GRB080319B: GRB080319B: a high resolution spectroscopic viewa high resolution spectroscopic view

Valerio D’Elia(INAF – OAR)

&

F. Fiore, F. Nicastro (Rome),R. Perna (Colorado), Y. Krongold (Mexico),

E.Meurs, L.Norci (Dublin) + MISTICI group (Rome/Milan)

June, 24th - 2008Nanjing - China

• GRB080319B wih UVES

• Absorbing systems (main + intervening)

• Main system gas separation in components

• Fine structure absorbing features

• Fine structure line variability

• Conclusions and perspectives

OUTLINE

OBSERVATION LOG

19 March 2008, 06:12:49 UT: the brightest GRB ever (z=0.937)

• Observed before, during and after the GRB worldwide• R=5 at about 20 s and H=4.2 at about 50 s from the GRB: naked eye GRB!

UVES observations began just 8m30s after the GRB (fastest response and higest S/N ever)Two RRM and one ToO observations of the event (8.5m, 2h and 3h time delay)

Obs. UT Time from burst (s)

Exp. (s) S/N Dich. Arms Rmag

RRM1 06:21:26 517 600 3050 2 B+R 12 13

RRM2 08:06:42 6833 1800 7 12 1+2 B+R 16 17

ToO 09:07:22 10482 1200 5 8 1+2 B+R 16 17

GRB explosion site

Circumburstenvironment

To Earth

Host gasfar away

Intergalactic matter

ABSORBING SYSTEMS

Fe II

Cr IIMg I

Ni II

1) Main system z=0.9372) Intervening #1z = 0.76

Fe II

Mg II

3) Intervening #2z = 0.71

Fe II

Mg II Mg I

5) Intervening #4z = 0.53

Fe II

Mg II Mg IMn II

4) Intervening #3z = 0.57

Fe II

Mg II

Unidentified lines

?

??

MAIN SYSTEM GAS SEPARATION IN COMPONENTS

Six components clearly identified

MAIN SYSTEM GAS SEPARATION IN COMPONENTS

Although some lines are saturated, we have so many transitions that the six components fit results to be very robust

Component I shows strong Mg II absorption but no evidence of Mg I: this is possibly the closest component to the GRB

IIIIIIIVVVI

MAIN SYSTEM GAS SEPARATION IN COMPONENTS

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

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

Selectionrule: J=0,±1

(Si II, C II) (Fe II)

Incominge-

(O I)J=0

J=1

J=2

n

n

STRONG VARIABILITY EXPECTED!

Fine structure lines nearly disappear

in less than 2 hours (less than 1h rest frame at

z=0.937)!

FINE STRUCTURE LINE VARIABILITY

Fe II 2374

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)

Fine structure of component III and IV drops faster than that of component I

FINE STRUCTURE LINE VARIABILITY

Possible explanation:

Component I experiences higher fluxes for longer times, i.e., is the closest component to the GRB.

In addition, component I can receive contribution from collisional excitation (hints of a higher temperatures)

The ratio between the fine structure and ground levels are linked to the UV flux experienced by the GRB

FINE STRUCTURE LINE VARIABILITY

Component I

Components III/IV

Using GRB080319B lightcurve and spectral energy distribution, we can compute the UV luminosity of the GRB and thus the distance of the absorber in a steady-state approximation.

This yields

d = 18-34 kpc:

outside the host galaxy!

Prochaska et al. 2006

Diffuse emission elongated south of the AG

Two faint clumps at 1.5’’ and 3’’ 12 and 24 kpc at z=0.937

Tanvir et al. 2008

CONCLUSIONS

GRB080319B at z = 0.937 is the brightest GRB ever and the brightest and fastest-observed UVES GRB ever: 8.5 mins after the trigger and S/N = 50

• At least five different absorption systems identified, (main system at the GRB redshift + 4 intervening), between z = 0.937 and z = 0.5

• The main system can be resolved into six components, constituted by different shells of gas absorbing the radiation at different distances from the GRB

• The strongest fine structure line variation ever found in a GRB (optical depth reduced by a factor of 4 – 20 in less than 1 hour rest frame), withnesses UV pumping as excitation mechanism. d=18-34 kpc, well outside the host galaxy!

• Time-dependent photoexitation models are under construction. Preliminary results confirm that the distance from the GRB is greater than 1 kpc