© 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