VSOP polarization observations of the BL Lacertae object OJ 287

VLBI total intensity and linear polarization images of the BL Lacertae object OJ 287 have been obtained at     using a global ground array and the HALCA orbiting antenna, and at     two weeks earlier using the VLBA. In the ground-based 6-cm images, the source is dominated by a core–jet double structure the components of which are essentially unresolved. The baselines to the orbiting antenna resolve both of these compact components. In the VSOP images, the ground-based 'core' breaks up into several distinct components, demonstrating that this region is dominated by the contribution of bright, optically thin knots of jet emission. A very similar structure is observed in the 1.3-cm image. The magnetic field in the core is transverse, becomes longitudinal in the inner jet, then makes a sharp transition to a region of transverse field further from the core. This suggests that the field in the outer jet has become highly ordered in the transverse direction owing to the action of a shock; the physical nature of the extended region of longitudinal field closer to the core is not clear. The availability of nearly simultaneous observations with comparable resolution at widely spaced frequencies enabled detection of a ≃90° rotation in polarization position angle for the core, owing to the transition from the optically thick (6 cm) to the optically thin (1.3 cm) regime.     

Computing the Periods of Light Variation of Blazar Objects 3C279 and OJ 287 with Autoregressive Spectral Analysis Method

The light variability is one of the main characteristics of blazar objects. Because of the complexity of their light curves, the present periodicity analysis methods are not yet perfect. Based on the modern spectral estimate theory, this paper has described in details the principles of the maximum entropy spectral estimate and autoregressive (AR) spectral estimate, analyzed the effect of the order number selection on the resultant model. Applying these methods to the periodicity analysis of the quasar 3C 279 and BL Lac object OJ 287, their light periods are obtained to be 7.14 and 11.76 yr, respectively. As is verified by experiments, the AR spectral estimate has a high resolution and is a rather good periodicity analysis method. Finally, the items noteworthy for the application of these spectrum estimation methods to the periodicity analysis of the light variations of blazars are mentioned.     

BL Lac天体喷流物理参数的限定

同步+同步自康普顿(Synchrotron+Synchrotron Self Compton (SSC))模型用于拟合蝎虎天体(BL Lac object, BL Lac)准同时性多波段观测数据,可以获得相关的喷流物理参数,从而能对BL Lac天体喷流的物理性质进行解释.在同步自康普顿模型中,较多的自由参数给计算结果带来很大的不确定性,同时,由于这些模型参数范围太大降低了能谱拟合效率.利用多波段观测数据获得的物理量值,对双幂律电子分布情况下的单区、均匀SSC模型中涉及到的8个模型参数进行限定.另外,还利用模型计算两个典型的BL Lac天体多波段能谱对参数限定的结果进行检验.结果表明:在8个模型参数限定的范围内,选取的模型参数值计算出的理论光子谱与两个BL Lac天体的多波段准同时性观测数据符合较好.     

A Research on the Gravitational Wave Radiation of OJ 287

The research on quasar OJ 287 has lasted over 100 years. OJ 287 exhibits the phenomenon of periodic two-peak outbursts with the eruptive period of 12 years. Observations are rather well interpreted with the black hole binary model. In this model, the secondary black hole moves around the primary black hole and crashes against the accretion disk of the primary black hole, causing outbursts. This model reasonably explains the light curves of OJ 287 and correctly predicts the time of future outbursts. These indirectly justify the precessional effect of general relativity and the existence of gravitational waves. The massive black hole in the center of galaxy is an important kind of gravitational wave source. However, the number of the galaxies with precisely determined kinematical equations of inner components is quite small. The precise kinematic orbits of black holes are provided by the black hole binary model, so the radiation of gravitational waves can be studied on the basis of these kinematic orbits. Based on the existing work, the evolutionary relations of the radiation power and waveform of gravitational waves with time are first derived by using the post-Newtonian approximation method. According to the current progress of the detection equipment of gravitational waves, i.e., IPTA (International Pulsar Timing Array), the direct detection of gravitational waves from OJ 287 may be possible within the future decade.     

The BL Lac heart of Centaurus A

We have observed emission from the nucleus of the closest radio galaxy, Centaurus A, from the radio to the gamma-ray band. We construct, for the first time, its overall spectral energy distribution (SED) which appears to be intriguingly similar to those of blazars, showing two broad peaks located in the far-infrared band and at ∼0.1 MeV respectively. The whole nuclear emission of Centaurus A is successfully reproduced with a synchrotron self-Compton model. The estimated physical parameters of the emitting source are similar to those of BL Lacs, except for a much smaller beaming factor, as qualitatively expected when a relativistic jet is orientated at a large angle to the line of sight. These results represent strong evidence that Centaurus A is indeed a misoriented BL Lac, and provide strong support in favour of the unification scheme for low-luminosity radio-loud active galactic nuclei. Modelling of the SED of Centaurus A also provides further and independent indications of the presence of velocity structures in sub-parsec-scale jets.     

On the variability timescales and magnetic field in OJ 287

The continuum spectrum of OJ 287, like most other BL Lac objects, is featureless- no emission or absorption lines are observed. However, OJ 287 shows variations at different timescales in flux and polarization at various wavelength bands. Using the available variability data one can estimate the sizes of the emission regions in the source from light travel time arguments. We assume the emission mechanism to be synchrotron radiation by high energy electrons with single power law energy distribution. Theoretical synchrotron spectrum in the frequency range 10¹¹–10¹⁷ H _z is compared with the observed spectral shape, obtained from new multifrequency quasi-simultaneous observations, to estimate the lower and upper cut off frequencies. These frequencies are used to obtain theoretical values of the variability timescales and magnetic field in the emission region. We obtain a value of 0.93 G for the magnetic field and 5.184×10⁴ sec for the cooling time from the quiescent continuum spectrum. The shock-in-jet model explains the spectrum where shocks accelerate the particles and amplify the magnetic field in the jet. This timescale is compared with the one obtained from observed short timescale variability (20 minutes) with proper beaming correction. The short timescale variations (200 minutes in the source frame), possibly caused by an additional, flaring, component of the source, are also used to calculate compressed magnetic field. The observed and theoretically estimated variability timescales and the shape of the spectrum suggest that there are more than one emission components in OJ 287.     

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.     

OJ287双黑洞轨道计算:3.5阶后牛顿近似

大质量双黑洞OJ287是一个强引力辐射源.为了探测其引力波信号,需要知道波形,而这主要是由轨道运动所决定.为此,从广义相对论3.5阶后牛顿近似的运动方程出发对OJ287的轨道进行仔细研究,取大黑洞位置固定作为近似,给出了后牛顿近似下3.5阶的次黑洞轨道解,比他人2.5阶的工作高了一阶.次黑洞撞击吸积盘面到光学爆发存在时间延迟,这对于确定轨道参数有很大影响.利用径向距离与爆发时间关系的线性模型,对最近7次爆发时刻的观测值拟合,给出了更精确的OJ287双黑洞的轨道参数及其运动轨道.分析了计算结果,研究运动特征,并且发现了两个新性质:次黑洞进动在初期增加,在晚期接近并和时,进动达到最大值,然后减小并越过0而趋于负值.尚不能确定晚期的这个行为是否由3.5阶近似不够准确所造成.运动方程中耗散性的辐射项,后牛顿2.5阶和3.5阶的系数具有相反的符号.这意味着3.5阶项反而是从外界吸收能量.但2.5阶与3.5阶之和仍然是向外辐射引力波的,体系能量变化率为负.这个工作的计算结果可以用来更精确地计算OJ287的引力辐射.     

Unusual optical quiescence of the classical BL Lac object PKS 0735+178 on intranight time-scale

We present the result of our extensive intranight optical monitoring of the well-known low-energy peaked BL Lac (LBL) object PKS 0735+178. This long-term follow-up consists of R -band monitoring for a minimum duration of ∼4 hours, on 17 nights spanning 11 years (1998–2008). Using the CCD as an N-star photometer, a detection limit of around 1 per cent was attained for the intranight optical variability (INOV). Remarkably, an INOV amplitude of  ≥3 per cent  on hour-like time-scale was not observed on any of the 17 nights, even though the likelihood of a typical LBL showing such INOV levels in a single session of  ≳4  hours duration is known to be high  (∼50 per cent)  . Our observations have thus established a peculiar long-term INOV quiescence of this radio-selected BL Lac object. Moreover, the access to unpublished optical monitoring data of similarly high sensitivity, acquired in another programme, has allowed us to confirm the same anomalous INOV quiescence of this LBL all the way back to 1989, the epoch of its historically largest radio outburst. Here, we present observational evidence revealing the very unusual INOV behaviour of this classical BL Lac object and discuss this briefly in the context of its other known exceptional properties.     

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     

VSOP observations of the BL Lac object 2007+777

A numerical model of magnetic turbulence allows us to compute the diffusion coefficients for different degrees of anisotropy, and in particular for the cases corresponding to the solar wind. We consider here the problem of the propagation of the energetic particles accelerated by corotating interaction regions (CIRs) and observed by Ulysses at high heliographic latitudes. The excursion in latitude of a magnetic field line located at large heliospheric distance is evaluated with a Monte Carlo simulation. In the calculations, the random `force' terms are proportional to the square root of the diffusion coefficient in each direction. By tracing a large number of field lines, we can devise how the particles would be seen by Ulysses. Good agreement with the actual observations is found. This revised version was published online in July 2006 with corrections to the Cover Date.     

Faraday rotation in the VLBI core of BL Lacertae

External Faraday rotation has been detected in both the core and the parsec-scale jet of BL Lac in a four-frequency very long baseline interferometry (VLBI) experiment. This unexpected result indicates the presence of significant amounts of thermal gas close to the nucleus of this object. The rotation measure (RM) in the jet components is constant, and differs from the currently accepted Galactic RM, indicating that this value (−205 rad m⁻²) is not applicable to the components in the parsec-scale jet. The similarity of the RM in these jet components leads us to suspect that the jet RM is caused by a foreground screen in our Galaxy, although we cannot rule out a combination of Galactic RM and RM local to the jet. If the jet RM is due solely to the Galaxy, this would mean that the currently accepted value of the foreground RM (−205 rad m⁻² ) is not correct, either because the value changed between 1982 and 1997, or because the assumption of no intrinsic source rotation was incorrect, as it was at our later epoch of observation. Our observations suggest a value of     .
After correcting for the foreground RM, the core value is −427 rad m⁻², which is unexpected since, owing to the weakness of their line emission, BL Lac objects are often assumed to be depleted in gas. The core RM appears to be variable, probably because of the presence of at least two polarized components close to the core the relative contributions of which vary with time.     

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.     

The unusual host galaxy of the BL Lac object PKS 1413+135

The BL Lacertae object PKS 1413+135 is associated with a disc-dominated galaxy that heavily absorbs the BL Lac nucleus at optical and X-ray wavelengths. It has been argued whether this galaxy is actually the host galaxy of PKS 1413+135 or whether the BL Lac is a background QSO, gravitationally lensed by the apparent host galaxy. We have obtained deep high-resolution H -band images of this unusual BL Lac object using the UKIRT IRCAM3. Our observations show that the BL Lac nucleus is centred within 0.05 arcsec of the galaxy. Based on this result, we assess the probability for the lensing scenario and come to the conclusion that the disc galaxy is indeed the host of PKS 1413+135. The galaxy shows peanut-shaped isophotes, suggesting the presence of a central bar, which is a common feature of AGN.     

On the Geometry of Broad-Line Regions in BL Lac Objects

The geometry of broad-line regions (BLRs) in active galactic nuclei (AGNs) is still controversial. We use a sample of BL Lac objects, of which the black hole masses Mbh are estimated from their host galaxy absolute magnitude at R-band, MR, by using the empirical relation between MR and black hole mass Mbh. The sizes of the broad-line regions for Mgn are derived from the widths of Mgn lines and the black hole masses. Compared with the empirical relation between BLR size RBLR and MgII line luminosity LMgII, it is found the BLR sizes in the BL Lac objects derived in this paper are 2-3 orders of magnitude higher. If the BLR geometry of these sources is disklike, then the viewing angle between the axis and the line of sight is in the range of ~ 2°-15°, which is consistent with the unification scheme.