On the difference between the X-ray selected and the radio selected BL Lac objects in the near infrared

By using results from the 2MASS observation, the differences between the X-ray selected BL Lac objects (XBLs) and the radio selected ones (RBLs) in the near infrared are discussed in this paper. It is found that, statistically, the RBLs have redder near infrared colors and steeper spectral indices in the near infrared than XBLs, whereas the XBLs have more influences from their host galaxies than the RBLs. It is also seen that the RBLs have much brighter luminosity in the near infrared than the XBLs. In addition, the high polarization characteristics are found in common for RBLs, but not for XBLs.     

BL Lac天体中心黑洞周围的物质分布

本文从Ｘ光度与短时标出发讨论了两种选择ＢＬＬａｃ体的统一及它们中心黑洞的周围物质分布     

BL Lac天体和FR-I射电星系的统一模型(Ⅰ)

从射电结构的角度评述了BLLac天体和FR-I射电星系的统一模型的最新进展，内容包括射电结构、延展射电光度（Pext）、最大角尺度（LAS）、射电核主导系数（f）的比较以及对相对论聚束的测试。最新的射电观测资料表明XBLs是介于FR-I和RBLs之间的过渡型天体，这为BLLac天体是聚束的FR-I射电星系提供了一个很好的证据。     

Anti-correlation between the extended power and the core dominance parameter of BL Lac objects

The XBL and RBL samples presented recently by Kollgaardet al. (1996) are employed to investigate whether the extended power of BL Lac objects is correlated with the core dominance parameter. The results show that the two quantities are statistically anti-corelated at a rather high level of confidence for both XBLs and RBLs. Since the core dominance parameter is generally used as a relative measure of orientation and the extended emission of radio sources is assumed unbeamed and isotropic, the results obviously contradict common inference from unified schemes. By adopting and combining some basic concepts of unified schemes and the different SED scenario, a possible explanation for the results is suggested.     

Characteristics of the Radiation from Blazars

We have collected short-timescale variability data of 47 blazars, estimated the masses of their central black holes and the sizes of their radiation regions at different wavebands, and made a statistical analysis on the calculated results. It is found that the central black hole mass of blazars falls in the range 10⁷M to 10¹⁰M, and that the BL Lac objects and the flat-spectrum radio quasars have very different central black hole masses (the latter being generally greater), while they have very similar sizes of radiation regions in the infrared and γ-ray wavebands. Also, using the collected bolometric luminosity data, we have analyzed the relationship between the bolometric luminosity of blazars and their short-timescale variability, and it is concluded that the radiations from the radio-selected BL Lac objects (RBLs) and flat-spectrum radio quasars (FSRQs) are strongly beam-confined, while the effect of relativistic beaming is relatively small for the X-ray-selected BL Lac objects (XBLs).     

The unification of RBLs with XBLs

Many astronomers have discussed the property of BL Lacertae objects, including the variation of spectrum, the correlation of multi-wave bands and the property of polarization, which could give good information for studying intrinsic correlation of components and position of BL Lacertae objects. In the paper, we investigated the properties of RBLs and XBLs and RBLs/XBLs (these objects can also be found by radio survey as by X-ray survey). Firstly, we collected the light curve of 28 BL Lacertae objects, and gained their short timescales by Structure Function (SF). Secondly, we analyzed the distribution of the short timescales of some BL Lacertae objects, including 28 objects’ timescales calculated by SF, as well as the distribution of the redshift and the black hole mass and the flux densities of the multi-wavebands (radio, near-infrared, optical and Gamma-ray). Based on statistical analysis, the result of the paper support the unification model of RBLs and XBLs.     

Variations in the broad-band spectra of BL Lac objects: millimetre observations of an X-ray-selected sample

Observations at millimetre wavelengths are presented for a representative sample of 22 X-ray-selected BL Lac objects (XBLs). This sample comprises 19 high-energy cut-off BL Lac objects (HBLs), 1 low-energy cut-off BL Lac object (LBL) and 2 'intermediate' sources. Data for LBLs, which are mostly radio-selected BL Lac objects (RBLs), are taken from the literature. It is shown that the radio–millimetre spectral indices of HBLs     are slightly steeper than those of the LBLs     . A correlation exists between α _5–230 and 230 GHz luminosity. While this correlation could be an artefact of comparing two populations of BL Lac objects with intrinsically different radio properties, it is also consistent with the predictions of existing unified schemes that relate BL Lac objects to Fanaroff–Riley class I radio galaxies.
The HBLs have significantly flatter submillimetre–X-ray spectral indices     than the LBLs     although the two intermediate sources also have intermediate values of α _230–X∼−0.9. It is argued that this difference cannot be explained entirely by the viewing-angle hypothesis and requires a difference in physical-source parameters. The α _230–X values for the HBLs are close to the canonical value found for large samples of radio sources and thus suggest that synchrotron radiation is the mechanism that produces the X-ray emission. As suggested by Padovani & Giommi, the inverse-Compton mechanism is likely to dominate in the LBLs requiring the synchrotron spectra of these sources to steepen or cut off at lower frequencies than those of the HBLs.     

X选BL Lac天体的某些性质

本文给出了包含５６个Ｘ选择的ＢＬＬａｃ天体（ＸＢＬｓ）的完备样品，得到了一些相互关系并讨论了ＸＢＬｓ的一些性质。     

X选BL Lac天体的某些性质(Ⅱ)

本文对X射线选择BLLac天体（XBLs）的完备样品进行了较细致的研究，讨论了该样品各观测量的分布、最新发现的几例X射线和ROSAT选择的BLLac天体的Hubble关系及BLLac天体的红移起源问题．     

射电选BL Lac（RBLs）天体的观测胜质

本文给出了射电进ＢＬＬａｃ天体的完备样品及它们的观测数据并讨论了它们的各种性质。     

A Statistical Study of XBLs, RBLs and FSRQs at 1.5 GHz

BL Lac objects are similar to the fiat spectrum radio quasars in many aspects except regarding the emission lines.In order to study their relationship,we selected 56 BL Lacertae objects(33 X-ray-selected,23 radio-selected)and 45 flat spectrum radio quasars,analyzed their radio luminosities and core-dominance parameters.We found that the radio luminosities of the radio selected BL Lac objects located in between the X-ray selected BL Lac objects and the fiat spectrum radio quasars.However,this intermediate position does not bold for the core-dominance parameter:the RBLs have the largest core-dominance parameters.This sug- gests that the core-dominance parameter can not be taken as a sequencing criterion.We also investigated the correlation between the luminosity and the core-dominance parameter for the three subclasses.We concluded that,here,the sequence XBL-RBL—FSRQ still exists.     

A list of ultraviolet excess galaxies

We present a list of 412 faint galaxies selected for their apparent ultraviolet excess (UVX) from an area of 30 square degrees centered on Kapteyn Selected Area 28. The objects have been selected from a three-colour (U, B, V) plate taken with the Palomar 48-inch Schmidt telescope. The same plate was used to select a companion list of predominantly starlike UVX objects (Usher and Mitchell, 1982).Operated by Associated Universities, Inc., under contract with the National Science Foundation.     

Properties of BL Lac Objects with Redshift < 0.2

We present a large sample which includes 82 BL Lac objects with redshifts below 0.2 from recent literature. We find strong correlations in both flux and luminosity between the radio (5 GHz) and optical bands (5500 A). The correlations in other bands are very weak. Five TeV BL Lacs and two suspect sources are found to have similar properties as high-frequency-peaked BL Lacs (HBLs). Our results suggest that both the radio and optical emissions are from the same radiation mech anism in the SSC model. The TeV BL Lac candidates should be HBLs or HBL-like objects with small redshifts.     

Short-term variations of some UV-ceti type flare stars

Sinusoidal variations in bothV - andB-bands were detected in some flare stars of the UV Ceti type outside of flares. This detection has confirmed the light variation detection in Johnson'sV -band in EV Lac at quiet-state luminosity by Pettersen (1980) with a cyclic period equals about 4 _. ^d 378 and an amplitude of about 0 _. ^m 07. An interpretation of these short cyclic periods is that they are due to intensity modulations from a photospheric spot group as a result of stellar equatorial rotations. A short period of 14 days with an amplitude of 0 _. ^m 099 was detected inB-band in AD Leo. For the two flare stars, BD+55° 1823 and DO Cep in bothV- andB-bands, cyclic periods of more than 3 days and less than 17 days with amplitudes more than 0 _. ^m 090 and less than 0 _. ^m 250 have been registered. A significant contribution has been found in the flare star EV Lac in bothV- andB-bands at its quiescent-state luminosity where the detected cyclic periods are agreed with that which was detected by Pettersen (1980) in the same flare star in Johnson'sV-band, about 4 days. Furthermore, we found the same cyclic period in the colour index,B - V (about 4 days) which strengthens starspot phenomenon. This colour index period could not be detected by Pettersen (1980).     

Intermediate BL Lac objects

The 200-mJy sample, defined by Marchã et al., contains about 60 nearby, northern, flat-spectrum radio sources. In particular, the sample has proved effective at finding nearby radio-selected BL Lac objects with radio luminosities comparable to those of X-ray-selected objects, and low-luminosity flat-spectrum weak emission-line radio galaxies (WLRGs). The 200-mJy sample contains 23 BL Lac objects (including 6 BL Lac candidates) and 19 WLRGs. We will refer to these subsamples as the 200-mJy BL Lac sample and the 200-mJy WLRG sample, respectively. We have started a systematic analysis of the morphological pc-scale properties of the 200-mJy radio sources using VLBI observations.
This paper presents VLBI observations at 5 and 1.6 GHz of 14 BL Lac objects and WLRGs selected from the 200-mJy sample. The pc-scale morphology of these objects is briefly discussed. We derive the radio beaming parameters of the 200-mJy BL Lac objects and WLRGs and compare them with those of other BL Lac samples and with a sample of FR I radio galaxies. The overall broad-band radio, optical and X-ray properties of the 200-mJy BL Lac sample are discussed and compared with those of other BL Lac samples, radio- and X-ray-selected. We find that the 200-mJy BL Lac objects fill the gap between HBL and LBL objects in the colour–colour plot, and have intermediate α _XOX as expected in the spectral energy distribution unification scenario. Finally, we briefly discuss the role of the WLRGs.     

A Multiwavelength Study of the Jets in FR-I Radio Galaxies： I. Data and Analysis

We compile a sample of 11 Fanaroff-Riley type I Radio Galaxies(FR-I RGs)with multi-wavelength observations to address the dynamic behavior of jets in these objects.Optical images acquired by the Hubble Space Telescope(HST)are carefully analyzed.The method and reduction procedure are described in detail. Unresolved optical cores emerge after having properly removed starlight from the host galaxies in eight of the FR-I RGs,of which five are new identifications.Broad band spectral properties of these newly identified compact cores are compared with that previously found in FR-I RGs,as well as the low-energy-peaked BL Lac objects. The similarity between them argues for the same non-thermal synchrotron origin. Well-resolved optical jets with knotty morphologies are found in three FR-I RGs in our sample,namely 3C 15,3C 66B and B20755 37.The optical counterparts to the inner radio/X-ray jets are identified and a clear one-to-one correspondence between the optical,radio and X-ray knots is found.The structure and information on the optical jets are discussed.Physical parameters such as the knots position,flux and size are also presented.Detailed comparison between the multi-wavelength data and radiative and dynamic models of jet will be made in a forthcoming paper.     

Optical Spectra Evolution of BL Lac Objects

Many quasi-simultaneous optical observations of nine BL Lac objects are obtained from literature. We study the relationship between the optical spectral index and the luminosity of BL Lac objects, and are tempted to exploit spectral evolution in the optical frequency ranges. Our results show that: (i) The optical spectra index of the low-frequency peaked BL Lac objects (LBLs) is steeper than the high-frequency peaked BL Lac objects (HBLs); (ii) The spectra tend to be softer when the source becomes brighter for LBLs and the intermediate BL Lac objects (IBLs) (i.e., bluer-when-brighter), and the spectra of HBLs does not vary when the brightness of HBLs changes. Possible explanations are briefly discussed for this phenomenon.     

Viewing angles in some selected BL Lac objects

A selected subset of BL Lac objects were monitored with VLBI techniques at multifrequencies. Apparent superluminal motion and X-rays were detected in all of the monitored sources. From the observational data, kinematic parameters, geometric parameters and beaming effects for the core of the selected BL Lac objects have been estimated. Our results indicate that some BL Lacs have large Lorentz factors and small viewing angles; relativistic beaming effects are strongly present in these sources. On the other hand, some BL Lacs have small Lorentz factors; their viewing angles are quite large. It does not seem possible to avoid the fact that some BL Lac objects are not oriented particularly close to the line of sight. It conflicts with the basic assumption of the accepted theories for explaining the characteristics of BL Lac objects at present. Also, it seems that the BL Lac objects in the class I of the Burdige and Hewitt's classification have larger viewing angles than those int he class II. The whole BL Lac classification could be a misnomer: the sources classified as BL Lac objects are a mixture of physically different populations with different parent objects.     

Properties of BL Lac Objects with Redshift≤0.2

1 INTRODUCTIONBlazars, including BL Lacertae objects and Optical Violently Variable (OVV) quasars,belong to a class of active ga1actic nuclei (AGNs), the spectrum of which is dominated bynontherma1 radiation from re1ativistic electrons in jets pointing at us (e.g. BlandfOrd & Rees1978; Urry & Padovani 1995). The entire electromagnetic spectraI energy distributions (SEDs)usually reveal two broad components, the low energy part, peaking in the IR to soft X-rayrange, and the high en…     

Photopolarimetry of BL Lac

This paper presents results of our monitoring of BL Lac at the 2.56 m Nordic Optical Telescope in La Palma and at the 0.91 m telescope at National Astronomical Observatory in Dodaira, Japan. On La Palma we used aUBVRI-photopolarimeter which gives truly simultaneous observations in all five colour bands. At Dodaira we used a photopolarimeter which gives simultaneously photometry atB, V andR-bands and polarimetry atV-band. We have observations of BL Lac during flare behaviour and during these times we have observed a clear rotation of the polarization position angle. During these flares the polarization level has also been strongly variable and the object has lost its preferred direction of the polarization position angle. The reason for this behaviour remains still unclear, but there are two possible explanations: shocks in the helical magnetic field in the jet pointing nearly towards us or polarized synchrotron flares rotating within the accretion disk (seen nearly face on) of the supermassive central black hole.