The λ=6 cm VLBI polarization structure of nine BL Lacertae objects

VLBI total intensity ( I ) and linear polarization ( P ) images at λ =6 cm have been obtained for nine radio-bright BL Lacertae objects. These are the first VLBI P images for these sources, and, in a number of cases, the first I images as well. They confirm the previously noted tendency for the jet magnetic fields of BL Lacertae objects to be transverse to the local jet direction, but also provide new evidence that a sizeable minority of BL Lacertae objects have VLBI jet components with longitudinal magnetic fields. In addition, two sources have VLBI jet components in which the direction of the electric vector χ bears no obvious relation to the apparent local jet direction; the origin of these arbitrary χ offsets is unclear. A new tentative superluminal speed of β =6.3  h ⁻¹ has been determined for 0828+493; tentative speeds for two knots in 1418+546 are β =4.3 and 2.5  h ⁻¹. This work is part of an ongoing programme to determine the VLBI I and P structure of all 34 sources in the 1-Jy sample of northern BL Lacertae objects defined by Kühr & Schmidt.     

Analysis of λ = 6 cm VLBI polarization observations of a complete sample of northern BL Lacertae objects

The results of very long baseline interferometry (VLBI) total intensity ( I ) and linear polarization ( P ) observations at     are presented for 10 radio bright BL Lacertae objects. These images complete first-epoch polarization observations for the 1-Jy sample of northern BL Lacertae objects defined by Kühr & Schmidt. Estimates of superluminal speeds are presented for several sources, bringing the total number of sources in the sample for which such estimates are available to 16. Second-epoch observations currently being reduced should yield speed estimates for VLBI features in essentially all the sources in the sample. The jet magnetic fields of these BL Lacertae objects are usually transverse to the local jet direction, but a sizeable minority (about 30 per cent) have VLBI jet components with longitudinal magnetic fields. This may suggest that the conditions in the VLBI jets of BL Lacertae objects are favourable for the formation of relativistic shocks; alternatively, it may be that the toroidal component of the intrinsic jet magnetic field is characteristically dominant in these sources.     

Optical multiband observations of BL Lacertae during the outburst of 2005

The aim of our observations is to investigate the intranight variability properties and the spectral variability of BL Lacertae. 799 optical multiband observations were intensively made with the Beijing–Arizona–Taiwan–Connecticut (BATC) 60/90 cm Schmidt telescope during the outburst composed of two subsequent flares in 2005. The second flare, whose rising phase lasted at least 44 d, was observed with amplitudes of more than 1.1 mag in three BATC optical bands. In general, on intranight time-scale the amplitude of variability and the variation rate are larger at the shorter wavelength, and the variation rate is comparable in the rising and decaying phases within each band. A possible time-lag between the light curves in bands e and m , around 11.6 min, was obtained. Based on the analysis of the colour index variation with the source brightness, the variability of BL Lacertae can be interpreted as having two components: a 'strongly chromatic' intranight component and a 'mildly chromatic' internight component, which may be the results of both intrinsic physical mechanism and geometric effects.     

Search for hard lags with intra‐night optical observations of BL Lacertae

It is generally believed that the high energy end of synchrotron emission, generated by the most energetic tail of relativistic electrons in the jets, account for the X‐ray emission of high‐energy peaked BL Lac objects (HBLs) and the optical emission of intermediate‐energy peaked BL Lac objects (IBLs). It is thus expected that both should show similar variability characteristics. One of the important variability parameters is the inter‐band time lag which probes the acceleration and cooling of relativistic particles responsible for the emission. The switches between soft and hard lags have been detected in the intra‐day X‐ray variability of a few HBLs, which is not the case for the intra‐day optical variability of IBLs yet. We present the results of our intra‐night optical observations for BL Lacertae, aiming at searching for hard lags of its optical variations, performed with the 80 cm telescope in fourteen nights of 2010 September‐November. Intra‐night changes of ∼0.2 mag were detected in most of nights. The intra‐night variability amplitude tends to become larger from red to blue wavelength, and the optical spectrum hardens with increasing brightness. The intra‐night variations correlate between different wavebands, but we did not find significant time lags, either soft or hard. Nevertheless, on November 2, the B band variations showed a sign of lagging the R band ones by 317±214 s. The claim of this hard lag is strongly limited by the photometric precision and time resolution. Therefore, the switches between soft and hard lags of IBLs in the optical bands needs further demonstration with more higher quality observations. (© 2016 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Intraday polarization variability outside the VLBI core of the active galactic nucleus 0716+714

Rapid ('intraday') cm-wavelength variations in both total and polarized flux density have been observed in a number of strong extragalactic radio sources. It is difficult to explain these variations purely as propagation effects, but if they are intrinsic to the sources, implausibly high brightness temperatures are required. We discuss here rapid polarization variability during our λ =6 cm global very long-baseline interferometry (VLBI) observations of the active galactic nucleus 0716+714. Measurements made with the Very Large Array (VLA) during the VLBI observations indicate a ≃50° swing in the position angle χ of the VLA core polarization in 12 hours. Corresponding variations were observed only for short VLBI baselines, so that they could not have occurred in the VLBI core (the only feature detected in our VLBI polarization map). The fact that the variations appear both in the VLA data and in the VLBI data for short baselines makes it difficult to explain them as instrumental effects. This leads us to conclude that the rapid variations occurred outside the area covered by our VLBI map; we estimate that the variability occurred in some compact feature roughly 25 milliarcseconds from the nucleus. It is clear that compact structures on a wide range of scales must be taken into account in studies of intraday variability in AGN.     

Fast variability of BL Lacertae at 1 μm

We present the results of photometric observations of BL Lacertae at 1 μm performed during the strong outburst that occurred in the summer of 1997. Measurements were carried out using the F99 filter of the Arizona system. Simultaneous observations in the B V bands were made with other telescopes. Large and fast variations were observed on the night of August 2 when the brightness of BL Lacertae changed by ∼ 0.5 mag in about 2 h: the variation amplitude is greater at higher frequency. Some implications of these results for the origin of the fast variations are discussed. We also report the first F99 calibration for three reference stars in the BL Lac field.     

Evidence for parsec-scale absorption from VSOP observations of the BL Lacertae object 0735+178

VLBI total intensity and linear polarization images of the BL Lacertae object 0735+178 have been obtained at     using the     VLA as a ground array together with the HALCA orbiting antenna, and at     about a month later using the VLBA. The most surprising result to come from a comparison of these two images, which have nearly equivalent resolution, is that the path of the jet appears to be appreciably different at the two wavelengths. We have interpreted this as evidence for free–free absorption to the east of the core, near a bend of about 90° in the 2-cm image where the emission is very weak in the 6-cm image. Overall, the polarization structures seen in corresponding regions at the two wavelengths are in good agreement. We find tentative evidence for enhanced rotation measures near two sharp bends in the VLBI jet.     

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.     

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.     

Optical R-band variability of seven X-ray-selected BL Lacertae objects

This paper presents the results of optical CCD photometry in the Cousins R band for seven X-ray-selected BL Lacertae objects. Observations have been performed with the 70-cm meniscus telescope of Abastumani Astrophysical Observatory, Georgia. The data obtained provide important information on sources that have been rarely observed in the optical band. The targets show clear long-term variability with time-scales of 1.5–5 yr. The maximum amplitudes are slightly greater than 1 mag. 1ES 0323+022, 0502+675 and 0806+524 show clear non-periodical behaviour. 1ES 1028+511 changes quasi-periodically but further monitoring for periodicity confirmation and period derivation is needed. Overall brightness variation is maximal for 1ES 0647+250 with  Δ R = 1.29  mag. Two-peak maxima, indicating the existence of reverse shocks in blazar jets, are observed for 1ES 0502+675, 0647+250, 0806+524 and 1517+656. The targets do not show intra-night variability. Day-to-day changes with amplitudes of 0.1–0.32 mag coincide mainly with short-term bursts. The latter are seen at faint (quiet) stages as during flaring activities. This fact leads to the suggestion that both hot-spot and shock-in-jet models explaining the origin of short-term bursts should be reasonable.     

非均匀喷流模型及其在BL Lac天体中的应用

在非均匀锥形喷流模型中,电子数密度、磁场强度随着到喷流顶点的距离呈幂律分布.该模型能成功解释活动星系核喷流核心区域的平谱射电辐射,但已有的模型计算只适用于喷流运动方向与视线夹角很大的情况,所以需要建立适用于任何视角情况的非均匀锥形喷流辐射计算公式.普遍认为BL Lac天体中喷流的运动方向与视线夹角很小,推广后的非均匀喷流模型拟合了3个BL Lac天体的射电观测谱,确定了它们喷流中电子数密度、磁场强度等物理参数.研究结果表明观测辐射谱拐折频率确定出锥形喷流离黑洞最近距离,对于这3个BL Lac天体,它们的锥形喷流离黑洞最近距离约为Schwarzschild半径.     

BeppoSAX observations of 1-Jy BL Lacertae objects – I

We present new BeppoSAX observations of seven BL Lacertae objects selected from the 1-Jy sample plus one additional source. The collected data cover the energy range     (observer's frame), reaching ∼50 keV for one source (BL Lac). All sources characterized by a peak in their multifrequency spectra at infrared/optical energies (i.e., of the low-energy peaked BL Lac type, LBL) display a relatively flat     X-ray spectrum, which we interpret as inverse Compton emission. Four objects (two-thirds of the LBLs) show some evidence for a low-energy steepening, which is probably due to the synchrotron tail merging into the inverse Compton component around ∼     . If this were generally the case with LBLs, it would explain why the     ROSAT spectra of our sources are systematically steeper than the BeppoSAX ones     . The broad-band spectral energy distributions fully confirm this picture, and a synchrotron inverse Compton model allows us to derive the physical parameters (intrinsic power, magnetic field, etc.) of our sources. Combining our results with those obtained by BeppoSAX on BL Lacs covering a wide range of synchrotron peak frequency, ν _peak, we confirm and clarify the dependence of the X-ray spectral index on ν _peak originally found in ROSAT data.