For astrophysics research, traditional methods include observations and theoretical simulations. Large telescopes on the ground or space telescope are used to observe the celestial bodies in different wave bands. However, for some celestial and astronomical phenomena, the studies are limited sometimes since the observational data are scarce, objects are too far away from the Earth to observe, or evolution time of some objects is too long to get a comprehensive understanding in a limited period. Therefore, observation and theoretical simulation alone is not enough for astrophysics research.
  With high-power laser systems, scientists are able to create the extreme conditions of physical experiments in the laboratory presently. Such experimental conditions are unprecedented, and can be used to simulate some representational celestial objects, which allow scientists to study many important and critical issues in the laboratory. A new field of research, high energy density laboratory astrophysics (HEDLA), is developing rapidly. HEDLA has made some exciting explorations in many aspects, for example, the equation of states for the internal structure of planets, hydrodynamics of supernova explosions, physical mechanism of jets in astronomy, etc.
  In China, as early as 2000, Prof. Zhang Jie (Institute of Physics, Chinese Academy of Sciences) and Gang Zhao (National Astronomical Observatories) jointly proposed to study the astrophysical phenomena in the laboratory, by using high power lasers to produce similar environment to the astrophysical conditions. During the past few years, the two research groups, together with international collaborators, have been carrying out experiments, theory and numerical simulations on some related topics. For example, they have studied the collsionless shockwaves created by counter-streaming plasmas, the high Mach plasma jets produced through energy release by the reconnection of self-generated magnetic fields.
  Recently, compact objects (black holes or neutron stars) become one of hot topics in HEDLA. To study the black holes, astronomers usually observe the x-rays emitted from the surrounding plasmsas photonized by the black holes. The photoionization processes have also been studed by the groups, in collabration with Japanese and Korean scientists, with the “Shenguang-II” (National Laboratory on High Power Lasers and Physics, Shanghai) and “GEKKO-XII” (Osaka University, Japan) high power laser facilitiess.
  In the Shengguang experimetns, the photoionized SiO2 aerogel plasmas generated under a near-Planckian radiation field in gold hohlraum targets irradiated by high power laser pulses (with a total energy of 2.4 kJ in 1 ns) are measured by observing the absorption spectra and line emissions in the range between 0.64 and 0.74 nm. The experimental results are simulated by theoretical calculations under local thermodynamic equilibrium (LTE) using a detailed-level-accounting (DLA) model. The contributions of different Si ions to the specific components of the measured absorption spectra are identified. The results has been published on Physics of Plasmas  [Physics of Plasmas 17, 012701 (2009)] and presented in the HEDLA-08 conference [Jie Zhang, Plenary Talk on the HEDP/HEDLA-08 Program Committee and The American Physical Society (APS), April 11-15, 2008, St. Louis, Missouri. USA.]. 
  In the GEKKO experiments, the radiation filed is generated by CH shell implosion. Figure 1 shows the schematic view of the photoionized plasma experiment. A spherical hollow CH plastic shell (diameter is 505±5μm, wall thickness is 6.4±0.1μm) is imploded with 12 laser beams (3TW, 4±0.2kJ) from the GEKKO-XII facility, resulting a Planckian X-ray radiator with  radiation temperature of 0.48keV and peak temperature of 1keV, which achieve the conditions of radiation field around the black hole and other compact objects, as shown in Figure 2. The gas around the compact objects is simulated by the plasma produced by a Si plate target irradiated by a laser pulse with 10ns and low intensity of 5×10¹⁰ Wcm^-2. After expansion of 8 to10 ns, the electron temperature of plasma reduced to about 25eV, and the electron density reduced to about 5×10¹⁹cm^-3. Under the condition of such electron temperature and density, the collision process between particle and particle is not dominative in a short time scale, and the average ionized stage of plasma is from Si⁶⁺ to Si⁸⁺. The X-ray emission spectra were measured, which is the Kα line (1.860 keV) of He-like Si ion mainly, as shown in Figure 3. The X-ray spectra emitted from photoionized silicon plasmas resemble those observed from the binary stars Cygnus X-3 and Vela X-1 with the Chandra X-ray satellite. However, experimental scientists give different interpretations with astrophysicists, by using Non-LTE model to analyze the characteristics of the spectrum components carefully. According to experimental results scientists believe that understandings of physical structure around compact objects may need to do some revises.
  The results on “GEKKO-XII” laser has been published on Nature Physics [Nature Physics 5, 821 (2009)] in October 2009. Prof. R.P. Drake gives a specialized comment, titled “How to see a black hole”, on the achievement in the same journal. 

Figure 1. Schematic view of the photoionized plasma experiment.

Figure 2. Average temperature of the radiation field.

Figure 3. Comparison between the experimental and time-integrated computed X-ray spectra.