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

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

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

MAIN SYSTEM GAS SEPARATION IN COMPONENTS

Two components identified at z = 2.1071

Si IV

C IV

BI 30km/s

III

BII 90km/s

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.

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

Identified:

- CII, SiII, FeII and OI fine structure transitions

- FeII and NiII excited features

FINE STRUCTURE AND EXCITED LINES

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

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)

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

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