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GCN Circular 33580

Subject
GRB 230307A: JWST NIRSpec observations, possible higher redshift
Date
2023-04-06T17:00:47Z (a year ago)
From
Andrew Levan at U.of Leicester <a.levan@astro.ru.nl>
A.J. Levan (Radboud), D. Watson (DAWN/NBI), J. Hjorth (DARK/NBI), N. R. Tanvir (Leicester), D. B. Malesani (Radboud, DAWN/NBI), E. Burns (LSU), B. P. Schneider (MIT), J. P. U. Fynbo (DAWN/NBI), S. D. Vergani (CNRS-Paris Observatory), W. Fong (Northwestern), A. Fruchter (STScI), G. Pugliese (API/UvA), S. Smartt (Oxford) report for a larger collaboration:

���In addition to the JWST/NIRCam observations of GRB 230307A (Fermi GBM team, GCN 33405) reported in GCN 33569, we also obtained observations with NIRSpec on 5 April 2023. The observations were obtained for a total integration time of approximately 1 hour, using the prism to obtain a spectrum in the range 0.5-5.5 um. The trace confirms the spectral shape measured from NIRCam observations. Blueward of 2 um, the source has a flat spectrum, but low signal to noise, and a strong upturn redward of 2 um. 

We note the presence of two weak, narrow emission features in the spectrum, both of which are offset from the trace by approximately 0.1-0.2". These are consistent with the H-alpha and [O III] (5007 AA) emission lines at z = 3.87. Excess flux in the spectrum is also visible at the location of H-beta at this redshift. If the GRB is from a low redshift merger, as suggested by GCN 33569, then this could only be explained by a chance alignment with an unrelated background galaxy. It is hard to quantify this possibility, given that we could be seeing both transient and galaxy light, however the chances are clearly low.

Alternatively, the lines could be from the host galaxy, implying that GRB 230307A lies at z = 3.87. The very red colours of the continuum source would not naturally match the expectations of any afterglow models at z = 3.87, but in fact could be consistent with those of a dusty, red galaxy. Given only one epoch in the mid-IR is available, we cannot rule out this possibility, although such a galaxy would be extremely compact, and the transient would have to decay very steeply since the last ground-based observations. We note that, at this redshift, GRB 230307A would be by far the most energetic burst ever detected, with E_iso ~ 10^56 erg. This would be an order of magnitude more energetic than any of the ~500 GRBs with available isotropic energy releases. This may require a unique progenitor system.���
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