Transcript Fermi Observations of Gamma-ray Bursts Masanori Ohno(ISAS/JAXA) on behalf of Fermi LAT/GBM collaborations April 19, 2010 Deciphering the Ancient Universe with Gamma-Ray Bursts.

Fermi Observations of
Gamma-ray Bursts
Masanori Ohno(ISAS/JAXA)
on behalf of Fermi LAT/GBM
collaborations
April 19, 2010
Deciphering the Ancient Universe with
Gamma-Ray Bursts
1
HE emission from GRBs : Pre-Fermi Era
GRB940217(Hurley et al. 94)
GeV photons
up to 90min after the trigger
GRB941017
(Gonzaletz et al. 03)
Temporary distinct
HE spectral component
-18 to 14 sec
14 to 47 sec
47 to 80 sec
80-113 sec
113-211 sec
April 19, 2010
- Many observations in keV-MeV
- Little is known about HE (>100 MeV)
emission from GRBs
1) Distinct HE spectral component ?
2) Maximum photon energy (cut-off ?)
3) Long-lived HE emission ?
Important key for emission mechanism
and environment of GRBs
Need more sensitivity, larger FoV
Deciphering the Ancient Universe with Gamma-Ray Bursts
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Fermi Gamma-ray Space Telescope
LAT
Silicon-Strip detectors
- Identification &direction
measurement of γ-rays
CsI calolimetor
- Energy measurement
ACD (plastic scintillators)
- background rejection
Gamma-ray Burst Monitor (GBM)
12 NaI detectors (8keV-1MeV)
- onboard trigger , localization
- spectroscopy
2 BGO detectors (150keV-40MeV)
- spectroscopy (overlapping LAT
band)
-Efficient observing mode
-Wide FoV
-Low deadtime
-Large effective area
-Good angular resolution
-Energy coverage
April 19, 2010
Deciphering the Ancient Universe with Gamma-Ray Bursts
More photons
from
Many GRBs
3
Fermi GRBs
Detections as of 090904
• The GBM detects ~250 GRBs/year (~400 total)
– ~18% short
– ~50% in the LAT FoV
• The LAT detects ~10 GRBs/year
– 17 total as of today (recent detection :100225A, 100325A, and 100414A)
– ~10% of GBM GRBs observed
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What we have seen from Fermi GRB observations
1. Extra component of the prompt emission ?
Different emission mechanism: Synchrotron self Compton ? Hadronic origin ?
GRB941017 shows the sign of extra component
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Extra PL component in short and long GRBs
GRB 090510 (short)
Abdo, A. A. et al., ApJ submitted
GRB 090902B (long)
Abdo, A. A. et al., ApJL 706, 138 (2009)
T0+4.6s to T0+9.6s
First extra component by Fermi
At > 5 sigma level
• First time a low-energy extension of the
PL component has been seen
3 LAT GRBs shows extra PL component
(090510, 090902B, 090926A)
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What we have seen from Fermi GRB observations
2. What is the maximum energy of HE emission ?
Constrain the bulk Lorentz factor of the relativistic jet
No evidence of the cut-off so far.
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Limit on bulk Lorentz factor
Due to large luminosity and small emitting region, optical depth for the
γ-γ -> e+e- pair production is too large to observe the non-thermal
emission from GRB  compactness problem.
Relativistic motion (Γ>>1) could avoid this compactness problem
Γmin can be derived using observed
highest energy photon
Γmin
090510
E=31 GeV
090902B
E=33 GeV
080916C
E=3 GeV
z
Γmin~1000 for short and long GRBs
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GRB 090926A: the first HE spectral cutoff
Preliminary !
8-14.3keV
14.3-260 keV
0.26-5 MeV
νFν(erg/cm2/s)
Time-integrated photon
spectrum(3.3-21.6s)
LAT all event
10
>100 MeV
>1GeV
102
103
104
Energy (keV)
105
106
- Delay in HE onset: ~3 s
- The extra component shows at >5 σ
spectral break at ~1.4 GeV
- First direct measurement of Γ ~ 630
(if cutoff due to γ-γ absorption)
(See Uehara’s poster #095)
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What we have seen from Fermi GRB observations
3. HE emission is delayed and/or long-lived ?
Suggests another emission mechanism
A few GRBs show delayed high energy emission (GRB940217, GRB080714)
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Deciphering the Ancient Universe with Gamma-Ray Bursts
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Long-lived GeV emission
~ Swift and Fermi view of GRB 090510 ~
GRB 090510 (short GRB)
UVOT XRT
Fermi/LAT
De Pasquale et al., ApJL 709, 146 (2010)
t1.380.07
LAT emission until 200 s
No spectral evolution
(photon index -2.1 ± 0.1)
Simultaneous fit
of the SED at 5 different times
• Forward shock model can
reproduce the spectrum from
the optical up to GeV energies
• Extensions needed to arrange
the temporal properties
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HE delayed onset in short and long GRBs
GRB 080916C (long)
GRB 090510 (short)
Abdo et al. 2009, Science 323, 1688
Abdo et al. 2009, Nature 462, 331
8-260keV
0.26-5MeV
LAT all events
>100 MeV
>1GeV
The first few GBM peaks are missing in
the LAT but later peaks coincide
Delay in HE onset: 0.1-0.2 s
The first LAT peak coincides with the
second GBM peak
Delay in HE onset: ~4-5 s
HE delayed onset can be seen from almost all LAT GRBs
April 19, 2010
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Constraint on QG and EBL models
GRB 090510
Abdo et al. 2009, Nature 462, 331
GRB 090902B
Abdo, A. A. et al., ApJL 706, 138 (2009)
GBM NaI
GBM BGO
LAT (>1MeV)
31 GeV
0.83 s
Constraints on the quantum gravity
mass (MQG) by direct measurement
of photon arrival time
MQG,1/Mplank > 1.19
Most models are optically thin for
33 GeV photon from GRB 090902B
(z=1.822)
“baseline” and “fast evolution” models
are rejected at 3.6 σ level
Disfavors quantum gravity models which
linearly alters the speed of light (n=1)
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Models for HE delayed onset and extra-PL
• Leptonic models (inverse-Compton or SSC) (Toma et al., 2009)
– Hard to produce a delayed onset longer than spike widths
– Hard to produce a low-energy (<50 keV) power-law excess
– Hard to account for the different photon index values of the Band spectrum at low energie
(but photospheric models can) and of the HE component
– But, photospheric models could explain these properties (Toma et al. 2010)
• Hadronic models (pair cascades, proton synchrotron) (Asano et al., 2009)
– GRBs as possible sources of Ultra-High Energy Cosmic Rays
– Late onset: time to accelerate protons & develop cascades?
– Proton synchrotron radiation (requires large B-fields)
– Synchrotron emission from secondary e± pairs produced via photo-hadron interactions
• can naturally explain the power-law at low energies
• require substantially more energy than observed (GRB 090510: Etotal / Eiso ~ 100-1000)
– Hard to produce correlated variability at low- and high-energies (e.g. spikes of GRB 090926A) ?
• Early Afterglow (e+e- synchrotron from external shock) (Kumar et al, 2009)
– Can account for possible delayed (~9 s) onset of power-law component in GRB 090902B
– Short variability time scales in LAT data (e.g. GRB 090926A) argues against external shock
– Requires larger bulk Lorentz factor than measured for GRB 090926A
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Summary of LAT GRBs
Angle
Duration # of events
from
(or class) > 100 MeV
LAT
GRB
Detections as of 090904
# of events
> 1 GeV
Delayed Long-lived
Extra
HE
HE
spectral
onset
emission
comp.
Highest
photon
Energy
Redshift
080825C
~ 60°
long
~ 10
0
?
✔
X
~ 600 MeV
080916C
49°
long
145
14
✔
✔
?
~ 13.2 GeV
081024B
21°
short
~ 10
2
✔
✔
?
3 GeV
081215A
~ 86°
long
—
—
—
—
--
—
090217
~ 34°
long
~ 10
0
X
X
X
~ 1 GeV
090323
~ 55°
long
~ 20
>0
?
✔
?
3.57
090328
~ 64°
long
~ 20
>0
?
✔
?
0.736
090510
~ 14°
short
> 150
> 20
✔
✔
✔
090626
~ 15°
long
~ 20
>0
?
✔
?
090902B
51°
long
> 200
> 30
✔
✔
090926
~ 52°
long
> 150
> 50
✔
091003A
~ 13°
long
~ 20
>0
091031
~ 22°
long
~ 20
100116A
~ 29°
long
~ 10
April 19, 2010
~ 4.35
~ 31 GeV
0.903
✔
~ 33 GeV
1.822
✔
✔
~ 20 GeV
2.1062
?
?
?
>0
?
?
?
~ 1.2 GeV
3
?
?
?
~ 2.2 GeV
Deciphering the Ancient Universe with Gamma-Ray Bursts
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Summary of LAT GRBs
Angle
Duration # of events
from
(or class) > 100 MeV
LAT
GRB
Detections as of 090904
# of events
> 1 GeV
Delayed Long-lived
Extra
HE
HE
spectral
onset
emission
comp.
Highest
photon
Energy
Redshift
080825C
~ 60°
long
~ 10
0
?
✔
X
~ 600 MeV
080916C
49°
long
145
14
✔
✔
?
~ 13.2 GeV
081024B
21°
short
~ 10
2
✔
✔
?
3 GeV
081215A
~ 86°
long
—
—
—
—
--
—
090217
~ 34°
long
~ 10
0
X
X
X
~ 1 GeV
090323
~ 55°
long
~ 20
>0
?
✔
?
3.57
090328
~ 64°
long
~ 20
>0
?
✔
?
0.736
090510
~ 14°
short
> 150
> 20
✔
✔
✔
090626
~ 15°
long
~ 20
>0
?
✔
?
090902B
51°
long
> 200
> 30
✔
✔
090926
~ 52°
long
> 150
> 50
✔
091003A
~ 13°
long
~ 20
>0
?
~ 31 GeV
0.903
✔
~ 33 GeV
1.822
✔
✔
~ 20 GeV
2.1062
?
?
Delayed
onset
and~ long-lived
HE emission
~ 22°
long
20
>0
?
?
is common long
feature~of
LAT GRBs
??
100116A ~ 29°
10
3
?
091031
April 19, 2010
~ 4.35
Deciphering the Ancient Universe with Gamma-Ray Bursts
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?
~ 1.2 GeV
?
~ 2.2 GeV
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Summary of LAT GRBs
Angle
Duration # of events
from
(or class) > 100 MeV
LAT
GRB
Detections as of 090904
# of events
> 1 GeV
Delayed Long-lived
Extra
HE
HE
spectral
onset
emission
comp.
Highest
photon
Energy
Redshift
080825C
~ 60°
long
~ 10
0
?
✔
X
~ 600 MeV
080916C
49°
long
145
14
✔
✔
?
~ 13.2 GeV
081024B
21°
short
~ 10
2
✔
✔
?
3 GeV
081215A
~ 86°
long
—
—
—
—
--
—
090217
~ 34°
long
~ 10
0
X
X
X
~ 1 GeV
090323
~ 55°
long
~ 20
>0
?
✔
?
3.57
090328
~ 64°
long
~ 20
>0
?
✔
?
0.736
090510
~ 14°
short
> 150
> 20
✔
✔
✔
090626
~ 15°
long
~ 20
>0
?
✔
?
090902B
51°
long
> 200
> 30
✔
✔
090926
~ 52°
long
> 150
> 50
✔
091003A
~ 13°
long
~ 20
>0
091031
~ 22°
long
~ 20
100116A
~ 29°
long
~ 10
April 19, 2010
~ 4.35
~ 31 GeV
0.903
✔
~ 33 GeV
1.822
✔
✔
~ 20 GeV
2.1062
?
?
?
>0
?
?
?
~ 1.2 GeV
3
?
?
?
~ 2.2 GeV
Deciphering the Ancient Universe with Gamma-Ray Bursts
0.8969
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Summary of LAT GRBs
Angle
Duration # of events
from
(or class) > 100 MeV
LAT
GRB
Detections as of 090904
# of events
> 1 GeV
Delayed Long-lived
Extra
HE
HE
spectral
onset
emission
comp.
Highest
photon
Energy
Redshift
080825C
~ 60°
long
~ 10
0
?
✔
X
~ 600 MeV
080916C
49°
long
145
14
✔
✔
?
~ 13.2 GeV
081024B
21°
short
~ 10
2
✔
✔
?
3 GeV
081215A
~ 86°
long
—
—
—
—
--
—
090217
~ 34°
long
~ 10
0
X
X
X
~ 1 GeV
090323
~ 55°
long
~ 20
>0
?
✔
?
3.57
090328
~ 64°
long
~ 20
>0
?
✔
?
0.736
090510
~ 14°
short
> 150
> 20
✔
✔
✔
090626
~ 15°
long
~ 20
>0
?
✔
?
090902B
51°
long
> 200
> 30
✔
✔
090926
~ 52°
long
> 150
> 50
✔
091003A
~ 13°
long
~ 20
>0
091031
~ 22°
long
~ 20
100116A
~ 29°
long
~ 10
April 19, 2010
~ 4.35
~ 31 GeV
0.903
✔
~ 33 GeV
1.822
✔
✔
~ 20 GeV
2.1062
?
?
?
>0
?
?
?
~ 1.2 GeV
3
?
?
?
~ 2.2 GeV
Deciphering the Ancient Universe with Gamma-Ray Bursts
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Long vs Short GRBs
Abdo, A. A. et al., ApJ 712, 558 (2010)
Preliminary !
short
short
short
• Comparable LE and HE gamma-ray outputs for short GRBs
• Long GRBs seem to emit ~5-20 times less at HE than at LE w.r.t.
short GRBs
April 19, 2010
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Summary
Fermi detected ~400 GRBs including 17 LAT GRBs in ~1.5 years
=> 250 GRBs/year for GBM and ~10 GRBs/year for LAT
Extra component of the prompt emission ?
-Clear evidence of extra PL component from 3 LAT GRBs
-Low-energy excess is also seen
 What is the maximum energy of HE emission ?
-Constraint lower limit of bulk Lorentz factor: Γmin ~1000
-GRB 090926A, first detection of HE spectral cutoff : Γ ~ 630
 HE emission is delayed and/or long-lived ?
-Many LAT GRBs show delayed and long-lived high energy emission
Many leptonic or hadronic models are proposed for LAT high energy emission
No difference of high energy properties between short and long GRBs
(but lower energy in high energy for long GRBs ?)
Constraint on QG and EBL models
April 19, 2010
Deciphering the Ancient Universe with Gamma-Ray Bursts
20