Gamma-ray Burst Afterglow Spectroscopy

J. P. U. Fynbo, Niels Bohr Institute / Dark Cosmology Centre

What is a gamma-ray burst?Brief (ms - min) and intense (~10-7 erg cm–2 s–1) burst of

soft (~100 keV) gamma-ray radiation

*Discovered by chance in the 60ies

*Rapid variability*Non-thermal

spectra*About 1 per day

with current sensitivities

*The key problem in revealing their nature was getting precise positions.

GRB Afterglows

Wijers et al. (1999); Covino et al. (1999); Beuerman et al. (1999); Vreeswijk et al. (2001)

VLT/FORS1:Spectroscopic redshift: z = 1.62

Host: R(AB)>26

Detection of polarization

Halo

HI, HII, H2, metals, dustDM, supermassive BH

GRB sightline

protodisk

HII regions

QSO sightline

Pontzen et al. (2008)

(Cold) accretion

IGM

Outflows driven

by SNe or AGN

GRB afterglow spectroscopy provides a unique probe of star-

forming galaxies HI H2 Metallicities Extinction curves UV-photon escape fraction Unique selection of star-forming

galaxies (#massive stars per LF bin)

GRB030323

VLT + HST/ACS

z = 3.371

Host: V(AB)=28.0SFR = 1 M/yr

AV<0.5 mag

[Fe/H]=-1.5[S/H]=-1.3

Vreeswijk et al. (2004)

Building a representative sampleSample definition Jakobsson et al. 2006, A&A; Fynbo et al. 2009, ApJS)

1. T90 > 2 sec2. XRT localized within 12 hr.3. Galactic AV < 0.5.4. -70o < declination < +70o

5. θSun > 55o.

March 2005 - September 2008: • 148 bursts• 75% optical/near-IR afterglows• 45% afterglow-based spectroscopic redshifts

Redshifts from optical afterglow spectroscopy

High or medium resolution is preferred as much more information is available.

But most OAs are too faint for high-res spectroscopy with available spectrographs

Elíasdóttir et al. (2009);Krühler et al. (2008)

X-shooter spectrum revealing emission lines from the host

(M. Sparre)

FORS spectrum of the afterglow with 2175 Å extinction bump (z=2.45, AV~1mag)

Zafar et al. (2011,2012)

X-shooter spectrum of the host SFR = 30 M/yrOxygen abundance 40-110% solar Krühler et al. (2012)

Afterglow derived extinction curve with 2175 Å extinction bump (z=1.65, AV=0.5 mag)

GRB080607Spectroscopy started 20 min post burst – Bloom and PerleyR>24 when observable from La Palma (12 hr later)

GRB070306

Prochaska et al. (2009)

Keck

z = 3.04logNHI=22.7H2 and CO Forest of metal lines!Solar metallicity

AV=3.3 mag2175Å extinction bump.

Bright/massive and dusty hostSFR = 10 M/yr

Imagine E-ELT

GRB090323

GRB070306

Savaglio et al. (2012)

VLT/FORS z = 3.57logNHI=20.7 Super-solar metallicityAV=0.1-0.2 mag

GRB120815ASpectroscopy started 1.67 hr post burst

GRB070306

Krühler, Ledoux, Fynbo, et al. in prep (2012)

VLT / X-shooter(40 min exp.)

z = 2.358logNHI=22.1H2

Low metallicity

AV≈0.2 magNo 2175Å extinction bump.

Tentative detec. host emisson lines.

GRB07306, z=1.50, VLT, strong reddening, but blue host.

Jaunsen et al. 2008

GRB070306

Jaunsen et al. (2008)

Metallicities (Examples from X-shooter GTO)

D’Elia et al. (2010), Thöne et al. (2012); Sparre et al. in prep.; D’Elia et al., in prep; Savaglio et al (2012)

What we need for the E-ELT HIRES• An instrument that will allow a spectroscopic

study of the underrepresented faint, dusty, metal- and likely molecular-rich GRB sightlines.

• Specs: ideally 3200-24000 Å, high efficiency (need to reach AB>22), R>10000, AO assisted where possible to gain the depth

• The blue/near-UV is important (HI, H2). Rather loose K and the blue.

• Decent flux calibration• RRM/ToO observing mode

GRB090926A SFR = <2 M/yr[S/H] = -1.9

D’Elia et al. (2010)

The mean escape fraction of UV photons from star-forming galaxies: fescape<7% (95% confidence)

Chen et al. (2007); Fynbo et al. (2009)

DLA at z=2.354 towards Q2222-0946

[Zn/H]=-0.46, SFR=20M/yr

z=2.58 DLA towards Q0918+1636: [Zn/H]=-0.12, H2, CI, SFR≈20M/yr