A VLBI search for pulsar wind nebulae in supernovae

We examine recent supernovae which have been observed with very-long-baseline interferometry in order to detect or limit the emission from a possible compact remnant of the explosion. Such a remnant could be a neutron star, generating a pulsar wind nebula, or a black hole with an accretion disk and jets. Four supernovae, and also more than a dozen supernovae or their young remnants in M82, have structure sufficiently resolved to allow useful conclusions as to the strength of the emission from such young neutron stars or black holes. We recently discovered a compact component in the center of SN 1986J’s shell with a spectral luminosity at 15 GHz 200 times that of the Crab Nebula. This is most likely the compact remnant of the explosion, the first and only one found in any modern supernova. For other modern supernovae, the upper limits on the radio spectral luminosities of such young compact remnants range from 180 times that of the Crab Nebula for SN 1979C in M100 in the Virgo cluster to 0.001 times that of the Crab Nebula for SN 1987A in the Large Magellanic Cloud.     

MHD interaction of pulsar wind nebulae with SNRs and with the ISM

In the late 1960s the discovery of the Crab pulsar in its associated supernova remnant, launched a new field in supernova remnant research: the study of pulsar-driven or plerionic supernova remnants. In these type of remnants, the relativistic wind emitted by the pulsar, blows a pulsar wind nebula into the interior of its supernova remnant. Now, more then forty years after the discovery of the Crab pulsar, there are more then fifty plerionic supernova remnants known, due to the ever-increasing capacity of observational facilities. These observational studies reveal a Zoo of complex morphologies over a wide range of frequencies, indicating the significance of the interaction between a pulsar wind nebula with its surrounding supernova remnant. A pulsar which gained a kick velocity at birth, will ultimately break outside of its remnant, after which the pulsar wind nebula interacts directly with the interstellar medium. In general these pulsar wind nebulae are bounded by a bow shock, due to the supersonic motion of the pulsar. There are a few examples known of these pulsar-powered bow shocks, a number which is slowly increasing.I will review our current understanding of the different evolutionary stages of a pulsar wind nebula as it is interacting with its associated supernova remnant. Therefore, I will discuss both analytical and more recent numerical (M)HD models. The four main stages of a pulsar wind nebula are: the supersonic expansion stage, the reverse shock interaction stage, the subsonic expansion stage and ultimately the stage when the head of the bubble is bounded by a bow shock, due to the supersonic motion of the pulsar. Ultimately this pulsar wind nebula bow shock will break through its associated remnant, after which the pulsar-powered bow shock will interact directly with the interstellar medium. I will discuss recent numerical models from these type of pulsar wind nebulae and their morphology.     

The Crab pulsar seen with AquEYE at Asiago Cima Ekar observatory

We are developing fast photon-counter instruments to study the rapid variability of astrophysical sources by time tagging photon arrival times with unprecedented accuracy, making use of a Rubidium clock and GPS receiver. The first realization of such optical photon-counters, dubbed AquEYE (the Asiago Quantum Eye), was mounted in 2008 at the 182 cm Copernicus Observatory in Asiago. AquEYE observed the Crab pulsar several times and collected data of extraordinary quality that allowed us to perform accurate optical timing of the Crab pulsar and to study the pulse shape stability on a timescale from days to years with an excellent definition. Our results reinforce the evidence for decadal stability of the inclination angle between the spin and magnetic axis of the Crab pulsar. Future realizations of our instrument will make use of the Galileo Global Navigation Satellite System (GNSS) time signal.     

May 1980 low energy gamma-ray observations with the ‘MISO’ telescope

During a balloon flight of the MISO telescope on 1980 May 17, the Crab Nebula and the Seyfert galaxy NGC 4151 were studied over the photon energy range 0.03 –16 MeV. The photon spectrum of the Crab Nebula was measured up to ～ 2 MeV. No gamma-ray emission from NGC 4151 was detected on this occasion.     

Optical,ultraviolet, and infrared observations of isolated neutron stars

Forty years passed since the optical identification of the first isolated neutron star (INS), the Crab pulsar. 25 INSs have been now identified in the optical (O), near-ultraviolet (nUV), or near-infrared (nIR), hereafter UVOIR, including rotation-powered pulsars (RPPs), magnetars, and X-ray-dim INSs (XDINSs), while deep investigations have been carried out for compact central objects (CCOs), Rotating RAdio transients (RRATs), and high-magnetic field radio pulsars (HBRPs). In this review I describe the status of UVOIR observations of INSs, their emission properties, and I present the results from recent observations.     

The devil is in the details: Compact structures in pulsar wind nebulae

The large-scale structure of pulsar wind nebulae (PWNe) tells us a considerable amount about their average magnetic fields, the total particle input from the pulsar winds, and the confining pressure at their outer boundaries. However, the details of the pulsar outflow, the sites of shocks and particle acceleration, the effects of instabilities in the magnetic field, and the interaction between the relativistic wind and the surrounding ejecta are contained in small-scale structures, where we observe jets and toroidal structures, time-varying emission from compact clumps, and filaments in both the inner and outer regions of the nebulae. Here, I review recent observational studies of compact structures in PWNe and present current scenarios (and questions) regarding their origin.     

Hard X-ray observation of galactic X-ray sources

A large (1455 cm²) hard X-ray telescope was successfully launched aboard a stratospheric balloon on October 4, 1980. During this flight four galactic X-ray sources were observed, namely the transient recurrent X-ray pulsar A0535+26, the Crab Nebula, Cygnus X-1 and X Persei. Here we report the results on the latter two sources. From Cygnus X-1 we measured a photon flux in the band 30 to 200 keV, of 3.5 × 10^?2 photons cm^?2 which is 6.5 times lower than that recieved from the source in a “low” intensity state in the same energy band. In addition, the photon spectrum in the same energy band was very soft and consistent with a power law with photon index α = 2.71 ± 0.14. Even if a simultaneous observation of the source at lower energies was not available, our data strongly suggest that we observed the source during a “high” intensity state. We report also positive detection in the band 30 to 200 keV of the low luminosity X-ray pulsar X Persei. In its spectrum we confirm the presence of a hard X-ray tail consistent with a power law (photon index α = 2.17 ± 0.42).     

The termination shock in a striped pulsar wind

I present observational and theoretical evidence that most of the pulsar spin-down energy is transferred away as a striped pulsar wind and that this energy is released by annihilation of the alternating magnetic field at the pulsar wind termination shock. One-dimensional particle-in-cells (PIC) simulations show that the alternating fields do annihilate at the termination shock in a striped wind. The particle acceleration should be studied in multidimensional simulations. As a first step, I simulated driven annihilation of alternating fields undergoing compression by an external force. It is shown that that in the course of this process, a particle distribution function is formed, which resembles that observed in plerions.     

Bow shocks around pulsars and neutron stars

Pulsar wind nebulae are now well-established as important probes both of neutron stars’ relativistic winds and of the surrounding interstellar medium. Amongst this diverse group of objects, pulsar bow shocks have long been regarded as an oddity, only seen around a handful of rapidly moving neutron stars. However, recent efforts at optical, radio and X-ray wavelengths have identified many new pulsar bow shocks, and these results have consequently motivated renewed theoretical efforts to model these systems. Here, I review the new results and ideas which have emerged on these spectacular systems, and explain how bow shocks and “Crab-like” nebulae now form a consistent picture within our understanding of pulsar winds.     

Sporadic emission of ultra high energy gamma rays from crab pulsar

In a further study of sporadic emission from pulsars we find evidence for short lived intense emission from the Crab pulsar.     

Evidence of a cyclotron line at 70 keV from 4U0531+21

The HXR79 hard X-Ray experiment was flown the August 26th 1979 from the Milo Base (Sicily, Italy) and a 27-hour flight was achieved. The scientific payload consisted of two Multiwire Spectroscopic Proportional Counters (MWSPC) with 900 cm² sensitive geometric area each, in the hard X-Ray range (16–180 keV). A single drift scan was performed on the Crab Nebula region after the balloon reached the flotation point (2.9 mbar). The observation gave a maximum counting rate of about 25 counts/s superimposed on a background of about 75 counts/s. The Crab power-law spectrum was observed, and these data provide evidence for a line emission enhancement around 70 keV.     

IUE observations of supernova remnants

Ultraviolet spectra of supernova remnants obtained with the IUE satellite provide unique information concerning the shock conditions and elemental abundances in the optically bright filaments. High temperature species such as N V provide diagnostics for shock velocities above 100 km s^?1, and strong lines of carbon and silicon in the IUE spectral range make it possible to study the destruction of refractory grains in shocked interstellar gas. Observations of a non-radiative shock at the edge of the Cygnus Loop provide constraints on the physics of the shock front itself. Most of the very young remnants whose optical spectra show anomalous elemental abundances are too highly reddened for IUE observations, but extensive observations of the Crab Nebula and a spectrum of the supernova remnant in NGC 4449 yield carbon to oxygen ratios from which the mass of the progenitor may be estimated.     

Inner core wobble and free core nutation of pulsar PSR B1828-11

PSR B1828-11 has long-term, highly periodic and correlated variations in pulse shape and a slow-down rate with period variations of approximately 1000, 500 and 250 days [Stairs, I.H., Lyne, A.G., Shemar, S.L. Evidence for free precession in a pulsar. Nature 406, 484–486, 2000]. There are three potential explanations of pulses time-of-arrival from a pulsar. These are related to the interior of the neutron star, planetary bodies, free precession and nutation. We use the Hamiltonian canonical method of Getino (1995) for analyzing the dynamically symmetric pulsar PSR B1828-11, consisting of a rigid crust, elliptical liquid outer core and solid inner core. Using the theory of differential rotation of a pulsar, we investigate the dependence on Chandler wobble period, inner core wobble, retrograde free core nutation and prograde free inner core nutation from ellipticity of the inner crystal core, outer liquid core and total pulsar.     

On the morphology of supernova remnants

The difference in morphology between filled and shell type supernova remnants is attributed to differences in the activity of the neutron stars left by the supernovae. Pulsar activity leads to centrally concentrated remnants similar to the Crab. Non-activity as a pulsar results in all of the rotational energy loss going into dipole radiation. The pressure of this radiation creates shell-like objects with hollow interiors such as Cas A.     

基于X射线脉冲星导航试验卫星观测数据的到达时间估计

中国首颗X射线脉冲星导航试验卫星（X-ray Pulsar Navigation-1, XPNAV-1）搭载了两种体制的X射线探测器,其主要任务是在轨开展X射线脉冲星的探测以及进行脉冲星导航体制的验证。为了实现到达时间（Time of Arrival,TOA）的高精度估计,提出了利用阵列信号处理领域的多重信号子空间分类（Multiple Signal Classification,MUSIC）方法进行脉冲星导航的TOA估计,进行了试验仿真验证,并且对XPNAV-1观测到的蟹状星云（Crab）脉冲星的光子数据进行处理。在观测时间为协调世界时（Coordinate Universal Time,UTC）57727.0约简儒略日（Modified Julian Day,MJD）到UTC57812.0MJD内,选取了0.5~10keV频段内的121段光子观测数据,对这些数据进行了脉冲轮廓折叠,得到了Crab脉冲星的折叠轮廓,然后分别利用互相关法和MUSIC方法进行折叠轮廓的TOA估计,最后对比了两种方法的估计结果。     

Pulsars and their nebulae as EGRET sources

At the end of the EGRET mission, only 6–8 Galactic sources had been identified as young pulsars. Since then, several energetic pulsars have been detected in EGRET error boxes along the Galactic plane, as well as several pulsar wind nebulae from which pulsations have not yet been discovered. Some of these nebulae are associated with moderately variable EGRET sources, suggesting that the γ-ray emission might be coming from the nebula rather than from the pulsar magnetosphere. There is also a population of unidentified EGRET sources at mid-Galactic latitudes which have been proposed to be either nearby middle-aged pulsars or millisecond pulsars. I review the current status of observational studies of pulsars associated with EGRET sources and what they suggest the upcoming AGILE and GLAST γ-ray missions might observe.     

TeV gamma-ray observations of pulsar wind nebulae with the HESS detector

High energy stereoscopic system (HESS) is a recent operational detector dedicated to the observation of γ-rays in the very high energy range. Situated in Namibia, it is composed of four Imaging Atmospheric Cherenkov Telescopes and gives a significant improvement in sensitivity and in accuracy of the reconstructed γ-ray parameters. First results on observations of pulsar wind nebulae are reported here. The binary system PSR B1259-63/SS 2883 has been detected in 2004 around the periastron, showing clear flux variations. The pulsar wind nebula around PSR B1706-44 has been observed and upper limits on its flux have been derived.     

X-ray imaging techniques— modulation collimator and coded mask

Imaging over the hard X-ray energy band may be achieved by masking the flux with proper obstacles. The imaging modulation collimator has been developed and has been applied thus far, e.g., to produce hard X-ray pictures of the solar flare and to construct the X-ray image of the Crab Nebula up to the angular resolution of ～ 10 arcsec. Variations of the concept such as the Fourier Transform Telescope are discussed. Virtue of the modulation collimator is that high angular resolution may be achieved with a relatively simple detector system and that a wide field of view may be accomodated. Among several proposed coded masks, the techniques of Hadamard transform are discussed in some details. The coded mask is provided with a better total transmission, but its angular resolution is limited due to presently limited spatial resolution of the detector. Developments of the technique of the position sensitive detector are awaited for further improvements of the resolution.     

Characteristics of plerions powered by pulsars

It is generally believed that energy flows from a pulsar initially as magnetic dipole radiation and then largely as a wind of relativistic particles and magnetic field which fill out an expanding spherical volume around the pulsar. We calculate here the nature and evolution of the non-thermal radiation from such a plerion, giving adequate attention to the betatron effect on the electrons induced by the non-adiabatic behaviour of the magnetic field. These calculations are of interest in the modelling of supernova remnants (SNR) like the Crab-nebula, in discussing pulsar-SNR associations and in predicting the observational features expected if a pulsar is embedded inside the remnant of SN1987A when the debris becomes transparent.     

电铸镍Wolter I型光学系统制造技术发展综述

摘要： 为满足未来脉冲星导航和空间天文观测任务对X射线望远镜的载荷需求，Wolter I光学系统的研制正逐渐成为新的研究热点，电铸镍方案是当前国内外X射线光学系统镜筒制造的主要技术方案.文章对电铸镍X射线光学系统制造工艺、国内外研制现状、未来应用需求进行了介绍和整理，梳理了电铸镍X射线光学系统研制遇到的关键技术和难点，提出了后续研究发展建议，以促进中国在 X射线脉冲星自主导航、空间探测领域的快速发展.