银河系光行差及其对天文参考架的影响

银河系光行差,或称为长期光行差漂移,是由于太阳系质心绕着银河系中心做轨道运动的加速度引起的视自行效应,量级大约为5μas·yr~(-1).在21世纪之前,由于观测精度尚未达到如此高的程度,人们很少讨论银河系光行差效应.随着甚长基线干涉(Very Long Baseline Interferometer,VLBI)在基本天文学中的广泛应用和欧洲空间局(European Space Agency,ESA)的第2代微角秒天体测量卫星Gaia的问世,该效应显得逐渐重要.由于河外源的分布不均匀,银河系光行差效应会使得河外源天球参考架缓慢旋转,进而需要修正地球岁差参数,其中岁差速率的改正值大约为1μas·yr~(-1).对于微角秒精度的VLBI和Gaia参考架,银河系光行差将会引起框架扭曲,在两者的连接过程中,也是必须考虑的系统效应.     

The General Method for Fixing the Gauges of Relativistic Astronomical Reference Systems

This article applies a new scheme of the first post-Newtonian theory (Damour et al., 1991–1994) to the problem of gauge in relativistic reference systems. Choosing and fixing gauge are necessary when the precision of time measurement and application needs to reach the 2PN level (10⁻¹⁶ or better). We present a general method for fixing the gauges of both the global and local coordinate systems, and for determining the expressions of gravitational potentials and coordinate transformations. The results relevant are consistent with the newest IAU resolutions, therefore they can be applied to astronomical practice.     

Microarcsecond instability of the celestial reference frame

The fluctuation of the angular positions of reference extragalactic radio and optical sources under the influence of the irregular gravitational field of visible Galactic stars is considered. It is shown that these angular fluctuations range from a few up to hundreds of microarcseconds. This leads to a small rotation of the celestial reference frame. The non-diagonal coefficients of the rotation matrix are of the order of a microarcsecond. The temporal variation of these coefficients due to the proper motion of the foreground stars is of the order of one microsecond per 20 years. The celestial reference frame can therefore be considered inertial and homogeneous only to microarcsecond accuracy. Astrometric catalogues with microarcsecond accuracy will be unstable, and must be re-established every 20 years.     

The dependence of quasar variability on black hole mass

In order to investigate the dependence of quasar variability on fundamental physical parameters like black hole mass, we have matched quasars from the Quasar Equatorial Survey Team, Phase 1 (QUEST1) variability survey with broad-lined objects from the Sloan Digital Sky Survey. The matched sample contains ≈100 quasars, and the Sloan spectra are used to estimate black hole masses and bolometric luminosities. Variability amplitudes are measured from the QUEST1 light curves. We find that black hole mass correlates with several measures of the variability amplitude at the 99 per cent significance level or better. The correlation does not appear to be caused by obvious selection effects inherent to flux-limited quasar samples, host galaxy contamination or other well-known correlations between quasar variability and luminosity/redshift. We evaluate variability as a function of rest-frame time lag using structure functions and find further support for the variability–black hole mass correlation. The correlation is strongest for time lags of the order of a few months up to the QUEST1 maximum temporal resolution of ≈2 yr, and may provide important clues for understanding the long-standing problem of the origin of quasar optical variability. We discuss whether our result is a manifestation of a relation between characteristic variability time-scale and black hole mass, where the variability time-scale is typical for accretion disc thermal time-scales, but find little support for this. Our favoured explanation is that more massive black holes have larger variability amplitudes, and we highlight the need for larger samples with more complete temporal sampling to test the robustness of this result.     

Micro-arcsecond Celestial Reference Frames: definition and realization - Impact of the recent IAU Resolutions

The adoption of the International Celestial Reference System (ICRS),based on Very Long Baseline Interferometry (VLBI) observations of extragalactic radiosources by the International Astronomical Union ...     

Geometry of Broad Line Regions of Active Galactic Nuclei

It has long remained an open question as to the geometry of the broad line region (BLR) in active galactic nuclei (AGNs). The reverberation mapping technique which mea-sures the response of the broad emission lines to the ionizing continuum, when combined with multiwavelength continuum fitted by sophisticated accretion disks, provides a way of probing the BLR geometry. We analyze a sample of 35 AGNs, which have been monitored by the reverberation mapping campaign. In view of energy budget, the reverberation-based BH masses are found to be in agreement with those obtained by accretion disk models in two thirds of the present sample while the reverberation mapping methods underestimate the BH masses in about one third of objects, as also suggested by Collin et al. in a recent work. We point out that there are obviously two kinds of BLR geometry, which are strongly dependent on the Eddington ratio, and separated by the value LBol/LEdd ～ 0.1. These results prefer a scenario of the disk and wind configuration of the BLR and identify the Eddington ratio as the physical driver regulating the wind in the BLR.     

Parallax distortion by the weak microlensing effect

Parallax measurements allow distances to celestial objects to be determined. Together with measurements of their position on the celestial sphere, they give a full three-dimensional picture of the location of the objects relative to the observer. The distortion of the parallax value of a distant source affected by weak microlensing is considered. This means that the weak microlensing leads to distortion of the distance scale. The gravitational deflection causes a retrograde apparent motion of the image, which is revealed as a negative parallax. It is shown that the distortions may significantly change the parallax values when they amount to several microseconds of arc. In particular, at this level many measured values of parallaxes should be negative.     

Astronomical units and constants in the general relativity framework

An attempt is made to analyze the existing system of astronomical constants within the general relativity theory (GRT) framework. The general conclusion is that, to avoid any confusion in the GRT compatible interpretation of units and constants, one should give precisely, with full post-Newtonian accuracy, the expressions of the metric forms describing the astronomical barycentric and geocentric reference systems used, for example, in IERS analysis of observations.Institute of Applied Astronomy, St. Petersburg, 197042, Russia     

Spectral Preprocessing and Its Effect on Galaxy/Quasar Classification

Because of the effects of noise, distortion, observational environment and other factors, some appropriate preprocessing should be made in advance of automatic classification of celestial spectra. We have studied the effect of data format and flux standardization on the automatic classification of sky survey spectra. A basic model adaptable for the order-of-magnitude variation of fluxes is proposed, and the corresponding standardization methods are given. Our experimental results on galaxy and quasar classification show that the logarithmic wavelength data format is better for the automatic spectral classification. By these experiments, the reasonableness of the proposed model and the performances of the given flux standardization methods are verified. Especially, it is noted that the commonly used flux standardization is the worst, among other standardizations, for automatic spectral classification.     

The Doppler Effect and Spectral Energy Distribution of Blazars

The relativistic beaming model is adopted to discuss quantitatively the observational differences between radio-selected BL Lac objects (RBLs) and X-ray-selected BL Lac objects (XBLs), and between BL Lac objects and flat spectrum radio quasars (FSRQs). The main results are the following:(1) In the Doppler corrected color-color(αro^in-αox^in) diagram,XBLs and FSRQs occupy separated regions, while RBLs bridge the gap between them. These properties suggest that similar intrinsic physical processes operate in all the objects under a range of intrinsic physical conditions. (2) Our results are consistent with the results of Sambruna, Maraschi and Urry (1996) from other methods. We show the αxox introduced by Sambruna to be a good index for describing the energy distribution because it represents the intrinsic energy distribution and includes the Doppler correction. (3)The Doppler effect of relativistic beaming is the main mechanism, and the physical differences(such as magnetic fields, electron energies) are also important complementary factors for understanding the relation between XBLs and RBLS.     

The cluster environments of powerful radio-loud and radio-quiet active galactic nuclei

The spatial clustering amplitude ( B _gq) is determined for a sample of 44 powerful active galactic nuclei (AGN) at z ≃0.2. No significant difference is detected in the richness of the cluster environments of the radio-loud and radio-quiet subsamples, both of which typically inhabit environments as rich as Abell class ≃0. Comparison with radio luminosity-matched samples from Hill & Lilly and Wold et al. suggests that there is no epoch-dependent change in environment richness out to at least z ≥0.5 for either radio galaxies or radio quasars. Comparison with the APM cluster survey shows that, contrary to current folklore, powerful AGN do not avoid rich clusters, but rather display a spread in cluster environment, which is perfectly consistent with being drawn at random from the massive elliptical population. Finally, we argue that virtually all Abell class ≃0 clusters contained an active galaxy during the epoch of peak quasar activity at z ∼2.5.