grb absorption spectroscopy - Inaf

Absorption studies of GRB afterglows
with high resolution spectroscopy
Napoli, III Congresso Nazionale GRB
Valerio D’Elia
(ASI/ASDC & INAF/OAR)
21 Settembre 2012

OUTLINE
• What can we learn from GRB absorption spectroscopy
• The need for high resolution spectra
• Absorption features in high resolution
• X-shooter’s results: GRB090926A and GRB120327A
• Conclusions and perspectives

Suitable to:
GRB ABSORPTION SPECTROSCOPY
• Find redshifts and build the GRB luminosity function
• Estimate the metal content in high redshift galaxies
• Characterize the circumburst environment
• Explore the interaction between the GRB and the surrounding
medium
• Study the intervening absorbers along GRB sightlines

LOW – RES GRBs
GRBs represent a new tool to study high-z galaxies
The ISM gives us precious information on the metal enrichment history of the
galaxies, which in turn is linked to the mass function evolution.
In the past, metal enrichment in galaxies at high z has been studied using:
• Lyman Break Galaxies
• Galaxies along the line of sight of quasars (Damped Lyman-α systems)
The first class cannot be representative of the true galaxy population
The second one is entangled by selection effects: the radiation from the
QSOs probes preferentially the halos of the galaxies.

LOW – RES GRBs
GRBs provide an independent way of studing the metal enrichment of
galaxies at z > 1.
Advantages:
• Probing central galaxy regions
• No luminosity bias
• ISM can be studied up to higher
redshift than DLA systems.
GRB-DLAs appear to be more
metal rich than QSO-DLAs
(Savaglio 2005 and 2010;
Prochaska et al. 2007)
Savaglio 2010

HIGH RESOLUTION SPECTROSCOPY
Advantages of hi-res spectroscopy:
• Separates the GRB surrounding medium in components,
allowing a more accurate study of the composition, density,
cinematics and physics of the absorbing gas.
• Is necessary to disentangle the ISM from the absorption
coming from the GRB surroundings.
• Is our only tool to fully explore the wealth of information
carried by the lines absorbed by the excited (in particular fine
structure) levels.
Disadvantages
• Suitable for high luminous afterglows only
• A fast reaction to the trigger is needed
Now achievable thanks to Swift and RRM

Advantages:
Piranomonte
et al. 2008
HIGH RESOLUTION SPECTROSCOPY
2) 1) disentangles Separates the the GRB ISM surrounding and the absorption medium coming in components, from the allowing GRB surroundings a more
accurate study of the composition, density, kinematics and physics of the
absorbing gas.
GRB 050922C
GRB 021004
Fiore et al. 2005
FORS, R=1000 UVES, R=40000

Average [C/Fe] ratio is
0.08±0.24, consistent with
values predicted for a
galaxy younger than 1 Gyr
undergoing star formation
[C/Fe] of component 3 is
0.53±0.23, larger than in 2
(-0.15±0.13), with [C]
roughly constant.
GRB 050730
Since Fe dust grains are more efficiently destroyed than C dust grains by
the GRB UV flux and blast wave (Perna, Lazzati & Fiore 2003), this
suggests that component 2 is closer to the GRB than component 3.

FINE STRUCTURE FEATURES
The gross structure of an atom is due to the principal quantum number
n, giving the main electron shells of atoms. However, electron shells
exhibit fine structure, and levels are split due to spin-orbit
coupling (the
energy difference
between the
electron spin
being parallel or
antiparallel to
the electron's orbital
moment).
First fine structure
excited level
Fine structure splitting

FINE STRUCTURE FEATURES
How to populate fine structure excited levels:
1. Collisional processes:
2. Radiative processes:
Incident UV
radiation
Incoming
e -
2a. Indirect UV pumping
Photoexcitation
n + 1
Radiative
de-excitation
n
J=3/2
J=1/2
Incident IR
radiation
(Si II, C II) (Fe II)
n
(O I)
2b. Direct IR pumping
STRONG VARIABILITY EXPECTED!
J=2
J=1
J=0
Selection
rule: ΔJ=0,±1
n
J=1/2
J=3/2
J=5/2
J=7/2
J=9/2

GRB 060418: VARIABILITY
Fine structure variability: hint of UV pumping…
Vreeswijk et
al. 2007

High ionization lines such as
NV could be produced by
the GRB UV flux
SIV is far from the explosion
site in GRB050730
Fox et al. 2008
HIGH IONIZATION LINES
(Prochaska et al. 2008)
If this is the case, the NV
should be close to the GRB
(10pc)
d (SIV) > 130 pc
Multi-epoch spectra of a z>2 GRB is needed to search for NV variability
Variation not observed in GRB080310 (De Cia et al. 2012)

CrII groud state
line is decreasing
in GRB 080310
log N epII (Cr II) =
13.07 ± 0.06
log N epIV (Cr II) =
12.37 ± 0.14
(De Cia et al. 2012)
PHOTOIONIZATION IN ACTION

A NEW INSTRUMENT @ THE VLT: X-SHOOTER
High efficiency spectrograph at the UT2 Cassegrain focus
Intermediate resolution (R = 4000-14000)
Wide spectral coverage (3000-25000Å)
Three arms splitting: UVB, VIS, NIR
First light: Nov 2008
In operation: Oct 2009
Suitable to:
• spot GRBs up to z ~ 20
• study host metallicity in a wide redshift
range
• follow line variations with higher temporal
resolution and longer times
• collect good quality GRB spectra up to R ~
21-21.5 (short GRBs)

OVI
Lyβ SII
NiII
NV
NiII
Lyα
OI/SiII
2 - CIV
SiII
SiII 3 - CII CIV SiIV
1 - CIV
CIV
FeII
CaII
FeII
Al II
FeII MgII
Intervening Main absorbers System
SiII
MgI
Al III 4 - MgII/MgI
1) z=1.9466
2) z=1.7986
3) z=1.7483
4) z=1.2456

MAIN SYSTEM GAS SEPARATION IN COMPONENTS
B I ≈ 30km/s
B II ≈ 90km/s
Two components identified at z = 2.1071
II
I
Si IV
C IV

MAIN SYSTEM METALLICITY
Ly_α
Missing lines
Saturation
Ly_β
Contamination
Metallicity is in the range
4.2X10 -3 -1.4X10 -2 , i.e., among the
lowest in GRB hosts.
N H =21.60±0.07 cm -2

GRB/ABSORBER DISTANCE
Component I
Component I - FeII+SiII: d=680±40 pc
Component II - SiII: d ~ 1.4 kpc

CONCLUSIONS
GRBs are a new tool to study the metal enrichment history of the galaxies, up to z > 5.
Medium-to-high resolution, absorption spectroscopy is necessary to resolve the GRB
environment in components and to separate it from the ISM, and to fully explore the
information carried by the excited transitions.
The ISM of the GRB hosts is complex, with many components contributing to the main
system absorption.
Interesting GRB surrounding medium characteristics, such as different properties of
different components, high ionization lines, photoionization/photoexcitation needs
medium-to high resolution in order to be properly addressed.
Fine structure lines are commonly observed in GRB afterglows and are produced by
UV pumping. They can give us constraints on the distance of the absorbing gas from
the GRB. The prompt/afterglow emission affects the ISM up to hundreds of pc
(GRB080330), or even some kpcs (GRB080319B, GRB090926A).
X-shooter is is gathering exciting data on GRBs since more than 3 years.

CONCLUSIONS
GRB090809/GRB100425A: Skuladottir et al. "
"
GRB091018A: Wiersema et al. "
"
GRB091127A: Vergani et al. "
"
GRB100219A: Thoene et al. "
"
GRB100316D: Bufano et al., Flores et al."
"
GRB100418A: de Ugarte Postigo et al. "
"
GRB100621A: Watson et al. "
"
GRB100727A: Vreeswijk et al. "
"
GRB100814A: Piranomonte et al. "
"
GRB100816A: Antonelli/Tanvir et al. "
"
GRB100925A: DʼAvanzo/Kaper et al."
⎬⎬List of X-shooter GRB "
"
papers in preparation"

GRB explosion
site
Host gas
far away
Circumburst
environment
Intergalactic matter
To
Earth

Swift satellite locates
GRBs with
arcsec precision in a few
tens of seconds
HIGH RESOLUTION SPECTROSCOPY
GRB
Coordinate
Network (GCN)
releases these
positions in a
few seconds
VLT Rapid
Response
Mode (RRM)
allows to point
such
coordinates in
about 8
minutes

GRB 050730
The main system presents 5 ( +1) components
C IV: 5 components
1) +32.6 2) +2.4
3) -44.0 4) -90.2
5) -154.6
Si IV: 4 components
2) +2.4 3a) -44.0 a
3b) -44.0 b 4) -90.2
CIV and Si IV components used as reference to fit the other ions

GRB 050730
Fine structure transitions - the ion C II
C II 1036 and C II 1335 doublets
Only components 2 and 3 are present in the excited fine
structure features

GRB 050730
Fine structure transitions - the ion Si II and the atom O I
Si II 1260, Si II 1304, Si II 1526 doublets and O I 1302 triplet
SiII*
OI*
Only components 2 and 3 are present in the excited fine
structure features

GRB 050730
Fine structure transitions - the Fe II
Fe II 1608 - 1611 multiplet
Only component 2 is present in the fine structure multiplet

GRB 050730
Second component: UV pumping scenario favoured
G/G0 = 105 ÷ 106 where G0 = 1.6 X 10-3 erg cm-2 s-1 .
Third component: possibly electron-ion collisions at work
From C II fine
structure doublet:
10 3 < T< 10 4 K
10 < n e < 60 cm -3

1) 2) 3) 5) 4) Unidentified Main Intervening system lines #1 #2 #4 #3
z=0.937 z = 0.76 0.71 0.53 0.57
Ni II
?
ABSORBING SYSTEMS
?
Mg I Fe Mn II II Cr II
Mg II Mg I
Fe II
?
Fe II
Mg II
Fe II
Fe II
Mg II Mg I Mg II

GRB080319B - MAIN SYSTEM: COMPONENTS
Six components clearly identified

GRB080319B - MAIN SYSTEM: COMPONENTS
Some lines are saturated, but we have so many transitions that
the six component fit is very robust

GRB080319B - MAIN SYSTEM: COMPONENTS
VI
V
Component I shows strong Mg II absorption but no evidence of
Mg I: this is possibly the closest component to the GRB
IV
III
II
I

GRB080319B - FINE STRUCTURE LINE VARIABILITY
Fine structure lines nearly
disappear
in less than 2 hours
(less than 1h rest frame at
z=0.937)!
Fe II*
2396
Ground state lines remain
constant (slight increment
compatible with the
decreasing of the excited
levels)
The strongest fine structure line variation
ever found
(optical depth reduced by a factor of 4 – 20)
Fe II
2374

FINE STRUCTURE LINE VARIABILITY
Fine structure of components III and IV drops faster than that of component I
Explanation:
Component I experiences
higher fluxes for longer
times, i.e., is the closest
component to the GRB.
I
III+IV

Balance equation:
dN t
u
dt
( )
TIME DEPENDENT MODELING
= N ( ) ( ) ( ) ( )
l t Blu F! t $ Nu t "
% Aul + BulF t #
! &
% ! % " % 2
& t ' & $ ' & d '
( ) ( ) ( )
* tc + * $ c + * dL
+
were: F ( t) = C [ 1+
z]
hν
Absorption
#
up
low
hν
Spontaneous emission
% 1
up up
hν
low low
Stimulated emission

GRB080319B - TIME DEPENDENT MODELING
d = 2kpc
Component I Component III
d = 6kpc
Possible extragalactic origin!

GRB080330 - MAIN SYSTEM: COMPONENTS
Four components identified
Only component IV
shows up in excited
levels

GRB080330 - MAIN SYSTEM: COMPONENTS
Four components identified

GRB080330 - TIME DEPENDENT MODELING
FeII
d = 280±50 pc
Component IV
SiII
d < 800 pc
For d=280pc and
n e = 1-10 3 cm -3
the fraction of FeII and
MgI during the first 40
minutes (rest frame) of
the afterglow is almost
unchanged
(self consistency)

INTERVENING ABSORBERS
GRB Strong MgII excess - possible explanations:
• Dust extintion bias
• Gravitational lensing
• Local, high velocity systems related to the GRBs
• Different beam size of the GRB and QSO emitting regions
Variability is predicted!

INTERVENING ABSORBERS
GRB080319B
NO VARIABILITY
MgII2796 FeII2600 MgI2852
1st observation
2nd observation
3rd observation
(Variability is less than 10% at 3σ:
a tighter constraint (a factor of 3) than that
for GRB060206, Thöne et al.2008, Aoki et al. 2008)

FINE STRUCTURE AND EXCITED LINES
Identified:
- CII, SiII, FeII and OI fine structure transitions
- FeII and NiII excited features

THE GRB090926A SIGHTLINE
Four, very weak intervening absorbers identified at 1.24