The line-of-sight towards GRB at z = 2.66: Probing matter at stellar, galactic and intergalactic scales Palli Jakobsson Astronomical Observatory University of Copenhagen Padova 21 April 2004

Jens Hjorth ( AO, Copenhagen ) Johan Fynbo ( Århus Univ. ) Michael Weidinger ( Århus Univ. ) Darach Watson ( AO, Copenhagen ) Javier Gorosabel ( STScI, Baltimore ) Kristian Pedersen ( AO, Copenhagen ) Cedric Ledoux ( ESO, Santiago ) Gulli Björnsson ( Science Institute, Reykjavík ) Ralph Wijers ( Astronomical Institute, Amsterdam ) GRACE Main Collaborators

Outline Burst environment: playing the ( α,β,p)-game Host galaxy (DLA): dust-to-gas ratio Mg II absorber along the line-of-sight: very small impact parameter, D = 1.2” Strong gravitational lensing?

Imaging of GRB Initially a 1’ error radius circulated Limiting magnitude from a GCN: >20 mags at 1.9 hours Fulfilled our ”dark burst” criteria 6 VLT epochs in R & VRIJK during 2 epochs A week later: correct error radius had actually been 2’ A galaxy only 1.2” away from the host  initially considered to be a probable host galaxy. VLT spectroscopy soon showed: z GRB = 2.66 z galaxy = 0.84

Spectral Energy Distribution K J I R V Best fit with SMC: β = ± 0.12 and A V = 0.34 ± C

Light Curves p(β) = 1.72 ≠ p(α 2 ) = 3.46 Electron energy power-law index

Burst Environment Closure relation (Price et al. 2002): only a jet expanding into a wind-stratified medium works For the fireball model to be consistent, a bright long-lived (2-3 days) bump must be present in the GRB light curve Achromatic light curve behaviour

The Host Galaxy A zoom in on the Ly α line from the VLT OA spectrum: Log N(H I ) = 21.6 ± 0.1  Damped Ly α Absorber (DLA)

The Host Galaxy A high value of the ratio between N(H I ) and the optical extinction N(H I )/E(B-V) = (34 ± 9) ∙ cm -2 mag -1 Fully consistent with SMC (metal-poor)

The Mg II Absorber At z galaxy = 0.84, 1.2” corresponds to an impact parameter of only D = 9 kpc In the ”QSO-business” D is frequently interpreted as the lower limit of the radii of suspected counterparts of absorption- selected galaxies At a redshift of ~1, the majority of counterparts are thought to have D > 50 kpc

The Mg II Absorber The real absorber could be very faint and/or hidden inside the glare of the QSO  overestimating the counterpart radius Possible to test this with OAs, they are high-z, and fade away completely within a few months All evidence from OAs suggests that D is indeed much smaller: Of the 6 GRBs that have Mg II absorption systems (besides the one produced in the host galaxy) in their OA spectra, at least 4 of them have nearby galaxies at 1”-4” (5-20 kpc)

Strong Gravitational Lensing? There are at least 2 facts that make it improbable: The Einstein radius has to be at least as large as the impact parameter. This corresponds to a mass of >5.4 · M sun. At z galaxy = 0.84, 1.2” corresponds to 9 kpc, so this mass is roughly 6 times larger than for Milky-Way like galaxies. For the OA-galaxy configuration, the critical surface mass density is 0.43 g cm -2, around 3-4 times larger than in normal galaxy lenses

Summary Mg II absorber Progenitor environment Host galaxy Strong lensing Afterglow appearance not affected by the nearby galaxy. A DLA with a dust-to-gas ratio fully consistent with that of the SMC. The SED also consistent with an SMC-like extinction law. Small impact parameter in contrast to identifications of most QSO absorption- selected galaxy counterparts. Shaped by the stellar wind of a massive star progenitor. Density? Refreshed shocks? Jet structure? Microlensing?