X-shooter spectroscopy of the GRB090926A afterglow
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X-shooter spectroscopy of the X-shooter spectroscopy of the GRB090926A afterglowGRB090926A afterglow
Valerio D’Elia(ASDC/INAF-OAR)
&
The X-shooter GRB collaboration
April, 22nd - 2010Kyoto - Japan
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• GRB090926A wih X-shooter
• Main absorption system• Profile fitting and components
• Metallicities• Excited levels
• Search for other features at the host redshift
• The extinction curve shape
• Intervening absorbers
• Conclusions and future work
OUTLINE
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OBSERVATION LOG
26 March 2009, 04:20:26 UT (Fermi burst, z=2.1071)
• Swift on target 13hr later• Skynet/PROMT reported R=18 20hr post burst
X-shooter observations began 22hr after the GRB: four spectra of ten minutes each acquired. All observations acquired with the 3 arms (UVB+VIS+NIR), /=10 000
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Ly SII
Ly
SiII
OI/SiII
OVICII SiIV
SiII
CIV
FeII
SiII
Al IIINV
FeII
FeII MgII
MgINiII
NiII
Al II
CaII
Main SystemIntervening absorbers
1 - CIV
2 - CIV
3 - CIV 4 - MgII/MgI
1) z=1.94662) z=1.79863) z=1.74834) z=1.2456
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MAIN SYSTEM GAS SEPARATION IN COMPONENTS
Two components identified at z = 2.1071
Si IV
C IV
BI 30km/s
III
BII 90km/s
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MAIN SYSTEM METALLICITY
Ly_
Ly_ NH=21.600.03 cm-2
Contamination
Missing lines
Metallicities are in the range 4.2X10-3-1.4X10-2, i.e., among the lowest in GRB hosts.
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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).
Fine structure splitting
First fine structure excited level
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Identified:
- CII, SiII, FeII and OI fine structure transitions
- FeII and NiII excited features
FINE STRUCTURE AND EXCITED LINES
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Ratio between excited and ground state abundances
FINE STRUCTURE LINES
Plot from Prochaska, Chen & Bloom, 2006
Assumptions• Excitation: indirect UV pumping• Steady state approximation
FeII*/FeII
SiII*/SiII
Flux experienced by the absorbing gas
Distance GRB/absorber
GRB redshift
Component I: d=82070pcComponent II: d=1.00.2kpc
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Emission lines from the host galaxy: Not detected H with 9X10−18 erg s−1 cm−22MSun/yr would have been detected
Molecular absorption features: Not detected NH2<15.3cm-2, NCO<14.3cm-2, logf<-4.5
Diffuse Interstellar Bands: Not detected EW<0.5 A (2 confidence)
OTHER FEATURES AT THE HOST REDSHIFT
EXTINCTION CURVE SHAPE
Assuming a power law model the spectral index of the continuum emission is: =0.890.06 (3 confidence) (F-)
Best fit to the continuum obtained assuming a SMC model with EB-V < 0.01, i.e., no extinction (3 upper limit)
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Four, very weak intervening absorbers identified at 1.24<z<1.95
THE GRB090926A SIGHTLINE
Ly_1) z=1.9466
CIV
EWrf(CIV1548)=0.15 0.04 A
2) z=1.7986
EWrf(CIV1548)=0.11 0.03 A
Ly_
CIV
3) z=1.7483
EWrf(CIV1548)=0.21 0.03 A
Ly_
CIV
MgI
MgII
4) z=1.2456
EWrf(MgII2796)=0.19 0.06 A
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CONCLUSIONS
GRB090926A at z = 2.1071 was detected by Fermi/LAT and observed 22hr later with X-shooter, when its magnitude was still 18.
• The main system (GRB host) detected in the X-shooter spectrum can be well described by a two component model.
• Metallicities are in the range 4.2X10-3-1.4X10-2, i.e., among the lowest in GRB hosts.
• Excited transitions allow us to derive a GRB/absorber distance in the steady state approximation of 0.9-1kpc.
• No other features (emission lines, molecules, DIBs) are detected
• The continuum fit does not allow any intrinsic extinction adopting a SMC extiction curve (EB-V < 0.01).
• Four very weak (CIV and MgII Ewrf < 0.21 A) intervening system are detected in the range 1.24<z<1.95
• To do list:• Check the GRB/absorber distances with time-dependent photoexitation codes• Characterize the host galaxy through the element abundance ratios