Fermi Observations of Gamma-ray Bursts

Fermi Observations of Gamma-ray Bursts

Masanori Ohno(ISAS/JAXA)

on behalf of Fermi LAT/GBM collaborations

April 19, 2010 1Deciphering the Ancient Universe with Gamma-Ray Bursts

HE emission from GRBs : Pre-Fermi EraHE emission from GRBs : Pre-Fermi Era

GRB940217(Hurley et al. 94)

-18 to 14 sec

14 to 47 sec

47 to 80 sec

80-113 sec

113-211 sec

GRB941017 (Gonzaletz et al. 03)

GeV photons up to 90min after the trigger

Temporary distinct HE spectral component


- 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 mechanismand environment of GRBs

Need more sensitivity, larger FoV

Fermi Gamma-ray Space TelescopeFermi Gamma-ray Space Telescope

GGamma-ray amma-ray BBursturst MMonitor (onitor (GBMGBM)) 12 NaI detectors (8keV-1MeV) - onboard trigger , localization - spectroscopy 2 BGO detectors (150keV-40MeV) - spectroscopy (overlapping LAT

band)

LATLATSilicon-Strip detectors - Identification &direction measurement of γ-raysCsI calolimetor - Energy measurementACD (plastic scintillators) - background rejection

-Efficient observing mode-Wide FoV-Low deadtime-Large effective area-Good angular resolution-Energy coverage

More photonsfrom Many GRBs


Fermi GRBs Fermi GRBs

April 19, 2010 4

Detections as of 090904

LAT FoV

GBM FoV

• 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

Deciphering the Ancient Universe with Gamma-Ray Bursts

What we have seen from Fermi GRB observations

1. Extra component of the prompt emission ?1. Extra component of the prompt emission ?Different emission mechanism: Synchrotron self Compton ? Hadronic origin ? GRB941017 shows the sign of extra component


Extra PL component in short and long GRBs

Abdo, A. A. et al., ApJL 706, 138 (2009)Abdo, A. A. et al., ApJ submitted

GRB 090902B (long)GRB 090510 (short)

• First time a low-energy extension of the PL component has been seen

April 19, 2010 6

3 LAT GRBs shows extra PL component(090510, 090902B, 090926A)

First extra component by FermiAt > 5 sigma level


T0+4.6s to T0+9.6s

2. What is the maximum energy of HE emission ?2. What is the maximum energy of HE emission ?

Constrain the bulk Lorentz factor of the relativistic jetNo evidence of the cut-off so far.



Limit on bulk Lorentz factorLimit 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

April 19, 2010 8

Γmin~1000 for short and long GRBs

z

Γmin


090510E=31 GeV

090902BE=33 GeV

080916CE=3 GeV

GRB 090926A: the first HE spectral cutoff

Preliminary !

- 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)

April 19, 2010 9

8-14.3keV

14.3-260 keV

0.26-5 MeV

LAT all event

>100 MeV

>1GeV


Time-integrated photon spectrum(3.3-21.6s)

νFν(

erg

/cm

2/s

)

Energy (keV)10 102 103 104 105 106

(See Uehara’s poster #095)

3. HE emission is delayed and/or long-lived ?3. HE emission is delayed and/or long-lived ?Suggests another emission mechanismA few GRBs show delayed high energy emission (GRB940217, GRB080714)



Long-lived GeV emission ~ Swift and Fermi view of GRB 090510 ~

De Pasquale et al., ApJL 709, 146 (2010)

• Forward shock model can reproduce the spectrum from the optical up to GeV energies• Extensions needed to arrange the temporal properties

t1.380.07

Simultaneous fitof the SED at 5 different times

LAT emission until 200 sNo spectral evolution(photon index -2.1 ± 0.1)

April 19, 2010 11

GRB 090510 (short GRB)UVOT XRT Fermi/LAT


HE delayed onset in short and long GRBs

The first few GBM peaks are missing in the LAT but later peaks coincideDelay in HE onset: 0.1-0.2 s

Abdo et al. 2009, Science 323, 1688

The first LAT peak coincides with thesecond GBM peakDelay in HE onset: ~4-5 s

Abdo et al. 2009, Nature 462, 331

GRB 080916C (long)GRB 090510 (short)

HE delayed onset can be seen from almost all LAT GRBsApril 19, 2010 12

8-260keV

0.26-5MeV

LAT all events

>100 MeV

>1GeV


Constraint on QG and EBL models


Constraints on the quantum gravity mass (MQG) by direct measurement of photon arrival time

MQG,1/Mplank > 1.19

Disfavors quantum gravity models which linearly alters the speed of light (n=1)

Most models are optically thin for33 GeV photon from GRB 090902B(z=1.822)

“baseline” and “fast evolution” modelsare rejected at 3.6 σ level

Abdo et al. 2009, Nature 462, 331GRB 090510 GRB 090902B

Abdo, A. A. et al., ApJL 706, 138 (2009)

31 GeV

GBM NaI

GBM BGO

LAT (>1MeV)

0.83 s

• 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

Models for HE delayed onset and extra-PL



Summary of LAT GRBs


GRBAngle from LAT

Duration(or class)

# of events> 100 MeV

# of events > 1 GeV

Delayed HE

onset

Long-lived HE

emission

Extra spectral comp.

Highest photon Energy

Redshift

080825C ~ 60° long ~ 10 0 ? ✔ X ~ 600 MeV

080916C 49° long 145 14 ✔ ✔ ? ~ 13.2 GeV ~ 4.35

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 ✔ ✔ ✔ ~ 31 GeV 0.903

090626 ~ 15° long ~ 20 > 0 ? ✔ ?

090902B 51° long > 200 > 30 ✔ ✔ ✔ ~ 33 GeV 1.822

090926 ~ 52° long > 150 > 50 ✔ ✔ ✔ ~ 20 GeV 2.1062

091003A ~ 13° long ~ 20 > 0 ? ? ? 0.8969

091031 ~ 22° long ~ 20 > 0 ? ? ? ~ 1.2 GeV

100116A ~ 29° long ~ 10 3 ? ? ? ~ 2.2 GeV



Summary of LAT GRBs


GRBAngle from LAT

Duration(or class)


# of events > 1 GeV

Delayed HE

onset

Long-lived HE

emission



Redshift

080825C ~ 60° long ~ 10 0 ? ✔ X ~ 600 MeV

080916C 49° long 145 14 ✔ ✔ ? ~ 13.2 GeV ~ 4.35

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 ✔ ✔ ✔ ~ 31 GeV 0.903

090626 ~ 15° long ~ 20 > 0 ? ✔ ?

090902B 51° long > 200 > 30 ✔ ✔ ✔ ~ 33 GeV 1.822

090926 ~ 52° long > 150 > 50 ✔ ✔ ✔ ~ 20 GeV 2.1062

091003A ~ 13° long ~ 20 > 0 ? ? ? 0.8969

091031 ~ 22° long ~ 20 > 0 ? ? ? ~ 1.2 GeV

100116A ~ 29° long ~ 10 3 ? ? ? ~ 2.2 GeV


Delayed onset and long-lived HE emission is common feature of LAT GRBs ?


Summary of LAT GRBs


GRBAngle from LAT

Duration(or class)


# of events > 1 GeV

Delayed HE

onset

Long-lived HE

emission



Redshift

080825C ~ 60° long ~ 10 0 ? ✔ X ~ 600 MeV

080916C 49° long 145 14 ✔ ✔ ? ~ 13.2 GeV ~ 4.35

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 ✔ ✔ ✔ ~ 31 GeV 0.903

090626 ~ 15° long ~ 20 > 0 ? ✔ ?

090902B 51° long > 200 > 30 ✔ ✔ ✔ ~ 33 GeV 1.822

090926 ~ 52° long > 150 > 50 ✔ ✔ ✔ ~ 20 GeV 2.1062

091003A ~ 13° long ~ 20 > 0 ? ? ? 0.8969

091031 ~ 22° long ~ 20 > 0 ? ? ? ~ 1.2 GeV

100116A ~ 29° long ~ 10 3 ? ? ? ~ 2.2 GeV


Long vs Short GRBs

April 19, 2010 19

• 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

short

Abdo, A. A. et al., ApJ 712, 558 (2010)

Preliminary !


short

short

SummarySummary

Fermi detected ~400 GRBs including 17 LAT GRBs in ~1.5 years => 250 GRBs/year for GBM and ~10 GRBs/year for LAT

April 19, 2010 20

Extra component of the prompt emission ?Extra component of the prompt emission ?

What is the maximum energy of HE emission ?What is the maximum energy of HE emission ?

HE emission is delayed and/or long-lived ?HE emission is delayed and/or long-lived ?

-Clear evidence of extra PL component from 3 LAT GRBs-Low-energy excess is also seen

-Constraint lower limit of bulk Lorentz factor: Γmin ~1000-GRB 090926A, first detection of HE spectral cutoff : Γ ~ 630

-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

Fermi Observations of Gamma-ray Bursts

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