Photospheric Emission inGRBs
Hirotaka Ito RIKENCollaborators
Shigehiro Nagataki (RIKEN), Jin Matsumoto (RIKEN),
Shiu-Hang Lee (JAXA), Masaomi Ono (Kyushu Univ.),

Jirong Mao (Kyushu Univ), Asaf Pe’er (UCC), Akira Mizuta (RIKEN),
Alexei Tolstov (IPMU), Maria Dainotti (RIKEN),
Shoichi Yamada (Waseda Univ.), Seiji Harikae (Mitubishi UFJ)＠RESCEU-RIKEN-IPMU joint meeting 2014 7/8

Gamma-Ray Burst (GRB)
Most luminous explosion in the universe

Lγ,iso ~ 1052 - 1054 erg/sBand function

・event rate~1000/yr・duration
T ~ 10ms ー 100s

・rapid variability
δt ～ msFν∝ν-α （hν< Ep）
Fν∝ν-β （hν> Ep）
Long GRB< Ep > ~ 160 keV<α>~ -0.9
Short GRB
< Ep > ~ 490 keV<α>~ -0.5<β>~ -2.3

Nava + 2011

Model for Emission Mechanism
Internal Shock Model
flaw

・Low efficiency for gamma-ray production
・too hard spectrum in low energy band (α)

Photospheric Emission Model
Natural consequence of fireball model(e.g., Rees & Meszaros 2005, Pe’er et al.2005, Thompson 2007)

γγ

photosphere

Internal shock
External shockModel for Emission Mechanism
Internal Shock Modelflaw

・Low efficiency for gamma-ray production
・too hard spectrum in low energy band (α)

Photospheric Emission Model
Natural consequence of fireball model(e.g., Rees & Meszaros 2005, Pe’er et al.2005, Thompson 2007)• High emission efficiency• Peak at ~1 MeV• Non-thermal appearance
E (MeV)Dissipative process
high energy tail is reproduced by the relativistic pairs produced by dissipative
processes

Magnetic recconection
Giannios & Spruit 2007, Giannios 2008

relativistic pairs upscatterthermal photons

Repeated ShockIoka + 2007, Lazzati & Begelman 2010

Proton-neutron collision
Derishev 1999, Beloborodov 2009, Vurm+2011

Beloborodov 2009

Geometrical brodening
Structure of the jet can give rise to the non-thermal spectra

Mizuta+2011

spectrum broadens even in theabsence of relativistic pairs

Ioka+2011

Multi-dimensional structure of jet may be a key to resolve the difficultyOur focus: Effect of the jet structure on the emission
Find the jet structure that can explain the observation

Stratified Jet structure

2 effects on the spectra(I) multi-color effectsee also Lundman & Pe’er (2013)t~1
photosphere

G0G1

νfν

G1SpineSheath

G0 > G1n

Our focus: Effect of the jet structure on the emission
Find the jet structure that can explain the observation

Stratified Jet structure

2 effects on the spectra(I) multi-color effectsee also Lundman & Pe’er (2013)(II) Fermi acceleration ofphotons

Accleration
regionPhotons gain energy bycrossing the boundary layer

photosphereνfν

t~1Spine

Sheath

G0 > G1n

Our focus: Effect of the jet structure on the emission
Find the jet structure that can explain the observation

Stratified Jet structure

2 effects on the spectra(I) multi-color effectsee also Lundman & Pe’er (2013)(II) Fermi acceleration ofphotons

Accleration
regionPhotons gain energy bycrossing the boundary layer

photosphere

νfν

t~1
SpineSheath

G0 > G1nPropagation of photons are solved by Monte=Carlo method

2-component (spine-sheath) jetSpine(θ<θ0 )

Sheath(θ0<θ<θj )rout (τ<<1)

Radial structrue
fireball model (e.g., Piran 2004)
h: baryon loading(terminal Lorentz factor)
L: Kinetic luminosity

ri: initial radiusrin (τ>>1)

ri0r= ri1 h0> h1irh0

GG0

ri

G1

h1

rs1

rs0r

2-component (spine-sheath) jet
Spine(θ<θ0 )Sheath(θ0<θ<θj )Calculation Range
rin ＝ rs1 << Rphrout (τ<<1)rout = 500Rph(τ~2×10-3):photospheric radiusrin (τ>>1)

rh0

GG0

ri

G1

h1

rs1

rs0r

2-component (spine-sheath) jet
Spine(θ<θ0 )Sheath(θ0<θ<θj )Inject thermal photons at theinner boundary

rout (τ<<1)rin (τ>>1)

Initial Condition

Tin = 0.9 r81/6G4008/3 L53-5/12 (rin/1011cm)-2/3 keV

Lin = 5.4×1052 r82/3G4008/3 L531/3 (rin/1011cm)-2/3 erg/s

rh0

GG0ri

G1

h1

rs1

rs0

r

2-component (spine-sheath) jet
Spine(θ<θ0 )

Sheath(θ0<θ<θj )Inject thermal photons at theinner boundary

rout (τ<<1)rin (τ>>1)

Initial Condition

Tin = 0.9 r81/6G4008/3 L53-5/12 (rin/1011cm)-2/3 keV

Lin = 5.4×1052 r82/3G4008/3 L531/3 (rin/1011cm)-2/3 erg/srh0GG0

riG1

h1

rs1

rs0r

Propagation of photons are solved by Monte=Carlo methodTwo-component jet
G0=400 qj =1° q0=0.5° qobs=0.4°
Thermal + non-thermal tail

Spine

SheathEmax = G0mec2
Klein-Nishina cut-off

Non-thermal tail becomes prominent as the relative
velocity becomes larger

Two-component jet
G0=400 qj =1° q0=0.5° qobs=0.4°
Thermal + non-thermal tail

Spine

SheathEmax = G0mec2
Klein-Nishina cut-off

Non-thermal tail becomes prominent as the relativevelocity becomes larger-1 ～0.14°G -1
But limited only for narrow range of |θθ|<Gobs0400
observer angle

Multi-component jet
G0=400 G1=100dθInterval of velocity shear dθ < 2G-1

Multi-component jet
G0=400 G1=100

β= -2.3dθInterval of velocity shear dθ < 2G-1

high energy spectra (β) is reproduced by accelerated photons

Multi-component jet
G0=400 G1=100

β= -2.3dθ

Emax = G0mec2 ~ 100MeV

Interval of velocity shear dθ < 2G-1

high energy spectra (β) is reproduced by accelerated photons
Cut off at ~ 100 MeV

Multi-component jet
G0=400 G1=100

β= -2.3
α=-1dθ

Emax = G0mec2 ~ 100MeV

Interval of velocity shear dθ < 2G-1

high energy spectra (β) is reproduced by accelerated photons
Cut off at ~ 100 MeV

Low energy spectra (α) is reproduced by multi-color effect(Lundman & Pe’er 2013)

Multi-component jet
G0=400 G1=100

β= -2.3
α=-1dθ

Emax = G0mec2 ~ 100MeV

Interval of velocity shear dθ < 2G-1

Face on view of jet
Simulation by Dr. Matsumotopolarization
two-component jet
DOP （％）＝(I+ - I-) / (I+ + I-) ×１００

Spine
edge

0%

qobs (degree)

Degree of polarization (DOP) becomes larger
as the relative velocity becomes large

SheathI+I-DOP（％）＝(I+ - I-) / (I+ + I-) ×１００

polarization
multi-component jet that reproduces Band spectra
G0=400 G1=1005%

edge

0%0%

dθ

5%

qobs (degree)

High polarization degree (>10%) is predicted not onlyin the off-axis region but also in the on-axis region
Future missions such as Tsubame and POLAR may probe such an emission

On-going project3D Hydrodymical simulation of relativistic jet as a background fluid
simulation by Dr. Matsumoto
Detail of spectra, polarizationand lightcurves for more
realistic case can be obtainedSummary
jet can produce a power-law non-thermal tail- Stratified
above the peak energy
non-thermal particle is not required- Multi-component jet can reproduce Band function irrespective tothe observer angle
β is reproduced by the accelerated photons
α is reproduced by the multi-color effect- Degree of polarization tends to increase as the relative velocity
increases
High DOP (>10%) is predicted for the jet structure that
reproduces Band function

Futrure works
・Photon accelerations in various structures
shocks, turbulence・Hydrodymical simulation of relativistic jet as a background fluid