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© Daisuke YONETOKU (Kanazawa Univ.)
Daisuke YONETOKU (Kanazawa Univ.) Toshio MURAKAMI (Kanazawa Univ.)Shuichi GUNJI ( Yamagata Univ. )Tatehiro MIHARA (RIKEN), Kenji TOMA (Osaka Univ.) & GAP team
SN & GRB ConferenceKyoto, Japan(11-15, Nov., 2013)
Study of prompt emission mechanism by gamma-ray polarization
with IKAROS-GAP
GRB polarimetry
How to release the huge amount of energy of 1052-54 erg in gamma-ray band in short time duration.
Direction (spatial distribution), Time Variability, Spectrum.The another information of E-M Wave “Polarization”.
Inter-StellarMedium
Internal Shock(prompt)
External Shock(afterglow)
□ X-ray ☑ Optical ☑ Radio
(Non-detect)
□ Gamma□ X-ray
☑ Optical
Γ > 100
Central engine Γ < 10
GRB090102 Steele et al. (2009)
バースト発生後 160.8 秒後から
P = 10.1±1.3%
Covino et al. (2004) Q (%)
U (%
)
■ Optical afterglows: P = 1 ~ 5% ■ Optical Flash P = 10 %
Polarization in Opt. afterglows
~ 160sec after the burst
GRB990510: Covino et al. (1999)
Optical Flash
P=1.7 +/- 0.2%
KANATA telescope (Hiroshima Univ.)
Early afterglowT = 149 ~ 706 secP = 10.4±2.5%
Uehara et al. (2012)GRB091208B
X-rayPolarization Vector
Bragg Reflection(OSO-8 : Crab Nebula, Weisskopf et al. (1978)
Rotate
BraggCrystal
(~ a few keV :)
detector
Compton Scattering(e.g. GAP, PoGO Lite, PHENEX)
(~ 100 keV)
drift plane
Photo-electron
Amplifier
Signal
readout ASIC
GEM
Photo-Electric Absorption(e.g. GEMS, PolariS)Polarization
Vector
(~ 10 keV)
polarization
Coburn & Boggs (2004)GRB021206A (RHESSI)
P = 80±20 %
P < 100 % Rutledge & Fox (2004)P = 41 (+57, -44) % Wigger et al. (2004)
Rutledge & Fox (2004)
RHESSI
X
■ Complex Geometry ■ No onboard Coincidence
for Compton events
RHESSICoburn & Boggs (2003)Rutledge & Fox (2004)Wigger et al. (2004)
Difficulty
INTEGRAL-SPI McGlynn et al. (2007)Kalemci et al. (2007)
P = 63±30%, 98±33%
(INTEGRAL-IBIS) Gotz et al. (2009)
P < 4%
GRB041219A (INTEGRAL)
INTEGRAL (SPI)
?
The situation is c
omplicated.
We need further observations with a reliable polarim
eter.SPI
0 100 300
Coun
t Rat
e of
Com
pton
Sca
t.
200Scattering Angle (deg)
GAmma-ray burst Polarimeter ■ Angular distribution of Compton Scat. ■ Geometrical symmetry
r0 : classical electron radiusE0 : energy of incident photonE : energy of scattered photon
GAP (sensor unit)
20 cm, 3700g
Polarimetry in 70 – 300 keV ■ KEK Experiment
□ Geant 4 simulation
Yonetoku et al. (2006, 2011)
IKAROS Launched May 21, 2010
Interplanetary Kite-craft Accelerated by Radiation Of the Sun
Systematic Uncertainty
Sys. Error does not depend onthe incident angle.
1.7% Average
Syst
emati
c un
cert
aint
y (%
)
Incident angle (degree)
Systematic Uncertainty for Off-Axis Incident Photon
1m
57Co (122keV)241Am (59.5keV)
Geant4
The systematics caused by imperfect tunings of parameters in the ground and in-orbit calibrations for the off-axis radiation.
Comparing the experimental and simulated modulation curves, we estimated the systematic uncertainty is ~ 1.7% ofthe total coincidence gamma-rays.
Proto-FM
No.
GRB Fluence(erg/cm2)
incident angle
OtherObs.
No. GRB Fluence(erg/cm2)
incident angle
OtherObs.
1 100707A a 8.8×10-5 93 K,F,W,M 16 110124A - K,W2 100715A 19 K,I,W,M 17 110301A a 3.7×10-5 48 K,F,W3 100719B 145 K,F 18 110406A b 4.8×10-5 133 K,W,I,Sw4 100722A 34 K,F 19 110423A - K5 100804A 63 K,F 20 110428A a 2.3×10-5 109 K,F,W,Sw6 100809A - K 21 110505? - ?7 100820A 34 K,F 22 110510? - ?8 100826A b 3.0×10-4 20 K,F,W,M 23 110514 - K9 101014A a 2.0×10-4 54 K,F 24 110604A c 3.1×10-5 43 K,W,Sw
10 101021A 41 K,F 25 110625A b 6.1×10-5 41 K,F,Sw11 101113A 26 K,F 26 110708A d 2×10-6 67 K,F,Sw12 101123A a 1.3×10-4 74 K,F,I,Sw 27 110715A b 2.3×10-5 88 K,W,Sw13 101126A 62 K,F 28 110717B 25 F,K14 101219A b 3.0×10-6 52 K,Sw 29 110721A a 3.5×10-5 30 K,F,I,M15 101231A 63 F 30 110825A a 5.4×10-5 29
a : 10-1000 keV
b : 20-10000 keV
c : 20-5000 keV
d : 20-200 keVData SamplesKonus, Fermi, Swift,WAM, Integral, Mess.
Modulation Curve
Coin
cide
nce
Coun
t (co
unts
)Scattering Angle (degree)
α = - 1.3β = - 2.1 Ep = 606 keV
Interval-14821 photons(polari. data)
Interval-22,733 photons(polari. data)
Modulation Curve
Coin
cide
nce
Coun
t (co
unts
)Scattering Angle (degree)
α = - 1.3β = - 2.1 Ep = 606 keV
Interval-22733 photons(polari. data)
Interval-14821 photons(polari. data)
Δχ2 マップConfidence Contour Δχ2
Pola
rizati
on A
ngle
(deg
ree)
Polarization Degree (%)
Polarization Degree(including sys. uncertainty)
PolarizationAngle
Interval-1 P = 25±15% (95.4% C.L.) PA = 159±18 deg
Interval-2 P = 31±21% (89.0% C.L.) PA = 75±20 deg
Combined Fit
P = 27±11% (99.4% C.L.)
3.5σ ConfidenceLevel
GRB100826A
Interval-1 Interval-2
Coun
t Rat
e (c
ount
s/se
c)
Time since GRB trigger (sec)
Very bright events with F = 3.0x10–4 erg/cm2
Yonetoku et al. (2011)
T90 ~ 100 sec χ2 = 21.8 for 19 d.o.f
GRB110301A & GRB110721AWe detected the polarization fromtwo bright GRBs with high significance.
The polarization angles did not changeduring the prompt GRBs.
GRB110301A
GRB110721A
1,820 photons(polari. data)
1,092 photons(polari. data)
T90 = 7 sec
T90 = 11 sec
GRB110301A
GRB110721AP = 84(+16, -28) %PA = 160±11 deg3.3σ
P = 70±22%PA = 73±11 deg3.7σ
Pola
rizati
on A
ngle
(deg
ree)
Pola
rizati
on A
ngle
(deg
ree)
Polarization Degree (%)
Polarization Degree (%)
χ2 = 14.0 for 10 d.o.f
χ2 = 7.3 for 10 d.o.f
Lu et al. (2012) Fermi-GBM Spectral Evolution
We simulated the model functions (detector responses) with the time resolved spectral parameters, a, b, Ep, flux,and combined them into one model.
Slide by Felix Ryde and Magnus Axelsson
GAP’s energy range
■ Significant Polarization was detected from bright 3 GRBs. ■ GRB100826A : Polarization angle changed (3.5s confidence level.) ■ GRB110721A & GRB110301A : Polarization angle was stable.
We need the emission model to explain both cases of change and no-change of polarization angle.
Results of Polarization Analyses
PolarizationDegree (%)GRB Duration
T90 (sec)IncidentAngle (deg)
90% upper-limit
100
Jet opening angle : θj Relativistic beaming effect : 1/Γ
Helical Magnetic Fields to drive the relativistic jet
Distribution of polarization
Distribution of polarizationGlobally Random Magnetic Fields,But Locally Coherent
The change of polarization angle can beexplained with patchy structures of smaller than 1/Γ.
Inner Structures may exist in the Jet.
Lazzati et al. (2003–2009)Toma et al. (2009)
Narrow: θj < 1/G ~ 0.01 rad Broad: θj > 1/G ~ 0.01 rad
Observing entire jet surface. Pol. angle can change.
Observing the part of jet surface.Pol. angle can not strongly change.
Jet Opening Angle Change/No-Change of Polarization Angle
1/G
1/G
We can explain both cases of change/no-change of pol. angle
with the relation between θj and 1/G, and also the patches.
PhotonField
highlypolarizedElectron
Rest Frame
Observer FrameNon-polarized
highlypolarized
Compton Drag Lazzati et al. (2005)
GRB Jet qj
1/GDimmer,
Highly Polarized
Brighter, Non-Polarized
& Photospheric Emission
The highly polarized gamma-rays:only when the observer is slightly outside of the jet. The prompt emission is dim.
It is difficult to explain the existence of bright & highly polarized GRBs in the frame work of Compton drag and Photospheric emission mode.
GAP 70 – 300 keV
Epeak – Polarization Degree
Both low- and high-energy part of spectra are polarized.
Pola
rizati
on D
egre
e (%
)
Epeak (keV)
100826ARapid Change of Polarization Angle
110301A
110721A101014A
110625A110825A
Average
GRB061122 INTEGRAL (IBIS) Gotz et al. (2013)
250 – 800 keV
250 – 350 keV
350 – 800 keV
250 – 800 keV
250 – 350 keV
350 – 800 keV
T90 = 12 sec (almost single peak)Fluence = 2x10-5 erg/cm2
68%
90%95%99%
Non-polarization was rejected with 4σ level
Gamma-Ray (Prompt – Forward shock)27±11 % (GRB100826A)70±22 % (GRB110301A)84(+16, -28)% (GRB110721A)
Optical Flash (Prompt – Reverse shock)10.1±1.3 (GRB090102) Steele et al. 2009
Early Optical Afterglow (Forward shock)< 8 % (GRB060418) Mundel et al.10.4±2.5% (GRB091208B) Uehara et al. 2012
After Jet Break?? %
Time
Brig
htne
ssEvolution of polarization
Late Optical Afterglow (Forward shock)1 – 3 % (summarized in Covino et al. 2004)
Prompt
Afterglow
Summary ■ We detected the g-ray polarization from 3 bright GRBs,
and set U.L. for 4 GRBs with GRB polarimeter “GAP”.
■ The emission mechanism of prompt GRBs are probably the synchrotron radiation in the coherent magnetic fields. (Our results favor theICMART model (Zhang’s group). We cannot exclude the photospheric and comptonized emission model)
■ Since the polarization angle rapidly changed, the multiple emission regions and/or the patchy structures with the scale of < 1/Γ may exist in the relativistic jet.
Next gamma-ray polarimeter aboard “TSUBAME” small satelliteby the Tokyo-Tech team will be launched next year.
© Daisuke YONETOKU (Kanazawa Univ.)
Inner structures & Magnetic Fields
Image of Gamma-ray bursts