"Wukong" these seven years

Today, the "Wukong" dark matter particle detection satellite turns 7 years old! Since its successful launch from the Jiuquan Satellite Launch Center on December 17, 2015, "Wukong" has been operating stably for 7 years, far exceeding its expected lifespan, and all detector indicators are still in good working condition.

As of December 17, 2022, Wukong has been in orbit for 2,557 days, has flown around the Earth 38,952 times, completed 14 full-sky scans, detected 13 billion high-energy particles, and recorded an average of about 5 million high-energy particles per day, generating 216 TB of advanced data products (level 2A).

Wukong has unique advantages in the direct detection of cosmic ray energy spectra due to its high energy resolution and large acceptance. During its seven years of operation, Wukong has achieved seven important scientific results, including precise measurements of cosmic ray electron, proton, helium nuclear energy spectra and the ratio of boron to carbon and boron to oxygen, and discovered a series of energy spectrum structures, and achieved internationally leading results in the search for monoenergetic gamma-ray line spectra and fractional charge.

1Discovered the TeV inflection in the electron energy spectrum

Comparison of Wukong's positron and electron energy spectrum (red dots) and other experimental results. Source: Wukong collaboration team (2017, Nature, 552, 63)

The precise measurement results of the energy spectrum structure of positrons and electrons in the energy range of 25 GeV - 4.6 TeV were obtained, and the inflection structure of the energy spectrum at 0.9 TeV was discovered with high confidence for the first time, providing important observational data for understanding the origin of cosmic positrons and electrons and limiting the parameters of the dark matter model. This achievement was selected as one of the top ten scientific advances in China in 2018.

2Discovery of the 14 TeV inflection in the proton energy spectrum

Comparison of Wukong proton spectrum (red dots) and other experimental results. Image source: Wukong collaboration team (2019, Science Advances, 5, eaax3793)

The precise measurement of the cosmic ray proton energy spectrum in the energy range of 40 GeV - 100 TeV revealed with high confidence that the proton energy spectrum softens at 14 TeV, which may be the mark left by a nearby cosmic ray acceleration source. This achievement was selected as one of the top ten astronomical science and technology advances in China in 2019.

3. Discovery of the 34 TeV inflection in the helium nuclear energy spectrum

Comparison of the Wukong helium nuclear energy spectrum (red dots) and other experimental results. Image source: Wukong collaboration group (2021, Physical Review Letters, 126, 201102)

The helium nuclear energy spectrum of cosmic rays in the energy range of 70 GeV - 80 TeV was accurately measured, revealing with high confidence the softening inflection behavior of the helium nuclear energy spectrum at 34 TeV. The structures of the helium nuclear energy spectrum and the proton energy spectrum show a high degree of similarity, indicating that they have a common physical origin.

4. Revealing new characteristics of e-Forbes decline recovery time

Comparison of the change of the positive and negative electron flux of Wukong with time (green dots) and the results of Wukong's no-trigger event (pink line) and ground neutron monitor (blue line). Source: Wukong collaboration team (2021, The Astrophysical Journal Letters, 920, L43)

Wukong observed the Forbes decline behavior of the positron and electron flux caused by a huge coronal mass ejection event in September 2017, and accurately measured the relationship between the recovery time after the flux decline and the change in energy, revealing new characteristics that are not completely consistent with traditional theoretical expectations, and providing new data for understanding the transport process of cosmic rays within the solar system and its relationship to solar activity.

5 Obtain the most sensitive detection results of gamma-ray line spectrum

Wukong's exclusion region (shaded) of dark matter decay lifetime that produces monoenergetic gamma-ray line spectra. Image source: Wukong collaboration team (2022, Science Bulletin, 67, 679)

With its very high energy resolution (~1%), Wukong conducted a highly sensitive search for gamma-ray line spectrum radiation and placed strict restrictions on models of dark matter annihilation or decay producing monoenergetic gamma-ray line spectra. The restrictions on annihilation model parameters are comparable to the results of Fermi-LAT, while the restrictions on decay model parameters are better than the results of Fermi-LAT, reaching the best level internationally.

6. Found the inflection of the boron-carbon and boron-oxygen ratio energy spectrum

Comparison of the boron-carbon ratio of the Wukong cosmic rays (red dots) with other experimental results. The blue dashed line is the traditional model expectation. Image source: Wukong collaboration team (2022, Science Bulletin, 67, 2162)

The energy spectra of the boron-carbon ratio and the boron-oxygen ratio in the energy range of 10 GeV/n - 5.6 TeV/n were accurately measured, revealing a significant turning behavior of the two at 100 GeV/n, which means that the propagation speed of cosmic rays in the high-energy band is slower than expected, posing a challenge to the traditional cosmic ray propagation model.

7 Obtained the most sensitive detection results of fractionally charged particles in space

The upper limit of the flux set by Wukong for 2/3e particles (red dot) is compared with other experimental results. The left picture shows the results of ground experiments, and the right picture shows the results of space experiments. Image source: Wukong collaboration team (2022, Physical Review D, 106, 063026)

The Wukong spacecraft conducted a highly sensitive search for fractionally charged particles that may exist in outer space, and placed strict restrictions on the flow rate of fractionally charged particles with an absolute charge of 2/3 of the charge of an electron. The upper limit is two to three orders of magnitude lower than previous space experiment results.

"Wukong" will continue to display its skills in space and add new colors to the grand picture of China's space science.

Rotating Editor-in-Chief: Zhang Shuinai

Editor: Wang Kechao