分类: 物理学 >> 核物理学 提交时间: 2016-08-30
摘要: The gamma-ray observation of dwarf spheroidal satellites (dSphs) is an ideal approach for probing the dark matter (DM) annihilation signature. The latest Fermi-LAT dSph searches have set stringent constraints on the velocity independent annihilation cross section in the small DM mass range, which gives very strong constraints on the scenario to explain the the AMS-02 positron excess by DM annihilation. However, the dSph constraints would change in the velocity dependent annihilation scenarios, because the velocity dispersion in the dSphs varies from that in the Milky Way. In this work, we use a likelihood map method to set constraints on the velocity dependent annihilation cross section from the Fermi-LAT observation of six dSphs. We consider three typical forms of the annihilation cross section, i.e. p-wave annihilation, Sommerfeld enhancement, and Breit-Wigner resonance. For the p-wave annihilation and Sommerfeld-enhancement, the dSph limits would become much weaker and stronger compared with those for the velocity independent annihilation, respectively. For the Breit-Wigner annihilation, the dSph limits would vary depending on the model parameters. We show that the scenario to explain the AMS-02 positron excess by DM annihilation is still viable in the velocity dependent cases
分类: 天文学 >> 天文学 提交时间: 2024-02-28 合作期刊: 《Research in Astronomy and Astrophysics》
摘要: Remote-sensing measurements indicate that heavy ions in the corona undergo an anisotropic and mass-charge dependent energization. A popular explanation to this phenomenon is the damping of the Alfvén/ion cyclotron waves. In this paper, we propose that the ion beam instability can be an important source of the Alfvén/ion cyclotron waves, and we study the excitation of the ion beam instability in the corona at the heliocentric distance ∼3R⊙ and the corresponding energy transfer process therein based on plasma kinetic theory. The results indicate that the existence of the motionless heavy ions inhibits the ion beam instability. However, the anisotropic beams of heavy ions promote the excitation of the ion beam instability. Besides, the existence of α beams can provide a second energy source for exciting beam instability. However, when both the proton beam and the α beam reach the instability excitation threshold, the proton beam driven instability excites preferentially. Moreover, the excitation threshold of the Alfvén/ion cyclotron instability driven by ion beam is of the local Alfvén speed or even less in the corona.
分类: 物理学 >> 核物理学 提交时间: 2016-08-31
摘要: A new hybrid experiment has been constructed to measure the chemical composition of cosmic rays around the "knee" in the wide energy range by the Tibet ASγ collaboration at Tibet, China, since 2014. They consist of a high-energy air-shower-core array (YAC-II), a high-density air-shower array (Tibet-III) and a large underground water-Cherenkov muon-detector array (MD). In order to obtain the primary proton, helium and iron spectra and their "knee" positions in the energy range lower than 1016 eV, each of PMTs equipped to the MD cell is required to measure the number of photons capable of covering a wide dynamic range of 100 - 106 photoelectrons (PEs) according to Monte Carlo simulations. In this paper, we firstly compare the characteristic features between R5912-PMT made by Japan Hamamatsu and CR365-PMT made by Beijing Hamamatsu. This is the first comparison between R5912-PMT and CR365-PMT. If there exists no serious difference, we will then add two 8-inch-in-diameter PMTs to meet our requirements in each MD cell, which are responsible for the range of 100 - 10000 PEs and 2000 - 1000000 PEs, respectively. That is, MD cell is expected to be able to measure the number of muons over 6 orders of magnitude.