2
V

6

M

2

5/γ

9

M

@

10

-

Outline

1. Introduction of GRB science
2. Theoretical models of g-ray polarization
3. Current data: GAP, INTEGRAL, POLAR
4. Discussion

Gamma-ray bursts (GRBs)
Prompt g-ray count rate

Prompt g-ray spectrum

6

Author(s)
54

10

53

Peak Luminosity Lp[erg/s]

10Time [s]Afterglow

52

10

51

10

50

10

49

10

long GRB
short GRBLL GRB48

10high-z GRB
outlier

47

10

10

55

Cosmological distances (z = 0.0085 - 8.2)
10 in the universe
Most luminous objects
1054

y Eiso[erg]

•
•

100
1000
Peak Energy Ep [keV]

53

10

52

Gehrels, Ramirez-Ruiz & Fox 2009;10De Pasquale+2010; Yonetoku+2012

Long GRBs & short GRBs
Compact stars mergers… probably•

Gravitational waves•

Heavy elements•

Population III GRBs?

•

VHE g-rays for CTA?

•

Low-luminosity GRBs•

HE n for IceCube?

Massive star collapses… probably
Kawaguchi+2018; KT, Yoon & Bromm 2016;
S. Inoue+2013; Murase+2006; & refs therein

GRB g-rays may be counterparts ofmany interesting phenomena

a 3.0
1 kpc

Issues on relativistic jets

Declination offset from 43-GHz core (mas)

2.5

2.0

1.5

Black Hole Shadow in M87

External
mediumB ﬁeld

e+

50 µas

Rela%vis%c
Jet

Accre%on Disk
Black Hole

Blandford & Znajek 1977; Koide et al. 2000; Komissarov 2001;
McKinney & Gammie 2004; Barkov & Komissarov 2008;
Tchekhovskoy et al. 2011; Ruiz et al. 2012; Contopoulos et al.
2013; Bromberg & Tchekhovskoy 2016; KT & Takahara 2014;2016;  2017 

–4–5

EHT Collaboration; Hada+2011

Disk windPhoton

100Rs (0.06 pc)

Declination offset from 43-GHz core (mas)e-

7

1.0flux (or accretion rate) across the horizon, reand mass
spectively. Since the GRMHD simulations scale with theblack-hole mass, M is set only at the time of producing
0.5
the synthetic
images with the GRRT codes. The magnetic flux is generally non-zero because magnetic field
is trapped in the black hole by the accretion flow andsustained0by currents in the surrounding plasma.
These two parameters allow us to describe accretion
disks that are either prograde (a⇤0) or retrograde(a⇤ < –0.50) with respect to the black hole spin axis, andwhose accretion flows are either “SANE” (from “Standard and Normal Evolution”, Narayan et al. 2012) with
⇠ 1,–1.0
or “MAD” (from “Magnetically Arrested Disk”,–1 SANE–2–3
Narayan et1 al. 2003) with 0⇠ 15. 2 In essence,
accretion flows are characterized by moderate dimensionless magnetic flux and result from initial magnetic
fields that are smaller than those in MAD flows. Furb
thermore, the opening angles of the magnetic funnel in
SANE flows are generically smaller than those in MAD
0.8
flows. Varying a⇤ and , we have performed 43 highresolution, three-dimensional
6Rs and long-term simulations
covering well the physicalof magnetized ac0.01properties
pc
cretion 0.6
flows onto Kerr black holes.
All GRMHD simulations are initialized with a weakly
magnetized torus orbiting around the black hole anddriven into a turbulent state by instabilities, including the 0.4
magnetorotational instability (Balbus & Hawley
1991), rapidly reaching a quasi-stationary state. Oncea simulation is completed, the relevant flow properties
at di↵erent times are collected to be employed for the
0.2
further post-processing of the GRRT codes. The generation of synthetic images requires, besides the properties of the fluid (magnetic field, velocity field, rest-mass
density), 0also the emission and absorption coefficients,
the inclination i (the angle between the accretion flow
angular-momentum vector and the line of sight), the position angle PA (the angle East of North, i.e., counter–0.2on our images, of the projection on the sky ofclockwise
the accretion-flow angular momentum), the black holemass M and distance D to the observer.
Because the photons at 1.3 mm wavelength observed–0.4
by the EHT are believed to be produced by synchrotron
emission, whose absorption and emission coefficients depend on the electron distribution function, we consider
0.4
–0.4
–0.6
–0.8
the plasma to be composedof0.2electrons 0and ions–0.2
thathave the same temperature in the magnetically
RAdomoffset from 43-GHz core (mas)inated regions of the flow (funnel), but a substantiallydi↵erent temperature in the gas dominated regions (disk
1 8 6 | NInA particular,
T U R E | we
V Oconsider
L 4 7 7the| plasma
8 S E PtoT be
EMBER 2011
midplane).

•
•
•
•
•

Energy source
Acceleration mechanism
Dissipation
Stability
Particle source•

Electromagnetic model or
Fireball model ??

Figure 3. Top: EHT image of M87* from observations on
April 11, 2017 as a representative example of the images collected in the 2017 campaign. The image is the average ofthree di↵erent imaging methods after convolving each witha circular Gaussian kernel to give matched resolutions. The
largest of the three kernels (20 µas FWHM) is shown in thelower right. The image is shown in units of brightness temperature, Tb = S 2 /2kB ⌦, where S is the flux density,
is the observing wavelength, kB is the Boltzmann constant,and ⌦ is the solid angle of the resolution element. Bottom:
Similar images taken over di↵erent days showing the stability of the basic image structure and the equivalence amongdi↵erent days. North is up, East to the left.

Gammie 2018; Bronzwaer et al. 2018; Younsi et al.
2019). We limit ourselves to providing here a brief description of the initial setups and the physical scenarios explored in the simulations; see Paper V for details
on both the GRMHD and GRRT codes, which havebeen cross-validated for accuracy and consistency (Porthet al. 2019; Gold et al. 2019).
A typical GRMHD simulation in the library is characterized by two parameters: the dimensionless spina⇤ ⌘ Jc/GM 2 , where J and M are the spin angular momentum and mass of the black hole, respectively, and thenet dimensionless magnetic flux over the event horizon⌘ /(Ṁ Rg2 )1/2 , where and Ṁ are the magnetic flux

–1.0

–1.2

– 1.4

©2012 Macmillan Publishers Limited. All rights reserv

2 We here use Heaviside units, where a factor of

into the definition of the magnetic field.

p4⇡ is absorbed

Frontier of GRB study: polarization
BH + accretion flow
Relativistic jet
Gamma-ray

Photosphere Internaldissipation
Prompt emission (~100 keV)
GAP, INTEGRAL, POLAR(Yonetoku, Murakami, Gunji, Mihara, KT+11;12;
Gotz+09;13; Zhang, Kole+ 2019)
Prompt Opt? (Troja+17)

Simply speaking:Electromagnetic jets -> synchrotron emission
Fireball jets -> photospheric emission

Reverseshock

Afterglow

Forwardshock

Early-time Opt afterglow: Liverpool, Kanata
PL ~ 30% (Mundell+13), PL ~ 10% (Steele+09),
PL ~ 10% (Uehara, KT, Kawabata+12)
PL < 8% (Mundell+07)
Late-time Opt afterglow
PL ~ 1-3% (Covino+03)
PC ~ 0.6 % (Wiersema, Covino, KT+14)
Radio: ALMA PL ~ 0.2% (Urata, KT+19)

Synchrotron emission polarization
dne
d e

e

<latexit sha1_base64="yjdFrM+Ln/RATUAsTSn4lp14ugs=">AAACcXichVFNSwJBGH7cvsw+tLoUXUQxgkDGCIpOUpeOfuQHqMnuNtrifrG7Cib+gf5AQSeFiOhndOkPdPAnREeDLh16XReionqHmXnmmfd555kZyVQV22Fs4BMmJqemZ/yzgbn5hcVgaGk5bxtNS+Y52VANqyiJNlcVneccxVF50bS4qEkqL0iNw9F+ocUtWzH0Y6dt8oom1nWlpsiiQ9RJuS5qmljtlC0trHWroSiLMzfCP0HCA1F4kTJCtyjjFAZkNKGBQ4dDWIUIm1oJCTCYxFXQIc4ipLj7HF0ESNukLE4ZIrENGuu0KnmsTutRTdtVy3SKSt0iZRgx9sTu2JA9snv2zN5/rdVxa4y8tGmWxlpuVoMXq9m3f1UazQ7OPlV/enZQw57rVSHvpsuMbiGP9a3zy2F2PxPrbLA+eyH/PTZgD3QDvfUq36R55hoB+oDE9+f+CfLb8QTh9E40eeB9hR/riGCT3nsXSRwhhRyda+EKPfR9Q2FNCAuRcarg8zQr+BLC1gcNWo+A</latexit>F⌫m⇧L = 50 %

p

e
p+1
⇧L =
75 %
7 ' 60p+ 3(p=1-3, ordered B)⌫m / B<latexit sha1_base64="y5P7vIIFpU2LcWBwpA8HpTvNzXQ=">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</latexit>

2m⌫
Rybicki & Lightman 1979

singqualaw.
andTo examine adiabatic cooling, one should consider the energy
flux of radiation in the lab frame F̃ = 4π I˜1 , where I˜1 =Γ2 [β(I + I ) + (1 + β 2 )I ] is the first moment of intensity in the

Beloborodov 2011; Ito+2014; Lundman+2014

Beloborodov

Low density
case

(11)

(1) At one boundary of the computational domain µ → −1
and the transfer equation gives (S − I )/I → 0. This requires
I = S = I0 at µ = −1. Note that the optical depth∆τray passed along the ray in one step ∆ ln r depends on µ:∆τray (µ) = τ (r) ∆ ln r/(1+µ). Numerical integration is possible
only if ∆τray < 1, which is violated close to the boundaryµ = −1. However, in this region τray = τ/(1 + µ) ≫ 1 simply
enforces I ≈ S. In the process of integration, we set I = S
wherever ∆τray (µ) > 0.7.
(2) The transfer equation contains the term (1 − µ2 ) ∂I /∂µ =sin θ ∂I /∂θ . We use a grid θi (i = 0, ..., n), where θ0 = 0
and θn = π. The term sin θ ∂I /∂θ is not needed at θ0 andθn (it vanishes). For all other θi we evaluate this term using∂I /∂θ = (Ii+1 − Ii−1 )/(θi+1 − θi−1 ).
The transfer Equation (12) has no free parameters and thesolution is unique. The result is shown in Figure 1. The striking
feature is the strong beaming of the radiation field in the fluid
frame, even at large optical depths τ ∼ 10. Beaming may bedescribed by the ratio of intensities at µ = 1 and µ = −1:b(r) ≡ I (1, r)/I (−1, r). This quantity is shown in Figure 2. It
significantly deviates from unity starting at τ ∼ 10. In the zoneof τ ≪ 1, I (1, r) = const and I (−1, r) = I0 ∝ r −2 . Therefore,b(r) ∝ r 2 at r ≫ R⋆ .

Lth / r 2/3

= S0
I1 is
and
ame.
0, 0)
F̃ =
onst
oton
Figure 1. Angular distribution of radiation intensity in the fluid frame at three
radii: r/R⋆ ≈ 0.03, 0.1, and 0.5, which correspond to optical depths τ ≈ 30,
10, and 2, respectively. For a better comparison, we plot (r/R⋆ )2 I , where thefactor (r/R⋆ )2 compensates for the photon dilution due to expansion. The overall
normalization of the transfer solution is chosen so that I (µ, r) = (r/R⋆ )−8/3 =τ 8/3 at radii r < 0.01R⋆ where radiation is nearly isotropic (I does not dependon µ) and follows the adiabatic cooling law I = τ 8/3 . Open circles show theintensity at τ = 2 that is obtained by the Monte Carlo code (Section 3.3).
(A color version of this figure is available in the online journal.)

Relatively dim,low Ep, PL <~40%

3.2. Adiabatic Cooling

=1
⌧⇠

Tn

0 rph

(12)

Photon intensity angular
distribution in the fluid framer
Optically-thickphoton dominated
jet

opic
t of
ross
(3)

High density
case

T / r 2/3

Lk

(13)

milar
rical
e is

s of
n for
−3
R⋆
dary
105 .

Bright, high Ep, Unpolarized

Lth

the
the
ives
hich
ated
that
onds

Photospheric emission polarization

Uniform jet model1

yj=100yj=10
yj=1
yj=0.1

Π(60-500keV)

0.8

0.6

0.4

0.2

0

4

5

0

1

2

3

4

5

q=θv/θjFigure 3: Left: Polarization degrees as functions of q = θv /θj in the SR model (seeSynchrotron
withOrdered(helical)BSchematic
field
Synchrotron
withnetRandom(transverse)
B isfield
rees as functions of q = θv /θ[4]theSO model,j in
fordetails).
Right:where
picture
of
the
jet.
The
polarization
property
emission
the similar
propertiesof sight and θj is the jet opening
angle.
(Γθj )2emission,
j ≡bright
determinedby ythefromPhotospheric
the points withθ ∼ Γ−1hasaroundthe lineof

or of the jet. Typical parameters
are adopted
for thesight, whosepolarization
vectors (represented by the thick arrows) are axisymmetric.
etails). Right: Schematic picture of the jet with theetic fields •(thinTypicallines). Onlya fraction
ofhighthe emitting
GRBs
have
pol
in SO model<
patches
observed
withθvpdegree∼ θp decrease the net Π. On the other hand, the bursts we
f sight is•brightbecauseofobserved
theevents
relativistic
beaming
Only
off-axis
havehigh which
pol degreein SR/Phmodelsare allverybright,implies thatsome patches
are seen with θ < θ .
vp ∼p> 30%.
Therefore, the SR model is not favored to explain the observed Π ∼KT 2013, arXiv:1308.5733

Monte Carlo simulation in a uniform jet model
TOMA
ET AL.
TOMA
ET AL.

Vol.
698
Vol.
698
1

-4

10

-7

10

-8

10

-6

-7

Π(60-500keV)-6

0.8 0.8

0.6

0.4

-8

10

0.2

0.6


Π(60-500keV)

-5

10

10

yj=100yj=10
yj=1
yj=0.1

1
0.8

10-5

10

1

10

Π(60-500keV)-4

10Time-Averaged Flux (2-400keV) [erg/cm 2/s]

Time-Averaged Flux (2-400keV) [erg/cm 2/s]

048 1048

0.6

0.4

0.2

0.4
0

0

1

2

3

4

5

q=θv/θj

0.2

Synchrotron with

Figure 1: Left: Polarization degrees as functions of q = θv /θj in the SO model, where-910
field yj ≡ (Γθj )2 ,0 θ is the viewing angle of the line of sight and θ is theOrdered
0
1
2
3
4
-9
jet openingBangle.
10
10
10
10
10
10
j4
1v
2
3
0
0
1
2
3
4
10
10
10
10
10
10
10 obs
10
10
3
obs
Ep
[keV]
the
Lorentz
factorof the jet.104Typical parameters are adopted for the[keV] 10101 where Γ is102 bulk Ep
1
obs
obs
y =100
Ep
[keV]
Ep
[keV]
y =10 for the
spectrum(see[4] fordetails).
Figure 5. Ep,obs − F diagram calculated in our Monte Carlo simulation. The
Figure 6.emission
Ep,obs − Π diagrams
simulated
eventsthat can be Right:
detected bySchematic picture of the jet with the
y =1gure 5.
Ep,obs −events
F diagram
in by
ourWXM
MonteonCarlosimulation.
The by Figure
simulatedthat cancalculated
be detected
HETE-2are representedy=0.1
6. Ep,obs−
Π
diagramsforthesimulated
eventsthatcanbedetected
by
GRAPE
in
the
SO
(red
open
circles),SR(greenfilledcircles),andCD
pluslines). Only a fraction of the emitting
toroidal 0.8
component of the magnetic fields (blue
(thin
dots.
Theythat
are cancomparedwith thedataHETE-2(points witherrorbars; Sakamoto
mulatedeventsbe detected
byHETE-2
WXM onare represented
bysigns)
models.
Theopenadoptedparametersareasfollows.
The
fixedparameters
GRAPE
in theSO (redcircles),SR(greenfilledcircles),
and
CD
(blue
plus
−1
θ <0.5,Γparametersthelineof1.2,
sightisbrightet al.
ots. They
are2005).
comparedwith
the HETE-2
data (points witherrorbars;
Sakamotoareγ = shell,100,
q 2 100,=around−2.0,=−0.2,β=andparameters
T =20 s. because of the relativistic beaming
signs)
models.
The qadopted
areαandasfollows.
The
fixed
1 = G
•
We
create10,000jets
withrandom
q
and
fixed
=
then
0.6
j
al. 2005).
The
distributionofthesourceredshiftz
isassumedtobeinproportiontoare γ = 100,eﬀect.
q 1 = 0.5, q 2 = −2.0, α = −0.2,β = 1.2, and T = 20 s.the
2
′
j

j

j

Π(60-500keV)jcosmic star offormation
rate.redshift
The parametersr Adegrees
γ νproportion
are distributed
0toanddistribution
the sourcez is assumed
be incalculate fluxes for random viewingTheangles,
andpolarization
forto theso00among GRBs and X-ray flashes in several models of geomet- cosmic
0.4 Thethatstar
the formation
simulated Erate.
− F parameters
diagram is consistent
data so(see
p,obs
r02 A0 andwith
γ ν0′ theareobserved
distributed
detectablemongrical
GRBs
and
X-ray
flashesin several
modelsgeomet-et al. that the
Figure
5).
See
text
for
the
cases
of
the
spectralindices
distributed
realistically,
structureof GRB
jetsevents(Zhanget al.
2004; ofYamazakisimulated
Ep,obs −overF diagramconsistent
the observed
data (see patches with characteristic angularuniformthe isshell,
butwithconsistsof multiplefor5).
−0.5
α<
0.9 < β < 1.8.
2004;
Dai
&
Zhang
2005;
Toma
et
al.
2005;
Donaghy2006).
We
FigureSee<text
for0.4
theandcal structure
of
GRB
jets
(Zhangetal.
2004;
Yamazakietal.
0.2cases of the spectral indices distributed realistically,• & Zhang
Ps
areGRBclustered
~40%
for
SO
model
while
~0%
for
SR/CD
models.
CD θp ≪ θj [6] (see Figure 2). In the case of
size
smaller
thaninjet
opening
angle,(A colorof thisis 1.8.
available
the online
journal.)for −0.5< α version< 0.4much
and0.9figure<β<
generated
10,000jetsetwith
Lorentz
factor, γ2006).
, and opening
004; Dai
2005;
Toma
al. 2005;
DonaghyWe−1Γ of this∼ figureθj0, itis isnaturalsees multiple patches with diﬀerent magnetic fieldcanhaveeventsPjet>according
70% (A color version
availablein thethat
onlineone
journal.)angle,
θjmodel, anda random
forγ each
enerated
10,000GRB
jets
withviewing
Lorentzangle
factor,, andopening
0
1
2
3
5
−1
directions,and thatobserves
significant
PA4 ξchanges.
≪ θj , oneof sindθv dφjetwith
θv < 0.22
et al. 2002).
We assume
the coefficients
ξ1 and2 obey the On the other hand, if Γngle, to
θj ,theandprobability
a randomdistribution
viewing angleforθveachaccordingq=θv/θj12
For eachburst generated
calculatethe νI spectrum to et al.lognormal
distribution(Ioka&coefficients
Nakamura
2002)
withaveragessees
a limitedof the ξcurvedo the rad.
probability
distributionof sinwe
θv dθ2002).onlyWeassumethattherangeobey
the fields, which leads to no significant PA
v dφ with θv ν < 0.22
1 and ξ2magnetic
This corresponds
to12 obtain the spectral peak energy, Ep,obs , and the fluence, I, in theof 1 anddistribution
logarithmic
variancesof 0.3 andis
0.15,
respectively.
Thead. For each burst generated we calculate the νIν spectrum tolognormal(Ioka& Nakamura
2002)with Inaverageschangeeven
if theemission
patchy.
such
a scenario,
GRB
100826A corresponds
Figure3:
Left:
Polarization
degreesasfunctionsofq=θ
v /θj in the SR modelKT,
al. 2009
2–400
keV range
by usingEet
Equation(4). Since E I, in’sthe
and Is of 1 lastparameters
are fixed
by αand= −0.2andβ = 1.2, whichemissionbtain
theSakamoto,spectralpeak Zhang
energy,andtwologarithmic
variances0.15,
respectively.
p,obs , and the fluence,p,obs
to theof Γ−1of∼0.3
θj , while
the
other
twoTheburstswithno PA
correspondto[4]casefordetails).
Right:
Schematic
pictureof thephotosphericjet. The
netchangepolarization
propertDissipative electromagnetic jets
1748

Magnetic instabilities in relativistic jets

O. Bromberg and A. Tchekhovskoy

1751

Figure 9. Meridional slices of log10 (σ ) through the x − z (left) and yenergy− z (right) planes, Figure
in a 3D 16.
jet atThe
timethermalt = 2984R≈ 1 s(solidin ourlines)
high-and the Poynting flux
L /c flux
resolution 3D simulation,
model
M3HR.
Note
that
the
horizontal
axis(dashed lines), as a function of z. We show thescale
fiducial model, M3, at t =has been stretched 1885R
by approximately
an ordermagnitude
to clearly
blueoflines)
and a highresolution model, M3HR, at
L /c ∼ 0.6 s (thickshow the jet structure.
The jets
highlymagnetized,with
0 ≃ to the same distance,t = 2515R∼ 0.8outs (thingreen
lines). Bothjetsσextend
L /cstart
8 cm, converge on to the9
25. They recollimate
at
z
≃
17R
≈
1.7
×
10
zh ≃ coll
150RL ≃ L1.5 × 10 cm, and have the same width, though the highaxis, and develop anresolution
internal kinkinstability, atwhich
is seenslower
as small-scale
jet propagatesa velocity
by 25 per cent due to somewhat
Byellow–red wigglesth
in
the
jets.
The
decreaseofthemagnetization(seen
as the jets show someincreased wobbling motions of the head. Even
thoughthe transition in colour
from
red
to
yellow)atthe
wigglesreflects
magdifferences at their heads, the dissipation rates at their recollimation points
netic energy dissipation via the internal kink instability. Some field lines,
Figure 8. A snapshot of the central region in our fiducial 3D model M3 atare essentially the same. This indicates that the dissipation could be driven
which have small enough opening angles to maintain strong causal cont = 4400RL /c ∼ 1.5 s, when the jet head is at z = 800RL ∼ 8 × 109 cm,
by a large-scale instability and not affected by Bromberg
the microphysics.& Tchekhovskoy 2016;
across the jets, become unstable earlier and dissipate their energy at
or about 10 perFigure
cent of the
The colour
schemethe left-hand
15.stellar
The radius.
morphology
of jets
with indifferent
powers. Thetact
left-handseeto also Porth & Komissarov 2015
These lines form a ‘wedge’ of lower σ that extends down
panel shows thepanel
log10shows(∇ × B),
which is a tracer
of conduction
and /c ∼lower
a low-power
jet in model
M3LP currents,
at t = 10995R
3.7 s.altitudes.
TheL|z| ≃ 10R ≈ 108 cm.
the right-hand panel shows the log (σ ), which is a tracer of magnetization.

•••

Recollimation of jet inside the starInternal kink instability & B field dissipation
L ~LPrompt emission spectra

Blackbodies in GRB prompt spectra

2

Vurm & Belobo

83

GRB 160625B

GRB 990123

Figure 1. Synchrotron spectra from an optically thin spherical shell with three electron distributions: mono-energetic (dottedline), Maxwellian (short-dashed line), and fast-cooling Maxwellian(long-dashed line). For comparison the Band fit of GRB 990123 isshown by the red thick curve.

Photospheric emission?

Synchrotron emission?
Figure 10. GRB 980425. Symbols are the same as in Fig. 7. In this case, we

In
vid
be
is
dep
Eic
ser
to
tru
we
bro
ing
to
B
of
neu
giv
ma
em
cha
in

been proposed over the years (Thompson 1994;
Eichler & Levinson 2000; Mészáros & Rees 2000;
Giannios & Spruit 2007; Beloborodov 2010, hereafter
B10; Levinson 2012; Thompson & Gill 2014). All sharea key feature: the jet is dissipative, i.e. significantly
heated as it propagates away from the central engine.
Th
This heating modifies the emitted photospheric radiFigure 11. GRB 990123. Symbols are the same as in Fig. 7.
buation from simple blackbody emission. The resulting
Ravasio+2017; Oganesyan+2017;& Beloborodov
2015 lep
spectrum wasGhirlanda+2007;
shown to have aVurm
nonthermal
shape thattions for the entire duration of each burst, we derive the contributionmaclosely resembles the phenomenological Band functionGAP data

GRB 100826A 27 ± 11%
GRB 110301A 70 ± 22%
GRB 110721A 84+16
−28 %

> 6%
> 31%
> 35%

2.9σ
3.7σ
3.3σ

yes
no
no

Table 1: Polarimetric data of the three GRBs obtained with GAP. The Polarization
2σlimit’limit means
Detection
significancePAatchange
degrees Π are Event
shownnamewith 1σ error.ΠThe ‘2σthe lower
limit on Π
theGRB
100826A
27
±
11%
>
6%
2.9σ
yes
2σ statistical significance level. The ‘detection significance’ means the significance22% > 31%3.7σ
nolevels for Π >GRB
0%. 110301A 70 ±GRB 110721A 84+16
> 35%
3.3σ
no
−28 %
TableEvent
1: Polarimetric
dataof thethree [ergGRBsobtainednameTfluencecm−2]
Epwith[keV]GAP. The Polarization
90 [s]−4 means +134
degrees
Π areshown with
1σ error.
The‘2σ×limit’
the lower limit on Π at theGRB
100826A
≃
150
(3.0
±
0.3)
10
606
−109
IKAROS solar power
2σ statisticalsignificance
The± ‘detection
significance’
means the significanceGRB 110301A≃ 5level.(3.650.03) × 10−5
106.8+1.85−1.75
sail demostrator
levels GRB
for Π110721A> 0%.
≃ 24 (3.52 ± 0.03) × 10−5
393+199
6000
4000

GRB 110301A

2000

Count rate (counts/s)

The Astrophysical Journal Letters, 758:L1 (5pp), 2012 October 10−104

The AstrophysicalJournal Letters, 758:L1 (5pp), 2012 October 10
Yonetoku et al.

Yonetoku et al.

3000
2000
1000

unt rate (counts/s)

Figure 1. Light curve of the prompt γ -ray emission of GRB100826A detected by the GAP. We divide the data into Interval-1 and -2 for the polarization analysis.

0

GRB 110721A

1000

2000

3000

−1

−1

0

Count rate (counts/s)

2000

4000

GRB 110301A

0

Count rate (counts/s)

The Astrophysical Journal Letters, 743:L30 (5pp), 2011 December 20

6000

Table 2: Light-curve and spectral data of the three GRBs takenfrom the GCN
for the polarization data is 60.0 counts s for GRB 110301A
Event nameT90 [s]fluence [erg cm−2 ] andp [keV]
51.6E
countss for GRB 110721A. The total numbers ofcirculars. Ep is the photon energy of the time-averaged νFν spectrum.
gamma-ray
photons
after subtracting the background are 1820
+134
photons for each burst, respectively.
GRB 100826A ≃ 150(3.0 ± 0.3) × 10−4 andTo1092
606−109
estimate the systematic uncertainty, we first consider−5the spin rate of the+1.85
IKAROS spacecraft. The rotation of the
GRB 110301A≃5(3.65 ± 0.03) × 10 instrument
106.8
−1.75
generally
reduces the systematic uncertainty because
differences of +199
each sensor and the geometrical skewness are
GRB 110721A ≃ 24 (3.52 ± 0.03) × 10−5theaveraged.393
However,in these case, the time durations of the−104
bursts are smaller than the period of rotation of the IKAROS
high axial symmetry in shape and high gain uniformity are keys forspacecraft.
reliable
The spinmeasurerate is 1.61 rpm and 0.22 rpm for theepoch of GRB 110301A and GRB 110721A, respectively. Using
2: Light-curveand fake
spectraldata ofdue
thetothree
GRBs
taken
from
GCN
the backgroundinterval ofthe data, wethe
created thehistory ofment of Table
polarization
and avoiding
modulation
backgroundgamma-rays.
background modulation curves with the same time interval weanalyzed,
and confirmed that each modulation was consistentcirculars.
Equitephotonenergyuncertaintyof the time-averagedνF
p is theν spectrum.
These
realizedthesmall
systematic
of
≃
1.8%
level
[16].
with
being
constant within the statistical error. We confirmed
70-300 keV
0

GRB 110721A

10

20Time since GRB trigger (second)

Figure 1. Light curves of the prompt gamma-ray emission of GRB 110301A(top) and GRB 110721A (bottom) detected by GAP. The vertical dashed lines
indicate the time interval of polarization analyses for each burst.

Yonetoku+11;12;KT2013,
systematic
error
duearXiv:1308.5733
to thetrend
data analysis
shorttotime
the thatbursttheandexhibitsa hard-to-softfrom 110ofkeV
duration
is aboutGAP
σsys,1mainly= 1.0%of the total
polarization
26 keV.
Therefore,
observed
the energy
rangesignalsofINTEGRAL data

T~150s

GRB 041219A

PA change
GRB 061122

T~5s• P>~30% at 2σ, non-zero at ~3σ,consistent with GAP results
Gotz+2009; 2013

Data vs theory• P > 30% (2s) & PA changes• The SO model is favored• The SR and photospheric emission models require fine tunings• More accurate data with more statistics needed• Any bright bursts with low P? -> Ph or SR model• Correlation between P and spectral shape/brightness?

Zi-Gao Dai, Mi-Xiang Lan

POLAR

LAUNCHING OPPORTUNITIE

referred to as the modulation curve. The relative amplitude, theamplitude divided by the mean, of this is linearly proportional
to the polarization degree through µ = µ100 P. Here µ is
the relative amplitude extracted from the modulation curve,
P is the polarization degree of the measured photon flux andµ100 is the relative amplitude for a 100% polarized beam.
• Chinese Space Lab, ∼2012
The µ100 parameter is an energy dependent instrument specificvalue which needs to be acquired through calibration with a• International Space Station
polarized beam or through detailed Monte Carlo simulations.
(Columbus external payload ISS)The POLAR detector [6] is designed to measure the Compton scattering angle of incoming photons using a low-Z target• Iridium next for a fee (16M$ x n)of 1600 plastic scintillator bars, divided into 25 modular units.
Each module is read out by a Hamamatsu H8500, a flat-panel
64 channel multi-anode photomultiplier (MAPMT). Due thefine segmentation of the scintillator target the Compton scattering angle can be measured with a relatively high precision,
resulting in a µ100 of around 40% at 140 keV which translates
into a high sensitivity to polarization. The use of a full plasticarray results in a high Compton scattering cross section forphotons in the 10’s of keV energy range, thereby making
the instrument sensitive to the lower energy component of
the GRB gamma-ray emission. Lastly the uniformity of thedetection surface results in a relatively large effective area and Fig. 1. Schematic overview of one single POLAR module together with thefull 5x5 module instrument [9].
a large field of view which covers approximately 1/3 of the sky,making the instrument optimal for performing polarimetricmeasurements on transient events such as GRBs [9].
The scintillator bars used in POLAR have a surface areaof 6 by 6 mm and a length of 176 mm. Optical cross talk2
between the scintillators is reduced both by placing a reflective
vikuiti foil [7] between the bars and by shaping the top andT90 (s)bottom Fluence*
Prob(PD<2%)
PDup (99%)
PA(deg.)PA Changeof the scintillator intoPD
a conical shapein which thesurface is reduced to 5 5by 5 mm. The scintillators are coupled
6.76 to the1.25
⇥ 10
9%
9%
45%
40
No
MAPMT using an optically transparent pad consisting5
2.82 of Mapsil
1.27QS1123⇥ 10[8]. This material
8% provides a good
13%
31%
164
No
opticalcoupling to the MAPMTand absorbs vibrations and shocks
6
0.21 which 7.4
⇥ 10
11%
5.8%
67%
38
Unknown
could cause damage to the PMT window. For further
5
1.2 shock1.34
⇥ 10
10%
31%
106
No
protection
a 3 mm thicklayer of rubber is12%
placed atthe top of the scintillators.
The 64 bars which make up a
5
8.0 module
1.93
⇥ 10
4%
14%
28%
164
Yes
are surrounded by a 1 mm thick layer of carbon fiber
N/A for mechanical
N/A
43%
122
N/A
support. Each15%
MAPMT is read out8%
using its
own
front-end
electronics(FEEs),
the
25
FEEs
are
read
out
by
N/A 3 central FPGAs.
N/AThe full effective
41%area of the instrument
0.49% is74%
17
N/A
Fig. 2. The full flight ready POLAR instrument.
covered by a 1 mm thick layer of carbon fiber for mechanicalstability. The outside layer is placed for mechanical support
while also serving as a passive shield for low energy charged or if the module has 1 triggering channel and at least one otherparticles. Additionally the full instrument is covered by a module has a coincident triggering channel. Modules without
multi-layer insulator (MLI) for thermal stability. A schematic triggering channels are not read out to reduce the data size ofeach event. The timing precision of trigger event is of the µs

Key Laboratory of Modern Astronomy and Astrophysics (Nanjing University), Ministry of Education, China
Dai• Zi-Gao
Swiss-China
collaboration• 50-500 keV
Scherrer• PaulFOV:~1/3Institut,of the5232,
sky Villigen, Switzerland
Hajdas,detected
Radoslaw inMarcinkowski,
Hua-Lin Xiao
• Wojtek
55 GRBs~ 8 months
Iridium next
• Pol analysis for 5 GRBs

Chinese Space Lab

ISS

Chinese Tiangong 2
Purple Mountain Observatory, Chinese Academy of Sciences, Nanjing 210008, ChinaSpace Lab

Mi-Xiang Lan, Xue-Feng Wu

Table 1: Summary of the five GRBs. The di↵erent properties of the 5 GRBs
and of 2 time bins of GRB 170114A (*in units of erg/cm in 10-1000 keV).
GRB
161218A
170101A
170127C
170206A
170114A
170114Ap1
170114Ap2

Kole et al. 2016, arXiv:1612.04098; Zhang, Kole et al. 2019

POLAR data

POLAR-2

Merlin Kole

Figure 1: The polarization
measurements
different timeof GRBof
170114A
using
a combination
Time-resolved
dataof polof 9degree
&bins
angle
GRB
170114A:of the POLAR and Fermi-GBM data as presented in [16]. The polarization degree is plotted along theradial axis whereas the polarization angle is along the theta axis. The polarization angle can be seen to varysignificantly during the GRB while the polarization degree remains around 40% throughout the GRB.

High degree (~ 40%) but angle change = low degree
forduration
consistent
withand
the
GAP
data
by a whole
factor 4. Additionallyseveral~components
will be replaced
upgradedin orderto improve

the scientific performance as well as to mitigate all problems experienced during the operationof POLAR and during its development. For more information on the issues encountered during
development the reader is referred to [17]. The overall design remains largely based on that of
POLAR which is described in detail in [17]. The POLAR-2 instrument will have a total mass ofapproximately 100 kg, power consumption of approximately 200 Watts, physical dimensions of
700 ⇥ 700 ⇥ 830 mm3 and produce 30 GBs of data per day.

The time-resolved spectra are also explained bysynchrotron model
Second half (3 sec)
GRB
170114A
Figure 2: Posterior distributions of the polarization
parameters for the 5of
GRB
170114A
GRBs and the second time intervals of GRB 170114A. The panels on theleft from top to bottom are 161218A (a), 170101A (b) and 170127C (c) and on

3.2 Polarimeter Design
Similar to the POLAR design the instrument will have a modular structure. POLAR-2 will
Zhang, Kole et al. 2019; Burgess+2019; Kole 2019consist of a total of 100 detector modules. Photons are detected through their interaction within

POLAR2••

ICRC2019)572

be clearly observed that with POLAR it was not possible to distinguish between an unpolarized
and a 10% polarized GRB with this fluence. POLAR-2 can however clearly distinguish these two
scenarios with more than 99% confidence. It was additionally shown that POLAR-2 will be capable of performing measurements with an equal precision as that published by POLAR for GRB
2 . POLAR-2 is therefore expected to produce
Recently
be launchedin 2024
170206A forselected
GRBs with atofluenceof 2 ⇥ 10 6 erg/cm~
50 polper withyr witha precisiona totalof 50measurements
measurements per year
a precisionhigher orhigher
equal to or
theequal
5 GRB polarization
to
POLAR events
measurements
published by POLAR to date.

Figure 2: Left: Schematic representation of a module in POLAR-2. The carbon cover is shown in bla
here covering the top parts of the transparent plastic scintillators. Below the black grid used to hold the stillators in place is shown, below this the SiPM array is shown in green above the FEE electronics containalso the power supply for the SiPM and connectors for communication with the back-end electronics
power. Right: The schematic representation of the full POLAR-2 instrument. The 100 modules can be s
in black underneath the semi-transparent carbon cover. The red object as well as the blue box at the bot
are mechanical parts required for mounting on the CSS.

3.3 Simulation Based Scientific Performance of POLAR-2

Preliminary studies of the POLAR-2 performance were performed using a MC package ba
on the POLAR MC software as presented in [20]. The effective area and the M100 (a parame
used to express the sensitivity to polarization) as a function of energy were simulated for a typ
GRB using the current baseline design of POLAR-2. The incoming angle of the GRB here w
taken
26 off-axis,
to that ofphoton
GRB 170114A
observed
by POLAR. The simulations w
Figure 3: The effective area (for polarization events)
astoa befunctionof equal
incoming
energyof POLAR-2
performed for 3 different instrument configurations, the original POLAR instrument, the POL(blue) compared to that of POLAR (green) and thatinstrumentof a versionof POLAR with a 4 times larger area (red)increased in size by a factor of 4 (referred to as POLARx4) and finally POLAR-2.
on the left. The ratio of the effective area of POLAR-2
over thatof POLAR
as a design
functionof energy
canis bethe POLAR-2
simulations
the POLAR
for detector
modulesused while the PMTs
replacedby SiPMs.
The performanceof the SiPMs
is based on that of the data sheets ofseen on the right. It can be seen that especially at low
energies
a significant
improvement
is found.
S13361-6075 MPPC from Hamamatsu as well as on preliminary lab studies. The replacemen
2019,
ICRC 572
the PMTs by SiPMs was taken into account in the simulationKole
softwareby increasing
the phoeASTROGAM
Figure 3: e-ASTROGAM spacecraft with solar panels deployed.

Type
GRBs

Nb sources⇠ 300

Blazars> 17

Other AGN

>6

crystals. The depth of interaction in each crystal is measured from the di↵erence of recorded scintil

at both ends. Accurately measuring the 3D position and deposited energy of each interaction isComments
a proper reconstruction of the Compton events. The Calorimeter thickness – 8 cm of CsI(Tl) – m
detector having an absorption probability of a 1-MeV photon on-axis of 88%.
Minimum detectable polarization of radiation-length
10%,
20%,
and
The third main detector of the e-ASTROGAM payload consists of an Anticoincidence system com
main parts: (1) a standard Anticoincidence, named Upper-AC, made of segmented panels of plasti
50% in ⇠ 16, 40, and 100 GRBs per year,
respectively
covering the top and four lateral sides of the instrument, requiring a total active area of about(2) a Time of Flight (ToF) system, aimed at rejecting the particle background produced by the p
13 FSRQ and 4 BL Lacs (PKS 0716+714,
PKS
Upper-ACdetector 2155-304,
is segmented in 33 plastic tiles (6 tiles per lateral side and 9 tiles for the top
silicon photomultipliers (SiPM) by optical fibers. The bottom side of the instrument is protected
Mkn 421 and Mkn 501)
unit, which is composed of two plastic scintillator layers separated by 50 cm, read out by SiPMs
Time Digital Converters. The required timing resolution is 300 ps (1 ).
Bright radio-quiet quasars and Seyfert
galaxies:
For best
environmental conditions, the e-ASTROGAM satellite should be launched into a qu(inclination i < 2.5 ) low-Earth orbit (LEO) at a typical altitude of 550 – 600 km. The backgroundin such an3C
orbit is120,
now well-known
NGC 4151, IC4329a, NGC 4945, NGC 1068,
Centhanks
Ato the Beppo-SAX and AGILE missions. In addition
is practically una↵ected by precipitating particles originating from solar flares, a virtue for backgrou
42
18 currently known soft gamma-rayExtensive
pulsars
simulations of the instrument performance using state-of-art numerical toolsanumerical mass model of the satellite together with a thorough model for the background envir
shown that
e-ASTROGAM will achieve:
Persistent gamma-ray emission of 1RXS
J1708-4009,• Broad energy coverage (⇠0.15 MeV to 3 GeV), with nearly two orders of magnitude improv
4U 0142+61, 1E 1841-045, and SGR
1806-20continuum
sensitivity in the range 0.15 – 100 MeV compared to previous missions (Fig. 1;•
Excellentsensitivity
for the detection of key gamma-ray lines e.g. sensitivity for the 847 k
Currently known -ray binaries: PSRthermonuclear
B1259-63,
supernovae 70 times better than that of the INTEGRAL spectrometer (SPI);• Unprecedented angular resolution both in the MeV domain and above a few hundreds of MeV
LS I +61 303, LS 5039, HESS J0632+057,
ing the angular resolution of the COMPTEL telescope on board the Compton Gamma Ray(CGRO) and that of the Fermi/LAT instrument by a factor of ⇠4 at 5 MeV and 1 GeV, resp
1FGL J1018.6-5856, and CXOU J053600.0-673507
the e-ASTROGAM Point Spread Function (68% containment radius) at 1 GeV is 9’).
Cyg X-1, Cyg X-3, GRS 1915+105, 1E1740.7-2942,
SS
433,• Large field of view (>2.5 sr),ideal to detect transient Galactic and extragalactic sources, sbinaries and GRBs;
GRS 1758-258, GX 339-4, XTE J1550-56,• Timing
GRO
J1655-40
accuracy of 1 µs (at 3 ), ideal to study the physics of magnetars and rotation-power
8

Pulsars & PWNe
Magnetars> 18>4-ray binaries>6

Microquasars>9910, 11

well as the properties of terrestrial gamma-ray flashes;• Pioneering polarimetric capability for both steady and transient sources.

Table 2: Estimated number of sources of various classes whose polarization properties will be measured with
e-ASTROGAM in three years of nominal mission duration.

e-ASTROGAM is also expected to detect the gamma-ray polarization signal of a handful of Seyfert galaxies,
Tatischeff et al. arXiv:1805.06435

the identification of additional spectral components. For example, Guiriec et al. (2010) fit three bright SGRB withof thesesearches is for vastly tighter constraints on the neutrino flux from these sources. If this
MERGERS
multiple components, including a thermalNEUTRON
component, STARthe mainnon-thermal component, and an arXiv:1909.06085
extra power law,t the launch of a relativistic outflow in the presence of baryons is not a sufficient condition forwhich has been seen in additional burstsAND(e.g. HOW
Tak etTO
al. STUDY
2019). These
components could originate from the threeTHEM
CRs or that UHECR production may not require significant neutrino production, which may
stages (photospheric, internal dissipation, external shock) which can have overlapping emission given the enormous
is search from other sources.
velocities involved. Alternatively, some explain these same features through synchrotron radiation (e.g. Ravasio et al.
1
Eric Burns
2018). The detector response of gamma-ray scintillators
is non-linear,requiring a forward-folding spectral analysis
method that still (usually) relies on empirical functions rather than theoretically motivated ones, which significantly
4.5. The Prompt Emission Mechanism(s) of Gamma-Ray Bursts
complicates these studies.
rarelativistically
expandingfireball.
Aare
basic
representationof theemissionstages is
There
are twocapabilities
thatGoddard
providing
newCenter,insight
intopotential
the MD
promptGRB
1 NASA
Postdoctoral
Program
Fellow,
Space Flight
Greenbelt,
20771,
USAemission mechanism. Polarizatione energy
densityis truly enormousand,despite fields,
gamma-rays
cannot escapeuntil thejet reaches
probesthe existenceof large-scale
magnetic
where significant
detectionof highpolarization implies Poynting flux
11
12
⇠ 10
10 et
cm.
the central
engineresultin shells
that propagate
jets (Tomaal. Inhomogeneities
2009). Populationfrom
analyses
have only
recently
becomeavailable,
as these require Compton telescope
ABSTRACT
12
13bulkobservations
Lorentz factors.
Fast-moving
shellsbursts,
catch slow-moving
shells at ⇠ scattering
10
10 angle.
cm, releasingof particularlybrightgiven the probabilistic
Reported measurements are not
Neutronstar
mergersarethecanonical
multimessenger
events:
they
have
been
observedthrough photons
half aemissionthrough internal
Lastly, (e.g.
the jetpropagates
until theyet conclusive,given theshocks.
varied results
Lyutikovet al.outwards
2003; Yonetoku
et interaction
al. 2012; Chattopadhyay
et al.for2017;
century,
gravitational
sinceContinued
2017, radiation.
and advancement
are thought tosourcesof isneutrinos
and method
cosmic rays.
Studiesent
createstheal.afterglow
emission
via2019).
synchrotron
Zhang throughet
2019;
Burgess
etwaves
al.
in be
these
studiesa promising
to advanceof ourthese
eventsthat
enableinsights
into ofastrophysics,
inatheultrarelativistic
regime,ofthe
heavyimplies
elementof uniquetheemissionmechanismof particles
GRBs.
We
note
that the origin
lower fluence
SGRBs
There
areunderstanding
those
argue
theprompt
mainemission
GRBs originatesfromphotospherichistory
throughcosmictime,
cosmology,
dense
and
fundamental
physics.
sciencepolarization
willthatnotabemeasured
time,
but,
under
generalthat Uncovering
GRBs have thisthe sameovenrichment& theirMészáros2017). Or
Poynting
fluxforjetsomecan releasethematter,
promptthe
signalonceassumption
turbulence
16 LGRBs
requiresvast
observational
resources,
coordination,
and advancementsin theory and simulation, whichemissionmechanism(s),frommaycentral
stillbeengine
informative.
ion
hit
a criticalpoint, closerresultsto⇠ 10
cmunparalleledfrom the(Zhang& Yan 2010), whichareasconstrained
our currentof nuclear,atomic,andastroparticle
articlean attemptTheother newparameter
is understanding
the 4.4).
time o↵set fromthe GW
to GRBemission.
These physics.
were firstThis
exploredfor isGW170817
Bs
the origin
ofbyUHECR(Sectionat anda phenomenologicalfor(2017a).
science withneutronstar mergers.
Wegeneralprovideconstraints
rough estimates
the Granot
detection
GRB 170817A inprescription
Abbott et al.
Severalwonderful
analyses onfollowedon(e.g.
rates
of 2017;these Shoemaker
events throughvarioussignalsovernext decade.
summarize,
develop,andquantify between
the future
et al.
& Murase
2018a;
Zhang
et theal. 2018),
as well asWe
thosethat sought
to test or
distinguishleadingmodels(e.g.starMeng
et al. 2018)
alternative
scenarios(e.g. capabilities
Kasliwal et al.
2017).utilize
The separate
intrinsic sourcestimesciencethatneutronmergers
enableoranddiscuss thenecessary
to fully
these enigmaticparameters
each provide unique information on these events (Zhang et al. 2019). With a large enough sample
to delayunderstandour universe.
we can independently constrain the separate parameters, providing tighter constrains on, e.g., the jet launch time andthe sizeof the emitting
region atwaves,emissiontime. stars, black holes, nucleosynthesis, cosmology, equation of
Keywords:
gravitational
neutron
These studies then require polarization measurements of GRBs, which will be difficult given there is no active