Introduction: more than a decade of GRB afterglow research
A little more than a decade ago, fairly little was known about gamma-ray bursts
(GRBs). They were split into two classes (Kouveliotou et al. 1993, ApJ, 413, L101) distinguished principally
by the duration of the prompt γ-ray emission: long (>2 s) and short (<2 s) burst. The discovery of the
so-called afterglows (in radio, optical and X-rays) of the long bursts in 1997 (e.g. van Paradijs et al.
1997, Nature, 368, 686) ushered in a new era in the GRB field. The afterglows pinpointed host galaxies, offered
the route to redshifts in many instances and provided a new window into the extreme physics of these powerful
explosions.
Now, mainly thanks to the study of optical/near-infrared (NIR) afterglows,
we have learnt that: GRBs are cosmological (the current record holder is GRB 090429B at z ~ 9.4:
Cucchiara et al., 2011, ApJ, 736, 7) and outshine every other known object in the Universe, both in
the optical/NIR (Bloom et al. 2009, ApJ, 691, 723) as well as in X-rays (Watson et al. 2006, ApJ, 637,
L69). Several long GRB afterglows have a spectroscopically confirmed supernova (SN) component (e.g. Hjorth
et al. 2003, Nature, 423, 847), providing evidence that at least some long GRBs are linked to the core
collapse of a massive star (Woosley 1993, ApJ, 405, 273); but see e.g. Fynbo et al. (2006, Nature, 444,
1047) for SN-less GRBs. Finally, the recently discovered afterglows of short GRBs indicate that they
have different progenitors than the long GRBs (e.g. Hjorth et al. 2005, ApJ, 630, L117; Hjorth et al.
2005, Nature, 437, 859).
With the launch of the GRB-dedicated satellite Swift, the understanding on GRBs and their afterglows
increased considerably. Swift has consistently detected an average of about 2 GRBs/week and transmitted
their accurate X-ray localisations (<3''error radius), via the X-Ray Telescope (XRT), to the ground
within minutes for rapid follow-up observations. The redshift distribution found is very different from
pre-Swift data and skewed to significantly larger redshift. Approximately 50% of the Swift bursts are
found to be located at z > 2, while the corresponding fraction was only 20% for pre-Swift bursts.
More observations are needed as well as sophisticated theoretical models before we can conclude whether
the GRB rate is proportional to the global star formation density in the Universe (Jakobsson et al. 2006,
A&A, 447, 897). To date, our GRB data obtained with the NOT have resulted in over 80 refereed publications.
The role and goals of GRB research at the NOT
The GRB NOT observations carried out by our group have contributed significantly to
the current understanding of the GRB phenomenon. We have detected several tens of afterglows with the NOT, and
we also secured redshifts of several of them (e.g. we detected the most distant GRB that was discovered with 2-4-m
class telescopes). In addition, our program plays an important role in setting up and optimising campaigns
with the ESO/VLT.
The primary objective of this proposal is to optimise the legacy value of the Swift
mission: to determine redshifts for as many long bursts as possible. This enables a proper investigation
of the true redshift distribution and corresponding luminosity function, and allows us to fully exploit the
potential of GRBs as tracers of the cosmic density of star formation. We will focus on afterglows that have
"observing conditions" favourable for redshift determination, i.e. fulfill a set of simple criteria regarding
the Galactic extinction (AV < 0.5 mag), angular separation from the Sun (θ > 55°) and the availability of the
XRT localisation (< 12 hr). This will complement an accepted long-term afterglow X-shooter spectroscopy
programme at the VLT. In 2009, we published a comprehensive paper (Fynbo et al. 2009, ApJS, 185, 526)
reporting spectroscopic follow-up observations of 77 optical afterglows (mainly VLT and NOT data).
Another important objective is to promptly follow up bursts within the first few hours,
or even minutes, after the burst trigger, with the aim of detecting the optical/NIR afterglow (if it has not
been reported by other groups). With the typical power-law decline of GRB afterglows, the advantage of being
early on the target can often far outweigh the disadvantage of having an order of magnitude less collecting
area, which is the difference between using the VLT and the NOT. The resulting sub-arcsecond localisation
allows a proper investigation of the host galaxy via deeper observations carried out when the afterglow has
completely faded away.
The third immediate science goal of this proposal is detection of short GRB afterglows
and securing their absorption-line redshifts. Although the number of short GRB afterglow identifications is increasing,
very few have an unambiguous distance determined. There are a handful of emission redshifts available from
associated host galaxies, some of which are far from secure. To date, only one short GRB has a reliable
absorption-line redshift (Sánchez-Ramírez et al. 2013, GCN Circ. 14747). Absorption redshifts are imperative as they
offer the most secure means of securing the GRB host distance, something that has been challenging for short
bursts (e.g. Levan et al. 2007, MNRAS, 378, 1439).
Special attention will be devoted to those few GRBs detected at high energies by
Fermi/LAT and AGILE/GRID; the known cases all have bright afterglows and are easy targets
for the NOT. Knowing their redshift is fundamental to interpret the high-energy properties (the new window
opened by these satellites), constrain the emission and progenitor models along with quantum gravity effects
(see e.g. Abdo et al. 2009, Nature, 462, 331).
The redshift is not the only important information we obtain from the spectra; they
are also of great importance for probing the distant Universe. The extreme brightness of some GRB afterglows
allows us to obtain high S/N spectra which give detailed information about the IGM. In addition, for the
determination of absolute metallicities the observation of Lyα absorption is essential, and it is therefore
natural to exploit the excellent blue sensitivity of the NOT.
