检索结果-南通市图书馆

Reduced spin-down rate of PSR J0738–4042 explained as due to an asteroid disruption event

在线全文

学校读者我要写书评

暂无评论

Research in Astronomy and Astrophysics 2016年第5期16卷 41-46页

作者： yong-Bo Yu yong-feng huang Department of Astronomy Nanjing UniversityNanjing 210046China Key Laboratory of Modern Astronomy and Astrophysics(Nanjing University) Ministry of EducationNanjing 2 1 0046China

Long term observations by Brook et al. reveal that the derivative of rotational frequency of PSR J0738-4042 changed abruptly in 2005. Originally, the spin-down rate was relatively stable, with the rotational frequency derivative being - 1.14 x 10-14 s-2. After September 2005, the derivative began to rise. About 1000 days later, it arrived at another relatively stable value of about -0.98 x 10-24 s-2, indicating that the pulsar is spinning-down relatively slowly. To explain the observed change in spin-down rate, we resort to an asteroid disrupted by PSR J0738-4042. In our model, the orbital angular momentum of the asteroid is assumed to be parallel to that of the rotating pulsar, so that the pronounced reduction in the spin-down rate can be naturally explained as due to the transfer of angular momentum from the disrupted material to the central pulsar. The derived magnetospheric radius is about 7.0 x 109 cm, which is smaller than the tidal disruption radius （8.7 x 10^10cm）. Our model is self-consistent. It is shown that the variability in the spin-down rate of PSR J0738-4042 can be quantitatively accounted for by accretion from the asteroid disrupted by the central pulsar.

关键词： stars: neutron -- planet-star interactions -- pulsars: individual (PSR J0738-4042)

Effect of rapid evolution of magnetic tilt angle on a newborn magnetar's dipole radiation

在线全文

学校读者我要写书评

暂无评论

Research in Astronomy and Astrophysics 2015年第7期15卷 986-992页

作者： Ming Xu yong-feng huang Department of Physics Jiangxi Science & Technology Normal University Department of Astronomy Nanjing University

We study the electromagnetic radiation from a newborn magnetar whose magnetic tilt angle decreases rapidly. We calculate the evolution of the angular spin frequency, the perpendicular component of the surface magnetic field strength, and the energy loss rate through magnetic dipole radiation. We show that the spin-down of the magnetar experiences two stages characterized by two different timescales. The apparent magnetic field decreases with the decrease of the tilt angle. We further show that the energy loss rate of the magnetar is very different from that in the case of a fixed tilt angle. The evolution of the energy loss rate is consistent with the overall light curves of gamma-ray bursts which show a plateau structure in their afterglow stage. Our model supports the idea that some gamma-ray bursts with a plateau phase in their afterglow stage may originate from newborn millisecond magnetars.

关键词： magnetic fields -- star: magnetars -- gamma-ray bursts: general

Long-term continuous energy injection in the afterglow of GRB 060729

在线全文

学校读者我要写书评

暂无评论

Research in Astronomy and Astrophysics 2009年第12期9卷 1317-1323页

作者： Ming Xu yong-feng huang Tan Lu Department of Astronomy Nanjing University Nanjing 210093 China Purple Mountain Observatory Chinese Academy of Sciences Nanjing 210008 China

A long plateau phase and an amazing level of brightness have been observed in the X-ray afterglow of GRB 060729. This peculiar light curve is likely due to longterm energy injection in external shock. Here, we present a detailed numerical study of the energy injection process of magnetic dipole radiation from a strongly magnetized millisecond pulsar and model the multi-band afterglow observations. It is found that this model can successfully explain the long plateaus in the observed X-ray and optical afterglow light curves. The sharp break following the plateaus could be due to the rapid decline of the emission power of the central pulsar. At an even later time （～ 5×10^6 s）, an obvious jet break appears, which implies a relatively large half opening angle of θ ～ 0.3 for the GRB ejecta. Due to the energy injection, the Lorentz factor of the outflow is still larger than two even at 10^7 s after the GRB trigger, making the X-ray afterglow of this burst detectable by Chandra even 642 d after the burst.

关键词： gamma rays: bursts -- ISM: jets and outflows

On the Evolution of the Apparent Size of Gamma-Ray Burst Remnants

在线全文

学校读者我要写书评

暂无评论

Chinese Journal of Astronomy and Astrophysics 2006年第3期6卷 305-311页

作者： Ting-Ting Gao yong-feng huang Department of Astronomy Nanjing University Nanjing 210093

The remnants of two gamma-ray bursts, GRB 030329 and GRB 041227, have been resolved by Very Long Baseline Interferometry observations. The radio counterparts were observed to expand with time. These observations provide an important way to test the dynamics of the standard fireball model. We show that the observed size evolution of these two events cannot be explained by a simple jet model, rather, it can be satisfactorily explained by the two-component jet model. It strongly hints that gamma-ray burst ejecta may have complicated structures.

关键词： gamma rays: bursts - ISM: jets and outflows

Measuring the beaming angle of GRB 030329 by fitting the rebrightenings in its multiband afterglow

在线全文

学校读者我要写书评

暂无评论

Research in Astronomy and Astrophysics 2010年第11期10卷 1119-1126页

作者： Wei Deng yong-feng huang Si-Wei Kong Department of Astronomy Nanjing University Nanjing 210093 China Key Laboratory of Modern Astronomy and Astrophysics （Nanjing Univ.） Ministry of EducationNanjing 210093 China

Multiple rebrightenings have been observed in the multiband afterglow of GRB 030329. In particular, a marked and quick rebrightening occurred at about t 1.2 × 10^5 s. Energy injection from late and slow shells seems to be the best interpretation for these rebrightenings. Usually it is assumed that the energy is injected into the whole external shock. However, in the case of GRB 030329, the rebrightenings are so quick that the usual consideration fails to give a satisfactory fit to the observed light curves. Actually, since these late/slow shells freely coast in the wake of the external shock, they should be cold and may not expand laterally. The energy injection then should only occur at the central region of the external shock. Considering this effect, we numerically re-fit the quick rebrightenings observed in GRB 030329. By doing this, we were able to derive the beaming angle of the energy injection process. Our result, with a relative residual of only 5% - 10% during the major rebrightening, is bet- ter than any previous modeling. The derived energy injection angle is about 0.035. We assume that these late shells are ejected by the central engine via the same mechanism as those early shells that produce the prompt gamma-ray burst. The main difference is that their velocities are much slower, so that they catch up with the external shock relatively late and are manifested as the observed quick rebrightenings. If this were true, then the derived energy injection angle can give a good measure of the beaming angle of the prompt γ-ray emission. Our study may hopefully provide a novel method to measure the beaming angle of gamma-ray bursts.

关键词： gamma-rays bursts-ISM jets and outflows

Ring-Shaped Jets in Gamma-Ray Bursts

在线全文

学校读者我要写书评

暂无评论

Chinese Journal of Astronomy and Astrophysics 2008年第4期8卷 411-420页

作者： Ming Xu yong-feng huang Si-Wei Kong Department of Astronomy Nanjing University Nanjing 210093 China

When the axis of a gamma-ray burst （GRB） does not coincide with the spin axis of its source, there may result a ring-shaped jet. Using some refined jet dynamics, we calculate multi-wavelength afterglow light curves for such ring-shaped jets. In the R-band we find an obvious break in the afterglow light curve due to the beaming effect and the break is affected by many parameters, such as the electron energy fraction ζe, the magnetic energy fraction ζB^2, the width of ring △θ and the medium number density n. The overall light curve can be divided into three power-law stages, i.e., an ultra-relativistic stage, an after-break stage and a deep Newtonian stage. For each stage the power-law index is larger in the ring-shaped jet than in the corresponding conical jet.

关键词： gamma rays bursts - ISM jets and outflows

Detection of quantization index modulation steganography in G.723.1 bit stream based on quantization index sequence analysis

在线全文

学校读者我要写书评

暂无评论

Journal of Zhejiang University-Science C(Computers and Electronics) 2012年第8期13卷 624-634页

作者： Song-bin LI Huai-zhou TAO yong-feng huang Department of Electronic Engineering Tsinghua UniversityBeijing 100084China Tsinghua National Laboratory for Information Science and Technology Beijing 100084China

This paper presents a method to detect the quantization index modulation(QIM) steganography in G.723.1 bit *** show that the distribution of each quantization index(codeword) in the quantization index sequence has unbalanced and correlated *** present the designs of statistical models to extract the quantitative feature vectors of these *** the extracted vectors with the support vector machine,we build the classifier for detecting the QIM steganography in G.723.1 bit *** experiment shows that the method has far better performance than the existing blind detection method which extracts the feature vector in an uncompressed *** recall and precision of our method are all more than 90% even for a compressed bit stream duration as low as 3.6 s.

关键词： Steganalysis Quantization index modulation (QIM) G.723.1 Codeword distribution characteristics

Improvement of tundish shape and optimization of flow control devices for sequence casting heavy steel ingots

在线全文

学校读者我要写书评

暂无评论

International Journal of Minerals,Metallurgy and Materials 2012年第1期19卷 15-20页

作者： Guang-hua Wen yong-feng huang Ping Tang Ming-mei Zhu College of Materials Science and Engineering Chongqing UniversityChongqing 400044China

The metallurgical effect of a round tundish used to cast heavy steel ingots in machine works at present was evaluated through water modeling experiments. The flow control devices of the improved oval tundish, which was used instead of the round tundish, had been optimized. The results show that the residence time of the round tundish is short, its inclusion removal efficiency is too low, and it has more dead zones and an unreasonable flow field. Compared with the round tundish, the improved oval tundish with the optimized weir and dam has a better effect： its minimum residence time is prolonged by 38.1 s, the average residence time is prolonged by 233.4 s, its dead volume fraction decreases from 26% to 15%, and the ratio of plug volume fraction to dead volume fraction increases from 0.54 to 1.27. The inclusion removal efficiency also increases by 17.5%.

关键词： tundish steel ingots water modeling flow control devices inclusions

The effects of parametrization of the dark energy equation of state

在线全文

学校读者我要写书评

暂无评论

Research in Astronomy and Astrophysics 2011年第12期11卷 1403-1412页

作者： Ke Shi yong-feng huang1,2 Tan Lu Department of Astronomy Nanjing University Nanjing 210093 China Key Laboratory of Modern Astronomy and Astrophysics Nanjing University Nanjing 210093 China Purple Mountain Observatory Chinese Academy of Sciences Nanjing 210008 China Joint Center for Particle Nuclear Physics and Cosmology Nanjing University - Purple Mountain Observatory Nanjing 210093 China

We investigate in detail the influence of parametrizations of the dark energy equation of state on reconstructing dark energy geometrical parameters, such as the deceleration parameter q（z） and Om diagnostic. We use a type Ia supernova sample, baryon acoustic oscillation data, cosmic microwave background information along with twelve observational Hubble data points to constrain cosmological parameters. With the joint analysis of these current datasets, we find that the parametrizations of w（z） have little influence on the reconstruction result of q（z） and Ore. The same is true for the transition （cosmic deceleration to acceleration） redshift zt, for which we find that for different parametrizations of w（z）, the best fitted values of zt are very close to each other （about 0.65）. All of our results are in good agreement with the ACDM model. Furthermore, using the combination of datasets, we do not find any signal of decreasing cosmic acceleration as suggested in some recent papers. The results suggest that the influence of the prior w（z） is not as severe as one may anticipate, and thus we can, to some extent, safely use a reasonable parametrization of w（z） to reconstruct some other dark energy parameters （e.g. q（z）, Ore） with a combination of datasets.

关键词： cosmology: observations -- cosmology: cosmological parameters