张力
从事致密天体高能过程的研究
个性化签名
 姓名：张力
 目前身份：
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 学位：

学术头衔：
博士生导师， 优秀教师/优秀教育工作者， 教育部“新世纪优秀人才支持计划”入选者
 职称：

学科领域：
天体物理学
 研究兴趣：从事致密天体高能过程的研究
张力，男，云南省高校特聘教授，博士生导师，教育部“跨世纪优秀人才”，全国优秀教师。1987年硕士研究生毕业后，一直从事宇宙线物理和高能天体物理方面的研究工作，讲授过多门本科生和研究生课程。1995年6月至2002年2月，在香港大学物理系攻读博士学位和从事博士后研究工作。1993年和1995年两次到英国Durham大学物理系从事学术访问。近年来，主要从事致密天体高能过程的研究，主持过多项国家和云南省自然科学基金项目, 已发表科研论文100篇, 其中在国际核心学术刊物如美国“天体物理学报”（ApJ）,“英国皇家天文学会月刊”（MNRAS）和“欧洲天文和天体物理学”（A&A）等期刊上发表SCI论文62篇, 这些论文已被320篇他人的论文引用。获2001年度“云南省自然科学奖”壹等奖(排名第二), 2002年度“云南省自然科学奖”壹等奖(排名第一), 2002年度云南大学“伍达观优秀教师奖”杰出贡献奖，2003年度“宝钢教育基金”优秀教师奖。目前是云南大学理论物理博士点学术带头人。

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10
【期刊论文】GAMMARAY LUMINOSITY AND DEATH LINES OF PULSARS WITH OUTER GAPS
张力， L. Zhang， ， K.S. Cheng， Z.J. Jiang， and P. Leung
The Astrophysical Journal, 604: 317327, 2004 March 20，0001，（）：
1年11月30日
We reexamine the outergap size by taking the geometry of the dipole magnetic field into account. Furthermore, we also consider that instead of taking the gap size at half of the light cylinder radius to represent the entire outer gap, it is more appropriate to average the entire outergap size over the distance. When these two factors are considered, the derived outergap size ƒ （P, B,〈r〉（α）） is a function not only of the period P and magnetic field B of the neutron star but also of the average radial distance to the neutron star, 〈r〉, which depends on the magnetic inclination angle α We use this new outergap model to study the γray luminosity of pulsars, which is given by Lγ= ƒ3(P, B, 〈r〉(α))Lsd, where Lsd is the pulsar spindown power, and to study the death lines of γray emission of the pulsars. Our model can predict the γray luminosity of an individual pulsar if its P, B, and α are known. Since different pulsars have different α, this explains why some γray pulsars We have very similar P and B but very different γray luminosities. In determining the death line of γray pulsars, we have used a new criterion based on a concrete physical property, i.e., that the fractional size of the outer gap at the nullcharge surface for a given pulsar cannot be larger than unity. In an estimate of the fractional size of the outer gap, two possible Xray fields are considered: (1) Xrays produced by neutron star cooling and polarcap heating, and (2) Xrays produced by the bombardment of relativistic particles from the outer gap onto the stellar surface (the outer gap is called a "selfsustained outer gap"). Since it is very difficult to measure α in general, we use a Monte Carlo method to simulate the properties of γray pulsars in our Galaxy. We find that this new outergap model predicts many more weak γray pulsars, which have a typical age between 0.3 and 3 Myr, than does the old model. For all simulated γray pulsars with selfsustained outer gaps, the γray luminosity Lγ satisfies Lγ ∝ Lδsd, where the value of δ depends on the sensitivity of the γray detector. For EGRET, δ～0.38, whereas δ～0.46 for GLAST. For γray pulsars with Lsd≤Lcrit sd, δ～1, and, Lcrit sd=1.5×10 34P1/3 ergs s1 is determined by ƒ (〈r〉～rL)=1. These results are roughly consistent with the observed luminosity of γray pulsars. These predictions are very different from those of the previous outergap model, which predicts a very flat relation between Lγ and Lsd.

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【期刊论文】Xray and gammaray emission from millisecond pulsars
张力， L. Zhang and K.S. Cheng
A&A 398, 639646 (2003)，0001，（）：
1年11月30日
We present a selfconsistent model to describe Xray and γray emission from millisecond pulsars (MSPs). The Xrays of MSPs are produced by the backflow of primary charged particles from the outer gap and most likely consist of three components, two thermal components and one power law component if there is a strong multipole magnetic field on the stellar surface. The backflow of ultrarelativistic particles emits photons with energies about several tens of GeV via curvature radiation. These photons cause an electromagnetic cascade about 23 stellar radii above the polar cap. The synchrotron radiation of these cascade e± pairs produces hard Xrays with a power law index～1.5. Near 10 5cm above the stellar surface, the primary charged particles encounter the strong surface magnetic field, which alters the local radius of curvature greatly, and they quickly loose more than half of their remaining energies to curvature radiation. These curvature photons heat up the polar cap area with a radius～10 5cm, which produce the softer thermal Xray component. Finally, the primary charged particles deposit their remaining energies in a much smaller polar cap area, which corresponds to the footprints of outer gap and produce the medium hard Xray component. γrays are produced in the outer gap through synchrocurvature radiation. We have applied this model to the MSPs which emit pulsed Xrays and likely γrays such as PSR J04374715, PSR J21243358, PSR J0218+4232 and PSR B182124. Our results give an agreement between predicted spectrum and the observed spectrum of MSP emission.
gamma rays: theorystars: pulsars: general

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【期刊论文】QUIESCENT HIGHENERGY GAMMARAY EMISSION FROM SOFT GAMMARAY REPEATERS
张力， L. Zhang， and K.S. Cheng
THE ASTROPHYSICAL JOURNAL, 579: 716724, 2002 November 10，0001，（）：
1年11月30日
We present a model for the highenergy γray emission from the outer gap of soft gammaray repeaters (SGRs) during their quiescent states. In this model, Xrays come from the stellar surface, but the emerging Xray spectrum will have a powerlaw tail because of the multiple scattering at the cyclotron resonance in the magnetosphere, as pointed out by Thompson, Lyukitov, & Kulkarni. The outer gap is sustained by the collision between these Xrays with the highenergy photons produced in the outer gap through the photonphoton pair production.We have taken a magnetic dipole geometry into account in estimating the fractional size of the outer gap. The fractional size of the outer gap depends on the period, surface magnetic field, average Xray energy, and the magnetic inclination angle of the neutron star. After the average fractional size outer gap is determined, the spectrum and luminosity of highenergy photons from the outer gap can be calculated.We apply this model to some SGRs, such as SGR 180620 and SGR 1900+14, and compare the expected integral fluxes with the sensitivities of EGRET, GLAST, MAGIC, and VERITAS.We predict that the integral flux of SGR 1900+14 may be greater than the sensitivity of GLAST, and especially that the integral flux for a large magnetic inclination angle (say 80°) may be greater than the sensitivities of GLAST and MAGIC. However, we predict that SGR 180620 would not be detected by GLAST because its distance is about 3 times of that of SGR 1900+14.

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【期刊论文】THE PHASERESOLVED XRAY SPECTRA OF THE CRAB PULSAR
张力， L. Zhang， ， and K.S. Cheng
THE ASTROPHYSICAL. JOURNAL, 569: 872877, 2002 April 20，0001，（）：
1年11月30日
We present a theoretical study of phasedependent Xray emission from the Crab pulsar. In the threedimensional outer gap model for the Crab pulsar, Xray emission is produced by synchrotron radiation of the secondary e

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【期刊论文】Gammaray pulsars: the pulse profiles and phaseresolved spectra of Geminga
张力， L. Zhang， ★ and K.S. Cheng★
Mon. Not. R. Astron. Soc. 320, 477484 (2001)，0001，（）：
1年11月30日
We present a calculation of a threedimensional pulsar magnetosphere model to explain highenergy emission from the Geminga pulsar with a thick outer gap. Highenergy γrays are produced by primary accelerated particles with a powerlaw energy distribution through curvature radiation inside the outer gap. We also calculate the emission pattern, pulse profile and phaseresolved spectra of highenergy γrays of the Geminga pulsar, and find that its pulse profile is consistent with the observed one if the magnetic inclination and viewing angle are～50° and～86° respectively. We describe the relative phases among soft (thermal) Xrays, hard (nonthermal) Xrays, and γrays. Our results indicate that Xray and γray emission from the Geminga pulsar may be explained by the single thick outer gap model Finally, we discuss the implications of the radio and optical emission of the Geminga pulsar.
radiation mechanisms: nonthermalpulsars: generalpulsars: individual: Gemingagammarays: theory.，

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【期刊论文】Cosmicray positrons from mature gammaray pulsars
张力， L. Zhang， and K.S. Cheng
A&A 368, 10631070 (2001)，0001，（）：
1年11月30日
We consider a possible contribution of mature γray pulsars (with ages of ≥10 5 yrs) to cosmic ray positrons. Within the framework of the γray pulsar outer gap model, e±pairs in the pulsar magnetosphere are produced by the cascade of e±pairs through synchrotron radiation of the return current from the outer gap. A good fraction of these cascade e±pairs are reflected by the hard Xrays from the polar cap via resonant scattering and escape from the pulsar through the light cylinder. The escaped pairs are accelerated to relativistic energies in the pulsar wind driven by lowfrequency electromagnetic waves. Using Monte Carlo simulations, we generate a sample of the mature γray pulsars in our Galaxy and calculate the positron production rate from these pulsars. In a simple "leaky box" model, we calculate the ratio of cosmicray positrons to total electrons. Our result indicates that the pulsar contribution to the cosmic ray positrons peaks at about 60GeV and the observed e+/(e−+ e+) ratio can be explained in this model.
stars: pulsars: generalacceleration of particlesISM: cosmic rays

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【期刊论文】Young pulsars and unidentified gammaray sources at the Galactic plane
张力， L. Zhang， ， Y.J. Zhang， and K.S. Cheng
Astron. Astrophys. 357, 957967 (2000)，0001，（）：
1年11月30日
We present a statistical study on possible association between the unidentified γray point sources at the Galactic (b≤5°) and the Galactic young pulsars with ages of less than 10 6 yr. Based on the possible positional coincidence of the unidentified EGRET sources in the third catalog with the Galactic objects given by Romero et al. (1999) as well as the outer gap model of γray pulsars proposed by Zhang & Cheng (1997), we have made a statistical analysis on whether most unidentified EGRET point sources in the Galactic plane have pulsar origin. In our analysis, we use an evolving γray solid angle which is the function of pulsar's parameters such as period, magnetic field and magnetic inclination angle. The simulated distributions of the distance and γray energy flux are compared to observed data using KolmogorovSmirnov (KS) test. Our results suggest that young pulsars may account for the majority of unidentified EGRET sources that are positionally associated with supernova remnants and OB star associations, in such a way that two thirds are young radio quiet pulsars and one third are young radio pulsars.We have studied the variability of these unidentified γray sources and found that most of them may be nonvariable. Furthermore, we expect that GLAST may detect～80 100 radio pulsars and～1100 100 unidentified γray point sources in the Galactic plane with b≤5°, where 100 is the birth rate of neutron stars in units of 100 years. This model predicts that γray point sources located at small distances are more than those predicted by aγray pulsar model with a constant beaming solid angle.
stars: neutronstars: pulsars: generalgamma rays: observations

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【期刊论文】THE STATISTICAL PROPERTIES OF XRAY EMISSION FROM GALACTIC YOUNG PULSARS
张力， L. ZHANG， ， AND K.S. CHENG
THE ASTROPHYSICAL JOURNAL, 526: 327335, 1999 November 20，0001，（）：
1年11月30日
We study statistical properties of the Galactic population of Xray pulsars with ages of less than 10 6 yr using the Monte Carlo method. Combining the initial magnetic

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【期刊论文】The gammaray conversion efficiency of rotationpowered pulsars
张力， L. Zhang and K.S. Cheng
Mon. Not. R. Astron. Soc. 294, 177181 (1998)，0001，（）：
1年11月30日
We use a new selfconsistent model to derive the conversion efficiency from rotation power to γray power for pulsars (ηth). Our result indicates that ηth; ∝τ6/7P2, where τ and P are the characteristic age and period of the pulsar, which shows that although the efficiency increases with the characteristic age of the pulsar, it also depends on the pulsar period. We test our model results with the survey of highenergy γrays from pulsars by EGRET. Our model not only successfully explains the efficiency of the confirmed γray pulsars but also explains why the γray efficiency of millisecond pulsars is so low.
pulsars: generalgammarays: theory.，

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【期刊论文】HIGHENERGY RADIATION FROM RAPIDLY SPINNING PULSARS WITH THICK OUTER GAPS
张力， L. ZHANG AND K.S. CHENG
THE ASTROPHYSICAL JOURNAL, 487: 370379, 1997 September 20，0001，（）：
1年11月30日
We propose a selfconsistent mechanism to estimate the size of the acceleration region in the outer magnetosphere of pulsars (outer gap) and calculate the highenergy radiation produced by the synchrocurvature mechanism from the outer gap. We Dnd that a powerlaw energy distribution of the accelerated particles can be obtained if the outer gap is thick enough that E•B inside the gap can be approximately proportional to (Ωr/c)1/2B(r). We apply our model to explain Xrays and γrays from Geminga and PSR B105552, whose outer gaps may occupy～70% of the outer magnetosphere region. If the radius of curvature near the light cylinder of the medium outer gap is larger than the dipolar structure, then perhaps this model may also apply to PSR B1951+32, PSR B170644, and others.

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