Gamma-ray Bursts in the E-ELT eraGamma-ray Bursts in the E-ELT era

Rhaana StarlingUniversity of Leicester

Gamma-ray Bursts (GRBs)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)

» Classification: Short / long GRBs» Long GRBs == Type Ib/c core

collapse SNe

Galama et al 1998

Radio X-rays

Redshift distribution of GRBsRedshift distribution of GRBs

Jakobsson et al. 2006 updated 1 April 2008

cumulative histogram of Swift GRBs with redshifts

E-ELT will give us: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)

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).

Host chemistry: redshift, abundances, metallicity, DLA (HI), densities and temperatures, search for H2 (1 tentative detection so far)

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.

DLA

metal lines

Lyman limit

z=3.97

GRB 050730

metallicity Z ~ 0.01 Zsun

redshift z = 3.97

WHT ISISStarling et al. 2005

Afterglow spectroscopy: Afterglow spectroscopy: host galaxies in absorptionhost galaxies in absorption

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

Time-resolved afterglow spectroscopyTime-resolved afterglow spectroscopy

Host galaxy spectroscopyHost galaxy spectroscopy

Hosts are faint and at high z

Want to derive properties of stellar population and metallicityto input into GRB progenitor models

Only feasible now forclosest/brightest subsample

GRBs select a population of galaxies independent of their luminosity

GRB 040924, Wiersema et al. 2008GRB 060206, Thöne et al 2008

VLT (PI:Hjorth) and Gemini (PI:Levan)host galaxy surveys: <R> ~25.5 (of 2/3 detected)

Usually…

Massive stars and stellar populations in Massive stars and stellar populations in GRB hostsGRB 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

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

Many more photons» Go after the afterglows of dark bursts

Spatial resolution: Are GRB-producing regions special?Spatial resolution: Are GRB-producing regions special?

The brightest host: GRB 980425 at z=0.008 with VLT VIMOS, Christensen et al. submitted

Afterglow lies in a region of average metallicity, not the expected low-Z WR region

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]