References: DK, M. Georganopoulos, A. Mastichiadis 2002 A. Mastichiadis, DK 2006 DK, A. Mastichiadis, M. Georaganopoulos 2007 A. Mastichiadis, DK 2009 Bulk Comptonization GRB Model and its Relation to the Fermi GRB Spectra Demosthenes Kazanas NASA/GSFC Apostolos Mastichiadis Un. Of Athens

Major GRB Facts/Issues GRB involve relativistic blast waves ( Ress & Meszaros ) GRB involve relativistic blast waves ( Ress & Meszaros ) This guarantees that most of the energy of the swept-up matter is stored in relativistic protons (accelerated or not; electrons carry ~1/2000 of available energy). This guarantees that most of the energy of the swept-up matter is stored in relativistic protons (accelerated or not; electrons carry ~1/2000 of available energy). PROBLEMS: PROBLEMS: Protons do not lose energy to radiation easily. Protons do not lose energy to radiation easily. The observed luminosity peaks at E~1 MeV in the lab frame! (Despite the blast wave LF of order ). The observed luminosity peaks at E~1 MeV in the lab frame! (Despite the blast wave LF of order ).

There are (at least) two outstanding issues with the prompt GRB emission (Piran 2004): There are (at least) two outstanding issues with the prompt GRB emission (Piran 2004): A. Dissipation of the RBW free energy. Energy stored in relativistic p’s or B-field. Sweeping of ambient protons stores significant amount of energy in p’s anyway. Necessary to store energy in non-radiant form, but hard to extract when needed. A. Dissipation of the RBW free energy. Energy stored in relativistic p’s or B-field. Sweeping of ambient protons stores significant amount of energy in p’s anyway. Necessary to store energy in non-radiant form, but hard to extract when needed. B. The presence of E peak ~0.1 – 1.0 MeV. If prompt emission is synchrotron by relativistic electrons of Lorentz factor (LF) same as shock E p ~  , much too strong to account for the observations. B. The presence of E peak ~0.1 – 1.0 MeV. If prompt emission is synchrotron by relativistic electrons of Lorentz factor (LF) same as shock E p ~  , much too strong to account for the observations.

We have proposed a model that can resolve both these issues simultaneously. The model relies: We have proposed a model that can resolve both these issues simultaneously. The model relies: 1. On a radiative instability of a relativistic proton plasma with B-fields due to the internally produced sychrotron radiation ( Kirk & Mastichiadis 1992 ). 1. On a radiative instability of a relativistic proton plasma with B-fields due to the internally produced sychrotron radiation ( Kirk & Mastichiadis 1992 ). 2. On the amplification of the instability by relativistic motion and scattering of the internally produced radiation by upstream located matter, a ‘mirror’ ( Kazanas & Mastichiadis 1999 ). 2. On the amplification of the instability by relativistic motion and scattering of the internally produced radiation by upstream located matter, a ‘mirror’ ( Kazanas & Mastichiadis 1999 ).

p  e + e - eB B   In the blast frame the width of the shock  ~ R/  is comparable to its observed lateral width thereby considering all processes taking place in a spherical volume of radius 

The instability involves 2 thresholds: The instability involves 2 thresholds: (All particles behind the shock are assumed to have Lorentz factors equal to the shock one , i.e. no accelerated particle populations!) 1. A kinematic threshold for the reaction p   e - e + The photon at the proton rest frame must have energy greater than 2 m e c 2. The synchrotron photon has an energy E S ~ b  2 at the plasma frame and ~ b  3 on the proton frame. So the condition reads b  3 > 2 m e c 2 or  b   b  3 > 2 m e c 2 or  b  

2. A dynamic threshold : At least one of the synchrotron photons must be able to produce an e - e + - pair before escaping the plasma volume. Because each electron produces     b    b  photons, we obtain the following condition for the plasma column density (note similarity with atomic bombs!) n  p  R  b  or n  p  R    b    n  p  R  b  or n  p  R    b   

The instability involves 2 thresholds: The instability involves 2 thresholds: 1. A kinematic one for the reaction p   e - e + b  5 > 2 m e c 2 b  5 > 2 m e c 2 (the prompt phase ends when this condition is not satisfied) 2. A dynamic one: At least one of the synchrotron photons must be able to reproduce before escaping the plasma volume. This leads to the following condition for the plasma column density (note similarity with atomic bombs!)

Similarity of GRB/Nuclear Piles-Bombs The similarity of GRB to a “Nuclear Pile” is more than incidental: The similarity of GRB to a “Nuclear Pile” is more than incidental: 1. They both contain lots of free energy stored in: 1. They both contain lots of free energy stored in: Nuclear Binding Energy (nuclear pile) Nuclear Binding Energy (nuclear pile) Relativistic Protons or Magnetic Field (GRB) Relativistic Protons or Magnetic Field (GRB) 2. The energy can be released explosively once certain condition on the fuel column density (and not mass) is fulfilled ( Note: no particle acceleration required!! ). 2. The energy can be released explosively once certain condition on the fuel column density (and not mass) is fulfilled ( Note: no particle acceleration required!! ).

The instability requirements are greatly reduced if the radiation produced at the shock is mirrored by upstream located matter (because the energy of each photon increases by  2 ; DK, A. Mastichiadis 1999) to the following conditions: The instability requirements are greatly reduced if the radiation produced at the shock is mirrored by upstream located matter (because the energy of each photon increases by  2 ; DK, A. Mastichiadis 1999) to the following conditions: b  5 > 2 b  5 > 2 n  p  R    b    n  p  R    b    For n ~ 1, R ~ R 17  ~ 211 (n R 17 ) 1/4, T ~ 40 sec

RBW Mirror b    GeV photons b    MeV photons b    eV O-UV photons R  

No accelerated population is invoked !!

RBW Mirror Min(m e c 2  , b  6+1  GeV photons b    MeV photons ( GBM photons are due to BC) b    eV O-UV photons R   Syn. BC RS BC, SSC THE SPECTRA

RBW Mirror b    GeV photons b    MeV photons b    eV O-UV photons

We have modeled this process numerically. We assume the presence of scattering medium at R ~10 16 cm and of finite radial extent. We have modeled this process numerically. We assume the presence of scattering medium at R ~10 16 cm and of finite radial extent. We follow the evolution of the proton, electron and photon distribution by solving the corresponding kinetic equations. We follow the evolution of the proton, electron and photon distribution by solving the corresponding kinetic equations. We obtain the spectra as a function of time for the prompt GRB emission. We obtain the spectra as a function of time for the prompt GRB emission. The time scales are given in units of the comoving blob crossing time  co /c ~ R   c ~ 2 R 16  2.6 sec  The time scales are given in units of the comoving blob crossing time  co /c ~ R   c ~ 2 R 16  2.6 sec 

The kinetic equations are solved on the RBW rest frame with pair production, synchrotron, IC losses, escape in a spherical geometry of radius R/  and proton density n = n 0  The protons are assumed to be injected at energy E p = m p c 2  These are the following:

Prompt GRB Spectra (Mastichiadis & Kazanas 2006) S : b  3 BC: b  5 SSC : m e  2

The “scattering screen” We consider scattering of the Synchrotron (O-UV) photons by the ambient medium at atomic cross sections (  cm 2 ) at R ~ cm to give us photon mfp ~ R. For L  ~10 51 erg/s, L S ~ erg/s, or F ~ 10 ( ) ~10 24 ph/cm 2 /s or r ~ F  10 7 excitations/s to be compared to the spontaneous rate of 10 8 /s. For higher luminosities (fluxes) the corresponding radii will be larger. The Lya photons may contribute to the reduction of the width of the E peak distribution.

We have also modeled the propagation of a relativistic blast wave through the wind of a WR star (that presumably collapses to produce the relativistic outflow that produces the GRB). We have also modeled the propagation of a relativistic blast wave through the wind of a WR star (that presumably collapses to produce the relativistic outflow that produces the GRB). In this case we also follow the development of the blast wave LF and the radiative feedback on it. In this case we also follow the development of the blast wave LF and the radiative feedback on it. In this scenario the “scattering screen” necessary for the model to work is provided by the pre- supernova star wind, the length scales smaller, R 0 ~10 13 cm and the GRB is a “short GRB”. In this scenario the “scattering screen” necessary for the model to work is provided by the pre- supernova star wind, the length scales smaller, R 0 ~10 13 cm and the GRB is a “short GRB”.

Evolution of the LF and the luminosity

Evolution of Spectra with Time (  B ~1)

Evolution of Luminosity with Time (  B ~1)

Evolution of Luminosity with Time (  B ~0.01) Ep decreases with decreasing Luminosity. The BC component Decreases faster Than the SSC leaving The LAT flux after The GBM one is Gone.

Aftreglow, GRB, XRF, Unification Inclusion of non-thermal particle populations and repeating the same arguments as above one obtains the evolution of Epeak with G or with time. A simple calculation gives (DK, AM, MG 2007) Inclusion of non-thermal particle populations and repeating the same arguments as above one obtains the evolution of Epeak with G or with time. A simple calculation gives (DK, AM, MG 2007) Ep ~ [  (t)/50] 2 ~ t -3/4 Ep ~ [  (t)/50] 2 ~ t -3/4 (talk/poster by R. Margutti on Monday)

Conclusions The “nuclear pile” GRB model provides an over all satisfactory description of several GRB features, including the dissipation process, Ep and the Fermi observations. The “nuclear pile” GRB model provides an over all satisfactory description of several GRB features, including the dissipation process, Ep and the Fermi observations. Provides an operational definition of the GRB prompt phase. Provides an operational definition of the GRB prompt phase. No particle acceleration necessary to account for most of prompt observations (but it is not forbidden!). No particle acceleration necessary to account for most of prompt observations (but it is not forbidden!). GBM photons due to bulk Comptonization ( => possibility of high polarization ~100%). GBM photons due to bulk Comptonization ( => possibility of high polarization ~100%). It can produce “short GRB” even in situations that do not involve neutron star mergers. It can produce “short GRB” even in situations that do not involve neutron star mergers. Exploration of the parameter space and attempt to systematize GRB phenomenology within this model is currently at work. Exploration of the parameter space and attempt to systematize GRB phenomenology within this model is currently at work.

Distribution of LE indices 

The spectra of doubly scattered component (Mastichiadis & DK (2005)) S=1/2,  S=1,  S=2, 

 G  G  G  G

E iso of the three different spectral components as a function of B for  =400 and n p =10 5 cm -3. x 10 3 denotes the relative  -ray – O-UV normalization of GRB , a. O-UV 100 GeV 1 MeV X 10 3

E peak as a function of the magnetic field B

Variations If the “mirror” is in relative motion to the RBW then the kinematic threshold is modified to b     rel ~ 2;  rel is the relative LF between the RBW and the “mirror”. If the “mirror” is in relative motion to the RBW then the kinematic threshold is modified to b     rel ~ 2;  rel is the relative LF between the RBW and the “mirror”. The value of E peak is again ~ 1 MeV, however the synchrotron and IC peaks are higher and lower by   rel than     The value of E peak is again ~ 1 MeV, however the synchrotron and IC peaks are higher and lower by   rel than     In the presence of accelerated particles the threshold condition is satisfied even for  (2/b) 1/5. This may explain the time evolution of GRB (Gonzalez et al. 04) In the presence of accelerated particles the threshold condition is satisfied even for  (2/b) 1/5. This may explain the time evolution of GRB (Gonzalez et al. 04) GRB flux is likely to be highly polarized (GRB , Coburn & Boggs 03). GRB flux is likely to be highly polarized (GRB , Coburn & Boggs 03). This model applicable to internal shock model (photons from downstream shell instead of “mirror”). This model applicable to internal shock model (photons from downstream shell instead of “mirror”).

Then....

Shock – Mirror Geometry

formation region, generally not much different than the