GRB 071003: Broadband Follow-up Observations of a Very Bright
Gamma-Ray Burst in a Galactic Halo
Perley, D. A., et al. (28 co-authors)
The optical afterglow of long-duration GRB 071003 is among the brightest
yet to be detected from any GRB, with R~12 mag in KAIT observations
starting 42 s after the GRB trigger, including filtered detections
during prompt emission.
However, our high-S/N afterglow spectrum displays only extremely weak
absorption lines at what we argue is the host redshift of z=1.60435,
in contrast to the three other, much stronger Mg II absorption systems
observed at lower redshifts.
Together with Keck adaptive optics observations, which fail to reveal
a host galaxy coincident with the burst position, our observations
suggest a halo progenitor and offer a cautionary tale about the use
of Mg II for GRB redshift determination.
We present early- through late-time observations spanning the
electromagnetic spectrum, constrain the connection between the prompt
emission and early variations in the light curve (we observe no
correlation), and discuss possible origins for an unusual, marked
rebrightening that occurs a few hours after the burst: likely either
a late-time refreshed shock or a wide-angle secondary jet.
Analysis of the late-time afterglow is most consistent with a wind
environment, suggesting a massive star progenitor.
Together with GRB 070125, this may indicate that a small but significant
portion of star formation in the early universe occurred far outside
what we consider a normal galactic disk.
Status:
2008, ApJ, 688, 470.
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