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Source:    Pubtime:2016-04-01    【Size:A A A

1. Indirect detection of dark matter

Thanks to rapid progresses made in the past few decades, now we are in the era of the so-called precision cosmology. In the standard cosmological model the normal matter, (cold) dark matter, and dark energy constitute about 5%, 28%, and 67% of the energy density of the current Universe, respectively. Dark matter is a form of matter necessary to account for gravitational effects observed in very large scale structures such as the flat rotation curves of galaxies and the gravitational lensing of light by galaxy clusters that cannot be accounted for by the amount of the observed/normal matter. The nature of dark matter is still unknown. They neither emit nor scatter photons effectively and are rather hard to detect. Nevertheless, current astrophysical observations have imposed tight constraints on their nature. Dark Matter particles are neutral and have neither strong nor electromagnetic interaction. Hence they cannot be mainly made up of Standard Model particles, since most leptons and baryons are charged. The only potential “normal” candidate is neutrinos, but they are too “hot” or light to be trapped in dark matter halos. Therefore, a successful detection of dark matter particles will be the smoking-gun signature of the so-called “new physics”. Currently, the leading candidate is the so-called weakly interacting massive particles (WIMPs).

In general there are three ways to “detect” dark matter particles and measure their rest mass, collision cross section with normal particles, annihilation cross section or decay timescale. (1) Direct detection: As the Earth passes through our galaxy’s dark matter halo, given the expected weak interaction scale of the WIMP-nucleon scattering, galactic WIMPs should deposit a measurable amount of energy in an appropriately sensitive detector apparatus. The measurement of the corresponding recoil energy impacted to the detector nuclei can be used to constrain the properties of dark matter particles. Such a kind of approaches is the so-called dark matter direct detection. (2) Indirect detection: such experiments look for the products of WIMP annihilations (or decays). If WIMPs are Majorana particles (for which the particles are their own antiparticles), then two colliding WIMPs would annihilate to produce gamma rays, and particle-antiparticle pairs, such as positrons-electrons and protons-antiprotons. (3) The third approach to the detection of WIMPs is to produce them in the laboratory, for example, with the Large Hadron Collider.

One of the groups in the KLDMSA focuses on the indirect detection of dark matter particles by measuring the spectrum of high energy gamma-rays and cosmic rays in space, which is also the essential aim of some international space missions like PAMELA, Fermi-LAT and AMS-02. The Dark Matter Particle Explorer (DAMPE) built by this group, is one of the first two astronomical satellites and the first one focusing on high energy particle detection in China. It works in the GeV-TeV energy range with an excellent energy resolution (better than 1.5% in a wide energy range) and will measure both gamma-rays and cosmic rays accurately for high energy astrophysics as well as new physics (i.e., the dark matter) exploration. The launch of DAMPE was on Dec. 17 2015.

 

2. High energy astrophysical radiation  

High energy photons, neutrinos, and cosmic rays from energetic sources such as accreting black holes or rotating neutron stars carry valuable information on the physical processes taking place at these “hidden” central engines. Due to their extremely weak interaction with the matter, neutrinos are usually collected by some huge underground-based detectors. High energy photons, instead, are severely attenuated by the atmosphere and should be measured in space. Direct detection of cosmic rays in the GeV-TeV energy range needs to be performed in space as well. In the past few decades, the rapidly developing space astronomy has opened a new window for high energy astrophysics. Violent explosions surrounding rotating neutron stars or accreting black holes, for example gamma-ray bursts and X-ray bursts, have been routinely observed, and have been used to reveal the physics under extreme conditions (extremely strong magnetic field and/or gravitational field). At the same time, accurate cosmic ray spectra have been obtained up to TeV energies, which lead to robust constraints on their propagation parameters in the Galaxy and the distribution of the sources. This is especially the case for cosmic ray electrons which lose their energy through synchrotron and inverse Compton radiation very quickly and thus cannot travel for a very-long distance. The source of TeV cosmic ray electrons should be within ~1 kpc and relatively young. Given the small number of accelerator candidates in such a limited volume, it is possible to identify the source.

Before 2010 we had mainly worked on gamma-ray bursts, the brief flashes in soft gamma-ray band. In 1990s we constructed the gamma-ray burst detector that was later onboard ShenZhou-2. This was the first time for Chinese astronomers to successfully detect high energy emission with our own instrument onboard satellite/spaceship. We had also carried out researches on the physics of gamma-ray bursts, including the physical origin of the multiwavelength afterglow emission (in particular the X-ray flares detected by BeppoSAX and Swift and the GeV emission detected by Fermi-LAT). We were also interested in constraining the physical composition of the GRB outflow (highly magnetized or not) and revealing the physical process of extracting energy from the central engine (i.e., via neutrino annihilation processes or magnetic activities). Recently our GRB studies focus on identifying possible gravitational wave radiation signal in the electromagnetic radiation data and signature of r-process process forming material heavier than iron in short GRB afterglow data (i.e., the so-called Macronova or kilonova). We also carry out researches on supernova remnants and Galactic supermassive black hole.

 

3. Solar activities 

In astrophysics, many fundamental problems remain unresolved, e.g., why do stars have a much hotter upper atmosphere? how is magnetic energy explosively released? and how are energetic particles so efficiently accelerated? For these problems, we may find their answers on our star — the Sun. For human being, the Sun is the nearest star to us, close enough to be spatially resolvable on its surface. The Sun provides us with a natural physical laboratory for magnetized plasma, which information is crucial for reproducing controlled nuclear fusion on earth. Many physical processes on the Sun may occur elsewhere in the universe, but these processes can only be examined in detail on the Sun. A deep understanding of the Sun and its activities has its irreplaceable role in helping us to understand other magnetized and variable stars.

As the Sun is a source of light and heat for life on Earth, it sometimes is also a source of trouble. When strong flares and/or coronal mass ejections (CMEs) occur on the Sun, huge amount of magnetized plasma is ejected into interplanetary space, affecting our space environment and bringing us disastrous space weather. This kind of space weather could destroy crucial technology in orbit, and cause a large-scale blackout. It is one of the natural disasters faced by modern human beings. As we become more dependent upon satellites in space we are increasingly feeling the need to predict space weather. A deep understanding of the physics of solar activities will certainly increase our predicting ability. Therefore, traditional solar physics has become a science for a technological society. Inspired by full awareness of the above knowledge, solar physics research has been carried out at PMO for generations, starting nearly 60 years ago.

We carry out ground observations by means of taking spectra lines, high-cadence microwave spectra and high-cadence H-alpha blue wing images of solar activities. We pay particular interest to high spatial resolution observations by establishing a close collaboration with the team of the 1.6-meter aperture New Solar Telescope, largest solar telescope in the world, run by Big Bear Solar Observatory (BBSO) located at California. Space data analyzed includes observations from SOHO, RHESSI, STEREO, SDO, HINODE and etc. Here, it is worth of mentioning that we are part of an international science team for RHESSI, with at least 2-3 members participating in each workshop every year. In 2012, we hosted the 12th RHESSI workshop at Nanjing.  

With ground-based observations made at home and abroad and space open data, we work at the frontier of solar physics. Solar physics research group has made many noticeable progresses toward understanding physics of solar activities. In addition, we are also actively involved in planning next generation observations in China, which are ground-based and space-based Advanced Solar Observatories (ASO-G and ASO-S). This concept represents solar physics roadmap in China.