J. P. U. Fynbo (DAWN/NBI), D. A. Perley (LJMU), A. de Ugarte Postigo
(HETH/IAA-CSIC; DARK/NBI), D. B. Malesani (DTU Space), N. E. Jannsen
(NOT), report on behalf of a larger collaboration:
Spectroscopy was secured of the optical counterpart (Reva et al., GCN
26036; Perley et al., GCN 26039) of GRB 191019A (Simpson et al., GCN
26031), using the ALFOSC instrument mounted on the Nordic Optical
Telescope (NOT). Observations started on 2019 Oct 19.873 UT (5.74 hr
after the GRB), for a total of 2x1800 s exposure.
Our spectrum, secured with grism #4, covers the approximate wavelength
range 3500-9400 AA. The continuum is well detected. No prominent
emission lines are visible. However, several absorption features can be
identified, including Ca H & K, Na I D, Hdelta, Hgamma, Hbeta, G band,
all at a common redshift z = 0.248. The 4000 AA break is also present.
The spectrum is typical of an old stellar population, and is unlike that
of long-duration GRB host galaxies. The association of this object with
GRB 191019A is therefore unclear.
Using the formula by Bloom et al. (2002, AJ, 123, 1111), the chance
association probability for a source with magnitude r = 19.14 AB (from
the Pan-STARRS catalog) and an error circle radius of 3.1" (Evans et
al., GCN 26034) is approximately 3*10^-3.