Abstract
We present the observations of Lyman continuum (LyC) emission in the
afterglow spectra of GRB 191004B at $z=3.5055$, together with those of the
other two previously known LyC-emitting long gamma-ray bursts (LGRB) (GRB
050908 at $z=3.3467$, and GRB 060607A at $z=3.0749$), to determine their LyC
escape fraction and compare their properties. From the afterglow spectrum of
GRB 191004B we determine a neutral hydrogen column density at the LGRB redshift
of $łog(N_HI/cm^-2)= 17.2 0.15$, and negligible extinction
($A_V=0.03 0.02$ mag). The only metal absorption lines detected are
CIV and SiIV. In contrast to GRB 050908 and GRB 060607A, the host galaxy of GRB
191004B displays significant Ly$\alpha$ emission. From its Ly$\alpha$ emission
and the non-detection of Balmer emission lines we constrain its star-formation
rate (SFR) to $1 łeq$ SFR $4.7$ M$_ødot\ yr^-1$. We fit the
Ly$\alpha$ emission with a shell model and find parameters values consistent
with the observed ones. The absolute LyC escape fractions we find for GRB
191004B, GRB 050908 and GRB 060607A are of $0.35^+0.10_-0.11$,
$0.08^+0.05_-0.04$ and $0.20^+0.05_-0.05$, respectively. We compare the
LyC escape fraction of LGRBs to the values of other LyC emitters found from the
literature, showing that LGRB afterglows can be powerful tools to study LyC
escape for faint high-redshift star-forming galaxies. Indeed we could push LyC
leakage studies to much higher absolute magnitudes. The host galaxies of the
three LGRB presented here have all $M_1600 > -19.5$ mag, with the GRB
060607A host at $M_1600 > -16$ mag. LGRB hosts may therefore be
particularly suitable for exploring the ionizing escape fraction in galaxies
that are too faint or distant for conventional techniques. Furthermore the time
investment is very small compared to galaxy studies. Abridged
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