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Research Result Report
ICRR Inter-University Research Program 2023
Research Subject: Study of high-energy cosmic rays at a high altitude in Tibet, China
Principal Investigator: Ding Chen, Professor, National Astronomical Observatories ofChina, CAS, (China)
Participating Researchers:

Summary of Research Result：
1. Tibet AS + MD experiment
The Tibet-AS+MD experiment (Tibet Air shower array + Muon Detector arrayexperiment), which aims to develop gamma-ray astronomy in the 100 TeV region (101000 TeV), has been active. In FY2023, we continued to search for 100 TeV gammaray sources. We focused on HESS J1849-000, which is a very high energy gamma-ray
source and is considered to be a middle-aged PWN. The statistical significance of thisobservation is 4  above 25 TeV and 4.4  above 100 TeV. The gamma-ray energy
spectrum measured for the first time at 25 TeV < E < 320 TeV (Fig.1). The energy
spectrum given by a synthetic fit to our measurement and other experimental (subPeV to sub-PeV region) energy spectra can be explained by the electron origin. On theother hand, it is also consistent with the proton origin scenario (Fig.1), where gamma
rays come from the region where the nearby SNRs and molecular cloud overlap, in
which case the cutoff energy of the gamma ray (Ep_cut) is log10(Ep_cut/TeV) =3.73(+2.98-0.66). This means that we have found a new PeVatron candidate,suggesting that protons may be accelerated to the PeV region (ApJ, 954:200, (2023)).
The detection of 23 sub-PeV diffuse gamma-ray (>398 TeV) events along theGalactic plane (galactic latitude<±10 degrees) observed with the Tibet AS+MD
experiment was published in PRL in 2021. The directions of the 23 events werecompared with the directions of 43 sub-PeV (>100 TeV) gamma-ray emitting objects
in the 1LHAASO catalog (Fig 2: ApJ 961:L13, (2023)). No overlapping between the 23

Form 8events and the 43 sources supports that the 23 gamma-ray events do not originate
from sources, i.e., they are diffuse gamma rays. Of course, the possibility that a large
number of very weak unresolved sources could be the origin of the diffuse gamma
rays cannot be ruled out, but the most natural interpretation remains that they areof neutral pions produced by collisions of PeV-region cosmic rays stored in the Galactic
cosmic-ray pool with the interstellar medium.
2. Tibet AS + YAC experiment
The Tibet AS + YAC (Tibet air shower core detector array) experiment, which aims
to observe the energy spectrum of each particle component in the knee region cosmicrays, is being promoted. The Tibet air shower core detector array (YAC-II), which
consists of 124 air shower core detectors [burst detectors], is installed near the centerof the Tibet Air Shower Observatory. In FY2014, electronics and data acquisition
software were implemented, and YAC-II, which focuses on proton discrimination incosmic rays in the Knee energy region, started data acquisition. In FY2023, softwaretools for analysis were actively developed using Monte Carlo simulations.

Cygn

12Fig.1 Gamma-ray energy spectrum

Fig. 2 Directions of the 23 gamma-ray

from HESS J1849-000 observed byevents (>398 TeV) detected by the Tibet

the Tibet AS+MD experiment (redAS+MD experiment (red points) and thecircles: This work). The black curve

gamma-ray (> 100 TeV) emitting objects

is(blue circles) in the 1LHAASO catalog intheprediction

assumingahadronic-origin scenario.

the galactic coordinates.

3. International Conferences
12 presentations at ICRC2023 and others.
4. Publications[1] M. Amenomori et al., ApJ, 954:200(7pp), (2023).
[2] M. Amenomori et al., ApJ, 961:L13 (4pp), (2023).
No.