Supernova remnants

Optical observations of young supernova remnants can give information on the abundance structure of the supernova ejecta, from which properties of the supernova explosion can be deduced. Young remnants also act as laboratories for fast shock wave phenomena. Observations of X-ray and optical line emission show that while a collisionless shock wave does form, it is far from thermal equilibrium. Some young remnants are promising candidates for particle acceleration in shock waves, while in other remnants a central compact object is probably responsible for particle acceleration.     

A physical interpretation of the jet-like X-ray emission from supernova remnant W49B

In the framework of the study of supernova remnants and their complex interaction with the interstellar medium and the circumstellar material, we focus on the galactic supernova remnant W49B. Its morphology exhibits an X-ray bright elongated nebula, terminated on its eastern end by a sharp perpendicular structure aligned with the radio shell. The X-ray spectrum of W49B is characterized by strong K emission lines from Si, S, Ar, Ca, and Fe. There is a variation of the temperature in the remnant with the highest temperature found in the eastern side and the lowest one in the western side. The analysis of the recent observations of W49B indicates that the remnant may be the result of an asymmetric bipolar explosion where the ejecta are collimated along a jet-like structure and the eastern jet is hotter and more Fe-rich than the western one. Another possible scenario associates the X-ray emission with a spherical explosion where parts of the ejecta are interacting with a dense belt of ambient material. To overcome this ambiguity we present new results of the analysis of an XMM-Newton observation and we perform estimates of the mass and energy of the remnant. We conclude that the scenario of an anisotropic jet-like explosion explains quite naturally our observation results, but the association of W49B with a hypernova and a γ-ray burst, although still possible, is not directly supported by any evidence.     

A preliminary Chandra X-ray spectroscopy of the supernova remnant N132D

We present the preliminary results of a Chandra X-ray study of N132D, a young shell-like supernova remnant (SNR) in the Large Magellanic Cloud. The equivalent width maps of emissions from O, Ne, Mg, Si, and S are provided. Spatially resolved spectral analysis for the small-scale regions were tentatively performed. X-ray spectra of the interior can be described with a single-thermal model. The faint interior regions have lower density, higher temperature above 1 keV than that of bright interior regions. The X-ray spectra along the shell can be phenomenally fitted with either a double-vpshock model or a vpshock + powerlaw model. If the non-thermal component is true, N132D would be listed as another X-ray synchrotron SNR.     

The history of SN 1986J and the emergence of a new compact radio component in its center

We review the history of the unusual Type II supernova 1986J in the nearby spiral galaxy NGC 891. A series of VLBI observations have shown the expanding shell structure and allowed the expansion curve to be determined. The integrated radio spectrum and radio lightcurve have also been monitored. The spectrum was a power-law before 1998, after which an inversion appeared above 5 GHz. Our recent high-frequency VLBI observations showed that this inverted-spectrum emission was associated not with the shell emission, but rather with a compact component almost precisely in the center of the expanding shell. The new component is likely radio emission associated with the black-hole or neutron star compact remnant of the explosion, which would mark the first direct observational link between a modern supernova and such a compact remnant.     

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.     

The properties of hard X-ray emitting symbiotic stars

Hard X-ray emitting symbiotic stars are candidates for SN Ia progenitors. The importance of Type Ia SNe as standard candles for cosmology makes the study of their progenitor systems particularly important. Additionally, they provide one of the most promising laboratories for the study of astrophysical jets. Typically, the X-ray emission in these systems is modeled with a collisional plasma model, sometimes with an emission measure distribution taken from a cooling flow model. The lack of any coherent periods in both X-rays and optical wave band strongly suggests that the accreting white dwarfs in the hard X-ray symbiotic stars are non-magnetic. Although relatively few have been discovered to date, but we believe that there are very many of them in our galaxy and could be possible candidates for the Galactic Ridge X-ray Emissions (GRXE).     

Alternate models to dark energy

One of the unresolved questions currently in cosmology is that of the non-linear accelerated expansion of the universe. This has been attributed to the so called Dark Energy (DE). The accelerated expansion of the universe is deduced from measurements of Type Ia supernovae. Here we propose alternate models to account for the Type Ia supernovae measurements without invoking dark energy.     

Radio continuum observations of the galactic supernova remnant Vela Z (G266.2-1.2)

We present multi frequency radio-continuum observations of Galactic SNR Vela Z (G266.2-1.2), associated with the prominent X-ray source RX J0852.0-4622 discovered in 1998. Our study of this Galactic supernova remnant (SNR) is based on low-resolution mosaic observations with the ATCA radio interferometer at 1384 and 2496 MHz, Parkes 4850 MHz and MOST 843 MHz survey data. We determine the radio spectral index for the most prominent features of this SNR and compare these features with X-ray observations. We note a sudden spectral turnover at higher then expected frequencies of ∼1384 MHz.     

The evolution of the central component in SN 1986J

The unusual core-collapse supernova 1986J, in the nearby spiral NGC 891, is the first modern supernova in which evidence of a compact remnant of the supernova has been seen. This evidence comes from recent VLBI images, which show the emergence of a new radio component in the center of the expanding radio shell. The new component shows an inverted radio spectrum contrasting with that of the shell. The new component is likely radio emission associated with the black-hole or neutron star compact remnant of the explosion, which would mark the first direct observational link between a modern supernova and such a compact remnant. We report here on our recent VLBI images at 22 and 5 GHz, as well as on our monitoring of the integrated radio spectrum of SN 1986J. In the 22 GHz image, the central component is marginally resolved.     

超新星爆发发射的γ射线的能量

本文用两箱模型导出了大气中的14C浓度与14C的产率、碳元素在大气和海洋中的贮存量及居留时间等量之间的关系,并进而给出根据树年轮14C数据计算超新星爆发后大气中增加的14C核子数ΔX₁（0）和估算超新星爆发发出的γ射线能量E_γ的公式。      

Einstein observations of supernova remnants

Recent observations of SNRs have produced X-ray images with resolution comparable to that routinely achieved at optical wavelengths. There has also been a great improvement in the quality of X-ray spectra. Since most of the energy radiated by SNRs appears as X-rays, these new data are crucial to studies of SNRs, the interaction of SNRs with the interstellar medium, and the SN explosion itself. Images show a variety of shapes ranging from shel-like remnants to those dominated by the influence of central objects which appear both as point sources and as centers of diffuse activity. Once the temperature and spatial distribution of X-ray emitting material is known, the mass of ejected material and the energy release of the SN explosion can sometimes be calculated. X-ray images and spectra of several remnants are shown, and some quantitative results are given.     

Compact X-ray sources in nearby galaxy nuclei

We have found compact, near-nuclear X-ray sources in 21 (54%) of a complete sample of 39 nearby face-on spiral and elliptical galaxies with available ROSAT HRI data. ROSAT X-ray luminosities (0.2 – 2.4 keV) of these compact X-ray sources are ∼10³⁷ – 10⁴⁰ erg s⁻¹. The mean displacement between the location of the compact X-ray source and the optical photometric center of the galaxy is ∼390 pc. ASCA spectra of six of the 21 galaxies show the presence of a hard component with relatively steep (Γ ≈ 2.5) spectral slope. A multicolor disk blackbody plus power-law model fits the data from the spiral galaxies well, suggesting that the X-ray objects in these galaxies may be similar to a black hole candidate (BHC) in its soft (high) state. ASCA data from the elliptical galaxies indicate that hot (kT ≈ 0.7 keV) gas dominates the emission. The fact that the spectral slope of the spiral galaxy sources is steeper than in normal type 1 active galactic nuclei (AGNs) and that relatively low absorbing columns (N_H ≈ 10²¹ cm⁻²) were found to the power-law component indicates that these objects are somehow geometrically and/or physically different from AGNs in normal active galaxies. The X-ray sources in the spiral galaxies may be BHCs, low-luminosity AGNs, or possibly X-ray luminous supernovae. We estimate the black hole masses of the X-ray sources in the spiral galaxies (if they are BHCs or AGNs) to be ∼10²–10³ M_⊙. The X-ray sources in the elliptical galaxies may be BHCs, AGNs or young X-ray supernova also.     

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.     

X-ray spectral properties of non-magnetic cataclysmic variables using Chandra HETG

Results from a study of high resolution spectra obtained with the Chandra X-ray observatory for a sample of 6 Cataclysmic Variables (CVs) are presented. A global fit approach has been employed to obtain the spectral characteristics of the sources. The line-rich high-resolution spectra of these sources clearly indicate multi-temperature nature of the emitting plasma. Multi-temperature APEC models describe the spectra very well. Detection of significantly broad emission lines, indicates the presence of high velocity gas in SS Cyg and U Gem during the optical outbursts.     

Gravitational-collapse supernovae

The gravitational collapse of the core of a massive star releases ample energy for a supernova explosion. The mechanisms by which this energy can be imparted to the outer parts of the stars to actually yield an explosion is outlined, and some of the related physical processes are examined. In particular the generation of entropy and the loss of neutrinos during the collapse of the core and the generation of lepton-driven buoyancy instabilities are discussed in some detail.     

X射线脉冲星导航技术研究进展

X射线脉冲星能够为近地轨道、深空和星际空间飞行航天器提供位置、速度、时间和姿态等丰富的导航信息,实现航天器高精度自主导航和运行管理,有着巨大的发展潜力.本文论述了X射线脉冲星导航技术研究历程;重点研究了X射线脉冲星导航的基本原理、信息处理流程和自主导航算法,进而提出了脉冲星导航的关键技术;分析了国内脉冲星观测研究的基础条件,以及开展X射线脉冲星导航技术研究的必要性和可行性.      

X-ray synchrotron nebulae and the origin of neutron stars

“With all reserve, we advance the view that a supernova represents the transition of an ordinary star into a neutron star.”This conclusion, reached just 50 years ago in a classic paper by Walter Baade and Fritz Zwicky (1934), was published three decades before the first direct observational evidence for the existence of neutron stars was uncovered. It still informs the standard picture of neutron star production in the Galaxy. We examine herein some recent evidence bearing on this question which has been derived from Observatory X-ray observations of supernova remnants and radio pulsars. In particular, the discovery that X-ray synchrotron nebulae are found surrounding most young ( 10⁶ yr) pulsars observed to date is discussed. We explore the implications of the lack of such nebulae in the majority of supernova remnants (SNR) for the properties and frequency of neutron star formation in supernova events.     

Highly clumpy structure of the thermal composite supernova remnant 3C 391 unveiled by Chandra

The nature of the internal thermal X-ray emission seen in “thermal composite” supernova remnants is still uncertain. Chandra observation of the 3C391 shows a southeast–northwest elongated morphology and unveils a highly clumpy structure of the remnant. Detailed spatially resolved spectral analysis for the small-scale features reveals normal metal abundance and uniform temperature for the interior gas. The properties of the hot gas comparatively favor the cloudlet evaporation model as a main mechanism for the “thermal composite” X-ray appearance, though radiative rim and thermal conduction may also be effective. A faint protrusion is found in Si and S lines out of the southwest radio border.     

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.     

Gamma-ray observations of explosive nucleosynthesis products

In this review I discuss the various γ-ray emission lines that can be expected and, in some cases have been observed, from radioactive explosive nucleosynthesis products. The most important γ-ray lines result from the decay chains of ⁵⁶Ni, ⁵⁷Ni, and ⁴⁴Ti. ⁵⁶Ni is the prime explosive nucleosynthesis product of Type Ia supernovae, and its decay determines to a large extent the Type Ia light curves. ⁵⁶Ni is also a product of core-collapse supernovae, and in fact, γ-ray line emission from its daughter product, ⁵⁶Co, has been detected from SN1987A by several instruments. The early occurrence of this emission was surprising and indicates that some fraction of ⁵⁶Ni, which is synthesized in the innermost supernova layers, must have mixed with the outermost supernova ejecta.Special attention is given to the γ-ray line emission of the decay chain of ⁴⁴Ti (⁴⁴Ti → ⁴⁴Sc → ⁴⁴Ca), which is accompanied by line emission at 68, 78, and 1157 keV. As the decay time of ⁴⁴Ti is ∼86 yr, one expects this line emission from young supernova remnants. Although the ⁴⁴Ti yield (typically 10⁻⁵–10⁻⁴M_⊙) is not very high, its production is very sensitive to the energetics and asymmetries of the supernova explosion, and to the mass cut, which defines the mass of the stellar remnant. This makes ⁴⁴Ti an ideal tool to study the inner layers of the supernova explosion. This is of particular interest in light of observational evidence for asymmetric supernova explosions.The γ-ray line emission from ⁴⁴Ti has so far only been detected from the supernova remnant Cas A. I discuss these detections, which were made by COMPTEL (the 1157 keV line) and BeppoSAX (the 68 and 78 keV lines), which, combined, give a flux of (2.6 ± 0.4 ± 0.5) × 10⁻⁵ ph cm⁻² s⁻¹ per line, suggesting a ⁴⁴Ti yield of (1.5 ± 1.0) × 10⁻⁴M_⊙. Moreover, I present some preliminary results of Cas A observations by INTEGRAL, which so far has yielded a 3σ detection of the 68 keV line with the ISGRI instrument with a flux that is consistent with the BeppoSAX detections. Future observations by INTEGRAL-ISGRI will be able to constrain the continuum flux above 90 keV, as the uncertainty about the continuum shape, is the main source of systematic error for the 68 and 78 keV line flux measurements. Moreover, with the INTEGRAL-SPI instrument it will be possible to measure or constrain the line broadening of the 1157 keV line. A preliminary analysis of the available data indicates that narrow line emission (i.e., Δv < 1000 km s⁻¹) can be almost excluded at the 2σ level, for an assumed line flux of 1.9 × 10⁻⁵ ph cm⁻² s⁻¹.