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Yizhong Fan
(Niels Bohr International Academy, Denmark
Purple Mountain Observatory, China)
Fan (2009, MNRAS) and Fan & Piran (2008, Phys. Fron. China)
MeV-GeV emission from GRBs (EAGRET)
GRB afterglow
detection for
the first time!
GRB 940217
(Hurley et al. 1994)
MeV-GeV emission from GRBs (EGRET)
GRB 941017
(Gonzalez et al. 2003)
Much longer high
energy emission
Quick evolution
Almost constant
Theoretical predictions—before Fermi
(see Fan & Piran 2008 for a review)
(Pe’er & Waxman 2004; Pilla & Loeb 1998;
Gupta & Zhang 2007)
(Fan, Piran, Narayan & Wei 2008)
Fermi GRBs with GeV emission
 GRB 080916C (Abdo et al. 2009)
 GRB 081024B (Omodei et al. 2008)
 GRB 090217 (Ohno et al. 2009a)
 GRB 090323 (Ohno et al. 2009b)
 GRB 090328 (Cutini et al. 2009)
 GRB 090510 (Ohno & Pelassa 2009c)
One detection once a month, as expected (assuming Band
spectrum of GRBs, i.e., no GeV excess in most GRBs)
The delayed onset of the >100 MeV emission
(Abdo et al. 2009)
Extended high energy emission (Abdo et al. 2009)
Main properties of the Fermi GRBs
 No GeV spectrum excess detected in almost all GRBs
 The delay of arrival of the >100 MeV photons
 The extended high energy emission from both short
and long bursts
Interpreting the non-detection of the GeV
spectrum excess in most GRBs (Fan 2009)
Poynting-flux dominated outflow model
 Gradual magnetic energy dissipation (e.g., Giannios 2007): The
strong magnetic field in the emitting region suppresses the SSC
and the electrons are only mildly-relativistic
 Sudden energy dissipation at R~1E16 cm (Lyutikov & Blandford
2003): the SSC is in the extreme Klein-Nishina regime
1/4 1/2
 e,m ~ 2.5  104 [(1  z ) p / 600keV]1/2 k 11/4 Lm1/4
[

/
(1


)]
R ,15.5 ,
,52
 e,m (1  z ) p / me c 2
where  (
1.
1) is the ratio between the magnetic energy density and the particle energy density, and k is the
fraction of the magnetic energy which has not been dissipated.
Interpreting the non-detection of the GeV
spectrum excess in most GRBs (Fan 2009)
Standard internal-shock model
 e,m ~ 2  103[(1  z ) p / 600keV]1/2 ( B /  e ) 1/4 [(1  Y ) Lsyn,52 ]1/4 R1/2,14 ,
 e,m (1  z ) p / mec 2 ~ 10.
(The SSC in the extreme
Klein-Nishina regime?)
Interpreting the non-detection of the GeV
spectrum excess in most GRBs (Fan 2009)
Mildly (0.1<sigma<1) magnetized internal shocks
 The strong magnetic field in the emitting region suppresses
the SSC, and the synchrotron spectrum may be very soft
Interpreting the non-detection of the GeV
spectrum excess in most GRBs (Fan 2009)
Photosphere-internal shock model
 The electrons are assumed to be only mildly-relativistic
(Thompson et al. 2007)
Interpreting the delayed onset of
the >100 MeV emission (Fan 2009)
 In both collapsar and compact star merger
models, the early outflow likely suffers more
serious baryon pollution and thus has a
smaller bulk Lorentz factor than the late
ejecta. The GeV photons can not escape from
the early outflow
 In the collapsar scenario, before the
breakout, the initial outflow is choked by the
envelope material of the progenitor (Zhang,
W. et al. 2003). The emission of the
breaking out material may be dominated
by the quasi-thermal radiation from the
photosphere and may last a few seconds
(~ R* / c, where R* ~ 1011 cm is the radius
of the progenitor!)
The extended high energy emission
from both short and long bursts
(see Fan & Piran 2008 for a review)
 Synchrotron and SSC radiation of the forward/reverse
shocks (e.g., Meszaros & Rees 1994; Dermer et al. 2000; Sari & Esin
2001; Zhang et al. 2001; Wang et al. 2001a, b; Wei & Fan 2007; Gou et al.
2007; Yu et al. 2008; Fan et al. 2008; Galli & Piro 2008; Zou et al. 2009)
 External inverse Compton in reverse/forward shock
regions (e.g., Beloborodov 2005; Fan et al. 2005; Wang et al. 2006;
Fan & Piran 2006; Wang & Meszaros 2006; Fan et al. 2008; Zou et al.
2009)
Predicted high energy emission from the
naked-eye
burstemission
GRB 080319B (e.g.,
(Zou, Fan
 SSC radiation of the extended
prompt
& Piran
Wei et al. 2006; Wang et al. 2006; Fan
et al. 2009)
2008; Galli & Guetta 2008;
Yu & Dai 2009; Zou et al. 2009)
Are the GRB outflows magnetic
rather than baryonic?
The non-detection of GeV spectrum excess by
Fermi in almost all GRBs: magnetic fireball?
(Fan & Piran 2008; Fan 2009: arXiv:0905.0908)
synchrotron
SSC
(magnetized fireball)
Possible evidence for the magnetized outflow model
phase
phenomena
Implication
References
Prompt
emission
Highly polarized
gamma-rays?
Ordered magnetic field in
the emitting region?
Lyutikov et al. 2003;
Granot 2003
Prompt
emission
Absence of thermal
emission component
Poynting flux dominated
outflow?
Daigne & Mochkovitch 2002;
Prompt
emission
Non-detection of the GeV
Spectrum excess
A strong magnetic field
component?
Fan 2009
Very early
afterglow
Bright flash outshining
the forward shock optical
emission
Weakly magnetized
reverse shock region?
Fan et al. 2002;
Zhang et al. 2003;
Kumar & Panaitescu 2003
Very early
afterglow
Absence of bright optical
flashes
Mildly magnetized
reverse shock region?
Fan et al. 2004, A&A;
Zhang & Kobayashi 2005;
Mimica et al. 2009
Zhang & Pe’er 2009
Prompt emission

Reverse shock emission?
r
 =magnetic energy density/particle energy density
The low energy spectrum crisis in the
case of a baryonic fireball
(Cohen et al. 1997; Preece et al. 1998)
Fv  v
1/2
Fv  v 0
The magnetic field generated
in the shocks is very, very
low or decays quickly?
A possible solution in the case of baryonic fireball?
(Derishev et al. 2001; Derishev 2007)
Pro: For typical GRB parameters, within the
synchrotron radiation model, the SSC of electrons
emitting X-rays is very likely in Klein-Nishina
regime
Cons:
1. Fine tuning of microphysical parameters
2009,
communication
2.Nakar
Only for
hardprivate
GRBs (not
for X-ray flashes
and X-ray flares)
Magnetic fireball: spectrum problem
GRB 080916C
(Giannios 2007: gradual magnetic dissipation)
Poynting flux dominated outflow model
(Lyutikov & Blandford 2003: sudden magnetic dissipation)
Synchrotron radiation: sudden magnetic
dissipation (in preparation)
Fv  v 1/2
Repeated acceleration?
Summary
 The non-detection of GeV spectrum excess in almost all
GRBs can be well understood in a number of scenarios. The
simplest interpretation may be the magnetized outflow
model.
 The delayed onset of the >100 MeV photons may reflect the
physical condition of the early outflow (in particular the
breaking out material in the collapsar scenario)
 For the magnetic fireball, there is a serious low-energy
spectrum problem. For the baryonic fireball, there might be
more freedom (for example, a hard low energy spectrum can
be obtained if the magnetic field generated in the shocks is
very, very low or decays quickly).