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Pharos: distant beacons as cosmological probes. The “ Pharos ” of Alexandria, one of the Seven Wonders of the ancient world, was the tallest building on Earth (120m). Its mysterious mirror, which reflection could be seen more than 55 km off-shore fascinated scientists for

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Pharos: distant beacons as cosmological probes

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distant beacons ascosmological probes

The “Pharos” of Alexandria, one

of the Seven Wonders of the

ancient world, was the tallest

building on Earth (120m). Its

mysterious mirror, which reflection

could be seen more than 55 km

off-shore fascinated scientists for


Fabrizio Fiore

and V. D’Elia, S. Piranomonte, R. Perna, D. Lazzati, D. Guetta,L. Stella,

A. Antonelli, P. Ward and many others

Gamma ray bursts: one of the great wonders of the Universe

  • GRBs combine 4 themes of

    early 21st Century astrophysics:

    • Among the most energetic events in the Universe

      1997 1st GRB redshift (thank to BeppoSAX)

    • Galaxies in the age of star formation

      metal abundances, dynamics, gas ionization, dust

    • The recombination epoch

      2000-200? Gunn-Peterson trough at z~6-?

    • The fate of the baryons & large scale structure

      (See Nicastro et al. poster)

      1999 Warm IGM simulations

      2001 1st Warm IGM detection (thank to Chandra)

  • Minutes after the GRB event their afterglows are the brightest

  • sources in the sky at cosmological redshift.

  • Afterglows can be used to probe the high redshift Universe :

  • Can we use GRB to track star-formation at high-z? through:

  • High resolution spectroscopy of UV lines

  • Statistical studies


10 Crab!


1mCrab i.e. a bright AGN

Galaxies in the Age of Star Formation

GRBs also probe normal high z galaxies

peak of star formation

  • Star formation in the Universe peaked at z~2

star formation rate

GRB Hosts

  • Studies of z=>1-2 galaxies are biased against dusty environments.

redshift, (1+z)

  • GRB hosts are normal galaxies

Mann et al. 2002 MNRAS, 332, 549

  • GRB afterglowswill reveal host

  • Galaxy dynamics, abundances, & dust content at z>1

The history of the metal enrichment in the Universe

Savaglio 2003

Prochaska et al. 2003

Spectroscopy of UV lines

Star-formation regions (and the interstellar matter in general) are complex.

Can we truly learn anything about star-forming regions from afterglow’s spectroscopy? Or, are we just doing meteorology?

Just like to try to understand the physics of the atmosphere from the observation of one (a few) lightning …

Statistical studies

Porciani & Madau 2001, Schmidt 2001,

Guetta et al. 2004, Jakobsson et al. 2005

and many others.

But…. Too few z, 30-50% only of well selected GRB samples.

Selection effects complex and

difficult to account for.

Redshift distributions

30 Swift GRB with spectroscopic redshift

27 BeppoSAX and HETE2 GRB with spectroscopic redshift

Peak flux distributions

NH distributions

118 Swift and 44 BeppoSAX+HETE2 GRBs localized in regions with NH(Gal)<41021

Due to the different Eband of localization? (BAT 15-150keV, WFC and WXC 2-20 keV)

Selection effectfor different localization bands

X-ray localization: biased against large z=0 obscuration

==> BSAX and HETE2 samples miss highly obscured, low-z GRB.

==> Swift localizes highly obscured, low-z GRB, but dust extinction makes their optical afterglow faint, and so more difficult z determinations




1 .5

0 NH=1024cm-2

Selection effects


1) Sensitivity: BSAX and HETE2 samples biased against X-ray faint GRB

2) Localization Eband: BSAX and HETE2 samples biased against highly obscured GRB (preferably at low z)

3) Spectroscopic identification: dust in the host galaxy plays a major role

Spectroscopy of UV lines

1- The GRB surrounding medium: its physical and dynamical state can be easily modified by the GRB and its afterglow. Geometry, density and physical state of the gas can be constrained by line variability and comparison of line ratios to expectation of time dependent photoionization codes.

2- The ISM of the host galaxy. Metal column densities, gas ionization and kinematics.These studies have so far relied upon either Lyman Break Galaxies or Damped Lyman Alpha systems, which may not truly be representative of the whole high-z galaxy population.

GRB afterglows can provide new, independent tools to study high z galaxies.


UVES spectra 3200-9400 A, slit 1”, resolution=42,000

FORS spectra 3800-9500 A, slit 1”, resolution=1000

GRB020813: z=1.245 - 24 hours after the GRB; R=20.4

GRB021004: z=2.328 - 12 hours after the GRB; R=18.6

GRB050730: z=3.968 - 4hr after the GRB; R18

GRB050922C: z=2.199 – 3.5hr after the GRB; R18

The ISM probed by GRB afterglow

Complex, with many components spanning a velocity ranges from a few hundred to a few thousands km/s

GRB0500922C UVES spectrum

Separating different components Piranomonte et al. 2006

GRB050730 UVES spectrum

Strong fine structure transition: CII*, SiII*, OI*, OI**, FeII*, FeII**, FeII***. Prochaska et al. 2006: Observed abundances of excited ions are well explained by UV pumping with the gas at r~a few hundred pc from the GRB

GRB050730: a slightly different approach:

Separating different components

D’Elia et al. 2006


Strong CII* for both components 2 and 3

3 2 1


Strong SiII*,OI* and OI** only for

component 2

3 2

4 3 2


3 2

Strong FeII and FeII* transitions only for component 2

Fine structure lines

  • Two main processes:

  • Radiative excitation; 2) Collisional exitation

GRB050730 Fine structure lines

component 2

T=2650+3000-1000 K

Ne>a few 100

Assuming collisional excitation


GRB050730 fine structure lines

Component 3

NCII/NCII*~1.8, higher than that of Component 2!

Ne = a few 10

Assuming collisional excitation

GRB050730 gas ionization

and distance of the clouds from the GRB

Huge (and variable) ionizing flux! CI<12.3

[CIV/CII] and [SiIV/SiII] of components 2 and 3 similar. If density is different by a factor 10 (100), the distance from the GRB of component 3 is 3 (10) times higher than that of component 2.

Similar conclusion reached assuming UV pumping as dominant mechanism for excited transitions

Need detailed time-dependent photoionization codes!

Time dependent photoionization

Nicastro et al.1999

Perna, Lazzati et al.

  • Strong variation with time of ion abundances

  • A tool to constrain gas geometry and density through ion ratio variations

Lazzati et al. 2006


In case of multiple components it is truly difficult to estimate H columns for each, even with high quality spectroscopy.

The spatial distribution of heavy elements can be very different from that of H.

The star-forming regions hosting the GRB are likely to be much more enriched than the outer galaxy regions

GRB host galaxy metallicities








GRB host galaxy metallicities

  • However… metallicity depends on:

  • Impact factor

  • Galaxy mass

  • Star-formation rate

  • Etc….

1) Metallicity depend on impact factor




2) Metallicity depends on galaxy mass:

Savaglio et al. 2005,2006 Berger et al. 2006

Host em. lines

050904, abs.

Statistical population studies

  • … should consider

  • Complex selection effects, different localization bands and NH distributions should be taken into account

  • SFR / metallicities / masses appropriate for the typical GRB hosts

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