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 power law decay » Redshifts: = 2.3 (highest 6.3, pre-Swift ~1.0) » Classification: Short / long GRBs » Long GRBs == Type Ib/c core collapse SNe Galama et al 1998 Radio X-rays

Redshift distribution of GRBs Jakobsson et al updated 1 April 2008 cumulative histogram of Swift GRBs with redshifts

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 km/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 H 2 (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 metallicity Z ~ 0.01 Z sun redshift z = 3.97 WHT ISIS Starling et al Afterglow spectroscopy: host 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 ( pc) Or work back to derive UV radiation from GRB ONLY BRIGHTEST FEW% GRB UVES RRM, Vreeswijk et al 2007 Time-resolved afterglow spectroscopy

Host galaxy spectroscopy Hosts are faint and at high z 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 , Wiersema et al GRB , Thöne et al 2008 VLT (PI:Hjorth) and Gemini (PI:Levan) host galaxy surveys: ~25.5 (of 2/3 detected) Usually…

Massive stars and stellar populations in GRB hosts Are GRB hosts WR galaxies? Search deep in GRB hosts , (Hammer et al,. 2006, right) and (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 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? The brightest host: GRB 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 ( Angstrom) » 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]