D. B. Liu

Further Analysis of Absorption Features in the X-Ray Spectrum of the Isolated Neutron Star 1E 1207.4–5209

Observation of the isolated neutron star 1E 1207.4-5209 shows three distinct absorption features, regularly spaced at 0.7, 1.4, and 2.1 keV, which vary in phase with the stars rotation. In this paper, we attempt to clarify the understanding of these absorption lines by using the quantum theory of the cyclotron radiation under the quadrupole approximation, given in our previous papers. We found that these lines are more likely to arise from the electron-cyclotron absorption, rather than from proton-cyclotron. We explain why the observed fundamental and the second-harmonic cyclotron features with significantly different absorption coefficients have nearly the same strength. Assuming that the absorption features arise from an approximately uniform narrow column at the bottom of the pole region of neutron star, we complete a simplified model calculation to fit the observational data; thus, the included angle between the spinning axis and the observers sight line, as well as that between the spinning and the magnetic axes of the neutron star, are determined by this method. The geometrical configuration inferred from the deduced directions of these axes is helpful to better understand the physics and some other relevant observation facts of star 1E 1207.4-5209.

Does the Prompt γ-Ray Emission of Gamma-Ray Bursts Arise from Resonant Inverse Compton Scattering?

We argue that the dominant radiation mechanism responsible for the early prompt γ-ray emission of gamma-ray bursts could be the resonant inverse Compton scattering of relativistic electrons in an intense magnetic field. By using this mechanism, some problems in the field of gamma-ray bursts could be clarified, e.g., the origin of the Amati relation, the formation of the observed broken power-law spectra, and the related deadline problem, among others. Our model also predicts that the emitted γ-rays could be highly polarized.

Simplified spectrum and power formulae for resonant inverse Compton scattering in a strong magnetic field

Abstract The resonant inverse Compton scattering (RICS) of a relativistic electron in an intense magnetic field is an important radiation mechanism in hard X-ray and γ -ray astrophysics. So far the available formulae describing RICS radiation are quite complicate in mathematics and not easy to understand in physics. In this Letter, we present the markedly simplified, analytical formulae for both the spectral and the total power of the RICS process. We will show that the RICS radiation has good monochromaticity which concentrates in hard X-ray and γ -ray wavebands, and has extremely high efficiency when compared with the coexistent, nonresonant inverse Compton scattering, if the “accommodation condition”, derived in this Letter, is satisfied.

The Cerenkov iron Kα line in Active Galactic Nuclei - calculation in the optically thin case

In this paper we continue the research on the Cerenkov origin of the iron Kα line in AGNs. We claim again that the newly recognized line emission mechanism, Cerenkov line-like radiation, could be significantly responsible for the observed iron K-lines of AGNs. We give a new model calculation of the luminosity of the iron Kα line in the optically thin condition, which is an important extension of our previous calculation in the optically thick case. The new calculation is also comparable to the typical observed luminosity of iron Kα line, thus confirming the effectiveness of the new mechanism. Besides, we analyze a possible negative influence of the hydrogen-dominated plasma upon the efficiency of Cerenkov line-like radiation in detail, which was ignored in our previous paper (You et al. 2003). We conclude that such a negative effect of the plasma oscillation is unimportant and can be neglected.

Does the prompt γ‐ray emission of GRB arise from RICS?

We argue that the dominant radiation mechanism responsible for the early prompt γ‐rayemission of gamma‐ray bursts could be the resonant inverse Compton scattering(RICS) of relativistic electrons in an intense magnetic field. By using this mechanism, some problems in the GRB study could be clarified, e.g., the origin of the Amati relation, the formation of the observed broken power law spectra, and the related “deadline problem,” the solution of the “compactness problem,” etc.. Our model also predicts that the emitted γ‐rayscould be highly polarized.

Evolution of X-ray spectra in down-Comptonization. A comparison of the extended Kompaneets equation with Monte Carlo simulation and the Ross-McCray equation

The original Kompaneets equation fails to describe down-Comptonization, which is the most important radiative transfer process in hard X-ray and γ-ray astronomy, compared with up-Comptonization. In this paper, we improve our previous derivation of the extended Kompaneets equation and present it more clearly. The new equation can be used to describe a more general Comptonization process, including up- and down-Comptonization, suitable for any case, hν � kTe, hν � kTe and hν ∼ kTe. The condition of the original Kompaneets equation hν � kTe is no longer necessary. Using the extended equation, we give some typical solutions in X-ray astronomy, and compare them with those of the prevailing Monte Carlo simulations and the Ross-McCray equation. The excellent consistency between the extended Kompaneets equation and Monte Carlo simulation or Ross-McCray equation confirms the correctness of our extended Kompaneets equation. The numerical solution of the extended Kompaneets equation is less expensive in terms of computational time than the Monte Carlo simulation. Another advantage of the equation method is the simplicity and the clarity in physics. The potential applications in X-ray and γ-ray astronomy are also emphasized.

Thermal Iron K and Alpha: Line Emission from the X-Ray Binary GX 339-4

Abstract The accretion rate of the black hole candidate X-ray binary GX 339-4 in the ‘off’ state is low, and an advection dominated accretion flow (ADAF) is present. Hydrogen-like and helium-like iron Kα emission lines at 6.7 and 6.95 keV from hot plasma of ADAF can be produced by recombination-cascade processes with moderately high intensities, which are markedly distinguished from the fluorescent iron Kα line at ∼6.4 keV. We show that the observational features of GX 339-4 can be explained by the ADAF model, if the iron abundance is more than 10 times the solar value, though the reason for such a high abundance is still unclear. We suggest that the increase of the accretion rate makes GX 339-4 change from off, low, intermediate, to high and very high states, and the line center of iron Kα will therefore shift from ∼6.83 to ∼6.4 keV, i.e. to the fluorescent disc-line, since the disappearance of the ADAF due to its high accretion rate.