Presentazione di PowerPoint

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GRB080319B a high resolution spectroscopic view
Valerio DElia (INAF OAR) F. Fiore, F.
Nicastro (Rome), R. Perna (Colorado), Y. Krongold
(Mexico), E.Meurs, L.Norci (Dublin) MISTICI
group (Rome/Milan)
Nanjing - China
June, 24th - 2008
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OUTLINE

• GRB080319B wih UVES
• Absorbing systems (main intervening)
• Main system gas separation in components
• Fine structure absorbing features
• Fine structure line variability
• Conclusions and perspectives

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OBSERVATION LOG

• 19 March 2008, 061249 UT the brightest GRB
  ever (z0.937)
• Observed before, during and after the GRB
  worldwide
• R5 at about 20 s and H4.2 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)

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Circumburst environment
Intergalactic matter
GRB explosion site
Host gas far away
To Earth
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ABSORBING SYSTEMS
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MAIN SYSTEM GAS SEPARATION IN COMPONENTS
Six components clearly identified
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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
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MAIN SYSTEM GAS SEPARATION IN COMPONENTS
I
II
III
IV
V
VI
Component I shows strong Mg II absorption but no
evidence of Mg I this is possibly the closest
component to the GRB
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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).
First fine structure excited level
Fine structure splitting
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FINE STRUCTURE FEATURES

• How to populate fine structure excited levels
• Collisional processes
• Radiative processes

Incoming e-
J2
J1
J0
n
(O I)
2a. Indirect UV pumping
2b. Direct IR pumping
n 1
Incident UV radiation
Incident IR radiation
Selection rule ?J0,1
Radiative de-excitation
J1/2
J3/2
Photoexcitation
J5/2
J7/2
J3/2
J9/2
n
J1/2
n
(Si II, C II)
(Fe II)
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FINE STRUCTURE LINE VARIABILITY

Fine structure lines nearly disappear in less
than 2 hours (less than 1h rest frame at
z0.937)!
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)
Fe II 2374
Fe II 2396
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FINE STRUCTURE LINE VARIABILITY
Fine structure of component III and IV drops
faster than that of component I
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)
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FINE STRUCTURE LINE VARIABILITY
The ratio between the fine structure and ground
levels are linked to the UV flux experienced by
the GRB
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.
Component I
Components III/IV
This yields d 18-34 kpc outside the host
galaxy!
Prochaska et al. 2006
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Diffuse emission elongated south of the AG
Tanvir et al. 2008
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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.
  d18-34 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

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TIME DEPENDENT MODELING
Balance equation
were
up
up
up
low
low
low
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TIME DEPENDENT MODELING
A toy model with just three levels and three
transition
6D09/2
? 2600Å
6D7/2
6D9/2
Work in progress (more than 2000 transitions need
to be considered)
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THE FUTURE
Fine structure UV pumping or collisions?
ne 109 cm-3
In GRB 060418 fine structure lines are produced
by UV pumping at r ? 1.7 0.2 kpc (Vreeswijk et
al. 2007)
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HOW TO ACHIEVE HIGH RESOLUTION SPECTRA
UVES high resolution spectroscopy
UVES (Ultraviolet-visual echelle spectrograph)
operate with high efficiency from the atmospheric
cut-off at 300 nm to the long wavelength limit of
the CCD detectors (about 1100 nm). The light
beam from the telescope is splitted into two arms
(UV to Blue, and Visual to Red). Arms can be
operated separately or in parallel via a dichroic
beam splitter.
Instrument mode ?range(nm)
Maximum resolution(?/??) Covered ?range
Magnitude limits Blue arm
300-500 80,000
80
17-18 Red arm
420-1100 110,000
200-400
18-19 Dichroic 1 300-400
80,000
80
17-18
500-1100 110,000
200
18-19 Dichroic 2
300-500 80,000
80
17-18
600-1100
110,000 400
18-19
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ABSORPTION SPECTROSCOPY
Why studying fine structure absorption features

• 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)
• ne electron density - T temperature - N
  density of the states
• INFORMATIONS ON T AND ne can be obtained.
• If indirect UV pumping is instead at work,
  we can gather informations on the strength of the
  radiation field G.