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