黑洞吸积理论及其天体物理学应用的近期发展(Ⅱ)

黑洞吸积理论是天体物理学的一个基础理论,是认识许多高能天体系统如活动星系核、黑洞X射线双星,以及伽马暴等的重要物理基础.该文评述近年来黑洞吸积理论尤其是径移主导吸积流模型(advection-dominated accretion flow)及其变种的主要发展,并介绍该理论在银河系中心、低光度活动星系核、黑洞X射线双星等方面的应用.共分为两篇,该文是第2篇,内容是关于黑洞热吸积流理论在低光度活动星系核以及黑洞X射线双星方面的应用.     

黑洞吸积理论及其天体物理学应用的近期发展(Ⅰ)

黑洞吸积理论是天体物理学的一个基础理论,是认识许多高能天体系统,如活动星系核、黑洞X射线双星、伽马射线暴等的重要物理基础。该文评述了近年来黑洞吸积理论,尤其是径移主导吸积流(advection-dominated accretion flow,ADAF)模型及其变种的主要发展,并介绍该理论在银河系中心、低光度活动星系核、黑洞X射线双星等方面的应用。全文分为两篇,该文是第一篇,内容是关于黑洞的热吸积流理论及其在银河系中心的应用。     

黑洞X射线双星的类氢类氦铁K_α发射线

用径移主导吸积流模型(ADAF)不仅可以成功解释很多低吸积率天体的连续谱辐射,也可以说明X射线波段的谱线发射,而后者目前尚少有讨论.以黑洞X射线双星GX339-4处在低吸积率的宁静态情况为例,计算了它的铁线发射,表明在ADAF情况下的确可以产生足够强的可以在观测上检测的类氢和类氦铁Kα线.     

黑洞X射线双星的类氢类氦铁Kа发射线

用径移主导吸积流模型(ADAF)不仅可以成功解释很多低吸积率天体的连续谱辐射,也可以说明X射线波段的谱线发射,而后者目前尚少有讨论.以黑洞X射线双星GX339-4处在低吸积率的宁静态情况为例,计算了它的铁线发射,表明在ADAF情况下的确可以产生足够强的可以在观测上检测的类氢和类氦铁Kа线.     

黑洞X射线双星的类氢类氦铁Kα发射线

用径移主导吸积流模型（ADAF）不仅可以成功解释很多低吸积率天体的连续谱辐射，也可以说明X射线波段的谱线发射，而后者目前尚少有讨论。以黑洞X射线双星GX339－4处在低吸积率的宁静态情况为例，计算了它的铁线发射，表明在ADAF情况下的确可以产生足够强的可以在观测上检测的类氢和类氦铁Kα线。     

中子星中心致密天体的双星起源

探讨了认为中心致密天体(CCO)起源于双星的可能性.首先, CCO与正常遗迹脉冲星有着相似的平均自旋周期,但CCO的平均表面磁场强度(B～5.4×10~(10)Gs)低于正常遗迹脉冲星(B～7.7×10~(12)Gs)～2个量级.同时,几乎所有的正常遗迹脉冲星均分布在爱丁顿吸积加速线以上,而CCO全部分布在自旋加速线以下.因此怀疑CCO可能起源于双星吸积加速过程.其次,基于中子星再加速理论,分析了CCO可能的双星演化过程:双星系统中, CCO以M～1017g·s~(-1)的吸积率,经过～106yr的时间共吸积△M～10~(-2)M⊙的物质,其自旋周期将会从P～10 s降低至P～0.1 s,表面磁场强度将会从B～10~(12)Gs降低至B～1010Gs.考虑到～106yr的演化时标远大于CCO遗迹的年龄(～0.3–7 kyr),猜想CCO可能是双星系统中第1颗恒星超新星爆发的产物,而第2颗恒星超新星爆发后双星解体,留下CCO和第2颗恒星的超新星遗迹.该模型预言在CCO附近可能存在一颗年轻的正常脉冲星(P～0.02 s, B～1012Gs),并期望未来的射电望远镜和高能探测器能够进行搜寻.     

CygX—3X光子场对γ射线的吸收效应

本文详细讨论了Ｘ射线双星ＣｙｇＸ－３的Ｘ光子场对γ射线的吸收效应，计算结果表明：当ＣｙｇＸ－３的Ｘ光子发射场处于高态时，从致密中子星附近发射的能量为１０＾２－１０＾４ＭｅＶ的γ射线约有６０％被其吸收，吸收过程产生的正负电子的逆Ｃｏｍｐｔｏｎ散射会使出射能量１０－７０ＭｅＶ的 γ射线强度平均增加约５０％，计算的γ射线能谱与实验观测结果基本相符。     

甚高能γ射线的地面观测设备和分析方法

甚高能γ射线(E≥100 GeV)是全波段天文学的重要组成部分,是研究宇宙射线高能粒子的重要窗口.由于甚高能γ射线光子本身携带着重要的物理信息,对它进行研究将极大地拓展人们对宇宙演化、宇宙高能粒子、高能粒子源、高能粒子产生机制、产生环境和发射区几何形状、高能粒子之间以及高能粒子与星际介质间相互作用的认识.随着大有效面积、宽视场、较低能量阈值、较高角分辨率切仑柯夫望远镜的问世,甚高能γ射线天文学的研究取得了极大的成功,目前发现的甚高能γ射线源已经超过90个,证认了一些甚高能γ射线源的类型,包括活动星系核(AGNs)、Wolf-Rayet星、巨型分子云(GMcs)、壳层超新星遗迹(SNRs)、脉冲星风星云(PWNe)、双星系统等.介绍了甚高能γ射线的地面观测设备,并对分析方法进行评述.     

X射线双星的形成与演化

<正>X射线双星是包含一颗吸积致密星(黑洞、中子星或者白矮星)的双星系统,是宇宙中的重要天体.对它们的研究不仅可以帮助人们理解双星演化、吸积盘和致密星物理,而且也有助于对星系的形成和演化历史甚至宇宙学模型的认识.本文的目的是讨论X射线双星的形成和演化过程,涉及到Be/X射线双星、低质量X射线双星、极亮X射线源和激变变星.第1章我们简要介绍了双星演化的相关知识.     

奇异星周围的中微子主导吸积流

为统一解释伽玛射线暴(简称伽玛暴)与暴后再活动,提出了一个新的伽玛暴中心引擎模型一“奇异星-NDAF”模型(NDAF:Neutrino Dominated Accretion Flow,中微子主导吸积流),并计算了奇异星周围NDAF的结构.与其他中心致密天体不同的是,奇异星会向吸积流反馈以中微子为载体的奇异化相变能量.不考虑NDAF与奇异星的摩擦,结果表明:奇异星周围NDAF的结构对吸积率非常敏感;当吸积率大于0.18 Mo.S-1时,“奇异星-NDAF”模型能统一解释伽玛暴与暴后再活动,这个范围大于无摩擦的“中子星-NDAF”模型能统一解释的范围;在统一解释的情形下, “奇异星-NDAF”模型湮灭总能量的分布非常宽阔,当吸积率大于0.3 M0.S--1时,湮灭总能量大于1051 erg;最后,当吸积率大于0.3 M0.S-1时,“奇异星-NDAF”模型的湮灭光度超过同等吸积率下“黑洞-NDAF”模型一个多量级,有利于解释某些光度极大的伽玛暴.     

Superconducting cosmic strings interpretation of some astronomical objects

The possible interpretation of such astrophysical objects as quasars, the new class of supernova remnants, 0.511 MeV intense annihilation line, and the exotic binary system SS 433 has been given based on the string theory.     

One-photon and two-photon annihilation lines in gamma-bursts,II

This paper, which is a continuation of Paper I (Zheleznyakov and Litvinchuk, 1985), studies the formation of a one-quantum annihilation line in the spectrum of gamma-ray bursts. The radiative transfer equation together with the positron density balance relation is solved, and the expected photon fluxes in the one- and two-quantum annihilation lines are calculated. The fractional luminosities of these lines versus source parameters are investigated. For a gamma-burst one-photon annihilation line, the luminosity is always much less than that for the two-photon line-i.e., the positron source power transforms almost entirely into radiation in the two-quantum annihilation line and the positron density depends solely on the specific source power and two-quantum annihilation probability. The flux of one-quantum annihilation quanta 1 MeV is estimated to be less than what modern detectors can resolve. This explains failure to detect the one-photon annihilation lines in gamma-bursts so far.     

One-photon and two-photon annihilation lines in gamma-bursts,I

This paper and subsequent Paper II are an investigation of the annihilation line formation in gamma-ray bursts based on the assumption of positron production in a strong magnetic field (because of one-photon absorption of hard gamma-quanta radiated in the neutron star hot polar spot). We discuss a two-photon annihilation line in this paper. It is shown that if the star magnetic field is greater than 3×10¹² G, the relative flux in the line depends solely on the hardness of the continuum and is, as a rule, less than or about 10–20% of the total flux. This is consistent with the spectral data recorded by ‘Venera-11’ and ‘Venera-12’ space probes. The annihilation region formation above the hot polar spot is discussed, and positron density and annihilation region dimensions are estimated.     

Dusty disk wind in young binary systems viewed pole-on

A model of a young binary system with companions of unequal mass whose orbital plane is inclined at a large angle to the line of sight is examined. The system components are assumed to accrete matter from the remains of the protostellar cloud. It is found that eclipsing of the primary component by the disk wind of the secondary can be observed when the plane of the orbit is inclined at a large angle to the line of sight or even when the binary system is observed pole-on. __________ Translated from Astrofizika, Vol. 51, No. 2, pp. 267–275 (May 2008).     

Annihilation of positrons in the Galaxy

Observations of electron-positron annihilation radiation from the Galactic Center region with the SPI instrument aboard INTEGRAL are summarized. The measured width of the 511 keV line and inferred fraction of positrons annihilating through positronium formation are consistent with the annihilation taking place in the warm ISM phase, although combinations of several ISM phases are also allowed by the data. The spatial distribution of 511 keV emission suggests that positron sources are concentrated toward the Galactic bulge and avoid the Galactic disk.     

On the theory of annihilation lines in gamma-ray bursts

The mechanism of formation of an annihilation line 0.5 MeV in gamma-ray bursts due to electron-positron pair production in strong magnetic fields of neutron stars is discussed. Bremsstrahlung from a hot polar spot is supposed to be a source of gamma-quanta which produce the pairs. It is shown that a great part of radiation with the energyE>2mc ² per quantum (except for directions along or close to the magnetic field) is consumed by pair production and does not excape from the gamma-burster. This indicates a possible strong gap in continuum radiation at energies higher than 1 MeV. At the same time effective creation of pairs enables one to give a simple estimate of the expected annihilation line intensity in gamma-ray burst spectra. This estimate coincides with the available observational data.     

Orbital evolution and orbital phase resolved spectroscopy of the HMXB pulsar 4U 1538-52 with RXTE-PCA and BeppoSAX

We report here results from detailed timing and spectral studies of the high mass X-ray binary pulsar 4U 1538-52 over several binary periods using observations made with the Rossi X-ray Timing Explorer (RXTE) and BeppoSAX satellites. Pulse timing analysis with the 2003 RXTE data over two binary orbits confirms an eccentric orbit of the system. Combining the orbitial parameters determined from this observation with the earlier measurements we did not find any evidence of orbital decay in this X-ray binary. We have carried out orbital phase resolved spectroscopy to measure changes in the spectral parameters with orbital phase, particularly the absorption column density and the iron line flux. The RXTE-PCA spectra in the 3–20 keV energy range were fitted ∼6.4 keV, whereas the BeppoSAX spectra needed only a power law and Gaussian emission line at ∼6.4 keV in the restricted energy range of 0.3–10.0 keV. An absorption along the line of sight was included for both the RXTE and BeppoSAX data. The variation of the free spectral parameters over the binary orbit was investigated and we found that the variation of the column density of absorbing material in the line of sight with orbital phase is in reasonable agreement with a simple model of a spherically symmetric stellar wind from the companion star.     

Rapid evolutionary changes in the WR binary HD 5980

Observations of the optical spectrum of the Wolf-Rayet eclipsing binary HD 5980 in the Small Magellanic Cloud show how one component of this binary system unexpectedly underwent an eruption transforming its spectral type from a hot WN3 to a cooler and more luminous WN11 type. Radial velocity analysis of the preoutburst spectra indicates that both components of the binary are massive stars with emission lines in their spectra. The component that erupted is found to be the one in front of the system during the light curve phase zero. Differential analysis of the H Balmer and Heii Pickering emission line strengths indicates that the hydrogen content changes with the cooling of the spectrum. The maximum light of the eruption of HD 5980 had a duration of a few months, but the slow change from a WNE spectrum to WNL type took about 10 years. The similarity of the present spectrum of HD 5980 to the galactic WR binary system WR 12 is noticed.     

Pair annihilation and radio emission from galactic jet sources: the case of Nova Muscae

In the hard X-ray spectra of some X-ray binaries line features at around 500 keV are detected. We interpret these as arising from pair annihilation in relativistic outflows leading to a significant Doppler shift of the frequencies of the lines. We show how this can be used to accurately determine the bulk velocity and orientation to the line of sight of the outflows. Constraints on the energy requirements of such outflows are also derived. Furthermore, we show that a small fraction of pairs escaping the annihilation region may give rise to the radio synchrotron emission observed in some of these objects. We apply these ideas to the hard X-ray and radio observations of Nova Muscae 1991. In this object, the energy requirements seem to rule out a large proton fraction in the outflows.     

A numerical experiment relevant to line-tied reconnection in two-ribbon flares

The nonlinear evolution of a reconnecting magnetic field configuration similar to that occurring just before the onset of ‘post’-flare loops in two-ribbon flares is determined. The evolution, which is obtained by numerically solving the resistive MHD equations, shows two new features that have not yet been incorporated into contemporary models of ‘post’-flare loops. The first of these new features is the formation of a nearly stationary fast-mode shock above the region corresponding to the top of the loops. This fast-mode shock occurs just below the magnetic neutral line and between the slow-mode shocks associated with fast magnetic reconnection at the neutral line. The second new feature is the creation and annihilation of large-scale magnetic islands in the current sheet above the loops. The annihilation of the islands occurs very rapidly and appears to be a manifestation of the coalescence instability. The creation and annihilation of magnetic islands could be important in understanding the energetics of ‘post’-flare loops since the coalescence instability can produce an intermittent energy release more than an order of magnitude faster than that predicted by steady-state reconnection theories.