Schwarzschild‒Couder 型のCTA 小・中口径望遠鏡の開発状況
奥村 曉 1, 2
黒田裕介 1、古田和浩 1、田島宏康 1、片桐秀明 3、
他 CTA Consortium
1 名古屋大学 宇宙地球環境研究所（ISEE）
2 名古屋大学 素粒子宇宙起源研究所 （KMI）
3 茨城大学 理学部

2020 年 9 月 9 日 @ 日本天文学会 2020 年秋季年会

oxon@mac.com

1 GeVCWI [84]2.17±0.21

1 GeV
Baksan [85]2.77±0.17

1 GeV
MACRO [86, 87]
2.29 ± 0.15

2 GeV
3 GeV
3 GeV
IMB [88, 89]
Kam [90]
SuperK [91]2.26±0.11
1.94±0.12
1.74±0.07
Galactic CRs ~3 × 1015 eV (3 PeV)
Modiﬁed Cronin+ (1997)

Cosmic-ray Hadron Spectrum
104

PDG 2020

cludes the surrounding
medium in the10This section focuses on neutrinos
1 particle (m below
sec)
10
on of atmospheric
neutrinos in the Tev–
ompt component produced by charmed
10
3].
10
ntained eventsreflect neutrinos in theKneeegion where the product of increasing
1 particle (m year)10
ing flux is maximum. In the GeV reits angular10 distribution depend on thee detector and, to a lesser extent, on the10aively, we expect‹µ /‹e = 2 from countavors coming from the chain of pion and10o expectation, however, the numbers ofAncle
re similar rather
than different
by year)a fac10
1 particle (kmderstood to be a consequence of neutrino
10
ee the article
on neutrino properties in
2

Knee

Knee
4

10

-1

-2

Flux (m 2 sr s GeV) -1

-5

-8

2

-1

-11

-14

-17

E 2.6 F(E) [GeV1.6 m-2 s-1 sr -1]

2nd Knee

3

10

102

10

HAWC
MSU
Tibet
Akeno
CASA-MIA
KascadeKascade Grande
IceTop

Ankle

Ankle

Extra Galactic

HiRes 1
HiRes 2
Auger
Telescope Array

-20

-23

2

-1

1
13
10

-26

9

10

10

10

13

1011 1012 10151014 1016

10Energy (eV)181017 10

19

10

1014

10

15

10

16

1017

10

18

10

19

20

10

E [eV]
20

10

Figure 30.8: The all-particle spectrum as a function of E (energy
operties of atmospheric cosmic rays pro[95–105]
parison of themeasurements of atmo- 20 per-nucleus) from air shower measurements
8
-2.7
-3.0
~10
eV
(~100
MeV)
to
>
10
eV,
with
a
power
law
of
dN/dE
=
E
to
E
tation. These are the “sec ◊ effect” and]. The former refers originally to theWhat is the origin (PeVatron) of
Galactic
CRs(<where~3 PeV)?
Supernova
remnants? wit
ergy
E0 > 100TeV,
the low
flux makes measurementsof > 10 GeV muons (and neutrinos) at
Galactic
center? more in the small detectors in balloons and satellites difficult.
e the parent
pions propagate
Greisen [106] gives the following approximate analytic expres
phere where decay is enhanced relative2

A* is linked to this PeVatron. Sagittarius A* went through active
phases in the past, as demonstrated by X-ray outbursts5 and an
outflow from the Galactic Centre6. Although its current rate ofparticle acceleration is not sufficient to provide a substantial
contribution
to Galactic
cosmic rays, Sagittarius A* could have
LETTERRESEARCH
plausibly been more active over the last 106–107 years, and therefore
should be considered as a viable alternative to supernova remnants
as a source of petaelectronvolt Galactic cosmic rays.
30 photon statistics accumulated over the last 10 years of
The large
observations with the High Energy Stereoscopic System (HESS),
H.E.S.S.improvements(2016) in the methods of data analysis, allow for
together with20
a deep study of the properties of the diffuse very-high-energy (VHE;

160.0

10 a

10–11
+00.4

1/r

Galactic latitude (degrees)

6.0 × local cosmic-ray density

+00.2

1/r 2

+00.0

+00.2

23.0

+00.0

Sgr A*–00.2

Sgr A*

–00.4

0

20

40

60

80

100

120

Projected distance (pc)

140

160

180

200

7.8

–12

10
–00.2

–00.4 2
–00.6

b
61.7

+00.4

E2 × flux (TeV cm–2 s–1)

+00.6

1.9

Counts per pixel

wCR(≥10 TeV) (10–3 eV cm–3)

Galactic Center Region

spatial distribution of these cosmic rays which, together with the gas
distribution, shape the morphology of the central molecular zone
seen in VHE γ-rays. Figure 2 shows the radial profile of the E ≥ 10 TeV
cosmic-ray energy density wCR up to r ≈ 200 pc (for a Galactic Centre
distance of 8.5 kpc), determined from the γ-ray luminosity and theamount of target gas (see Extended Data Tables 1 and 2). This highenergy density in the central molecular zone is found to be an order ofmagnitude larger
10–10than that of the ‘sea’ of cosmic rays that universally
fills the Galaxy, while the energy density of low energy (GeV) cosmic
rays in this region has a level comparable to it8. This requires the presence of one or more accelerators of multi-TeV particles operating in
the central molecular zone.

–0.5
Diffuse emission (×10)
Model (best fit): diffuse emission
Model: diffuse emission E 68%–1.4
cut,p = 2.9 PeV
00.0
359.5
90%
Model: diffuse emission E cut,p = 0.6 PeV
Galactic longitude (degrees) 95%
Model: diffuse emission E cut,p = 0.4 PeV

00.0
359.5
359.0
Figure 2 | Spatial01.0
distribution00.5of the cosmic-ray
density versus
Galactic longitude (degrees)
10–13
projected distance from Sgr A*. The vertical and horizontal error bars
molecular gas, as traced by its HESS
CS lineJ1745–290
emission30. Black star, location of
Figure1 | VHE
γ-ray
image plusof thesystematic
Galactic Centreshow
the 1σstatisticalerrorsregion.
and theThebincolour
size, respectively.
2
scaleFits
indicates
0.02°
× 0.02°a, Theblack lines
outline= 11.8/9),
a 1/r(blue line, Sgrχ2/ A*. Inset (bottom left), simulation of a point-like source. The part ofto thecounts
data ofper
a 1/r
(red
line,pixel.
χ2/d.o.f.
the image shown boxed is magnified in b. b, Zoomed view of the inner
the regions
to calculate
the cosmic-ray(black
energyline,
densitythroughout= 61.2/9) cosmicd.o.f. =used
73.2/9)and a homogeneous
χ2/d.o.f.
70 pc and the contour of the region usedof the1 to extract the spectrum 10
the central
molecular
A sectionof 66°alongis excluded
fromannuliray density
radialzone.
profileintegrated
the lineof thesight
are shown.∼The
diffuse emission.
(see Methods). White
contour lines indicate the density distribution ofEnergy, E (TeV)
best fit of a 1/rα profile to the data is found for α = 1.10 ± 0.12 (1σ). The
Figure 3 | VHE γ-ray spectra of the diffuse emission and HESS
radial profile
isaffiliations
clearly appear
preferred
the*Lists1/r
of participants
and theirat the forend oftheHESS
paper. data.
J1745−290.
The y axis shows fluxes multiplied by a factor E2, where E is the
6 ⦿*in units of TeV cm−2 s−1. The vertical and horizontal
If the accelerator injects particles (here we consider protons through- energy on the0 0x axis,
M O N T H 2 0 1 6 | VO L 0 0 0 | NAT U R E | 1
error
bars show the 1σ statistical error and the bin size, respectively. Arrows© distribution
2016 Macmillan Publishers
Limited.
reservedout) at a continuous rate, Q! p(E ), the radialof cosmic
raysAll rights
represent 2σ flux upper limits. The 1σ confidence bands of the best-fit
in the central molecular zone, in the case of diffusive propagation, is spectra of the diffuse and HESS J1745−290 are shown in red and blue
Q! (E )
described9 as wCR(E , r , t ) = 4πDp (E)r erfc(r/rdiff ), where D(E) and rdiff are shaded areas, respectively. Spectral parameters are given in Methods. The
the diffusion coefficient and radius, respectively. For timescales t red lines show the numerical computations assuming that γ-rays result fromthe decay of neutral pions produced by proton–proton interactions. The
smaller than the proton–proton interaction time (t pp ≈ fluxes of the diffuse emission spectrum and models are multiplied by 10 to

Massive black hole (4 × 10 M ) at Sgr A (bright radio source)

Point source HESS J1745–290 at Str A* and diﬀuse gamma-ray emission

Diﬀuse component has a cutoﬀ energy of 2.9 PeV (68% conf.) → PeVatron?

3

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Cherenkov Telescope Array (CTA)

cherenkov
telescope
array

Image Credit: G. Pérez, IAC, SMM
5

Cherenkov Telescope Array (CTA)

cherenkov
telescope
array

Image Credit: G. Pérez, IAC, SMM
5

Cherenkov Telescope Array (CTA)

cherenkov
telescope
array

Image Credit: G. Pérez, IAC, SMM
6

Cherenkov Telescope Array (CTA)

cherenkov
telescopearray

Large-Sized Telescope (LST)
Dia. : 23 m
Energy : 20–150 GeVNTel : 4 @ North, 4 @ South

Image Credit: G. Pérez, IAC, SMM
6

Cherenkov Telescope Array (CTA)

cherenkov
telescopearray

Medium-Sized Telescope (MST)Dia. : 12 m
Energy : 150 GeV–5 TeVNTel : 15 @ North, 25 @ South

Image Credit: G. Pérez, IAC, SMM
6

Cherenkov Telescope Array (CTA)

cherenkov
telescope
array

Small-Sized Telescope (SST)Dia. : 4 m
Energy : 5–300 TeVNTel : 0 @ North, 70 @ South

Image Credit: G. Pérez, IAC, SMM
6

Cherenkov Telescope Array (CTA)

cherenkov
telescopearray

Schwarzschild–Couder Telescope (SCT)Dia. : 10 m
Energy : 150 GeV–5 TeVNTel : 15 @ North, 25 @ South (incl. MSTs)

Image Credit: G. Pérez, IAC, SMM
6

Davies–Cotton Configuration
Spherical (R = 2f)
Parabolic

Focal Length f

Focal Plane

Another standard optics design in Cherenkov telescopes (Parabola for LSTs)
Initially proposed for solar power plants
Wide field-of-view (FOV), but thus low angular resolution
7

0
0

1

2

3
4
Field angle [deg]

5

6

7

Schwarzschild–Couder Configuration

nfigurations of OS summarized in Table 2. (Right) The effective diameter of theV. Vassiliev et al. / Astroparticle Physics 28 (2007) 10–27

2
1

7

sagittal

30 0−1

2 × max { RMS

20
−2

−3
10
−4
0−5
0 −1

9

, RMS

Effective area [m 2]
Cross − sectional plane X [m]403} [arcmin]

Conf 1
Conf 2
Conf 34
50

10tangential

60 5

01

1

22

3 34
5 4 6
75 8
Cross
−sectional
plane Z [m]Fieldangle [deg]

96

10

8

Conf 1
Conf 2
Conf 3

LST/MST pixel size

6
5
4
3
2
1
0
0

11
7

Vassiliev+ (2007)

1

2

3
4
Field angle [deg]

5

6

7

theThefocal
plane forrays
atsecondary
field angle
anglesfor
ofmirrors
zero(left)
and five (right)
eft)
effective
areatangentialasa function
of
field
the
three
of OS
summarized
in resolution
Table 2. (Right)
Thesameeffective
diameteAsphericalprimary
andtoconfigurations
achievewide
FOVand betterat thetime
e light distribution in the focal plane of OSs.

Wider FOV brings fast survey and wider eﬀective area for higher-energy photons

]

]

Finer
highercan
sensitivity)and compact camera (→ less expensive) areBoththeshower-image
primary and resolution
secondary(→
mirrors
be segmented
5 expected5
to reduce
the cost of the optical system. A possible arrange4 Initially
proposedby theCTA US groupfor MSTs
mentof mirror
facets,as ‘‘petals’’,is shown
in Fig. 7.4 This
schemehas the advantage of requiring a minimal number
3
38

Schwarzschild–Couder Proposals for CTA
Schwarzschild–Couder MST

Schwarzschild–Couder SST

D = 10 m
D=4m

Credit: Deivid Ribeiro

Schwarzschild–Couder MST (SCT) with 10-meter diameter for MST extension‣

US, Italy, Germany, Mexico, and Nagoya‣ Japanese group contributions in SiPM, electronics, MC, and software

4-meter SC SST × camera design chosen to be the final SST design from three proposals in June 2019‣

Similar Japanese contributions but more active in SST
9can be later upgraded to a full 8.0°diameter envisioned for the SCT. A FEE module= 64 pixels with a size of 6.5 ⇥ 6.5 mm2 each (0.067°on sky). The SiPM, Hamamatsu
Need
Compact
Cameraswith
SiPMs
42, is selected as the photon detector for 16 FEE modules of the pSCT camera.
SCT Prototype Camera (11328 pixels)

SST Prototype Camera (2048 pixels)~80 cm~40 cmVassiliev (2017)

Credit: Christian Föhr (MPIK)The concave secondary mirrors make the plate scales (≡ 1/f) large
e 6: Assembled
mechanicalstructureofthecamera(Left).
Eight
FEE
modulesinstalled~6mm
and enable us to build compact cameras

T camera (Right).

Silicone photomultipliers (SiPMs) are used instead of conventional
photomultipliser tubes (PMTs)
10

SST Optical System
E. Giro et al.: Optical validation of the ASTRI SST-2M Cherenkov telescope

Giro+ (2017)

Camera Pixel Size ~6 mm

Fig. 5. PSF of the ASTRI SST-2M telescope across the focal plane. Alignment and optical performance have been optimized at 3 . The Cherenkov
camera pixel size is overplotted for each PSF.

Achieved good enough optical resolution matching the SST pixel size

sharperFirst
than realization
the others. The ofdi↵erence
between images taken observations
at the higher energies
the Schwarzschild–Couder
configuration
ever(>1 TeV), showing a goodat the same angles is no more than 6%.
agreement with the expected performances.
To calculate the D80 parameter, a background image was acquired on the nearby sky patch without bright stars in the CCD Acknowledgements. The authors acknowledge the support of the local sta↵11of

https://www.media.inaf.it/2019/06/05/hamamatsu-astri/

Lombardi et al.: atFirst5.4
detection
SST Crab Observations:S. Detection
σ of Crab Nebula at VHECountsprevented
level. Ne
NON = 360 ± 19, NOFF = 233 ± 14, αON/OFF = 0.88
the sourc
200
NEXC = 127 ± 24
threshold
Significance (Li&Ma) = 5.4 σ
Altho
150
from theimportancal design100
The dualparticularlar resolu
50
dual-mirr
Lombardi+ (2020)
large field
0
0
10
20
30
40
50
60
70
80
90
a smaller|Alpha| [deg]to cover w
The s
Fig.camera
2. |Alpha|-distributionsof the Crab
Nebula (ON,black)and the signal“ASTRI”(lead by Italian groups)
successfully
detectedgamma-rayisforeseebackground
(OFF,
grey)
data
from
ASTRI-Hornobservations
takenfrom the Crab Nebula
between 5 and 11 December 2018 above an energy threshold of ⇠3 TeV. paigns on
Combination
the Schwarzschild–Couder
and a line
SiPM(atcamera
verified Nebula. I
The regionofbetween
zero and the vertical dashed10 ) represents
the fiducial signal region.
(mainly12 o
250

TimeON = 12.4 h

Another (Our) SST Camera Test Observations

https://www.cta-observatory.org/chec-achieves-first-light-on-astri/“Our” camera also succeeded in air-shower observations on the same prototype
telescope (replaceable with the Italian camera)
Additional test observations canceled due to mirror re-coating and COVID-19
13

Another (Our) SST Camera Test Observations

https://www.cta-observatory.org/chec-achieves-first-light-on-astri/“Our” camera also succeeded in air-shower observations on the same prototype
telescope (replaceable with the Italian camera)
Additional test observations canceled due to mirror re-coating and COVID-19
13

m

Schwarzschild–Couder MST Optical System

Rousselle+ (2015)

2.5 times larger than the SST optical system

Schwarzschild-Couder telescope optical system, which inMore
numberof segmented
thus morell as theirrespective
bafflesand the focalmirrors,
plane. Right:
CAD complex
d-Coudertelescope(pSCT) was
under
constructionfinishedat the Fred
Opticalalignment
successfully
14

m

Schwarzschild–Couder MST Optical System

Rousselle+ (2015)

2.5 times larger than the SST optical system

Schwarzschild-Couder telescope optical system, which inMore
numberof segmented
thus morell as theirrespective
bafflesand the focalmirrors,
plane. Right:
CAD complex
d-Coudertelescope(pSCT) was
under
constructionfinishedat the Fred
Opticalalignment
successfully
14

Crab Observations by Prototype Schwarzschild–Couder

Crab Nebular detection at 8.3σ, while the prototype FOV is still limited
15

Crab Observations by Prototype Schwarzschild–Couder

Crab Nebular detection at 8.3σ, while the prototype FOV is still limited
15

Summary and Plans
Prototype Schwarzschild–Couder (SC)telescopes for the CTA
Medium- and Small-sized Telescopes constructedFirst realization of the SC configuration
Both succeeded in air-shower Cherenkov observations andCrab Nebula detection with prototype SiPM cameras
Prototype SC-MST will be upgraded with full FOV coveragewith more SiPM tiles and new electronics
SST is going to finalize the optics and camera designs
New SST prototype(s) will be built and tested before thepre-production phase16