Palli Jakobsson Astronomical Observatory University of Copenhagen

The line-of-sight towards GRB 030429 at z = 2.66: Probing matter at stellar, galactic and intergalactic

scalesPalli Jakobsson

Astronomical ObservatoryUniversity 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 030429

• 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:

zGRB = 2.66

zgalaxy = 0.84

Spectral Energy Distribution

K

J I R V

Best fit with SMC: β = -0.36 ± 0.12 and AV = 0.34 ± 0.04

000301C000926021004011211020124

Light Curves

p(β) = 1.72 ≠ p(α2) = 3.46Electron 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 030429 light curve

• Achromatic light curve behaviour

The Host Galaxy

A zoom in on the Lyα line from the VLT OA spectrum:

Log N(H I) = 21.6 ± 0.1 Damped Lyα 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) ∙ 1021 cm-2 mag-1

• Fully consistent with SMC (metal-poor)

The Mg II Absorber

• At zgalaxy = 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 · 1011 Msun. At zgalaxy = 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 bythe nearby galaxy.

• A DLA with a dust-to-gas ratio fullyconsistent with that of the SMC. The SEDalso consistent with an SMC-likeextinction law.

• Small impact parameter in contrast toidentifications of most QSO absorption-selected galaxy counterparts.

• Shaped by the stellar wind of a massivestar progenitor. Density? Refreshed shocks? Jet structure? Microlensing?