The high energy
GRBs: lessons
learned from Fermi
Elena Moretti

KTH and OKC
Stockholm
On behalf of the Fermi GBM
and LAT teams

Photospheric emission in BATSE bursts
CGRO BATSE ERA (1994-2000)

Spectra from temporally resolved pulses observed by BATSE over theenergy range 20-2000 keV.
Spectral fit: Black body combined with a power law:

Photosphere (Planck function)

Additional non-thermal emission
Band onlyBB+pl

Ryde 2005
Ryde 2004 (see also Ghirlanda et al. 2003)

EGRET TASC peak at Ep = 1600 keV

High-Energy Emissions from GRB (Past)
Gonzalez, Nature 2003 424, 749

5 EGRET bursts with >50 MeV
observations in 7 years●●EGRET observed:- delayed HE gamma-ray emissions;- spectral extra component;

Hurley et al. 1994

... and the X-ray Afterglow● Discovered by BeppoSax (‘97)– Measurements of the distance● Swift (2004-*):– Connection to the “Prompt” emission– X-Ray Flashes in the afterglow– Steep-Shallow-Steep decay– Also short bursts have an afterglow!
– Fading to lower frequencies

4

The LAT and GBM on Fermi
The GBM detects ~250 GRBs/year~18% short~50% in the LAT FoV
The LAT detects ~10 GRBs/year
NaI: 8 keV - 1 MeV
BGO: 200 keV - 40 MeV
LAT: 30 MeV – 300 GeV

GBM NaI
GBM
BGO

LAT

The Large Area Telescope
Si Tracker

γpitch = 228 μm
8.8 105 channels
18 planes

ACDsegmented
scintillator tiles

e+ eCsI Calorimeterhodoscopic array (8 layers)
6.1 103 channels

LAT: 4 x 4 modular array
3000 kg, 650 W
20 MeV – 300 GeV
3

GBM GRBs

7

The prompt spectrum• Band model is favorite only for asubset of bursts, while COMPT andPL are the most favorite;

Goldstein et al, 2012

Additional “BlackBody” component overa Band function
improves the residualsof the fit.
8Guiriec et al 2011, ApJL 727,L33

Extra HE spectral component
GRB 090510 (short)
Abdo, A. A. et al., ApJ 716, 1178A (2010)

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 ofthe PL component has been seen

6 LAT GRBs show clear extra PL component

9

Cut-off on HE spectral component
GRB 090926A (long)
Ackermann et. al. 2011, ApJ 729, 114A

8-14.3keV

14.3-260 keV

0.26-5 MeV

LAT all event>100 MeV>1GeV- Extra component shows at >5 σ
- spectral break at ~1.4 GeV- First direct measurement ofΓ ~ 630 (if cutoff due to γ-γ 10absorption)

Joint LAT GBM spectral analysis
Ackermann et. al. 2013, ApJS 209, 11A● GRB spectrum in several cases

is NOT a simple “Band”
function● Deviation from the Band
function at low energy;● Additional power-law observed
at high energy;● High energy cut-off measured

in the spectrum;11Non-detected LAT GRB

Bright GBM/BGO GRBs, nondetected in the LAT:➔

the fux “expected”(extrapolated) exeedes theLAT fux UL;

Ackermann et. al. 2012, ApJ 754, 121F

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Non-detected LAT GRB

Bright GBM/BGO GRBs, nondetected in the LAT:➔the fux “expected”(extrapolated) exeedes theLAT fux UL;➔an intrinsic spectral cut off isrequired to reconcile the GBM
and LAT data.

13Ackermann et. al. 2012, ApJ 754, 121F

Non-detected LAT GRB
Bright GBM/BGO GRBs, non detected in the LAT:➔

the fux “expected” (extrapolated) exeedes the LAT fux UL;➔

an intrinsic spectral cut off is required to reconcile the GBM and LATdata.

14Ackermann et. al. 2012, ApJ 754, 121F

Non-detected LAT GRB
Bright GBM/BGO GRBs, non detected in the LAT:➔

It is possible to estimate the bulk Lorentz factor if the cut off isdue to γγ absorption.

15Ackermann et. al. 2012, ApJ 754, 121F

Delayed Onset
GRB 090510 (short)
Abdo et al. 2009, Nature 462, 331

GRB 080916C (long)8-260keV

Abdo et al. 2009, Science 323, 1688

0.26-5MeV
LAT all events>100 MeV>1GeV

Delay: ~0.5s

Delay: ~5s

Almost all GRBs show a delayed onset of the HE component!!!

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Prompt and temporally extended emission
GRB 090926A (long)• Clear onset of the highenergy• Spectral evolution in theprompt phase– Spectral index stable atlater times• Highest event notcoincident with lower
energy pulses• Time extended emission
clearly visible

Ackermann et. al. 2013, ApJS 209, 11A

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Prompt and temporally extended emission‣ The Spectral index is stable at later times and has very similarvalue in many GRBs of ~ -2.

Three time windows:● GBM;● LAT;● EXT;

Ackermann et. al. 2013, ApJS 209, 11A

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Temporally extended emission‣ High-energy emission (observed by the LAT) starts later andlasts longer then the low-energy emission (observed by theGBM).
‣ “Delayed onset” and “Temporally extended” emission‣ In three cases a significant (3σ) break is measured in theLight curve

LATGBM

α ~ 1.7-2.7
PROMPT

TEMPORALLYEXTENDED

19Ackermann et. al. 2013, ApJS 209, 11A

Ground telescope possible catches

20Ackermann et. al. 2013, ApJS 209, 11A

Long lived HE component
GRB 090510 (short GRB)De Pasquale et al., ApJL 709, 146 (2010)

t(-1.38±0.07)

UVOT XRT

Fermi/LATLAT emission until 200 s
No spectral evolution(photon index -2.1 ± 0.1)

Simultaneous fitof the SED at 5 different times• Forward shock model can
reproduce the spectrum fromthe optical up to GeV energies• Extensions needed to arrange
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the temporal properties
Several GRBs have been detected simultaneusly from Fermi and Swift

HE fuence

Hyper luminous GRBs

- Brightest GBM bursts,
are also the Brightest
LAT bursts- Large dispersion
Class of hyper luminous
bursts- statistical fuctuation?

Fermi LAT GRB Catalog (arXiv:1303.2908v1)

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Intrinsic energetic• The brightest GRBs
are also the mostenergetic GRBs (notthe closest)• In the tail of the Eiso
distribution

Ackermann et. al. 2013, ApJS 209, 11A

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Conclusions• Fermi has made new interesting observations on GRB:
– Prompt emission observed over a wider energy range:• Band model is no longer the best phenomenologicalmodel.
• More complex spectral shapes are needed to reproduce
the spectrum- High-energy emission not common in GRBs
- Long lasting-delayed high-energy emission common in LAT
detected GRB
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Thank you!

LAT detection during X-ray fareactivity
GRB100728A:★Fermi/GBM: Very bright burst:

GRB100728A✴ S (10-1000 keV) ~ 1.3 x 10-4
erg/cm2/s → Fermi ARR★Swift/BAT: T90~200 s, faint emission
seen up to ~750 s★Swift/XRT: 8 bright fares (from ~150 sto ~850 s)★Fermi LAT:
✴ No detection during the prompt
phase (large incident angle ~ 58o)
✴ Significant detection during thefaring activity (TS=32)✴ No significant temporal correlation(which does not mean significant
non correlation!)

GRB100728A
A. A. Abdo et al. 2011 ApJ 734 L27

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Simultaneous Swift detections• 6 GRBs have been simultaneously detected by LAT and Fermi– GRB090510 [de Pasquale et al 2010 +...]– GRB100728A [Fermi Collaboration (Abdo et al ApJ 2011)]– GRB110625A [Tam, Kong and Fan, ApJ 2012]– GRB110731A [Fermi Collaboration (Ackermann et al 2013)]– GRB 120624B [GCN]
GRB 090510– GRB 130427A [GCN, Fan et al. arXiv:1305.126]
GRB110731A• Picture consistent with afterglow model...

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Future Direction of relativistic jets - Skokloster

Nicola Omodei – Stanford/KIPAC

The “fireball” model

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Alternatives exists (electromagnetic model,...)

Properties: Γmin calculationcompactness problem: large luminosity + small emitting region = large
optical depth (γ-γ -> e+e- large)
Possible solution: relativistic motion (Γ>>1)

Γmin calculation from highest energy photon

Γmin090510
E=31 GeV090902B
E=33 GeVz080916C
E=3 GeV29

Γmin~1000 for short and long GRBs

Quantum gravity mass constrain
A Constraint on the quantum
gravity mass (MQG) can be derived
by direct measurement of photon
arrival time (assuming theemitted time is the same for allphotons):

MQG,1/Mplank > 1.19
This value disfavors quantum
gravity models which linearly
alters the speed of light (n=1)

GRB 090510

Abdo et al. 2009, Nature 462, 331
GBM NaI

GBM BGOLAT (>1MeV)

31 GeV

0.83 s30

And then GRB130427A
happen...

BeforeAfter• The brightest GRB in the LAT ever detected;• More than 80 circulars delivered to the archive from several
observatories:– GCN from the “usual suspects” + HAWC + IceCube• Concept proven! Discoveries rely on the fast delivery ofinformations (GCN) quick look analysis and possible data
sharing.

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Extremely bright GRB(close)•One of the brightest GRBs ingamma rays ever detected!
– Redshift: z = 0.34, Energy
released in gamma rays ~ 1054
erg
– The emission saturated GBM
detectors!
– The brightest burst everdetected by the LAT•LAT detected emission for ~20hours!

95 GeV32