Rapid X-ray flaring from the direction of the supermassive black hole at the Galactic Centre.

The nuclei of most galaxies are now believed to harbour supermassive black holes. The motions of stars in the central few light years of our Milky Way Galaxy indicate the presence of a dark object with a mass of about 2.6 x 106 solar masses (refs 2, 3). This object is spatially coincident with the compact radio source Sagittarius A* (Sgr A*) at the dynamical centre of the Galaxy, and the radio emission is thought to be powered by the gravitational potential energy released by matter as it accretes onto a supermassive black hole. Sgr A* is, however, much fainter than expected at all wavelengths, especially in X-rays, which has cast some doubt on this model. The first strong evidence for X-ray emission was found only recently. Here we report the discovery of rapid X-ray flaring from the direction of Sgr A*, which, together with the previously reported steady X-ray emission, provides compelling evidence that the emission is coming from the accretion of gas onto a supermassive black hole at the Galactic Centre.     

Event-horizon-scale structure in the supermassive black hole candidate at the Galactic Centre

The cores of most galaxies are thought to harbour supermassive black holes, which power galactic nuclei by converting the gravitational energy of accreting matter into radiation. Sagittarius A* (Sgr A*), the compact source of radio, infrared and X-ray emission at the centre of the Milky Way, is the closest example of this phenomenon, with an estimated black hole mass that is 4,000,000 times that of the Sun. A long-standing astronomical goal is to resolve structures in the innermost accretion flow surrounding Sgr A*, where strong gravitational fields will distort the appearance of radiation emitted near the black hole. Radio observations at wavelengths of 3.5 mm and 7 mm have detected intrinsic structure in Sgr A*, but the spatial resolution of observations at these wavelengths is limited by interstellar scattering. Here we report observations at a wavelength of 1.3 mm that set a size of 37(+16)(-10) microarcseconds on the intrinsic diameter of Sgr A*. This is less than the expected apparent size of the event horizon of the presumed black hole, suggesting that the bulk of Sgr A* emission may not be centred on the black hole, but arises in the surrounding accretion flow.     

A gas cloud on its way towards the supermassive black hole at the Galactic Centre

Measurements of stellar orbits provide compelling evidence that the compact radio source Sagittarius A* at the Galactic Centre is a black hole four million times the mass of the Sun. With the exception of modest X-ray and infrared flares, Sgr A* is surprisingly faint, suggesting that the accretion rate and radiation efficiency near the event horizon are currently very low. Here we report the presence of a dense gas cloud approximately three times the mass of Earth that is falling into the accretion zone of Sgr A*. Our observations tightly constrain the cloud's orbit to be highly eccentric, with an innermost radius of approach of only ～3,100 times the event horizon that will be reached in 2013. Over the past three years the cloud has begun to disrupt, probably mainly through tidal shearing arising from the black hole's gravitational force. The cloud's dynamic evolution and radiation in the next few years will probe the properties of the accretion flow and the feeding processes of the supermassive black hole. The kilo-electronvolt X-ray emission of Sgr A* may brighten significantly when the cloud reaches pericentre. There may also be a giant radiation flare several years from now if the cloud breaks up and its fragments feed gas into the central accretion zone.     

A size of approximately 1 au for the radio source Sgr A* at the centre of the Milky Way

Although it is widely accepted that most galaxies have supermassive black holes at their centres, concrete proof has proved elusive. Sagittarius A* (Sgr A*), an extremely compact radio source at the centre of our Galaxy, is the best candidate for proof, because it is the closest. Previous very-long-baseline interferometry observations (at 7 mm wavelength) reported that Sgr A* is approximately 2 astronomical units (au) in size, but this is still larger than the 'shadow' (a remarkably dim inner region encircled by a bright ring) that should arise from general relativistic effects near the event horizon of the black hole. Moreover, the measured size is wavelength dependent. Here we report a radio image of Sgr A* at a wavelength of 3.5 mm, demonstrating that its size is approximately 1 au. When combined with the lower limit on its mass, the lower limit on the mass density is 6.5 x 10(21)M(o) pc(-3) (where M(o) is the solar mass), which provides strong evidence that Sgr A* is a supermassive black hole. The power-law relationship between wavelength and intrinsic size (size proportional, variantwavelength(1.09)) explicitly rules out explanations other than those emission models with stratified structure, which predict a smaller emitting region observed at a shorter radio wavelength.     

Formation of massive black holes through runaway collisions in dense young star clusters

A luminous X-ray source is associated with MGG 11--a cluster of young stars approximately 200 pc from the centre of the starburst galaxy M 82 (refs 1, 2). The properties of this source are best explained by invoking a black hole with a mass of at least 350 solar masses (350 M(o)), which is intermediate between stellar-mass and supermassive black holes. A nearby but somewhat more massive cluster (MGG 9) shows no evidence of such an intermediate-mass black hole, raising the issue of just what physical characteristics of the clusters can account for this difference. Here we report numerical simulations of the evolution and motion of stars within the clusters, where stars are allowed to merge with each other. We find that for MGG 11 dynamical friction leads to the massive stars sinking rapidly to the centre of the cluster, where they participate in a runaway collision. This produces a star of 800-3,000 M(o) which ultimately collapses to a black hole of intermediate mass. No such runaway occurs in the cluster MGG 9, because the larger cluster radius leads to a mass segregation timescale a factor of five longer than for MGG 11.     

Near-infrared flares from accreting gas around the supermassive black hole at the Galactic Centre

Recent measurements of stellar orbits provide compelling evidence that the compact radio source Sagittarius A* (refs 4, 5) at the Galactic Centre is a 3.6-million-solar-mass black hole. Sgr A* is remarkably faint in all wavebands other than the radio region, however, which challenges current theories of matter accretion and radiation surrounding black holes. The black hole's rotation rate is not known, and therefore neither is the structure of space-time around it. Here we report high-resolution infrared observations of Sgr A* that reveal 'quiescent' emission and several flares. The infrared emission originates from within a few milliarcseconds of the black hole, and traces very energetic electrons or moderately hot gas within the innermost accretion region. Two flares exhibit a 17-minute quasi-periodic variability. If the periodicity arises from relativistic modulation of orbiting gas, the emission must come from just outside the event horizon, and the black hole must be rotating at about half of the maximum possible rate.     

A star in a 15.2-year orbit around the supermassive black hole at the centre of the Milky Way

Many galaxies are thought to have supermassive black holes at their centres-more than a million times the mass of the Sun. Measurements of stellar velocities and the discovery of variable X-ray emission have provided strong evidence in favour of such a black hole at the centre of the Milky Way, but have hitherto been unable to rule out conclusively the presence of alternative concentrations of mass. Here we report ten years of high-resolution astrometric imaging that allows us to trace two-thirds of the orbit of the star currently closest to the compact radio source (and massive black-hole candidate) Sagittarius A*. The observations, which include both pericentre and apocentre passages, show that the star is on a bound, highly elliptical keplerian orbit around Sgr A*, with an orbital period of 15.2 years and a pericentre distance of only 17 light hours. The orbit with the best fit to the observations requires a central point mass of (3.7 +/- 1.5) x 10(6) solar masses (M(*)). The data no longer allow for a central mass composed of a dense cluster of dark stellar objects or a ball of massive, degenerate fermions.     

坐标系选取对星体轨道理论预言的影响

广义相对论关于天体轨道的预言依赖于所选取的坐标系。对不同坐标系下围绕银心运动的星体轨道的差异进行了研究,并讨论这些差异对于确定银河系中心黑洞参数的影响。     

The central image of a gravitationally lensed quasar

A galaxy can act as a gravitational lens, producing multiple images of a background object. Theory predicts that there should be an odd number of images produced by the lens, but hitherto almost all lensed objects have two or four images. The missing 'central' images, which should be faint and appear near the centre of the lensing galaxy, have long been sought as probes of galactic cores too distant to resolve with ordinary observations. There are five candidates for central images, but in one case the third image is not necessarily the central one, and in the others the putative central images might be foreground sources. Here we report a secure identification of a central image, based on radio observations of one of the candidates. Lens models using the central image reveal that the massive black hole at the centre of the lensing galaxy has a mass of <2 x 10(8) solar masses (M(o)), and the galaxy's surface density at the location of the central image is > 20,000M(o) pc(-2), which is in agreement with expections based on observations of galaxies that are much closer to the Earth.     

Energy input from quasars regulates the growth and activity of black holes and their host galaxies

In the early Universe, while galaxies were still forming, black holes as massive as a billion solar masses powered quasars. Supermassive black holes are found at the centres of most galaxies today, where their masses are related to the velocity dispersions of stars in their host galaxies and hence to the mass of the central bulge of the galaxy. This suggests a link between the growth of the black holes and their host galaxies, which has indeed been assumed for a number of years. But the origin of the observed relation between black hole mass and stellar velocity dispersion, and its connection with the evolution of galaxies, have remained unclear. Here we report simulations that simultaneously follow star formation and the growth of black holes during galaxy-galaxy collisions. We find that, in addition to generating a burst of star formation, a merger leads to strong inflows that feed gas to the supermassive black hole and thereby power the quasar. The energy released by the quasar expels enough gas to quench both star formation and further black hole growth. This determines the lifetime of the quasar phase (approaching 100 million years) and explains the relationship between the black hole mass and the stellar velocity dispersion.     

核心坍缩超新星中心黑洞超吸积在星系尺度上的贡献

核心坍缩超新星(Core-Collapse Supernova,CCSN)是大质量恒星演化末期的爆发现象,产生了宇宙中大多数的中子星和恒星级黑洞等致密天体.爆发可能伴随着强磁场中子星或黑洞超吸积引发的剧烈长时标伽马射线暴.CCSN还被认为是宇宙重元素的主要来源之一.本综述介绍了我们近期对CCSN中心黑洞超吸积过程的系列研究成果,主要包括研究了大质量星系中心附近伽马射线暴余辉阶段,因大量暗物质粒子湮灭电子注入而引发的光变和能谱的形态变化,探讨了其作为暗物质探测手段的可能性;研究了坍缩星框架下,中微子主导吸积流外流对核合成的贡献,及对太阳临近空间、(活动)星系等化学组分和演化的影响;最后,从数值模拟角度讨论了CCSN起源的致密天体质量分布,给出了低质量间隙可能起源于CCSN爆发能量分布的结论.     

对银河系中心超大质量黑洞候选体Sgr A*的VLBI观测研究

位于银河系中央的极其致密的非热射电源Sagittari8us A^*（Sgr A^*），被公认为最佳的超大质量黑洞候选体，其质量约为400万个太阳质量。甚长基线干涉测量（BLBI）技术以其独有的高空间分辩率为我们提供了唯一可以探讨Sgr A^*位于几个史瓦西（Schwarzschild）半径区域内天体物理性质的手段。介绍了对 Sgr A^*的VLBI研究现状和最新进展，着重论述了毫米波和亚毫米波VLBI观测对研究Sgr A^*的辐射区域的真实形状和大小，以及探索其结构随时间变化的重要性。Sgr A^*无疑是检验广义相对论将就的亚亮米波VLBI实验的最佳目标。     

A neutron-star-driven X-ray flash associated with supernova SN 2006aj

Supernovae connected with long-duration gamma-ray bursts (GRBs) are hyper-energetic explosions resulting from the collapse of very massive stars ( approximately 40 M\circ, where M\circ is the mass of the Sun) stripped of their outer hydrogen and helium envelopes. A very massive progenitor, collapsing to a black hole, was thought to be a requirement for the launch of a GRB. Here we report the results of modelling the spectra and light curve of SN 2006aj (ref. 9), which demonstrate that the supernova had a much smaller explosion energy and ejected much less mass than the other GRB-supernovae, suggesting that it was produced by a star whose initial mass was only approximately 20 M\circ. A star of this mass is expected to form a neutron star rather than a black hole when its core collapses. The smaller explosion energy of SN 2006aj is matched by the weakness and softness of GRB 060218 (an X-ray flash), and the weakness of the radio flux of the supernova. Our results indicate that the supernova-GRB connection extends to a much broader range of stellar masses than previously thought, possibly involving different physical mechanisms: a 'collapsar' (ref. 8) for the more massive stars collapsing to a black hole, and magnetic activity of the nascent neutron star for the less massive stars.     

Three classical Cepheid variable stars in the nuclear bulge of the Milky Way

The nuclear bulge is a region with a radius of about 200 parsecs around the centre of the Milky Way. It contains stars with ages ranging from a few million years to over a billion years, yet its star-formation history and the triggering process for star formation remain to be resolved. Recently, episodic star formation, powered by changes in the gas content, has been suggested. Classical Cepheid variable stars have pulsation periods that decrease with increasing age, so it is possible to probe the star-formation history on the basis of the distribution of their periods. Here we report the presence of three classical Cepheids in the nuclear bulge with pulsation periods of approximately 20?days, within 40 parsecs (projected distance) of the central black hole. No Cepheids with longer or shorter periods were found. We infer that there was a period about 25 million years ago, and possibly lasting until recently, in which star formation increased relative to the period of 30-70 million years ago.     

A 15.65-solar-mass black hole in an eclipsing binary in the nearby spiral galaxy M 33

Stellar-mass black holes are found in X-ray-emitting binary systems, where their mass can be determined from the dynamics of their companion stars. Models of stellar evolution have difficulty producing black holes in close binaries with masses more than ten times that of the Sun (>10; ref. 4), which is consistent with the fact that the most massive stellar black holes known so far all have masses within one standard deviation of 10. Here we report a mass of (15.65 +/- 1.45) for the black hole in the recently discovered system M 33 X-7, which is located in the nearby galaxy Messier 33 (M 33) and is the only known black hole that is in an eclipsing binary. To produce such a massive black hole, the progenitor star must have retained much of its outer envelope until after helium fusion in the core was completed. On the other hand, in order for the black hole to be in its present 3.45-day orbit about its (70.0 +/- 6.9) companion, there must have been a 'common envelope' phase of evolution in which a significant amount of mass was lost from the system. We find that the common envelope phase could not have occurred in M 33 X-7 unless the amount of mass lost from the progenitor during its evolution was an order of magnitude less than what is usually assumed in evolutionary models of massive stars.     

原初典型恒星及其星核黑洞的形成、演化和结构的基本规律

本文在文献[1]、[2]的基础上,应用相对论性牛顿万有引力定律,对原初典型恒星及其星核M。黑洞的形成、演化和结构,作了更深入的探索研究．得到了一系列很有意义的结果：1、导出了原初典型恒星Mo及其星核Ms黑洞质量的上界和下界．进而导出了原初典型恒星在其简并中子黑涧（Mos=Mo）态下一分为二的死亡大爆炸中释放能量的上下界（即超新星爆发所释放能量的上下界）;2、导出了星核Ms黑洞独有的一系列鼎级极限物理特性．导出了黑洞无内阻理想流体的超流效应及黑洞吸集粒子的经纬分流的筛选效应．证明了星核Ms黑洞是具有不再爆炸、不再坍缩、不再发射的“三不”特性的稳定天体;3、在相对论性引力理论框架内,证明了光速最大原理;4、导出了原初典型恒星在一分为二的死亡大爆炸中静质量Mo、场（暗）质量△MG的结构和分布规律．揭示了星核Ms黑洞结构的奥秘;5、导出了恒星、星系、总星系等典型层次天体的真空中场（暗）质量的分布规律．     

The suppression of star formation by powerful active galactic nuclei

The old, red stars that constitute the bulges of galaxies, and the massive black holes at their centres, are the relics of a period in cosmic history when galaxies formed stars at remarkable rates and active galactic nuclei (AGN) shone brightly as a result of accretion onto black holes. It is widely suspected, but unproved, that the tight correlation between the mass of the black hole and the mass of the stellar bulge results from the AGN quenching the surrounding star formation as it approaches its peak luminosity. X-rays trace emission from AGN unambiguously, whereas powerful star-forming galaxies are usually dust-obscured and are brightest at infrared and submillimetre wavelengths. Here we report submillimetre and X-ray observations that show that rapid star formation was common in the host galaxies of AGN when the Universe was 2-6 billion years old, but that the most vigorous star formation is not observed around black holes above an X-ray luminosity of 10(44) ergs per second. This suppression of star formation in the host galaxy of a powerful AGN is a key prediction of models in which the AGN drives an outflow, expelling the interstellar medium of its host and transforming the galaxy's properties in a brief period of cosmic time.     

银河系中心超大质量黑洞Sgr A＊的光变研究进展

越来越多的观测证据表明,位于银河系中心的致密射电源SgrA*是一个质量约为400万个太阳质量的超大质量黑洞.本文介绍在射电、毫米/亚毫米、近红外、X射线波段对Sgr A*的光变观测和理论研究进展.自1974年被射电干涉测量发现伊始,Sgr A*就被检测到时标从天到年不等的射电光变,随着新的观测设备的出现和观测技术的发展,天文学家先后在所有的对银河系中心可观测波段上检测到了Sgr A*的光变,包括在毫米/亚毫米波段的时标为小时的快速光变,近红外波段上时标不到1 h的显著光变,以及空间X射线卫星记录到的幅度变化几十倍以上的巨大耀变.这些光变观测数据可以用于估算产生Sgr A*耀变辐射的区域范围(假设光变是由整个源产生的,辐射区域的尺度/与光变时标τ之间存在线性关系,/≤Cr),并对Sgr A*辐射机制研究提供了间接的限制.近年来开展的一些多波段联测显示,近红外和×射线光变是同步发生的,并且都比射电和毫米波光变先出现,这类时间延迟支持Sgr A*光变的绝热膨胀理论模型.但目前成功的多波段耀变观测非常有限,如何获得从射电厘米波到X射线的Sgr A*光变间的内在关联将是未来几年中Sgr A*光变的一个主要研究方向.     

星的盘状分布

在广义相对论框架内 ,运用相对论形式的无碰撞Boltzmann方程和Schwarzshild度规 ,求得了中心为Schwarzshild黑洞的星的盘状分布 .计算结果表明盘状分布时的星数衰减比只有径向速度时的球状分布慢