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GCN Circular 33521

GRB 230325A: X-shooter optical afterglow and redshift
2023-03-25T20:56:07Z (a year ago)
Daniele B Malesani at Radboud U <>
Danielle Pieterse (Radboud Univ.), Johan P. U. Fynbo (DAWN/NBI), Luca 
Izzo (DARK/NBI), Jesse Palmerio (GEPI, Obs. Paris and IAP), Andrea 
Saccardi (GEPI, Obs. Paris), Nial R. Tanvir (Leicester), Dong Xu 
(NAO/CAS), Zi-Pei Zhu (NAOC, HUST), Daniele B. Malesani (Radboud univ. 
and DAWN/NBI) and Pall Jakobsson (Univ. Iceland), report on behalf of 
the Stargate collabaration:

We observed the field of GRB 230325A (Moss et al., GCN 33516) using the 
ESO VLT UT3 (Melipal) equipped with the X-shooter spectrograph. Images 
were also taken with the acquisition camera. An object is visible 
consistent with the UVOT-enhanced XRT position (Goad et al., GCN 33518), 
at coordinates (0.5" error):

RA = 19:47:23.98�� (J2000)
Dec = -46:04:33.3�� (J2000)

For this object, we measure the following magnitudes (calibrated against 
nearby objects from the Legacy Survey; Dey et al. 2019, AJ, 157, 168):

Filter Mean date�������������� Time since GRB�� Exptime�� Magnitude
 �������������� (UT)������������������������ (days)������������������ (s)���������� (AB)
r���������� 2023 Mar 25.371�� 0.235�������������������� 45������������ 22.70 +- 0.08
z���������� 2023 Mar 25.372�� 0.236�������������������� 60������������ 22.35 +- 0.08
g���������� 2023 Mar 25.374�� 0.237�������������������� 45������������ 22.16 +- 0.23

An object is present at a consistent location in the Legacy Survey, but 
significantly fainter, and it is the likely GRB host galaxy. The 
consistency with the XRT position and the flux increase compared to the 
Legacy Survey confirm that the object we see in the X-shooter images is 
the optical afterglow of GRB 230325A.

Spectroscopy was carried out, covering the wavelength range 3000-21000 
AA, and consisting of 2 exposures by 600 s each. The observation mid 
time was 2023 Mar 25.383 UT (0.247 days after the GRB).

Only a faint continuum is detected from this source, which allows us to 
set a redshift upper limit z <~ 3. In addition, two emission lines are 
visible, which are consistent with Halpha and [O III] 5008 at a common 
redshift z = 1.664, which we suggest to be the GRB redshift.

We acknowledge excellent support from the ESO observing staff in 
Paranal, in particular Fuyan Bian and Konrad Tristram.
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