Gamma-ray Bursts in the E-ELT era
Rhaana Starling
University of Leicester
Gamma-ray Bursts (GRBs)
» Rates: Swift ~100 GRBs /yr
» Afterglow has a synchrotron
spectrum
» Broad luminosity function (R~16
to >24 @1hr), power law decay
» Redshifts: <z> = 2.3 (highest 6.3,
pre-Swift <z> ~1.0)
Galama et al 1998
» Classification: Short / long GRBs
» Long GRBs == Type Ib/c core
collapse SNe
Radio
X-rays
Redshift distribution of GRBs
cumulative
histogram of
Swift GRBs
with redshifts
Jakobsson et al. 2006
updated 1 April 2008
E-ELT will give us:
Many more photons (from day one)
» Obtain high resolution spectroscopy routinely
» Go after the afterglows of the more elusive GRB sub-groups ie dark
bursts, short bursts
» reach the faint end of the luminosity function for GRB hosts
» Allow polarimetry on a number of GRB afterglows to reveal jet
structure and physics
Greater spatial resolution (with laser guide star AO in place)
» Accurately locate GRBs within their host galaxies and study nearby
host galaxies in great detail (eg stellar population studies)
Afterglow spectroscopy:
host galaxies in absorption
Kinematics: outflows of 22-2900km/s observed (stellar
wind? Galactic winds? Halo gas?)
more structure beyond current resolution? (VLT UVES to
7.5km/s for brightest few which are likely to have highest
densities).
At current rate would
take ~10 years to
have sample size 50:
E-ELT  statistically
meaningful samples
in much shorter
timespan and sample
wider population of
afterglows and hosts.
Host chemistry: redshift, abundances, metallicity, DLA (HI),
densities and temperatures, search for H2 (1 tentative
detection so far)
GRB 050730
DLA
metallicity Z ~
0.01 Zsun
redshift z =
3.97
Lyman
limit
z=3.97
metal lines
WHT ISIS
Starling et al.
2005
Time-resolved afterglow spectroscopy
Indirect UV pumping of fine
structure lines  line
variability.
Used to derive important
parameters like
•ISM density
•temperature
•abundances
•GRB  absorber distance
(102-103pc)
•Or work back to derive UV
radiation from GRB
ONLY BRIGHTEST FEW%
GRB 060418 UVES RRM, Vreeswijk et al 2007
Host galaxy spectroscopy
Hosts are faint and at high z
Usually…
Want to derive properties of
stellar population and metallicity
to input into GRB progenitor
models
Only feasible now for
closest/brightest subsample
GRBs select a population of galaxies
independent of their luminosity
GRB 040924, Wiersema et al. 2008
GRB 060206, Thöne et al 2008
VLT (PI:Hjorth) and Gemini (PI:Levan)
host galaxy surveys:
<R> ~25.5 (of 2/3 detected)
Massive stars and stellar populations in
GRB hosts
Are GRB hosts WR galaxies?
Search deep in GRB hosts 980425,
020903 (Hammer et al,. 2006,
right) and 060218 (Wiersema et
al. 2008)
Wolf-Rayet stars may be
progenitors of GRBs
Compare the host stellar
populations to local galaxy stellar
pops.
Many more photons
» Go after the afterglows of short bursts
~25% of GRBs are short-duration
and likely have very different
origins from the long GRBs:
compact binary merger?
We do not know!
DSS and inset VLT images of the location
of the first short burst afterglow showing a
probable elliptical host. Gehrels et al.
2005; Hjorth et al. 2005.
Afterglows are few mags fainter
than for long GRBs – afterglow
spectroscopy so far impossible
Many more photons
» Go after the afterglows of dark bursts
Dark bursts have optical emission which is much fainter than expected
from the standard GRB model (eg Jakobsson et al. 2005; Rol et al. 2007).
Could be due to anomalously large dust columns (GRB sites usually
have low dust content, but some dark GRB hosts are EROs), or
high-z which can be probed with E-ELT.
Probe of dusty galaxies through afterglow spectroscopy
Swift: ~20% dark
Spatial resolution: Are GRB-producing regions special?
Afterglow lies in a region of
average metallicity, not the
expected low-Z WR region
The brightest host: GRB 980425 at z=0.008
with VLT VIMOS, Christensen et al.
submitted
How much spatial resolution will
we get with E-ELT?
E-ELT could resolve a single star
forming region of size say 100pc
up to z=0.1 with a resolution of
50mas per pixel.
Many more photons
» Obtain high resolution spectroscopy routinely
Complement
high resolution
X-ray spectral
studies using eg
Estremo, Xeus,
Con-X
Probe the
WHIM in
absorption,
backlit by GRB
afterglow.
GRBs with E-ELT
GRB science goals for E-ELT that we cannot do now:
» High resolution spectra for all GRB afterglows: fine-structure line variability studies
to derive local gas properties; comparison with local galaxy populations etc
» Probe faint end of luminosity function of GRB hosts
» Finally large statistical samples of host galaxies
» Studies of the faintest types of afterglows: Short burst afterglows to learn about their
origins; Dark burst afterglows to learn about the dust-enshrouded population
» Spatially resolved distributions of host galaxy properties for a large number of
nearby GRBs: is the GRB site special? Where are the massive stars located?
» WHIM studies in comparison with X-ray
» (Polarisation studies of afterglows to map the jet structure and physics)
Desirables:
» Broad wavelength coverage (3300-25000Angstrom)
» Medium-high resolution optical and nIR spectroscopy
» Fairly fast reaction time (~30mins ideal: very fast not necessary) : trade-off between
fast response + short exposure times and slow response + longer exposure time
[afterglows decay as a power law]