GeV GRBsGeV GRBs

Gabriele GhiselliniGabriele Ghisellini

With the collaboration of:With the collaboration of:Giancarlo Ghirlanda, Lara Nava, Annalisa Giancarlo Ghirlanda, Lara Nava, Annalisa CelottiCelotti

EGRET – GRB 940217EGRET – GRB 940217

18 GeV18 GeV

1.5 hours1.5 hours

It lasts much It lasts much longerlonger

It starts during the prompt at lower It starts during the prompt at lower energiesenergies

The most energetic photon arrives late The most energetic photon arrives late Prompt or afterglow? Compactness Prompt or afterglow? Compactness argument??argument??

GRB 090510GRB 090510

• ShortShort• Very hardVery hard• z=0.903z=0.903• Detected by the LAT up to 31 GeV!!Detected by the LAT up to 31 GeV!!• Well defined timingWell defined timing

Fermi-L

AT

Fermi-L

AT

0.6s0.5

s

Time since trigger (precursor)Time since trigger (precursor)

precursorprecursor 8-260 keV8-260 keV

0.26-5 0.26-5 MeVMeV

LAT all LAT all

>>100 MeV100 MeV

>>1 GeV1 GeV31 31 GeVGeV

Ab

do e

t al 2009

Ab

do e

t al 2009

Delay between GBM and Delay between GBM and LAT. LAT. Due to Lorentz invariance Due to Lorentz invariance violation?violation?

Different Different componentcomponent

30 GeV0.1 GeV

1

2

3

3

4

AveragAveragee

Time Time resolveresolvedd

0.5-1s0.5-1s

F

(F(

) ) [e

rg/c

m[e

rg/c

m22/s

]

/s]

Energy [keV]Energy [keV]

Ab

do e

t al 2009

Ab

do e

t al 2009

If LAT and GBM radiation are If LAT and GBM radiation are cospatial: cospatial: >1000 to avoid photon->1000 to avoid photon-photon absorption photon absorption

090510090510

ttdecdec ~ 0.4 ~ 0.4

(1+z)(1+z)

(Ek53 /n)(Ek53 /n) 1/31/3

3 3

8/38/3seconsecondsds

Different Different componentcomponent

30 GeV0.1 GeV

1

2

3

3

4

AveragAveragee

Time Time resolveresolvedd

0.5-1s0.5-1s

F

(F(

) ) [e

rg/c

m[e

rg/c

m22/s

]

/s]

Energy [keV]Energy [keV]

Ab

do e

t al 2009

Ab

do e

t al 2009

If LAT and GBM radiation are If LAT and GBM radiation are cospatial: cospatial: >1000 to avoid photon->1000 to avoid photon-photon absorption photon absorption If If >1000: deceleration of the fireball >1000: deceleration of the fireball occurs early occurs early early afterglow! early afterglow! (see also Kumar & Barniol Duran 2009)(see also Kumar & Barniol Duran 2009)

090510090510

No matter the origin of the No matter the origin of the GeV emission, the bulk GeV emission, the bulk Lorentz factor must be largeLorentz factor must be large

Ghirlanda+ Ghirlanda+ 20102010

tt22 tt-1.5-1.5

Fermi-

Fermi-

LATLAT

background levelbackground level

T*=0.6sT*=0.6s

0.1-1 GeV0.1-1 GeV

>1 GeV>1 GeV

T-T* [s]T-T* [s]

Gh

irla

nd

a+

2010

Gh

irla

nd

a+

2010~MeV and ~GeV emission are NOT ~MeV and ~GeV emission are NOT

cospatial. cospatial. But the ~GeV emission is…But the ~GeV emission is… No measurable 0.1-10 GeV delay in No measurable 0.1-10 GeV delay in arrival time: tarrival time: tdelaydelay<0.2 s <0.2 s

Strong limit to quantum gravity Strong limit to quantum gravity

MMQGQG > 4.7 M > 4.7 MPlanckPlanck

GG+ 2010GG+ 2010

LAT GRBsLAT GRBs

GG+ 2010GG+ 2010

LAT GRBsLAT GRBs

backgrounbackgroundd

backgrounbackgroundd

short

z

no z

LogLog

Log

Log

FF

GBMGBM

LAT

BandBand PLPL

Time integrated spectraTime integrated spectra

vs vs

vs vs

LogLog

Log

Log

FF

GBMGBM LAT

-values consistent -values consistent with Zhang+ 2011with Zhang+ 2011

The 8 brightest LAT GRBsThe 8 brightest LAT GRBsz=2, z=2, assumedassumedz=1, z=1, assumedassumedz=2, z=2, assumedassumed

tt--10/710/7

Rad

iative!

Rad

iative!

Rad

iative!

Rad

iative!

Spectrum and Spectrum and decay: afterglow; decay: afterglow;

LLGeVGeV~L~Lbolbol

Spectrum and Spectrum and decay: afterglow; decay: afterglow;

LLGeVGeV~L~Lbolbol

The 4 brightest LAT GRBsThe 4 brightest LAT GRBs

tt--10/710/7

Rad

iative?

Rad

iative?

Rad

iative?

Rad

iative?

The 4 brightest LAT GRBsThe 4 brightest LAT GRBs

e

From Beloborodov From Beloborodov (2002) (2002)

e

e+

e-

e

e+

e-

e

LATLAT

GBMGBM

OptOpt

Time [s] Time [s]

0.1 1 10 102 103

1 10 102 103

1 10 102 103 1 10 102 103

ProbleProblemsmsFast variability of the GeV emission (Abdo+ Fast variability of the GeV emission (Abdo+ 2009)2009)

”…”…the observed large amplitude variability on short timescales the observed large amplitude variability on short timescales (≈90 ms) (≈90 ms) in the LAT data, which is usually attributed to prompt in the LAT data, which is usually attributed to prompt emission, emission, argues against such modelsargues against such models.”.”

Abdo+ 2009, ApJ, 706, L138

FERMI observations of GRB 090902B: a distinct spectral component in the prompt and delayed emission

090902B090902B

5s5s

Counts

per

Counts

per

bin

bin

ProbleProblemsmsFast variability of the GeV emission (Abdo+ Fast variability of the GeV emission (Abdo+ 2009). 2009). No evidenceNo evidence

Simultaneous GBM-LAT spikes (Ackermann+ Simultaneous GBM-LAT spikes (Ackermann+ 2011; Zhang+ 20112011; Zhang+ 2011

Ack

erm

ann+

A

ckerm

ann+

201

1201

1

090926A090926A

e+

e-

e

ProbleProblemsmsFast variability of the GeV emission (Abdo+ Fast variability of the GeV emission (Abdo+ 2009). 2009). No evidenceNo evidence

Simultaneous GBM-LAT spikes (Ackermann+ Simultaneous GBM-LAT spikes (Ackermann+ 2011; Zhang+ 2011 2011; Zhang+ 2011 EC scattering of prompt EC scattering of prompt photons? Numbers are okphotons? Numbers are ok

LAT spectra on the extrapolation of GBM LAT spectra on the extrapolation of GBM spectra (Zhang+ 2011; with exceptions) spectra (Zhang+ 2011; with exceptions) if fitted if fitted together (but LAT emission lasts longer…)together (but LAT emission lasts longer…)

Highest energy photons that arrive after the Highest energy photons that arrive after the peak of the LAT light curve are too energetic to peak of the LAT light curve are too energetic to be synchro(Piran & Nakar 2010).be synchro(Piran & Nakar 2010).

GG+ 2010GG+ 2010

LAT GRBsLAT GRBs

13 GeV13 GeV

33 GeV33 GeV

ProbleProblemsmsFast variability of the GeV emission (Abdo+ Fast variability of the GeV emission (Abdo+ 2009). 2009). No evidenceNo evidence

Simultaneous GBM-LAT spikes (Ackermann+ Simultaneous GBM-LAT spikes (Ackermann+ 2011; Zhang+ 2011 2011; Zhang+ 2011 EC scattering of prompt EC scattering of prompt photons? Numbers are okphotons? Numbers are ok

LAT spectra on the extrapolation of GBM LAT spectra on the extrapolation of GBM spectra (Zhang+ 2011; with exceptions) spectra (Zhang+ 2011; with exceptions) if fitted if fitted together (but LAT emission lasts longer…)together (but LAT emission lasts longer…)

Highest energy photons that arrive after the Highest energy photons that arrive after the peak of the LAT light curve are too energetic to peak of the LAT light curve are too energetic to be synchro (Piran & Nakar 2010). be synchro (Piran & Nakar 2010). Very few, Very few, possible additional component (SSC)?possible additional component (SSC)?

Bulk Lorentz Bulk Lorentz factorsfactors

=2000=2000

= 630= 630= 670= 670

= 900= 900

ttdecdec ~ 420 (1+z) ~ 420 (1+z) (Ek54 /n)(Ek54 /n) 1/31/3

2 2

8/38/3secondsecondss

A factor ~10A factor ~1033 dimmer in luminosity, but if dimmer in luminosity, but if nearby…nearby…

GeV detected GRBS could be the ones with the GeV detected GRBS could be the ones with the largest Lorentz factors… For smaller largest Lorentz factors… For smaller ……

If pair enrichment is required, GeV detected If pair enrichment is required, GeV detected GRBs could be the ones with GRBs could be the ones with

EEpeakpeak(1+z)>m(1+z)>meecc22

If EIf Epeakpeak < 511 keV and t < 511 keV and t-1-1: adiabatic because : adiabatic because

of no pairsof no pairs

Ghirlanda 2009

090510905100

511 keV511 keV

ConclusionsConclusions

• GeV preferentially in EGeV preferentially in Epeakpeak>511 keV >511 keV

GRBs GRBs

• GeV when GeV when is large is large early onset of early onset of the afterglow the afterglow very bright very bright

• Large ELarge EAftAft: helps to understand : helps to understand

EEpromptprompt/E/EAftAft

Internal shocks: relative kinetic energy of the shellsInternal shocks: relative kinetic energy of the shells

External shocks: entire kinetic energy of the fireballExternal shocks: entire kinetic energy of the fireball

Afterglows should be more energetic than the Afterglows should be more energetic than the promptprompt

Different Different componentcomponent

30 GeV0.1 GeV

1

2

3

3

4

AveragAveragee

Time Time resolveresolvedd

0.5-1s0.5-1s

F

(F(

) ) [e

rg/c

m[e

rg/c

m22/s

]

/s]

Energy [keV]Energy [keV]

Ab

do e

t al 2009

Ab

do e

t al 2009

If LAT and GBM radiation are If LAT and GBM radiation are cospatial: cospatial: >1000 to avoid photon->1000 to avoid photon-photon absorption photon absorption If If >1000: deceleration of the fireball >1000: deceleration of the fireball occurs early occurs early early afterglow! early afterglow!

If If >1000: large electron energies >1000: large electron energies synchrotron afterglow!synchrotron afterglow!

EEafterglowafterglow < E < Epromptprompt

EE afterg

low

afterg

low ~

0.1 E

~ 0.1

E prompt

prompt

Willingale+ 2007Willingale+ 2007

X-ray and optical X-ray and optical often often behave behave differentlydifferently

Late Late

prompt?

prompt?

X-ray

opticaloptical

We expected the opposite, if the We expected the opposite, if the efficiency of prompt is ~ 0.1.efficiency of prompt is ~ 0.1.

Why is the afterglow so faint?Why is the afterglow so faint?

Can it be hidden in some Can it be hidden in some “unexplored” frequency range, i.e. “unexplored” frequency range, i.e. GeV-TeV?GeV-TeV?

EE aft aft ~ E~ E prompt

prompt/10/10

Willingale+ 2007Willingale+ 2007

In GRB 080916C (Abdo et al. 2009a), there is evidence that the spectrum from 8 keV to 10 GeV can be described by the same Band function (i.e. two smoothly connected power laws), suggesting that the LAT flux has the same origin of the low energy flux.On the other hand, the flux level of the LAT emission, its spectrum and its long lasting nature match the expectations from a forward shock, leading Kumar & Barniol–Duran (2009) to prefer the “standard afterglow” interpretation (see also Razzaque, Dermer & Finke 2009 for an hadronic model; Zhang & Peer 2009 for a magnetically dominated fireball

model and Zou et al. 2009 for a synchrotron self–Compton origin).

In the short bursts GRB 090510 the spectrum in the LAT energy range is not the extrapolation of the flux from lower energies, but is harder, leading Abdo et al. (2009b) to propose a synchrotron self–Compton interpretation for its origin. Instead we (Ghirlanda, Ghisellini & Nava 2009) proposed that the LAT flux is afterglow synchrotron emission, on the basis of its time profile and spectrum (see also Gao et al. 2009; De Pasquale et al. 2009).

Finally, the LAT flux of GRB 090902B decays as t−1.5 (Abdo et al. 2009c), it lasts longer than the flux detected by the GBM, and its spectrum is harder than the extrapolation from lower frequen- cies, making it a good candidate for an afterglow interpretation, despite the arguments against put forward by Abdo et al. (2009c), that we will discuss in this paper. Moreover, in GRB 090902B there is evidence of a soft excess (observed in the GBM spectrum below 50 keV) which is spectrally consistent with the extrapolation at these energies of the LAT spectrum.

InterpretationsInterpretations

Ackermann Ackermann 2010: 090510 coincidental peaks in GBM and LAT. SSC code to explain LAT: disfavored, afterglow has less problems. Confusing. Too many indices.De Pasquale De Pasquale 2010 : 090510 Curva di luce e confronto con SwiftAckermann Ackermann 2011: 090926A: break a 1.4 GeV. Confusione sugli alpha del solo LAT: ripido nel time integrated (come noi) e piattozzo nel time resolved. The delay timescale of the extra spectral component would correspond to the time needed for the forward shock to sweep up material and brighten (Kumar & Barniol Duran 2009; Ghisellini et al. 2010; Razzaque 2010). The rapid variability observed in GRB 090926A is contrary to expectations from an external shock model, unless it is produced by emission from a small portion of the blast wave within the Doppler beaming cone. This could occur, for instance, if the external medium is clumpy on length scale ≈Γf cΔT /(1 + z) ≃ 1012 (Γf /103 )(ΔT /0.2 s) cm, where Γf is the Lorentz factor of the forward shock and ΔT is the pulse duration (Dermer & Mitman 1999; Dermer 2008).Cenko 2010: Cenko 2010: analysis of afterglows of a few LAT bursts. Ioka 2010 Ioka 2010 fa tutto, ma non ho capito nente…Kumar- Barniol Duran 2010: Kumar- Barniol Duran 2010: fanno LAT e resto dell’afterglow, con closure relations… + calcolo del flusso external shock a 100 MeV + confronto 100 MeV / X-ray e ottico. Tutto adiabatico. B molto molto piccolo (1e-5). Dicono che se fosse radiative si sballerebbe l’X early.

Kumar Barniol Duran 2009: Kumar Barniol Duran 2009: I primi a dire external shock. Il lavoro e’ complicato. B~2e-5 Gauss, non ho capito perche’.Larsson+ 2011: Larsson+ 2011: There have been many suggestions for the origin of the extra component, including external shocks (Ghisellini et al. 2010; Kumar & Barniol Duran 2010), hadronic processes (Asano et al. 2009; Razzaque 2010), Compton upscattering of a photospheric component (Toma et al. 2010) as well as a combination of different emission mechanisms (Pe’er et al. 2010).Liu 2010: Liu 2010: A partially radiative blast wave model, which though is able to produce a sufficiently steep decay slope, can not explain the broadband data of GRB 090902B. The two-component jet model can.Maxham, Zhang 2011Maxham, Zhang 2011: Detailed modelling adia/radia: Fit is good only after the peak. I think they do not include pairs. In any case fit is reasonably good, even if not perfect.McBreen+ 2010: McBreen+ 2010: GROND data for 4 LAT: they go on Amati, but not on Ghirlanda (no jet break or too late) Toma+ 2010: Toma+ 2010: photospheric emission scattered by relat. e- in internal shocks (ma come fanno a farla durare piu’ del prompt? E poi anche loo dicno che ci sono problemi nella parte a bassa energia, piu’soft di un BB ma piu’ hard di un sincro coolato).

Wang+ 2010: Wang+ 2010: importance of KN: at early times suppresses the IC cooling, at later times it becomes more important synchro decays faster because at late times it competes with IC.Zhang+ 2011Zhang+ 2011: strongly favors internal origin: time resolved GBM+LAT fits yield a single component (LAT on the extrapolation of beta). If LAT data are fitted separately, the slopes are all consistent with us within the errors (that they do not give…)