Magnetar-like X-ray bursts from an anomalous X-ray pulsar

Anomalous X-ray pulsars (AXPs) are a class of rare X-ray emitting pulsars whose energy source has been perplexing for some 20 years. Unlike other X-ray emitting pulsars, AXPs cannot be powered by rotational energy or by accretion of matter from a binary companion star, hence the designation 'anomalous'. Many of the rotational and radiative properties of the AXPs are strikingly similar to those of another class of exotic objects, the soft-gamma-ray repeaters (SGRs). But the defining property of the SGRs--their low-energy-gamma-ray and X-ray bursts--has not hitherto been observed for AXPs. Soft-gamma-ray repeaters are thought to be 'magnetars', which are young neutron stars whose emission is powered by the decay of an ultra-high magnetic field; the suggestion that AXPs might also be magnetars has been controversial. Here we report two X-ray bursts, with properties similar to those of SGRs, from the direction of the anomalous X-ray pulsar 1E1048.1 - 5937. These events imply a close relationship (perhaps evolutionary) between AXPs and SGRs, with both being magnetars.     

Optical pulsations from the anomalous X-ray pulsar 4U0142+61

Anomalous X-ray pulsars (AXPs) differ from ordinary radio pulsars in that their X-ray luminosity is orders of magnitude greater than their rate of rotational energy loss, and so they require an additional energy source. One possibility is that AXPs are highly magnetized neuron stars or 'magnetars' having surface magnetic fields greater than 10(14) G. This would make them similar to the soft gamma-ray repeaters (SGRs), but alternative models that do not require extreme magnetic fields also exist. An optical counterpart to the AXP 4U0142+61 was recently discovered, consistent with emission from a magnetar, but also from a magnetized hot white dwarf, or an accreting isolated neutron star. Here we report the detection of optical pulsations from 4U0142+61. The pulsed fraction of optical light (27 per cent) is five to ten times greater than that of soft X-rays, from which we conclude that 4U0142+61 is a magnetar. Although this establishes a direct relationship between AXPs and the soft gamma-ray repeaters, the evolutionary connection between AXPs, SGRs and radio pulsars remains controversial.     

Transient pulsed radio emission from a magnetar

Anomalous X-ray pulsars (AXPs) are slowly rotating neutron stars with very bright and highly variable X-ray emission that are believed to be powered by ultra-strong magnetic fields of >10(14) G, according to the 'magnetar' model. The radio pulsations that have been observed from more than 1,700 neutron stars with weaker magnetic fields have never been detected from any of the dozen known magnetars. The X-ray pulsar XTE J1810-197 was revealed (in 2003) as the first AXP with transient emission when its luminosity increased 100-fold from the quiescent level; a coincident radio source of unknown origin was detected one year later. Here we show that XTE J1810-197 emits bright, narrow, highly linearly polarized radio pulses, observed at every rotation, thereby establishing that magnetars can be radio pulsars. There is no evidence of radio emission before the 2003 X-ray outburst (unlike ordinary pulsars, which emit radio pulses all the time), and the flux varies from day to day. The flux at all radio frequencies is approximately equal--and at >20 GHz XTE J1810-197 is currently the brightest neutron star known. These observations link magnetars to ordinary radio pulsars, rule out alternative accretion models for AXPs, and provide a new window into the coronae of magnetars.     

反常X射线脉冲星可能的高能辐射

 利用新的外间隙模型研究了反常X射线脉冲星可能的高能辐射.在该模型中,考虑了磁场几何效应对高能辐射的影响,同时利用平均半径〈r〉处的外间隙大小表示外间隙的特征大小,〈r〉是磁倾角α的函数.这样外间隙大小可以表示为f(P,B,〈r〉(α)),它是周期P,磁场B和平均距离〈r〉的函数.利用该模型计算了反常X射线脉冲星的γ射线的光度和流量,并与以前的计算结果进行比较,分析了考虑磁场几何效应所产生的结果.最后利用该模型计算了几颗典型反常X射线脉冲星(如:4U 0142+615(AXP 0142+615),1E 1841-045,PSR J1809-1943)可能的高能辐射.     

Flares from a candidate Galactic magnetar suggest a missing link to dim isolated neutron stars

Magnetars are young neutron stars with very strong magnetic fields of the order of 10(14)-10(15) G. They are detected in our Galaxy either as soft gamma-ray repeaters or anomalous X-ray pulsars. Soft gamma-ray repeaters are a rare type of gamma-ray transient sources that are occasionally detected as bursters in the high-energy sky. No optical counterpart to the gamma-ray flares or the quiescent source has yet been identified. Here we report multi-wavelength observations of a puzzling source, SWIFT J195509+261406. We detected more than 40 flaring episodes in the optical band over a time span of three days, and a faint infrared flare 11 days later, after which the source returned to quiescence. Our radio observations confirm a Galactic nature and establish a lower distance limit of approximately 3.7 kpc. We suggest that SWIFT J195509+261406 could be an isolated magnetar whose bursting activity has been detected at optical wavelengths, and for which the long-term X-ray emission is short-lived. In this case, a new manifestation of magnetar activity has been recorded and we can consider SWIFT J195509+261406 to be a link between the 'persistent' soft gamma-ray repeaters/anomalous X-ray pulsars and dim isolated neutron stars.     

gamma Rays from close binaries in the quiet stage

THE presence of rotating neutron stars in binary systems is revealed by the existence of X-ray pulsators which are generally associated with massive companions. The evolutionary history of these systems is now relatively clear (see ref. 1 and refs therein). The X-ray phase, which corresponds to accretion on the neutron star of a strong stellar wind \M approximately 10(-6) M(\circ) yr(-1), is preceded by a much longer quiet state, where the primary is unevolved, possibly with a weak wind, \M approximately 10(-9) M(\circ) yr(-1) and the rotational energy loss of the neutron star inhibits accretion. However, with the only exception of PSR1913 + 16, radio pulsars are not found in binary systems. Here, motivated by the recent discovery of gamma-ray emission from slow pulsars(2,3), we suggest (gamma)-ray observations as a way of detecting binaries in the quiet state and compare the expected number with the COS B results. We refer to the review of van den Heuvel(1) where, as a representative case, a system of initial mass 20 + 8 M(\circ) and orbital period 4.7 d is considered. After the first stage of mass exchange and the supernova explosion, one has a neutron star with an unevolved companion of 22.7 M(\circ). The period is now P = 12.6 d. The primary remains on the main sequence (quiet stage) for t(ms) = 3.6 x 10(6)yr. The X-ray phase occurs after the star has left the main sequence, but before it fills its Roche lobe, as an excessive mass transfer absorbs the X-ray emission. Its duration is t(x) approximately 10(4)yr.     

M87的TeV辐射光变

2005年第一次在M87中观测到TeV辐射爆发,分析表明TeV爆发的光变时标非常短且相对孤立.M87的喷流的视向倾角较大(～30?),是银河系外观测到的第一个非blazar类TeV源,这暗示着新的产生TeV辐射过程.考虑到TeV源有可能是产生于黑洞吸积盘上的等离子体云团,刚诞生的时候可能围绕中心黑洞作开普勒运动,本文在完全广义相对论框架下计算了这种情况下TeV辐射的光变.模型中不仅考虑了TeV源作开普勒运动的效应,还计算了轨道上不同点发出的TeV光子在到达无穷远处观测者的过程中被来自吸积盘上的软光子吸收的光深差异.计算表明TeV源的光变主要由不同轨道上的TeV光子的光深变化主导,TeV源作开普勒运动的效应基本可以忽略.在TeV源距离黑洞比较近的时候,随着黑洞自旋的增大光深显著减小,在黑洞自旋为零时,光深变化曲线明显不对称,TeV源越靠近黑洞这些效应越显著.     

Correlated fast X-ray and optical variability in the black-hole candidate XTE J1118+480.

Black holes become visible when they accrete gas, a common source of which is a close stellar companion. The standard theory for this process (invoking a 'thin accretion disk') does not explain some spectacular phenomena associated with these systems, such as their X-ray variability and relativistic outflows, indicating some lack of understanding of the actual physical conditions. Simultaneous observations at multiple wavelengths can provide strong constraints on these conditions. Here we report simultaneous high-time-resolution X-ray and optical observations of the transient source XTE J1118+480, which show a strong but puzzling correlation between the emissions. The optical emission rises suddenly following an increase in the X-ray output, but with a dip 2-5 seconds in advance of the X-rays. This result is not easy to understand within the simplest model of the optical emission, where the light comes from reprocessed X-rays. It is probably more consistent with an earlier suggestion that the optical light is cyclosynchrotron emission that originates in a region about 20,000 km from the black hole. We propose that the time dependence is evidence for a relatively slow (<0.1c), magnetically controlled outflow.     

The rotating wind of the quasar PG 1700+518

It is now widely accepted that most galaxies undergo an active phase, during which a central super-massive black hole generates vast radiant luminosities through the gravitational accretion of gas. Winds launched from a rotating accretion disk surrounding the black hole are thought to play a critical role, allowing the disk to shed angular momentum that would otherwise inhibit accretion. Such winds are capable of depositing large amounts of mechanical energy in the host galaxy and its environs, profoundly affecting its formation and evolution, and perhaps regulating the formation of large-scale cosmological structures in the early Universe. Although there are good theoretical grounds for believing that outflows from active galactic nuclei originate as disk winds, observational verification has proven elusive. Here we show that structures observed in polarized light across the broad Halpha emission line in the quasar PG 1700+518 originate close to the accretion disk in an electron scattering wind. The wind has large rotational motions (approximately 4,000 km s(-1)), providing direct observational evidence that outflows from active galactic nuclei are launched from the disks. Moreover, the wind rises nearly vertically from the disk, favouring launch mechanisms that impart an initial acceleration perpendicular to the disk plane.     

An X-ray outburst from the rapidly accreting young star that illuminates McNeil's nebula

Young, low-mass stars are luminous X-ray sources whose powerful X-ray flares may exert a profound influence over the process of planet formation. The origin of the X-ray emission is uncertain. Although many (or perhaps most) recently formed, low-mass stars emit X-rays as a consequence of solar-like coronal activity, it has also been suggested that X-ray emission may be a direct result of mass accretion onto the forming star. Here we report X-ray imaging spectroscopy observations which reveal a factor approximately 50 increase in the X-ray flux from a young star that is at present undergoing a spectacular optical/infrared outburst (this star illuminates McNeil's nebula). The outburst seems to be due to the sudden onset of a phase of rapid accretion. The coincidence of a surge in X-ray brightness with the optical/infrared eruption demonstrates that strongly enhanced high-energy emission from young stars can occur as a consequence of high accretion rates. We suggest that such accretion-enhanced X-ray emission from erupting young stars may be short-lived, because intense star-disk magnetospheric interactions are quenched rapidly by the subsequent flood of new material onto the star.     

反常粘滞吸积薄盘的稳定结构

根据描述吸积盘的流体动力学方程,给出无磁场情况下的质量、动量和能量守恒方程。在所给定方程的基础上,利用新的反常粘滞计算了黑洞周围吸积盘内区的结构。数值结果表明盘内区的气体不是做开普勒旋转,温度随吸积率变化比较明显,在M=10M_⊙,M ^.=100M ^._E的情况下,吸积盘的最大温度可达到3.35×10⁷K。 同时文中也给出了面密度和径向速度的分布规律。     

Direct detection of a magnetic field in the innermost regions of an accretion disk

Models predict that magnetic fields play a crucial role in the physics of astrophysical accretion disks and their associated winds and jets. For example, the rotation of the disk twists around the rotation axis the initially vertical magnetic field, which responds by slowing down the plasma in the disk and by causing it to fall towards the central star. The magnetic energy flux produced in this process points away from the disk, pushing the surface plasma outwards, leading to a wind from the disk and sometimes a collimated jet. But these predictions have hitherto not been supported by observations. Here we report the direct detection of the magnetic field in the core of the protostellar accretion disk FU Orionis. The surface field reaches strengths of about 1 kG close to the centre of the disk, and it includes a significant azimuthal component, in good agreement with recent models. But we find that the field is very filamentary and slows down the disk plasma much more than models predict, which may explain why FU Ori fails to collimate its wind into a jet.     

具有垂向结构的反常粘滞吸积盘的不稳定性研究

天体物理吸积盘是具有反常粘滞的流体盘．利用一种全新的反常粘滞,主要研究了具有垂向结构的吸积盘．在盘的内区发现两种声速模：O-mode,I-mode．由于反常粘滞的存在,导致I-mode在盘的内区是始终不稳定的．另外,在讨论高频不稳定模时,必须注意到垂向结构是不可忽略的．     

自引力均分吸积盘的不稳定性

采用微扰方法导了磁粘滞情况下自引力均分薄吸积盘的色散方程研究了吸积秀斩不稳定性，并分析了自引力，磁场，粘滞对吸积盘不稳定性的影响，数值计算表明，在同时考虑自引力和磁场时，吸积盘存在着３种振荡模式（总是稳定的粘滞横模式和通常不稳定的磁声模式及中性模式）这些结果为解释ＢＬ．Ｌａｃ天体，Ｓｅｙｆｅｒｔ星系，类星体等活动星系核的光变现象提供了理论依据。     

磁场从黑洞吸积盘提取能量的解析模型

提出了一种磁场提取黑洞吸积盘能量的解析模型,其中涉及到3种能量机制: (1) Blandford-Znajek (BZ)与连接黑洞与天体物理负载的开放磁力线有关, (2) 磁耦合(MC)过程与连接黑洞与盘负载的闭合磁力线有关, (3) DL过程与连接吸积盘与天体物理负载的开放磁力线有关.利用等效电路导出上述能量机制的电磁功率和力矩表达式.此外还讨论了这个模型潜在的天体物理应用.     

相对论电子在螺旋磁场中的辐射

从运动荷电粒子产生的流出发 ,导出相对论电子和相对论电子集在等离子体及真空情况下曲率辐射的能量谱及辐射总能量 ,分析了等离子体对辐射的影响 .并着重用曲率辐射公式讨论了在磁流体吸积盘的特殊螺旋强磁场中的辐射特点 .     

The unusual gamma-ray burst GRB 101225A explained as a minor body falling onto a neutron star

The tidal disruption of a solar-mass star around a supermassive black hole has been extensively studied analytically and numerically. In these events, the star develops into an elongated banana-shaped structure. After completing an eccentric orbit, the bound debris falls into the black hole, forming an accretion disk and emitting radiation. The same process may occur on planetary scales if a minor body passes too close to its star. In the Solar System, comets fall directly into our Sun or onto planets. If the star is a compact object, the minor body can become tidally disrupted. Indeed, one of the first mechanisms invoked to produce strong gamma-ray emission involved accretion of comets onto neutron stars in our Galaxy. Here we report that the peculiarities of the 'Christmas' gamma-ray burst (GRB 101225A) can be explained by a tidal disruption event of a minor body around an isolated Galactic neutron star. This would indicate either that minor bodies can be captured by compact stellar remnants more frequently than occurs in the Solar System or that minor-body formation is relatively easy around millisecond radio pulsars. A peculiar supernova associated with a gamma-ray burst provides an alternative explanation.     

Very fast optical flaring from a possible new Galactic magnetar

Highly luminous rapid flares are characteristic of processes around compact objects like white dwarfs, neutron stars and black holes. In the high-energy regime of X-rays and gamma-rays, outbursts with variabilities on timescales of seconds or less are routinely observed, for example in gamma-ray bursts or soft gamma-ray repeaters. At optical wavelengths, flaring activity on such timescales has not been observed, other than from the prompt phase of one exceptional gamma-ray burst. This is mostly due to the fact that outbursts with strong, fast flaring are usually discovered in the high-energy regime; most optical follow-up observations of such transients use instruments with integration times exceeding tens of seconds, which are therefore unable to resolve fast variability. Here we show the observation of extremely bright and rapid optical flaring in the Galactic transient SWIFT J195509.6+261406. Our optical light curves are phenomenologically similar to high-energy light curves of soft gamma-ray repeaters and anomalous X-ray pulsars, which are thought to be neutron stars with extremely high magnetic fields (magnetars). This suggests that similar processes are in operation, but with strong emission in the optical, unlike in the case of other known magnetars.     

A debris disk around an isolated young neutron star

Pulsars are rotating, magnetized neutron stars that are born in supernova explosions following the collapse of the cores of massive stars. If some of the explosion ejecta fails to escape, it may fall back onto the neutron star or it may possess sufficient angular momentum to form a disk. Such 'fallback' is both a general prediction of current supernova models and, if the material pushes the neutron star over its stability limit, a possible mode of black hole formation. Fallback disks could dramatically affect the early evolution of pulsars, yet there are few observational constraints on whether significant fallback occurs or even the actual existence of such disks. Here we report the discovery of mid-infrared emission from a cool disk around an isolated young X-ray pulsar. The disk does not power the pulsar's X-ray emission but is passively illuminated by these X-rays. The estimated mass of the disk is of the order of 10 Earth masses, and its lifetime (> or = 10(6) years) significantly exceeds the spin-down age of the pulsar, supporting a supernova fallback origin. The disk resembles protoplanetary disks seen around ordinary young stars, suggesting the possibility of planet formation around young neutron stars.     

Active galactic nuclei as scaled-up Galactic black holes

A long-standing question is whether active galactic nuclei (AGN) vary like Galactic black hole systems when appropriately scaled up by mass. If so, we can then determine how AGN should behave on cosmological timescales by studying the brighter and much faster varying Galactic systems. As X-ray emission is produced very close to the black holes, it provides one of the best diagnostics of their behaviour. A characteristic timescale--which potentially could tell us about the mass of the black hole--is found in the X-ray variations from both AGN and Galactic black holes, but whether it is physically meaningful to compare the two has been questioned. Here we report that, after correcting for variations in the accretion rate, the timescales can be physically linked, revealing that the accretion process is exactly the same for small and large black holes. Strong support for this linkage comes, perhaps surprisingly, from the permitted optical emission lines in AGN whose widths (in both broad-line AGN and narrow-emission-line Seyfert 1 galaxies) correlate strongly with the characteristic X-ray timescale, exactly as expected from the AGN black hole masses and accretion rates. So AGN really are just scaled-up Galactic black holes.