X-shooter spectroscopy of the GRB090926A afterglow

1. X-shooter spectroscopy of the GRB090926A afterglow Valerio D’Elia (ASDC/INAF-OAR) & The X-shooter GRB collaboration Kyoto - Japan April, 22nd - 2010

2. OUTLINE • 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

3. 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

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

5. II I BI 30km/s BII  90km/s MAIN SYSTEM GAS SEPARATION IN COMPONENTS Two components identified at z = 2.1071 Si IV C IV

6. Contamination Missing lines MAIN SYSTEM METALLICITY NH=21.600.03 cm-2 Ly_ Ly_ Metallicities are in the range 4.2X10-3-1.4X10-2, i.e., among the lowest in GRB hosts.

7. 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

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

9. FINE STRUCTURE LINES • Assumptions • Excitation: indirect UV pumping • Steady state approximation Ratio between excited and ground state abundances Flux experienced by the absorbing gas GRB redshift SiII*/SiII FeII*/FeII Distance GRB/absorber Component I: d=82070pc Component II: d=1.00.2kpc Plot from Prochaska, Chen & Bloom, 2006

10. OTHER FEATURES AT THE HOST REDSHIFT Emission lines from the host galaxy:Not detected H with 9X10−18 erg s−1 cm−22MSun/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) EXTINCTION CURVE SHAPE Assuming a power law model the spectral index of the continuum emission is: =0.890.06 (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)

11. MgI 1) z=1.9466 2) z=1.7986 4) z=1.2456 3) z=1.7483 Ly_ Ly_ Ly_ CIV MgII CIV CIV EWrf(MgII2796)= 0.19 0.06 A EWrf(CIV1548)= 0.11 0.03 A EWrf(CIV1548)= 0.21 0.03 A EWrf(CIV1548)= 0.15 0.04 A THE GRB090926A SIGHTLINE Four, very weak intervening absorbers identified at 1.24<z<1.95

12. 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