Prsentation PowerPoint

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Chandra observation of GRB 031220
Bruce Gendre (IASF/CNR Roma)
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20 December 2003, somewhere in the Universe
Intermediate X-ray rich/X-ray flash event
3 h 29 m 56 s UT HETE-2 FREGATE trigger
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HETE-2 Burst H2976 GRB031220

• X-Ray Rich burst
• Epeak 49.24 keV
• Fluence 1.946 x 10-6 erg.cm-2
• Duration 23.7 s

1st Chandra observation 5.7324 days after the
burst 2nd Chandra observation 28.5859 days
after the burst
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First Chandra Observation

• 40 kilo-seconds of observation
• No flare background

• 14 sources detected within the GRB error box
• Detection limit 1.5 x 10-15 erg s-1 cm-2

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Second Chandra observation
20 kilo-seconds of observation, without any flare
background 9 sources varied from the 1st
observation (1 located within the GRB error
box) Source 7 count rates 8 ( 2) x 10-4
(1st observation) c.s-1 3 ( 2) x 10-4
(2nd observation) c.s-1 1 sources disappeared
from the 1st observation (located within the GRB
error box) Source 1 count rate 8 ( 2) x
10-4 (1st observation) c.s-1 lt 2 x 10-4
(2nd observation limit) c.s-1
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X-ray decay
Prompt emission from HETE2-FREGATE (Barraud,
private communication)
Chandra observation at two epoch
Source 1
X-ray power law decay fit -1.3 0.1 (source 1)
and -0.70.1-0.5 (source 7)
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Color intensity diagram
0.2-8.0 keV count rate (10-2 count s-1)
Color 0.2-1.5 keV/1.5-8.0 keV
Majority of sources compatibles with a power law
model possible afterglow candidates
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Optical search of an afterglow

• Two sets of observation CTIO (1.04 days,
  Gorosabel et al. GCN 2513) and TNG (7.92 days,
  Antonelli et al. GCN 2503)
• Observations made in R band

CTIO TNG 4m 3.58m 1 24.4
? 7 23.03 23.48

• Other upper limits
• R gt 21 at 0.222 days ( Fox et al. GCN 2499)

? Dark burst ? Two possible candidates with R gt
23 at t 1.04 days
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Combination of the results

• X-ray colors sources 1 and 7 compatible with a
  power law spectra
• X-ray decays sources 1 and 7 decaying (decays
  1.2 and 0.7 for sources 1 and 7 respectively)
• Optical decays sources 1 and 7 decaying.
  Source 7 decay is rather low for a GRB afterglow
  (-0.21, Gorosabel et al. GCN 2513)

? Best candidate is source 1. ? Candidate source
7 has unusual afterglow properties, but cannot be
ruled out.
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X-ray Rich GRB
2 main possibilities to explain the X-ray rich
GRB (see Zhang Meszaros 2003)

• Models effect (either geometrical jet models
  or parameters e.g. dirty fireball)

• Normal GRB at high redshift
• ? if z is high enough, the Lymann alpha forest is
  observed in the optical band and extinguishes the
  optical afterglow
• The X-ray rich and dark GRBs are suspected to be
  those high redshift normal GRBs

The knowledge of the distance to X-ray rich dark
GRBs can comfort this latter hypothesis and give
clues about the stars at very high redshift
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Distance scale estimators

• 2 distance estimators used
• Pseudo-Z estimation method (Atteia 2003)
• Boër Gendre (2000) estimation method

Boër Gendre, 2000
Z estimated 2.3 ( 0.2) Pseudo-Z value 1.94
(Atteia, private communication)
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Conclusions
1 likely X-ray afterglow candidate Very faint
optical afterglow, if any ! Possibly redshift
greater than 2
For the future deep optical/IR observation
with larger telescopes X-ray afterglow
systematic analysis