Nobuyuki Kawai Tokyo Tech

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GRBs as a probe for the Universe in the
re-ionization Era

• Advantage of GRB as a probe for high-z Universe
• Science goals of high-z GRBs
• Instrument requirements

• Nobuyuki Kawai (Tokyo Tech)

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The Big Question
How and when the Universe was formed?

• IGM was re-ionized by the first generation
  luminous objects
• reionization at (Page et
  al. 2006)
• the universe reionized twice? (e.g. Cen 03)
• rapid increase of tIGM at z5 (QSO spectra)
  indicates the end of reionization at z6 (e.g.,
  Fan et al. 2006)
• z6-20, by first stars/quasars?
• need to study zgt6 !!!

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Objects at high redshifts

• AGN and galaxies
• Sample of only luminous objects
• massive systems (BH or stellar)
• special condensed region in the Universe
• large ionizing flux in X-ray (AGN) or UV (LAE)
• strong effects on ISM and IGM
• Difficult to find
• Require wide field survey
• not only rare, but also cosmic variance
• Less than 10 QSOs at zgt6 out of 200 million
  objects in SDSS (1/6 sky)

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GRB advantage at high redshifts

• GRBs
• No luminosity bias for the host galaxy
• more ordinary galaxy population
• simply tracing SFR
• No proximity effects
• Easy to detect in X/g
• gt15 at zgt5 out of 100 Swift GRBs/yr

• AGN and galaxies
• Sample of only luminous objects
• massive systems (BH or stellar)
• large ionizing flux in X-ray (AGN) or UV (LAE)
• Difficult to find
• Require wide field survey
• Less than 10 QSOs at zgt6 out of 180 million
  objects in SDSS (1/6 sky)

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Record redshifts
(just relatively uncontroversial ones)
quasars
galaxies
Tanvir 2005
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GRB vs. quasar
weak Ly a emission
GRB 050904 z6.295 Subaru FOCAS
White, R.L. et al., AJ 126, 1 (2003)
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not-so-luminous host galaxy
i-band (24 ks)
NB921 (56.7 ks)
gt 26.4 AB mag (3s) M1260 gt -20.4 mag ? LltL SFR lt
7.5 Msun/yr
Dec 27 05--Jan 01 06 (t0115119d)
Aoki et al. 2006 (in preparation)
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Swift is detecting more high-z GRBs

• Swift is detecting 100 GRBs/yr
• 50 with optical/NIR afterglow
• 25 has known redshifts
• out of 42 with known z
• 7 at zgt4
• 3 at zgt5
• 1 at zgt6
• 15 (or more) of detected GRBs likely at zgt5

pre-Swift ázñ 1.4
Swift ázñ 2.8
Lamb, Venice conference (2006)
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GRB 050904X-ray afterglow
Watson, D. et al. astro-ph/0509640
Watson, D. et al. astro-ph/0509640
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GRBs probe

• Metallicity
• S/H -1.3 for GRB050904 at z6.3
• xHI IGM neutral hydrogen fraction
• consistent with 0.0 at z6.3 (lt0.6 at 3s)
• SFR as a function of z
• need large sample with known redshift
• Nature of the pop-III stars
• how do they explode?
• SN products (Fe, a-elements) similar to low-z
  thermonuclear/core-collapse SNe?

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GRB 050904 at t3.4 d
z 6.3
Log NHI21.6
z 6.2950.002
S/H-1.3
Kawai et al. (2006) Totani et al. (2006)
Subaru FOCAS 4.0 hrs, l/Dl1000
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Metalicity vs. Redshift
GRBs DLAs D GRB hosts
M/H
GRB 050904 Kawai et al. 2006
Fynbo et al. 2006 Prochaska et al. 2003 Sollerman
et al. 2005
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X-ray vs. optical/NIR

• Optical/NIR afterglow detection rate only 50
• X-ray afterglow detection 100
• optical metallicity measurements affected by
  dust
• some elements condensed in dusts
• X-ray NH tend to be larger than those derived
  from optical extinction
• low dust formation? metallicity effects?
• Difficult to cover the right wavelengths
• no early NIR spectroscopy of afterglow yet
• e.g. GRB 050904 (S, Si, C, O, but no Fe)

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X-ray observation goals

• absorption ? Fe/O, redshift
• low-E absorption vs. Fe edge
• low-E NH1022 ? 1-2 keV restframe
• Fe edge 7 keV rest ? 1 keV at z6
• Energy coverage 0.2 keV 1.2 keV
• DE/E 1 eV or less desirable for Fe edge
• Fe line ? redshift, GRB physics, progenitor and
  environment
• need extremely special setting(geometry, Fe
  abundance, )

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simulation with XRS RMF
0.8
1.2
0.5
2.0

• photon statistics (GRB 050904 with XRS ) 10
• photon index2
• NH 1023 cm-2
• E.W10 eV
• solar abundance

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GRB070828 ASCA
Yoshida et al. 1999, 2001
Emission-like 5keV

• Consistent z, if interpreted as the radiation
  recombination edge

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GRB970508 Fe line(?)
Piro et al. 1999
E3.400.3 keV (6.20.6 keV at source)

• 97.3 confidence level
• Consistent with optical redshift z0.835
• Before reburst
• Mmin 0.5 Mo AFe1

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Fe line

• fluorescence from spherical medium

continuum
Fe line
EW 10 (NH/1022) eV (Zsun, 6.4 keV)
1.5 (NH/1022) eV (at z6)
Makino, Leahy and Kawai 1985
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Fe line

• fluorescence from spherical medium

• Maybe detectable
• at a late phase
• as a orphan afterglow(Fe line transient)

beaming factor 1/100
EW 1.5 (NH/1022) eV x(1/100) (at z6)
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orphan afterglow event rate

• 2 GRB/day/sky

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Instrument requirements

• Burst detector
• Trigger capability for long, low-flux, soft
  bursts
• Spacecraft
• Slew time lt 100 s
• prompt communication to/from ground (lt10 s)
• Anti-sun poiting desirable for ground coverage
• Focal place detector
• lower energy threshold lt0.2 keV (NH at z6)
• upper energy threshold gt1.2 keV (Fe edge at z6)
• DE/E 1 eV (Fe edge and Fe line)
• measurable flux gt 100 mCrab or 1 c/s/cm2 at 1 keV
• low background (for extended observation)
• large W gt 1 deg2 (orphan Fe line transient)
• additional detector (optical/NIR camera?)