savaglio (Univ. Calabria) report on behalf of the Stargate collaboration:
We observed the optical afterglow (Qiu et al., GCN 38600) of the SVOM GRB 241217A / EP241217b (Brunet et al., GCN 38594; William et al., GCN 38599; Zhou et al., GCN 38606) with the X-shooter spectrograph mounted on the ESO VLT UT3 (Melipal). The observation consisted of 4x1200 s with a mid-time at 2024-12-18 06:52.46 UT (13.93 hr after the trigger).
In images taken with the acquisition camera, the counterpart is detected with AB magnitude r = 23.44 +/- 0.11 (mid time 13.1 hr after the trigger), which is about 1 mag brighter than the archival value from the Legacy Survey (r = 24.35 +/- 0.14). The target contained thus a mix of both afterglow and host galaxy light.
From a preliminary reduction of the spectra, a faint continuum is detected in the NIR arm from 10500 to 18000 AA, likely due to the afterglow. A single emission line is very significantly detected in the near-infrared arm at 14410 AA. We note that this wavelength region is severely affected by telluric absorption, whose effect could slightly shift the observed line centroid. No other strong lines are confidently detected.
The likely interpretation is that the emission line is from [O III] 5007 AA at z = 1.879. At this redshift, other commonly observed emission lines, Halpha, Hbeta and [O III] 4959 all fall in regions of strong telluric absorption. A marginal detection of an emission line at 18990 AA is consistent with Halpha strongly attenuated by telluric absorption at this redshift. A low-significance emission feature can also be seen corresponding to [O II] 3729.
An alternative scenario would be that the emission line is due to Halpha at z = 1.196. In this case, we would expect to detect [O III] 5007 around 11000 AA, a region free from telluric absorption. Significant dust extinction must be invoked to suppress it and the other nebular lines.
The photometric redshift determination of the extended object visible in the Legacy Survey (z = 1.20 +/- 0.48; Zhou et al. 2021, MNRAS, 501, 3309) does not allow to strongly discriminate between the two values. We also warn that for such a faint object the Legacy Survey photometric redshifts are prone to systematic errors.
Even if the detection of a single line is ambiguous for redshift determination, we view the option z = 1.879 as the more likely scenario and we propose it as being the redshift of GRB 241217A / EP241217b. Further analysis is ongoing, and we will update the community should this reveal a stronger conclusion regarding the burst redshift.
We acknowledge the expert support from the ESO observing staff at Paranal, in particular Jesus Corral-Santana.