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GRB emission models and jet properties. Gabriele Ghisellini INAF-Osservatorio Astronomico di Brera - Italy. with the help of: Z. Bosniak, D. Burlon, A. Celotti, C. Firmani, G. Ghirlanda, D. Lazzati, M. Nardini, L. Nava, F. Tavecchio. The “standard” model. “The” model: Internal/External Shocks.

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grb emission models and jet properties
GRB emission modelsand jet properties

Gabriele Ghisellini

INAF-Osservatorio Astronomico di Brera - Italy

with the help of: Z. Bosniak, D. Burlon, A. Celotti, C. Firmani, G. Ghirlanda, D. Lazzati, M. Nardini, L. Nava, F. Tavecchio

the model internal external shocks
“The” model:Internal/External Shocks

Rees-Meszaros-Piran

Shell still opaque

Relativ. e- + B: synchrotron??

Relativ. e- + B: synchrotron

spectra
SpectraSpectra

na

nb

E F(E)

Fishman & Meegan 1995

Epeak

Energy [MeV]

slide6
Synchrotron g-ray emission?

The shell itself carries B-field

B can also be produced and amplified by the internal shock

The shock can accelerate e- to relativistic energies

Synchrotron seems a good choice

hnsyn ~ a few hundreds keV seems reasonable

slide7
Synchrotron g-ray emission?

ee (G/100)

3

tcool ~ 10-7

sec

2

nMeV

Radiative cooling is (and must be) very rapid

  • Extremely short - No way to make it longer
  • tcool << tdynamical ~ 10-2 sec
  • It must be short: if not, how can the flux vary?
slide8
Energy spectrum of a cooling electron

Fast cooling + synchro:

E(n) n-1/2

N(n) n-3/2

Photon index a

slide10
Kaneko+ 2006

Nava PhD thesis 2009

Line of death for cooling e-

Line of death for non cooling e-

seeking alternatives
Seeking alternatives…
  • Bulk Compton (Lazzati+ 2000; GG+ 2000)
  • Quasi-thermal Comptonization (GG & Celotti 1999)
  • Black-body from “deep impacts” (Thompson+2007; GG+ 2007; Lazzati+ 2009)
  • Reconnection and continuous heating (Giannios 2008)
seeking alternatives1
Seeking alternatives…
  • Bulk Compton (Lazzati+ 2000; GG+ 2000)
  • Quasi-thermal Comptonization (GG & Celotti 1999)
  • Black-body from “deep impacts” (Thompson+2007; GG+ 2007; Lazzati+ 2009)
  • Reconnection and continuous heating (Giannios 2008)
thompson meszaros rees 2007 lazzati morsony begelman 2008 gg 2007
“Deep impacts”Thompson, Meszaros & Rees 2007Lazzati, Morsony Begelman 2008GG+ 2007

At R ~ Rstar the fireball dissipates part of its energy  BB

slide14
Dissipation

Wolf Rayet (about to explode)

BB

Transparency

G~1/qj (i.e very small)

problems
Problems
  • G fine tuned (and small) in Thompson+ 2008
  • BB??
slide16
There can be a Black Body … but

BATSE

Time integrated spectrum

BB

Time resolved spectra

power law

Cut off pl

The same occurs for ALL GRBs detected by BATSE and with WFC

30 keV

Ghirlanda+ 2007

slide17
FERMI-GBM

Cutoff PL

Ghirlanda+ 2009

30 keV

slide18
FERMI-GBM

BB+PL

Ghirlanda+ 2009

30 keV

slide19
FERMI-GBM

Cutoff PL

Ghirlanda+ 2009

slide20
FERMI-GBM

BB+PL

The same occurs for all time slices

Ghirlanda+ 2009

first conclusion
First conclusion
  • We do not know yet what is the emission mechanism of the prompt
gev emission
GeV emission

Prompt or afterglow?

slide23
All LAT GRBs

GG+ 2009

slide25
a

b

G

Log nFn

GBM

LAT

Log n

slide26
a

G

b

Log nFn

Log n

LAT

GBM

b vs G

a vs G

G

slide27
The 4 brightest LAT GRBs

t-10/7

Spectrum and decay: afterglow

Radiative!

slide28
The 4 brightest LAT GRBs

t-10/7

Radiative?

slide31
e

e-

e+

slide32
e

e-

e+

slide33
Time Time
grb 090510
GRB 090510

Fermi-LAT

  • Short
  • Very hard
  • z=0.903
  • Detected by the LAT up to 31 GeV!!
  • Well defined timing
  • Delay: ~GeV arrive after ~MeV (fraction of seconds)
  • Quantum Gravity? Violation of Lorentz invariance?
slide35
precursor

8-260 keV

0.26-5 MeV

Delay between GBM and LAT

Due to Lorentz invariance violation?

LAT all

Abdo et al 2009

>100 MeV

>1 GeV

31 GeV

0.6s

0.5s

Time since trigger (precursor)

slide36
0.1 GeV

30 GeV

Average

Different component

Time resolved

0.5-1s

2

3

nF(n) [erg/cm2/s]

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

3

4

Abdo et al 2009

1

If G>1000: deceleration of the fireball occurs early  early afterglow!

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

Energy [keV]

slide37
Fermi-LAT

t2

t-1.5

Ghirlanda+ 2009

slide38
0.1-1 GeV

Ghirlanda+ 2009

>1 GeV

T-T* [s]

slide39
~MeV and ~GeV emission are NOT cospatial. But the ~GeV emission is… No measurable delay in arrival time of high energy photons: tdelay<0.2 s 

Strong limit to quantum gravity 

MQG > 4.7 MPlanck

Ghirlanda+ 2009

T-T* [s]

conclusions
Conclusions

“Paradigm”: internal+external shocks, synchrotron for both: it does not work

Fermi/LAT detection  large G Early high energy (and powerful) afterglow

Decay suggests radiative afterglows

GRB 090510: Violation of the Lorentz invariance? No (not yet)

why internal shocks
Why internal shocks?

A process that repeats itself

Spikes have same duration

Counts/s

Time [seconds]

slide45
Nava PhD thesis 2009

Kaneko+ 2006

[keV]

can it be rescued by
Can it be rescued by:
  • Reacceleration?No, in IS e- are accelerated only once. More generally, only few selected e- (and always the same) must be (re)accelerated
  • Adiabatic losses?No, too small regions would be involved, large e- densities, too much IC
  • Fast decaying B-field?No, synchro not efficient and too much IC
  • Self absorption?No, lots of e- needed, too much IC
  • Self Compton?No, tcool too small even in this case
  • Small pitch angles? No,Very very small to avoid cooling, becomes inefficient
  • External shocks to avoid cooling? No,same reason
slide48
Bulk Compton

Seeds n

Wolf Rayet (about to explode)

G2n

G~100

problems1
Problems
  • Time to refill the funnel of seeds
  • Needs a lot of seeds, not clear if the funnel is sufficient
  • Does not work for short (they do not have a SN)
quasi thermal componization
Quasi thermal Componization
  • Heating for r/c, not instantaneous acceleration
  • Cooling=Heating sub-relat. “T”
  • Synchro is self absorbed and produces seeds for quasi saturated Comptonization
  • y needs to be >10
problems2
Problems
  • Epeak ~ G kT too high
  • Needs time (t must be large)
  • Wien peak not observed
  • Seeds should be distributed in the center, e- more externally, to explain a >-1
reconnection and continuous heating giannios 2008
Reconnection and continuous heating (Giannios 2008)

1 MeV

Very promising, especially for magnetized fireballs, but….

slide56
Rate

Ghirlanda+ 2009

Epeak =k L1/2

Epeak [keV]

FERMI-GBM

Luminosity [erg/s]

how to explain it
How to explain it?

It is NOT DUE to selection effects!!!!!!

It indicates something fundamental and very robust… that we do not understand yet.

Geometrical? Difficult

Sequence of G ? Difficult

Radiation process? Likely, but which one?

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