Transcript ppt

GeV GRBs
Gabriele Ghisellini
With the collaboration of:
Giancarlo Ghirlanda, Lara Nava, Annalisa Celotti
EGRET – GRB 940217
It starts during the prompt at lower energies
It lasts much longer
The most energetic photon arrives late
Prompt or afterglow? Compactness argument??
1.5 hours
18 GeV
GRB 090510
•
•
•
•
•
Short
Very hard
z=0.903
Detected by the LAT up to 31 GeV!!
Well defined timing
precursor
8-260 keV
0.26-5 MeV
Delay between GBM and LAT.
LAT all
>100 MeV
0.6s
0.5s
31 GeV
Time since trigger (precursor)
>1 GeV
Abdo et al 2009
Due to Lorentz invariance violation?
0.1 GeV
30 GeV
090510
Time
resolved
2
Different component
3
0.5-1s
3
If LAT and GBM radiation are cospatial:
4
G>1000 to1 avoid photon-photon absorption
Energy [keV]
Abdo et al 2009
nF(n) [erg/cm2/s]
Average
tdec ~ 0.4
(1+z)
(Ek53 /n)
G3
8/3
1/3
second
s
0.1 GeV
30 GeV
090510
Time
resolved
2
Different component
3
0.5-1s
3
If LAT and GBM radiation are cospatial:
4
G>1000 to1 avoid photon-photon absorption
If G>1000: deceleration of the fireball
occurs early  early afterglow!
(see also Kumar & Barniol Duran 2009)
Energy [keV]
Abdo et al 2009
nF(n) [erg/cm2/s]
Average
No matter the origin of the GeV
emission, the bulk Lorentz factor
must be large
t2
t-1.5
background level
Ghirlanda+ 2010
T*=0.6s
Strong limit to quantum gravity 
GeV
M>1
QG > 4.7 MPlanck
T-T* [s]
Ghirlanda+ 2010
~MeV and ~GeV emission are NOT cospatial.
0.1-1 GeV
But the ~GeV emission is… No measurable
0.1-10 GeV delay in arrival time:
tdelay<0.2 s 
LAT GRBs
GG+ 2010
background
background
LAT GRBs
GG+ 2010
z
no z
short
Time integrated spectra
Log nFn
a
b
GBM
G
LAT
Band
PL
Log n
Log nFn
b vs G
a
b
LAT
GBM
Log n
a vs G
G-values consistent
with Zhang+ 2011
G
G
The 8 brightest LAT GRBs
z=2, assumed
z=1, assumed
z=2, assumed
The 4 brightest LAT GRBs
t-10/7
Spectrum and decay:
afterglow; LGeV~Lbol
The 4 brightest LAT GRBs
t-10/7
From Beloborodov (2002)
e
e
e
ee+
e
ee+
LAT
GBM
Opt
0.1
1
1
10
10
Time [s]
102
102
103
103
1
10
102
103
1
10
102
103
Time [s]
Problems
Fast variability of the GeV emission (Abdo+ 2009)
Abdo+ 2009, ApJ, 706, L138 FERMI observations of GRB 090902B: a distinct
spectral component in the prompt and delayed emission
”…the observed large amplitude variability on short timescales (≈90 ms) in the
LAT data, which is usually attributed to prompt emission, argues against such
models.”
090902B
Counts per bin
5s
Problems
Fast variability of the GeV emission (Abdo+ 2009).
No evidence
Simultaneous GBM-LAT spikes (Ackermann+ 2011;
Zhang+ 2011
Ackermann+ 2011
090926A
e
LEC
Lsyn
~
Lg,iso,54
2
4
R17 G3 eB,-1 n
ee+
Problems
Fast variability of the GeV emission (Abdo+ 2009).
No evidence
Simultaneous GBM-LAT spikes (Ackermann+ 2011;
Zhang+ 2011 EC scattering of prompt photons? Numbers
are ok
LAT spectra on the extrapolation of GBM spectra
(Zhang+ 2011; with exceptions) if fitted together (but
LAT emission lasts longer…)
Highest energy photons that arrive after the peak of
the LAT light curve are too energetic to be
synchro(Piran & Nakar 2010).
13 GeV
33 GeV
LAT GRBs
GG+ 2010
Problems
Fast variability of the GeV emission (Abdo+ 2009).
No evidence
Simultaneous GBM-LAT spikes (Ackermann+ 2011;
Zhang+ 2011 EC scattering of prompt photons? Numbers
are ok
LAT spectra on the extrapolation of GBM spectra
(Zhang+ 2011; with exceptions) if fitted together (but
LAT emission lasts longer…)
Highest energy photons that arrive after the peak of
the LAT light curve are too energetic to be synchro
(Piran & Nakar 2010). Very few, possible additional
component (SSC)?
Bulk Lorentz factors
G= 900
G=2000
G= 630
G= 670
GeV detected GRBS could be the ones with the
largest Lorentz factors… For smaller G…
tdec ~ 420 (1+z) (Ek54 /n)
G2
8/3
1/3
seconds
A factor ~103 dimmer in luminosity, but if nearby…
If pair enrichment is required, GeV detected
GRBs could be the ones with Epeak(1+z)>mec2
If Epeak < 511 keV and t-1: adiabatic because of
no pairs
090510
511 keV
Ghirlanda 2009
Conclusions
• GeV preferentially in Epeak>511 keV GRBs
• GeV when G is large  early onset of the
afterglow  very bright
• Large EAft: helps to understand Eprompt/EAft
Internal shocks: relative kinetic energy of the shells
External shocks: entire kinetic energy of the fireball
Afterglows should be more energetic than the prompt
0.1 GeV
Time
resolved
2
30 GeV
Different component
3
0.5-1s
3
If LAT and GBM radiation are cospatial:
4
G>1000 to1 avoid photon-photon absorption
If G>1000: deceleration of the fireball
occurs early  early afterglow!
If G>1000: large electron energies 
synchrotron afterglow!
Energy [keV]
Abdo et al 2009
nF(n) [erg/cm2/s]
Average
Eafterglow < Eprompt
Willingale+ 2007
X-ray and optical often behave differently
X-ray
We expected the opposite, if the efficiency
of prompt is ~ 0.1.
Why is the afterglow so faint?
Can it be hidden in some “unexplored”
frequency range, i.e. GeV-TeV?
Willingale+ 2007
Interpretations
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 frequencies, 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.
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 2010 : 090510 Curva di luce e confronto con Swift
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: analysis of afterglows of a few LAT bursts.
Ioka 2010 fa tutto, ma non ho capito nente…
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: I primi a dire external shock. Il lavoro e’
complicato. B~2e-5 Gauss, non ho capito perche’.
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: 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 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: GROND data for 4 LAT: they go on Amati, but not on
Ghirlanda (no jet break or too late)
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: 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+ 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…)