Early results from the Chandra X-ray observatory

We present some early results on AGN from the Chandra X-ray Observatory, highlighting high resolution spectroscopy using the High Energy Transmission Grating Spectrometer (HETGS). The quasar PKS 0637–752 was found to have a very bright X-ray jet whose shape is remarkably similar to that of the radio jet on a size scale of 100 kpc, but the X-ray emission is still inexplicably bright. Two BL Lac objects, PKS 2155–304 and Mrk 421, observed with the spectrometer were found to have no strong absorption or emission features. Other radio loud AGN observed with the HETGS show simple power law spectra without obvious features.     

Cosmological constraints from the X-ray gas mass fraction in relaxed lensing clusters observed with Chandra

We present precise measurements of the X-ray gas mass fraction for a sample of luminous, relatively relaxed clusters of galaxies observed with the Chandra observatory, for which independent confirmation of the mass results is available from gravitational lensing studies. Parametrizing the total (luminous plus dark matter) mass profiles using the model of Navarro, Frenk & White, we show that the X-ray gas mass fractions in the clusters asymptote towards an approximately constant value at a radius r ₂₅₀₀, where the mean interior density is 2500 times the critical density of the Universe at the redshifts of the clusters. Combining the Chandra results on the X-ray gas mass fraction and its apparent redshift dependence with recent measurements of the mean baryonic matter density in the Universe and the Hubble constant determined from the Hubble Key Project, we obtain a tight constraint on the mean total matter density of the Universe,     , and measure a positive cosmological constant,     . Our results are in good agreement with recent, independent findings based on analyses of anisotropies in the cosmic microwave background radiation, the properties of distant supernovae, and the large-scale distribution of galaxies.     

Chandra X-ray observations of two unusual BAL quasars

We report sensitive Chandra X-ray non-detections of two unusual, luminous Iron Low-Ionization Broad Absorption Line Quasars (FeLoBALs). The observations do detect a non-BAL, wide-binary companion quasar to one of the FeLoBAL quasars. We combine X-ray-derived column density lower limits (assuming solar metallicity) with column densities measured from ultraviolet spectra and CLOUDY photoionization simulations to explore whether constant-density slabs at broad-line region densities can match the physical parameters of these two BAL outflows, and find that they cannot. In the “overlapping-trough” object SDSS J0300+0048, we measure the column density of the X-ray absorbing gas to be N_H ? 1.8 × 10²⁴ cm^?2. From the presence of Fe ii UV78 absorption but lack of Fe ii UV195/UV196 absorption, we infer the density in that part of the absorbing region to be n_e ? 10⁶ cm^?3. We do find that a slab of gas at that density might be able to explain this object’s absorption. In the Fe iii-dominant object SDSS J2215–0045, the X-ray absorbing column density of N_H ? 3.4 × 10²⁴ cm^?2 is consistent with the Fe iii-derived N_H ? 2 × 10²² cm^?2 provided the ionization parameter is log U > 1.0 for both the n_e = 10¹¹ cm^?3 and n_e = 10¹² cm^?3 scenarios considered (such densities are required to produce Fe iii absorption without Fe iiabsorption). However, the velocity width of the absorption rules out its being concentrated in a single slab at these densities. Instead, this object’s spectrum can be explained by a low density, high ionization and high temperature disk wind that encounters and ablates higher density, lower ionization Fe iii-emitting clumps.     

A Chandra X-ray observation of the globular cluster Terzan 1

We present a ∼19-ks Chandra Advanced CCD Imaging Spectrometer (ACIS)-S observation of the globular cluster Terzan 1. 14 sources are detected within 1.4 arcmin of the cluster centre with two of these sources predicted to be not associated with the cluster (background active galactic nuclei or foreground objects). The neutron star X-ray transient, X1732−304, has previously been observed in outburst within this globular cluster with the outburst seen to last for at least 12 yr. Here, we find four sources that are consistent with the ROSAT position for this transient, but none of the sources are fully consistent with the position of a radio source detected with the Very Large Array that is likely associated with the transient. The most likely candidate for the quiescent counterpart of the transient has a relatively soft spectrum and an unabsorbed 0.5–10 keV luminosity of  2.6 × 10³² erg s⁻¹  , quite typical of other quiescent neutron stars. Assuming standard core cooling, from the quiescent flux of this source we predict long (>400 yr) quiescent episodes to allow the neutron star to cool. Alternatively, enhanced core cooling processes are needed to cool down the core. However, if we do not detect the quiescent counterpart of the transient this gives an unabsorbed 0.5–10 keV luminosity upper limit of  8 × 10³¹ erg s⁻¹  . We also discuss other X-ray sources within the globular cluster. From the estimated stellar encounter rate of this cluster we find that the number of sources we detect is significantly higher than expected by the relationship of Pooley et al.     

Chandra X-ray observations of the 3C 295 cluster core

We examine the properties of the X-ray gas in the central regions of the distant ( z =0.46) , X-ray luminous cluster of galaxies surrounding the powerful radio source 3C 295, using observations made with the Chandra Observatory . Between radii of 50 and 500 kpc, the cluster gas is approximately isothermal with an emission-weighted temperature, kT ∼5 keV . Within the central 50-kpc radius this value drops to kT ∼3.7 keV . The spectral and imaging Chandra data indicate the presence of a cooling flow within the central 50-kpc radius of the cluster, with a mass deposition rate of approximately 280 M_⊙ yr⁻¹. We estimate an age for the cooling flow of 1–2 Gyr , which is approximately 1000 times older than the central radio source. We find no evidence in the X-ray spectra or images for significant heating of the X-ray gas by the radio source. We report the detection of an edge-like absorption feature in the spectrum for the central 50-kpc region, which may be caused by oxygen-enriched dust grains. The implied mass in metals seen in absorption could have been accumulated by the cooling flow over its lifetime. Combining the results on the X-ray gas density profile with radio measurements of the Faraday rotation measure in 3C 295, we estimate the magnetic field strength in the region of the cluster core to be B ∼12 μG .     

Infrared observations of serendipitous hard Chandra X-ray sources

We present observations of a sample of optically faint, hard X-ray sources of the kind likely to be responsible for much of the hard X-ray background. We confirm that such sources are easily detected in the near-infrared, and find that they have a featureless continuum suggesting that the active nucleus is heavily obscured. The infrared colours of the majority of the targets observed are consistent with absorbed elliptical host galaxies at z =1–2. It is likely that we are observing some of the brighter members of the important new class of X-ray type II quasars.     

Hot dust in two hard Chandra X-ray sources

The two brightest hard X-ray sources discovered serendipitously by Chandra in the field of the lensing cluster A2390 are found to have ISOCAM counterparts at 6.7 and 15 μm. We use this fact, together with their non-detection by SCUBA at 850 μm, as the basis for dusty radiative transfer modelling of their infrared spectral energy distributions. For the best-fitting models, we find that the dust that reprocesses the optical–ultraviolet light in these Compton-thin active galactic nuclei (AGN) is heated to near its sublimation temperature (above 1000 K), with an inner radius within a parsec of the nucleus. Some warm-dust models with inner temperatures of 200 K are also formally acceptable. These findings strongly support the obscured AGN hypothesis for the new hard X-ray Chandra sources, which lack both strong emission lines and 850-μm SCUBA detections.     

Chandra imaging of the complex X-ray core of the Perseus cluster

We report subarcsec-resolution X-ray imaging of the core of the Perseus cluster around the galaxy NGC 1275 with the Chandra X-ray Observatory . The ROSAT -discovered holes associated with the radio lobes have X-ray bright rims which are cooler than the surrounding gas and not a result of shocks. The holes themselves may contain some hotter gas. We map strong photoelectric absorption across the northern lobe and rim owing to a small infalling irregular galaxy, known as the high-velocity system. Two outer holes, one of which was previously known, are identified with recently found spurs of low-frequency radio emission. The spiral appearance of the X-ray cooler gas and the outer optical parts of NGC 1275 may be due to angular momentum in the cooling flow.     

Scaling Properties of X-ray Clusters: The Chandra View

The study of X-ray clusters of galaxies, started 30 years ago, has revealed an increasing complexity in the thermodynamics of the X-ray emitting intracluster medium (ICM) as long as the sensitivity and the resolution of the X-ray satellites increased. At the same time, deep surveysdetected several, unexpected, high-z clusters. Here we focus on the Chandra observations of the most distant X-ray selected clusters (0.3 < z < 1.3), in order to constrain their thermodynamic evolution. The X-ray scaling properties show hints of negative evolution in the luminosity–temperature and M _gas–temperature relations, and a positive evolution in the entropy–temperature relation. We find that the mean iron abundance at 〈z〉 = 0.8 is Z _Fe = 0.25^+0.04 _−0.06 Z _⊙, and at 〈z〉 ∼ 1.2 is Z _Fe = 0.35^+0.06 _−0.05 Z _⊙, both measures consistent with no evolution with respect to the local value Z _Fe≃ 0.3 Z _⊙. These results can provide interesting constraints on the thermodynamics of the ICM at large look back times, pointing towards a redshift z ≳ 2 for the onset of non-gravitational processes.     

Cosmological constraints from Chandra X-ray observations of galaxy clusters

Chandra X-ray observations of rich, dynamically relaxed galaxy clusters allow the properties of the X-ray gas and the total gravitating mass to be determined precisely. Here, we discuss how Chandra observations may be used as a powerful tool for cosmological studies. By combining Chandra X-ray results on the X-ray gas mass fractions in clusters with independent measurements of the Hubble constant and the mean baryonic matter density of the universe, we obtain a tight constraint on the mean total matter density of the universe, Ο_m, and an interesting constraint on the cosmological constant, Ο_Λ. Using these results, together with the observed local X-ray luminosity function of the most X-ray luminous galaxy clusters, a mass-luminosity relation determined from Chandra and ROSAT X-ray data and weak gravitational lensing observations, and the mass function predicted by numerical simulations, we obtain a precise constraint on the normalization of the power spectrum of density fluctuations in the nearby universe,σ₈. We compare our results with those obtained from other, independent methods. This revised version was published online in July 2006 with corrections to the Cover Date.     

基于Chandra的活动星系核观测

活动星系核（AGN）是宇宙中最奇特的天体之一。它是真正意义上的全波天体，其中X射线波段的发射功率占到全波段功率的50％左右。AGN的X射线辐射研究涉及天体物理中的最基本问题，例如能量产生、辐射机制和宇宙论等，而Chandra X射线卫星的高分辨率图像和光谱对这一研究有着重要作用。以Chandra卫星的部分观测结果为例，简要介绍了几类不同类型AGN的X射线辐射研究进展：（1）宽吸收线类星体APM08279＋5255（z=3．91）的X射线谱分析，以及高红移类星体的观测概况；（2）Seyfert星系NGC 4151延展X射线发射问题的解决，及NGC 1068X射线辐射与光学波段的高激发态发射线（[OⅢ]λ5007）有很强相关性的发现；（3）6个BLLac天体样本的X射线环境分析；（4）射电星系X射线喷流的观测等。     

Chandra observations of the X-ray jet in 3C 66B

Our Chandra observation of the FR I radio galaxy 3C 66B has resulted in the first detection of an X-ray counterpart to the previously known radio, infrared and optical jet. The X-ray jet is detected up to 7 arcsec from the core and has a steep X-ray spectrum, α ≈1.3±0.1 . The overall X-ray flux density and spectrum of the jet are consistent with a synchrotron origin for the X-ray emission. However, the inner knot in the jet has a higher ratio of X-ray to radio emission than the others. This suggests that either two distinct emission processes are present or differences in the acceleration mechanism are required; there may be a contribution to the emission from the inner knot from an inverse Compton process or it may be the site of an early strong shock in the jet. The peak of the brightest radio and X-ray knot is significantly closer to the nucleus in the X-ray than in the radio, which may suggest that the knots are privileged sites for high-energy particle acceleration. 3C 66B's jet is similar both in overall spectral shape and in structural detail to those in more nearby sources such as M87 and Centaurus A.     

Localization of the X-ray burster KS 1731-260 from Chandra data

We analyze Chandra observatory images of the field of the X-ray burster KS 1731-260. A factor of 10 to 15 improvement in the localization accuracy (up to ～0.6″) has allowed a possible candidate for counterparts of KS 1731-260 to be determined from infrared sky images (Barret et al. 1998). The possible counterpart (the sky position difference is ～1.46″, i.e., less than 2σ) is a 16th magnitude star in the J band. If this star is actually an infrared counterpart of KS 1731-260, then we can estimate its luminosity and the lower limit on the counterpart total luminosity, L>L _J,H ~10L _⊙. The sharp decline in the X-ray flux from KS 1731-260 in 2001 offers an additional test of whether the proposed candidate is actually a counterpart of KS 1731-260. If the optical and infrared luminosities of this counterpart are largely attributable to reradiation of the X-ray flux from the neutron star, as is the case in low-mass X-ray binaries, then the brightness of the counterpart star must decrease sharply in 2001, after the X-ray source is turned off.     

Chandra monitoring observations of the ultraluminous X-ray source NGC 5204 X-1

We report the results of a two-month campaign conducted with the Chandra X-ray observatory to monitor the ultraluminous X-ray source (ULX) NGC 5204 X-1. This was composed of a 50-ks observation, followed by ten 5-ks follow-ups spaced initially at ∼3, then at ∼10-d intervals. The ULX flux is seen to vary by factors ∼5 on time-scales of a few days, but no strong variability is seen on time-scales shorter than an hour. There is no evidence for a periodic signal in the X-ray data. An examination of the X-ray colour variations over the period of the campaign shows the ULX emission consistently becomes spectrally harder as its flux increases. The X-ray spectrum from the 50-ks observation can be fitted by a number of disparate spectral models, all of which describe a smooth continuum with, unusually for a ULX, a broad emission feature evident at 0.96 keV. The spectral variations, both within the 50-ks observation and over the course of the whole campaign, can then be explained solely by variations in the continuum component. In the context of an optically thick corona model (as found in other recent results for ULXs) the spectral variations can be explained by the heating of the corona as the luminosity of the ULX increases, consistent with the behaviour of at least one Galactic black hole system in the strongly Comptonized very high state. We find no new evidence supporting the presence of an intermediate-mass black hole in this ULX.