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 Moreover, the access to unpublished optical monitoring data of similarly high sensitivity, acquired in another programme, has allowed us to confirm the same anomalous INOV quiescence of this LBL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.     

The Long-term Light Variation of BL Lac Object 1ES 1959+650

According to the data of optical observations of the Tuorla Observatory in Finland, using the power spectrum method, DCF (Discrete Correlation Function) method, and Jurkevich method, respectively, we analyzed the periodicity of the long-term light variation of the BL Lac object 1ES 1959+650, and obtained its light period to be P =1.4±0.3 yr. Assuming that the origin of the periodicity is concerned with the accretion disk, we obtained the region where the instability of this source occurs being R =9.65 R_g, here R_g represents the Schwarzschild radius.     

S5 2007+777的射电性质研究

S5 2007+777是一个典型的低峰值频率的蝎虎天体,该天体具有kpc尺度的X射线喷流,文献中利用模型估算的方法,得出X射线波段的多普勒因子达到13.0,从而喷流尺度可以达到Mpc量级.在此,搜集了有关S5 2007+777的欧洲甚长基线射电干涉网(European VLBI Network, EVN)高分辨率档案数据、美国甚长基线射电干涉网(Very Long Baseline Array, VLBA) 15 GHz观测数据等,研究了喷流的射电结构、亮温度、自行等方面的性质,发现该源的甚长基线干涉测量(Very Long Baseline Interferometry,VLBI)不同波段的喷流方向一致,但与文献中给出的kpc尺度的X射线喷流和甚大阵(Very Large Array, VLA)射电喷流方向存在一定的差异,说明该源的喷流辐射存在多普勒增亮效应.由VLBI观测得到的亮温度,估算了该源的射电多普勒因子的平均值及中值均为5.0,此值小于文献中X射线波段的多普勒因子,但与文献中利用其他方法得到的射电波段多普勒因子是一致的;另外,对多历元观测数据的拟合发现此源相同波段的各个成分在长历元上没有明显的自行,短历元上的自行甚至是视超光速运动.这可能是由低表面亮度成分中心位置的转移造成的.这同时也验证了之前估算的射电多普勒因子不是很大,小于X射线波段多普勒因子的结论.利用所得到的射电多普勒因子,发现该源具有较大尺度的本征射电喷流,可达到0.5 Mpc,由于这里使用的是均值,因此说明该源也有可能具有接近巨射电星系尺度的喷流.     

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.     

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.     

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.     

Radio Properties of BL Lac Object S5 2007+777

S5 2007+777 is a typical BL Lac object with a low peak frequency, and a kpc-scale extended X-ray jet. In the literature, the Doppler factor derived at the X-ray waveband by means of model estimation is about 13.0, then the intrinsic jet scale can reach the order of magnitude of 1 Mpc. In this work, we have studied the radio structure of the jet, the brightness temperature, proper motion and other properties of this source with the collected EVN (European VLBI Network) high-resolution archive data, and the American VLBA (Very Long Baseline Array) 15 GHz observed data, etc. It is found that the jet directions measured at the different wavebands of the VLBI (Very Long Baseline Interferometry) are consistent, but slightly different from the direction of the kpc-scale X-ray jet given by the literature and the direction of the VLA (Very Large Array) radio jet, which means that the Doppler brightening effect exists in the jet emission of the source. Both the mean value and median value of the radio Doppler factor of this source estimated from the brightness temperature that measured by the VLBI are 5.0, which is smaller than the Doppler factor at the X-ray waveband given by the literature, but consistent with the Doppler factor at the radio waveband obtained by using the other methods in the literature. Furthermore, by fitting the data observed in multiple epochs, it is found that the various components of the source at the same waveband do not have an evident proper motion in a long epoch, but the proper motion in short epochs exhibits even a super-luminal motion. This may be caused by the transfer of the central position of the component with a low surface brightness. Meanwhile, this also verifies the conclusion that the radio Doppler factor estimated previously is not very large, and smaller than the Doppler factor in the X-ray waveband. By using the obtained radio Doppler factor, it is found that the source has an intrinsic radio jet of rather large scale, which may reach 0.5 Mpc, because the mean value is adopted in this case, thus it indicates that the source also possibly possesses a jet close to the scale of a giant radio galaxy.     

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.     

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)     

Long-term optical and radio variability of BL Lacertae

Well-sampled optical and radio light curves of BL Lacertae in $B\text{,}V\text{,}R\text{,}I$ bands and 4.8, 8.0, 14.5 GHz from 1968 to 2014 were presented in this paper. A possible $1.26\pm 0.05$ yr period in optical bands and a $7.50\pm 0.15$ yr period in radio bands were detected based on discrete correlation function, structure function as well as Jurkevich method. Correlations among different bands were also analyzed and no reliable time delay was found between optical bands. Very weak correlations were detected between V band and radio bands. However, in radio bands the variation at low frequency lagged that at high frequency obviously. The spectrum of BL Lacertae turned mildly bluer when the object turned brighter, and stronger bluer-when-brighter trends were found for short flares. A scenario including a precessing helical jet and periodic shocks was put forward to interpret the variation characteristics of BL Lacertae.     

The 0.1–100 keV spectrum and variability of Mrk 421 in a high state

The results of a BeppoSAX target of opportunity (TOO) observation of the BL Lac object Mrk 421 during a high-intensity state are reported and compared with monitoring X-ray data collected with the BeppoSAX Wide Field Cameras (WFC) and the RXTE All Sky Monitor (ASM). The 0.1–100 keV spectrum of Mrk 421 shows continuous convex curvature that can be interpreted as the high-energy end of the synchrotron emission. The source shows significant short-term temporal and spectral variability, which can be interpreted in terms of synchrotron cooling. The comparison of our results with those of previous observations when the source was a factor 3–5 fainter shows evidence for strong spectral variability, with the maximum of the synchrotron power shifting to higher energy during high states. This behaviour suggests an increase in the number of energetic electrons during high states.     

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.