SciPost Phys. Proc. 13, 008 (2023)

The ALPACA experiment: The project of the first sub-PeV
gamma-ray observation in the southern sky
Teruyoshi Kawashima⋆ for the ALPACA collaboration
Institute for Cosmic Ray Research, University of Tokyo, 5-1-5,
Kashiwanoha, Kashiwa-shi, Chiba, 277-8582, Japan⋆ tkawa4ma@icrr.u-tokyo.ac.jp

21st International Symposium on Very High Energy Cosmic Ray Interactions(ISVHECRI 2022)
Online, 23-28 May 2022
doi:10.21468/SciPostPhysProc.13

Abstract
The ALPACA experiment is a project aiming to observe sub-PeV gamma rays for the firsttime in the southern hemisphere. The main goal of ALPACA is to identify PeVatrons, theaccelerators of Galactic PeV cosmic rays, by observing sub-PeV pion-decay gamma rays
generated in interactions between PeV cosmic rays and the interstellar medium. This
new air shower experiment is located at an altitude of 4,740 m above sea level in the
middle of Mt. Chacaltaya in Bolivia. The air shower array consists of 401 scintillation
counters covering an 83,000 m2 surface area. In addition, a water-Cherenkov-type muon
detector array with an area of 3,700 m2 is installed to discriminate gamma rays frombackground cosmic rays. The prototype array ALPAQUITA will start data taking in 2022
and will extend to ALPACA in 2024. We report on a general introduction to ALPACA, theprogress of the project, and the sensitivity to sub-PeV gamma rays.
Copyright T. Kawashima.
This work is licensed under the Creative Commons
Attribution 4.0 International License.
Published by the SciPost Foundation.

Received 31-08-2022
Accepted 08-09-2022
Check forupdatesPublished 28-09-2023
doi:10.21468/SciPostPhysProc.13.008

Contents
1 Introduction

2

2 Outline of the ALPACA project2.1 ALPACA
2.2 The prototype array: ALPAQUITA

22
3

3 Construction status of ALPAQUITA

44 Conclusion

5

References

6
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SciPost Phys. Proc. 13, 008 (2023)

1

Introduction

The identity of the PeV-cosmic-ray accelerators in our galaxy, called PeVatrons, is unknown, andidentifying them is an important challenge in astrophysics. When PeV cosmic rays interact with
the interstellar medium, neutral pions are produced, which immediately decay into two photons above 100 TeV (sub-PeV gamma rays). Therefore, observations of sub-PeV gamma rays
are essential to the search for PeVatrons and have been actively performed in recent years. The
Tibet ASγ collaboration opened the door to sub-PeV gamma-ray astronomy for the first timewith the detection of sub-PeV gamma rays from the Crab Nebula [1] followed by HAWC [2]and LHAASO [3]. Then, sub-PeV gamma-ray observations from several sources were reportedby these collaborators, such as SNR G106.3+2.7, Cygnus Cocoon, and J1908+0621 [4–6].
In addition, the detection of diffuse sub-PeV gamma rays by the Tibet ASγ collaboration has
established the existence of PeVatrons in our galaxy [7]. Thus, the sub-PeV gamma-ray astronomy in the Northern Hemisphere has been greatly developed by these collaborators. However,the direct identification of PeVatrons is still to be achieved.
As reported in the H.E.S.S. Galactic plane survey [8], more high-energy gamma-ray sources
exists in the southern sky including some promising PeVatron candidates. The H.E.S.S. groupreported gamma-ray emissions of up to 40 TeV (spectral index: −2.3) from the Galactic Center region, strongly pointing out the possibility of the Galactic Center being a PeVatron [9].
Furthermore, the H.E.S.S. group reported the existence of an unidentified object HESS J1702420A, a PeVatron candidate having a hard gamma-ray spectrum (spectral index: −1.5) extending above 100 TeV without cutoff [10]. However, in the southern sky, sub-PeV gamma-ray
observations are still almost untouched, so it is important to start surveys.
The Andes Large area PArticle detector for Cosmic ray physics and Astronomy (ALPACA)
collaboration aims to demonstrate the first sub-PeV gamma ray observations in the southernhemisphere using an air-shoer-array detector. The array is going to be construct in Bolivia
and plans to start observations in 2024. The construction of ALPAQUITA, a prototype arrayof ALPACA, is currently underway and is scheduled to start observations in 2022. This paper presents a general introduction to the ALPACA project, the sensitivites of ALPACA andALPAQUITA to celestial gamma rays, and the construction status of ALPAQUITA.

2

Outline of the ALPACA project

The ALPACA project is an international collaboration between Bolivia, Mexico, and Japan. A
new array based on Tibet ASγ design [11] is constructed at an altitude of 4,740 m above sea
level in the middle of Mt. Chacaltaya, Bolivia (16◦ 23’ S, 68◦ 08’ W and the atmospheric depthof 572.4 g/cm2 ).

2.1

ALPACA

ALPACA array consists of two main components; surface air shower detectors (SDs) and underground muon detectors (MDs) (see Figure 1). The array has a geometrical area of ∼ 83, 000 m2
and captures air showers produced by the interactions of primary gamma rays and cosmic rays
with the atmosphere. Each SD consists of a 1 m × 1 m × 5 cm plastic scintillator and a 2-inch
PMT covered by a steel box. 401 SDs are installed at 15 m intervals and can determine theenergy and direction of arrival of a primary particle. On the other hand, MDs comprises water Cherenkov detectors constructed 2 m underground and its total area is ∼ 3, 700 m2 . EachMD consists of 16 cells, each of which have a area of 56 m2 and contains a water layer witha 1.5 m depth. Most of the electromagnetic components in air showers are absorbed by the008.2

SciPost Phys. Proc. 13, 008 (2023)

soil before reaching MDs, but only muons with energies above 1 GeV penetrate into the waterlayer of the MDs and produce Cherenkov radiations, which are collected by a 20-inch PMT suspended downward at the cell’s ceiling after several reflection on the wall and floor. Counting
the number of muons in air showers allow us to distinguish gamma-ray-induced air showers
from (hadronic) cosmic-ray-induced ones.

Ver.1.0
M. Ohnishi

W

as

te

w

ate

r

Water

Power Line

Full ALPACA AS and MD

300 m
2

1 m 2 AS Detector x (97+304) (82,800 m )2

58 m2 Muon Detector x (16+48) (3,700 m )Figure 1: Schematic view of the ALPACA array. The small black and large grey squares
mean SDs and MDs, respectively. The gray rectangle in the center indicates the central electronics hut. The solid octagon in the lower left corner shows the location andarea of the prototype array ALPAQUITA (see Section 2.2).
ALPACA’s high duty cycle (∼ 100 %) and wide field of view (∼ 2 steradian) make it suitablefor detecting sub-PeV gamma rays with low arrival frequencies. For 100 TeV gamma rays, theangular and energy resolutions are estimated at about 0.2 degrees and 25 %, respectively. The
hadron rejection capability of MDs is more than 99.9 % at 100 TeV, while keeping about 90 %of the gamma-ray efficiency.
Figure 2 shows the sensitivity curve of ALPACA to gamma rays. The horizontal and verticalaxis indicates the energies of gamma rays and the integral gamma-ray flux multiplied by energy. Blue points show the diffuse gamma-ray spectra in the Galactic Center region measured
by H.E.S.S. telescope. The blue dashed and solid line indicates a power-law function (spectral
index: -2.3) with an exponential cutoff and the hadronic emission model, respectively, fitted
to the results of H.E.S.S., showing that ALPACA touches the Galactic Center flux above 100
TeV in one year.

2.2

The prototype array: ALPAQUITAThe prototype-array ALPAQUITA consists of 97 SDs and one MD as shown in Figure 1. The
ALPAQUITA construction is ongoing (see Section 3) and observations are scheduled to begin
in 2022. Figure 3 shows the sensitivity curve of ALPAQUITA together with that of ALPACA
sensitivity and the energy spectra of gamma-ray sources that resides in the ALPACA field ofview. The thick black curve indicates that ALPAQTUIA can detect several gamma-ray sources
including HESS J1702-420A in one year.
008.3

SciPost Phys. Proc. 13, 008 (2023)

HEGRA Crab
Tibet AS+MD Crab
Galactic Center Diffuse
ALPACA (1yr 5σ)

Int. Flux x E (TeV cm-2 s-1)

10-10

10-11

10-12

10-13

10-14

0.1

1

10

100

1000

Energy (TeV)

Figure 2: Sensitivity curve of ALPACA (thick red curve) overlaid on the integral fluxof the diffuse gamma rays from the Galactic Center region. The curve is estimatedby MC simulation based on Tibet AS+MD within a zenith angle of 60 degrees. Black
points indicates the Crab Nebula spectrum measured by Tibet ASγ [1] and the dashedline is apower-law function with an exponential cutoff
fitted to the spectrum.
#VNCPGKCLR?JϦQRPMLMKWϦϦ«

Fig. 17 Sensitivity curve of ALPAQUITA (the thick black curve) for a gamma-ray point source together

Figure 3:
curves
ALPAQUITA
ALPACAoverlaid
gamma-ray
withSensitivity
the energy spectraof theof
H.E.S.S.
[26, 46] andand
HAWC[17] gamma-ray
sourceson
thattheare inthe
ALPAQUITA
of view. The thick
purple curveshows
the estimatedsensitivityof ALPACA.
energy spectra
offield
gamma-raysourcesin theALPACA
fieldof view[12]. TheFor each
ALPACA curve is derived by scaling the sensitivity curve of Tibet ASγ [40] considering the ratio of theenergy spectrum,
theexperiments.
solid line
represents
theobserved
by HESS[10] andareas of these two
Regardingthe energy
spectra,different region
colors indicate
different[8]sourcespecies:supernova
remnants
(SNR)
in
red,
pulsarwind
nebulae
(PWN)
in
blue,
composite
SNRs(ComHAWC [13] and the dashed line shows the extrapolated region.

3

posite) in green, compact binary systems (Binary) in magenta and unidentified sources (UNID) in gray,respectively. The Crab Nebula spectrum modeled by M. Amenomori et al. (2019) [14] is shown in orange.
Solid and dashed lines show observed and extrapolated regions, respectively. In extrapolating the spectra,
the attenuation of gamma rays due to the e+ e− pair production with the interstellar radiation field is nottaken into consideration

Construction status of ALPAQUITAThe central electronics
hut hasalreadyofbeen
built
and
SDsthearealreadyaboutlocatedand the declination
dependence
exposure
does
not20
affectconclusion
the around thehut (see Figure4). detection.
Installations of the perimeters, cables trenches, lightining rods, and internet
sourceConnection are also completed. ALPACA powerline is supplied by the branch of the Chacaltaya
J1702-420A
a gamma-ray
pointofsource
discovered
Observatory HESS
power
line. WeHESS
haveJ1702-420A
restarted istheconstruction
ALPAQUITA
inby
June 2022 as
H.E.S.S.
along
with
the
surrounding
extendedsourceHESS
J1702-420B
[46].
The
the COVID-19 situation is improved.
relation between these two sources is not clear. The energy spectra of both sources
extend up to " 100 TeV without showing cutoff, and HESS J1702-420A dominates the total gamma-ray flux beyond 50 TeV with its extremely hard spectral index(" 1.5). Although SNR G344.7-0.1 and PSR J1702-4128 are in the vicinity of the
gamma-ray emission region, it is not easy to consider these objects as the origin of
the emission [48, 49]. The absence of X-ray flux [50, 51] and the observation of
gamma rays in 10 GeV to 30 TeV [52] do not favor the leptonic origin scenario of the008.4
VHE gamma-ray emission, but the hadronic
scenario is not conclusive because of thelack of clear correlation between the VHE gamma-ray emission region and the ISM
distribution [46, 53]. According to Figs. 17 and 18, ALPAQUITA will detect HESSSciPost Phys. Proc. 13, 008 (2023)

Figure 4: Photo of ALPACA (ALPAQUITA) site. Mt. Chacaltaya can be seen in the
background. The building with the red roof is the central electronics hut. The multiple white cubes around the hut are SDs.

4

Conclusion

Sub-PeV gamma-ray observaton is crutial to search PeV-cosmic-ray accelerators called PeVatrons. Tibet ASγ, HAWC, LHAASO Collaboration has pioneered sub-PeV energy gamma-ray
astronomy in the Northern Hemisphere. The ALPACA experiment is a project aiming to searchfor sub-PeV gamma-ray sources in the Southern Hemisphere for the first time using a new air
shower array in Bolivia. ALPACA is scheduled to start its observatio in 2024 and is expected toyield interesting results after one year of observations. ALPAQUITA, a prototype array underconstruction, can detect several bright sources in one-year observation such as HESS J1702420A. The infrastructure for ALPAQUITA is being built locally and its DAQ is going to begin in
2022. Ultimately, the ALPACA project will identify Pevatrons in the southern sky and will help
understand their acceleration mechanism of PeV comis rays.

Acknowledgements
Funding information The ALPACA project is supported by the Japan Society for the Promotion of Science (JSPS) through Grants-in-Aid for Scientific Research (A) 19H00678, Scientific
Research (B) 19H01922, Scientific Research (B) 20H01920, Scientific Research (S) 20H05640,
Scientific Research (B) 20H01234, Scientific Research (C) 22K03660, and Specially Promoted
Research 22H04912, the LeoAtrox supercomputer located at the facilities of the Centro de
Análisis de Datos (CADS), CGSAIT, Universidad de Guadalajara, México, and by the joint research program of the Institute for Cosmic Ray Research (ICRR), The University of Tokyo.
K. Kawata is supported by the Toray Science Foundation. E. de la Fuente thanks Coordinación
General Académica y de Innovación (CGAI-UDG), cuerpo académico PRODEP-UDG-CA-499,
Carlos Iván Moreno, Cynthia Ruano, Rosario Cedano, and Diana Naylleli, for financial andadministrative support during sabbatical year stay at the ICRR on 2021. I. Toledano-Juarez
acknowledges support from CONACyT, México; grant 754851.

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