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

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

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

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

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

MAIN SYSTEM METALLICITY Ly_  Ly_  N H =21.60  0.03 cm -2 Contamination Missing lines Metallicities are in the range 4.2X X10 -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=820  70pc Component II: d=1.0  0.2kpc

Emission lines from the host galaxy: Not detected H  with 9X10 −18 erg s −1 cm −2  2M Sun /yr would have been detected Molecular absorption features: Not detected N H2 <15.3cm -2, N CO <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:  =0.89  0.06 (3  confidence) (F  -  ) Best fit to the continuum obtained assuming a SMC model with E B-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= CIV EW rf (CIV1548)= 0.15  0.04 A 2) z= EW rf (CIV1548)= 0.11  0.03 A Ly_  CIV 3) z= EW rf (CIV1548)= 0.21  0.03 A Ly_  CIV MgI MgII 4) z= EW rf (MgII2796)= 0.19  0.06 A

CONCLUSIONS GRB090926A at z = 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 4.2X X10 -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 (E B-V < 0.01). Four very weak (CIV and MgII Ew rf < 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

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

TIME DEPENDENT MODELING Balance equation: were: h Absorption up low up low Spontaneous emission h Stimulated emission

Detailed balance equation for a two levels system: n: density of the states - w: radiative terms - Q: collisional terms Fine structure, assuming electron-ion collisions is main process: (For C II) (For Si II) n e : electron density - T: temperature - N: density of the states  INFORMATIONS ON T AND n e can be obtained. If indirect UV pumping is instead at work, we can gather informations on the strength of the radiation field G. ABSORPTION SPECTROSCOPY Why studying fine structure absorption features