The X-ray jet and halo of PKS 0521−365

Chandra ACIS observations of PKS 0521−365 find that the X-ray emission of this BL Lac object consists of emission from an unresolved core, a diffuse halo and a 2-arcsec jet feature coincident with the inner radio/optical jet. A comparison with a new ATCA 8.6-GHz map also finds X-ray emission from the bright hotspot south-east of the nucleus. The jet spectrum, from radio to X-ray, is probably synchrotron emission from an electron population with a broken power-law energy distribution, and resembles the spectra seen from the jets of low-power (FR I) radio galaxies. The hotspot X-ray flux is consistent with the expectations of synchrotron self-Compton emission from a plasma close to equipartition, as seen in studies of high-power (FR II) radio galaxies. While the angular structure of the halo is similar to that found by an analysis of the ROSAT High Resolution Imager image, its brightness is seen to be lower with Chandra , and the halo is best interpreted as thermal emission from an atmosphere of similar luminosity to the haloes around FR I radio galaxies. The X-ray properties of PKS 0521−365 are consistent with it being a foreshortened, beamed, radio galaxy.     

The final two redshifts for radio sources from the equatorial BRL sample

Best, Röttgering & Lehnert (BRL) defined a new sample of powerful radio sources from the Molonglo Reference Catalogue, for which redshifts were compiled or measured for 177 of the 178 objects. For the final object, MRC1059−010 (3C 249), the host galaxy is identified here using near-infrared imaging, and the redshift is determined from Very Large Telescope (VLT) spectroscopy. For one other object in the sample, MRC0320+053 (4C05.14), the literature redshift has been questioned: new spectroscopic observations of this object are presented, deriving a corrected redshift. With these two results, the spectroscopic completeness of this sample is now 100 per cent.
New redshifts are also presented for PKS0742+10 from the Wall & Peacock 2.7-GHz catalogue, and for PKS1336+003 from the Parkes Selected Regions. PKS0742+10 shows a strong neutral hydrogen absorption feature in its Lyman α emission profile.     

PKS 1117+146: a new large compact symmetric object

We present 23-GHz MERLIN observations of the high-luminosity radio galaxy PKS 1117+146. The radio image obtained from these data shows, for the first time, a central weak component (designated C) that we identify as the core of the radio galaxy, and two outer components (designated N and S) which are the hotspots of the extended lobes observed at lower frequencies. Extended emission in components C and S is an indication of a possible jet connecting the core to the strongest component. The overall optical and radio properties of PKS 1117+146 are consistent with the source being classified as a compact symmetric object (CSO). We discuss this hypothesis, which would make PKS 1117+146 the largest CSO known so far.     

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.     

Near-infrared imaging of the host galaxies of BL Lacertae objects

Near-infrared images have been obtained of nine BL Lacertae objects in order to investigate their host galaxy properties. From numerical modelling of the data we find that five of the nine BL Lacs have contributions from extended galaxy emission in their profiles. Tentative morphologies were derived for two of the BL Lacs, namely B2 0752+258, for which a disc morphology is slightly preferred, and S4 0954+65, for which an elliptical morphology is preferred. From our modelling we derive host galaxy absolute magnitudes of M_H =−25.6±0.5 for B2 0752+258, and M_H =−26.3±0.8 for S4 0954+65. We also find an average K -band absolute K -corrected host galaxy magnitude, for the BL Lacs, of 〈 M_K 〉=−26.3±0.6 asssuming an elliptical galaxy model, and 〈 M_K 〉=−26.1±0.9 assuming a disc galaxy model. The derived absolute magnitudes are similar to those found for the putative parent population of FRI radio galaxies, predicted by unified schemes. For those BL Lacs in which host galaxies have been previously detected at optical wavelengths we derive rest frame colours which are generally consistent with those of 'normal' galaxies. However, for TEX 0836+182 we find a rather blue colour which, together with the elongated morphology, may be indicative of interaction or spiral structure. Our results suggest that the host galaxies of BL Lacs are indeed similar to those of FRI radio galaxies, adding further weight to the unification scenario. In our present infrared frames of relatively modest resolution, we do not find that infrared observations are any more or less effective than optical observations at revealing the underlying host galaxy.     

The disc-dominated host galaxy of FR-I radio source B2 0722+30

We present new observational results that conclude that the nearby radio galaxy B2 0722+30 is one of the very few known disc galaxies in the low-redshift Universe that host a classical double-lobed radio source. In this paper, we use H  i observations, deep optical imaging, stellar population synthesis modelling and emission-line diagnostics to study the host galaxy, classify the active galactic nucleus (AGN) and investigate environmental properties under which a radio-loud AGN can occur in this system. Typical for spiral galaxies, B2 0722+30 has a regularly rotating gaseous disc throughout which star formation occurs. Dust heating by the ongoing star formation is likely responsible for the high infrared luminosity of the system. The optical emission-line properties of the central region identify a Low Ionization Nuclear Emission-line Region (LINER)-type nucleus with a relatively low [O  iii ] luminosity, in particular when compared with the total power of the Fanaroff & Riley type-I radio source that is present in this system. This classifies B2 0722+30 as a classical radio galaxy rather than a typical Seyfert galaxy. The environment of B2 0722+30 is extremely H  i -rich, with several nearby interacting galaxies. We argue that a gas-rich interaction involving B2 0722+30 is a likely cause for the triggering of the radio AGN and/or the fact that the radio source managed to escape the optical boundaries of the host galaxy.     

A search for optical polarization from the narrow-line Seyfert 1 galaxy RE J1034+396

We present spectropolarimetry data on RE J1034+396, an ultra-soft X-ray narrow-line Seyfert 1 galaxy, and find an upper limit of ∼ 0.4 to the linear polarization in the optical band. This suggests that there is no synchrotron emission in this AGN, and thus it is unlikely that RE J1034+396 is related to BL Lac objects. Furthermore, any other polarization arising from transmission through dust, or reflection from dust and electrons, must be cancelled out by geometrical effects or diluted to a high degree by unpolarized radiation.     

Calculating the Light Period of BL Lac Object S5 0716+714 by Wavelet Analysis Method

A kind of wavelet analysis method for identifying the light period of the BL Lac object S5 0716+714 is introduced. The rather complete observed data in the four optical wavebands B, V, R, I are collected, and the long-term light curves based on 10-day averaging are obtained. On these light curves, the periodicity analysis is performed by using a wavelet analysis method. The result demonstrates that the wavelet analysis method is preferable for searching and identifying the light periods of BL Lac objects. From the contour map of the real part of the wavelet transform coefficient, the periodical light variations of the BL Lac object S5 0716+714 can be precisely identified. By analyzing the wavelet variance curves at the 4 wavebands, it is found that the BL Lac Object S5 0716+174 has a stable light period of 1160 days. This result is consistent with the 3.3-year period given by Raiteri et al. It is predicted that the next outburst in this object will happen around the August of 2011.     

On the nature of EXO 0423.4−0840

We present a Chandra observation of the candidate BL Lac object EXO 0423.4−0840. The X-ray emission from EXO 0423.4−0840 is clearly extended, and is associated with an optical early-type galaxy, MCG-01-12-005, at the centre of cluster ClG 0422-09. We do not detect a point source that can be associated with a BL Lac, but we found a small radio source in the centre of MCG-01-12-005. The cluster gas temperature mapped by the Chandra observation drops continuously from 80 kpc towards the centre, and is locally single phase. We measure a metallicity profile that declines outwards with a value  0.8 Z_⊙  in the centre, dropping to  0.35 Z_⊙  at larger radius, which we interpret as a superposition of cluster gas and a dense interstellar medium (ISM) in the central galaxy. Although the temperature profile suggests that conduction is not efficient, the ISM and intra-cluster medium seem not to have mixed. The entropy profile declines continuously towards the centre, in agreement with recent results on groups and clusters. The radio source appears to have had some effect in terms of gas heating, as seen in the small-scale (∼10 kpc) entropy core, and the asymmetric hard emission on the same scale.     

Determining the nature of orbits in a three‐dimensional galaxy model hosting a BL Lacertae object

A three‐dimensional dynamical model for a galaxy hosting a BL Lacertae object is constructed. The model consists of a logarithmic potential representing an elliptical host galaxy with a bulge of radius cb and a dense massive nucleus. Using numerical experiments, we try to distinguish between regular and chaotic motion in both 2D and 3D system. In particular, we investigate how the basic parameters of our model, such as the mass of the nucleus, the internal perturbation and the flattening parameters influence the amount and the degree of chaos. Interesting correlations are presented for both 2D and 3D dynamical models. Our numerical results are explained and supported using elementary theoretical arguments and analytical calculations. Of particular interest is the local integral of motion which have been found to exist in the vicinity of stable periodic points. The obtained numerical outcomes of the present research are linked and also compared with several data derived from observations. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The CLASS blazar survey – II. Optical properties

This paper presents the optical properties of the objects selected in the CLASS blazar survey. Because an optical spectrum is now available for 70 per cent of the 325 sources present in the sample, a spectral classification, based on the appearance of the emission/absorption lines, is possible. A wide variety of optical spectral types is found. Besides 'classical' BL Lacs (42), BL Lac candidates (5) and high-power     flat spectrum radio quasars (67), a significant number of 'passive' elliptical galaxies (41) is also found. Moreover, 33 broad emission line objects with a low radio power     are discovered, suggesting that at least a fraction (∼     per cent) of low-power blazars have a broad line region. Finally, 34 objects showing only narrow emission lines, either as a result of some starburst activity in the host galaxy or as a result of the presence of an active galactic nucleus, appear in the sample.     

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.     

Active galactic nuclei and the minor merger hypothesis

We have investigated the dynamics of the merging process in the minor merger hypothesis for active galactic nuclei. We find that for a satellite galaxy to be able to merge directly with the nucleus of the host galaxy (for example, to give rise to the compact dust discs which are seen in early-type active galaxies), the initial orbit of the satellite is required to be well aimed. For the case of the host galaxy being a disc galaxy, if the initial orbits of the satellites are randomly oriented with respect to the host galaxy, then the orbits of those which reach the host nuclear regions in a reasonable time are also fairly randomly oriented once they reach the nucleus. We note that this result might be able to provide an explanation of why the jet directions in the nuclei of Seyfert galaxies are apparently unrelated to the plane of the galaxy discs.     

Optical and radio studies of radio sources

We present the classification of optical identifications and radio spectra of six radio sources from a complete (in flux density) sample in the declination range 10° to 12°30′ (J2000.0). The observations were carried out with the 6-m Special Astrophysical Observatory telescope (Russia) in the wavelength range 3600–10000 Å, the 2.1-m GHAO telescope (Mexico) in the range 4200–9000 Å, and the RATAN-600 radio telescope in the frequency range 0.97–21.7 GHz. Three of the six objects under study are classified as quasars, one is a BL Lac object, one is an absorption-line radio galaxy, and one is an emission-line radio galaxy. Five objects have flat radio spectra, and one object has a power-law radio spectrum. All of the radio sources identified as quasars or BL Lac objects show variable radio flux densities. The spectra of three objects were separated into extended and compact components.     

A triple nucleus in the brightest cluster galaxy in Abell 193

We present a ground-based, near-infrared, K -band image and a Hubble Space Telescope ( HST ) WFPC2 image of the brightest cluster galaxy in Abell 193 (IC 1695). This object was selected as the central cluster galaxy using X-ray information. Both images reveal a triple nucleus structure. Previously, this galaxy was thought to have only two nuclei. We present colours and magnitudes and a colour plot of the three nuclei. The nuclear structure and colours of the nuclei in this galaxy suggest that a merger may have taken place in its recent history.     

Radio jet interactions in the radio galaxy PKS 2152–699

We present radio observations of the radio galaxy PKS 2152–699 obtained with the Australia Telescope Compact Array. The much higher resolution and signal-to-noise ratio of the new radio maps reveal the presence of a bright radio component about 10 arcsec north-east of the nucleus. This lies close to the highly ionized cloud previously studied in the optical and here shown in a broad-band red snapshot image with the HST PC 2. It suggests that PKS 2152–699 may be a jet/cloud interaction similar to 3C 277.3. This could cause the change in the position angle (of ∼ 20°) of the radio emission from the inner to the outer regions. On the large scale, the source has Fanaroff & Riley type II morphology although the presence of the two hotspots in the centres of the lobes is unusual. The northern lobe shows a particularly relaxed structure while the southern one has an edge-brightened, arc-like structure.     

The AGN and gas disc in the low surface brightness galaxy PGC 045080

We present radio observations and optical spectroscopy of the giant low surface brightness (LSB) galaxy PGC 045080 (or 1300+0144). PGC 045080 is a moderately distant galaxy having a highly inclined optical disc and massive H  i gas content. Radio continuum observations of the galaxy were carried out at 320, 610 MHz and 1.4 GHz. Continuum emission was detected and mapped in the galaxy. The emission appears extended over the inner disc at all three frequencies. At 1.4 GHz and 610 MHz it appears to have two distinct lobes. We also did optical spectroscopy of the galaxy nucleus; the spectrum did not show any strong emission lines associated with active galactic nucleus (AGN) activity but the presence of a weak AGN cannot be ruled out. Furthermore, comparison of the Hα flux and radio continuum at 1.4 GHz suggests that a significant fraction of the emission is non-thermal in nature. Hence we conclude that a weak or hidden AGN may be present in PGC 045080. The extended radio emission represents lobes/jets from the AGN. These observations show that although LSB galaxies are metal poor and have very little star formation, their centres can host significant AGN activity. We also mapped the H  i gas disc and velocity field in PGC 045080. The H  i disc extends well beyond the optical disc and appears warped. In the H  i intensity maps, the disc appears distinctly lopsided. The velocity field is disturbed on the lopsided side of the disc but is fairly uniform in the other half. We derived the H  i rotation curve for the galaxy from the velocity field. The rotation curve has a flat rotation speed of ∼190 km s⁻¹.     

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 3.4-μm absorption feature towards the nucleus of NGC 5506

We report the results of 3-μm spectroscopy towards the nucleus of a nearby Seyfert 2 galaxy, NGC 5506. A weak absorption feature of carbonaceous dust is detected at ∼3.4 μm. The optical depth ratio of the 3.4-μm carbonaceous dust absorption to the 9.7-μm silicate dust absorption is smaller by more than a factor of 2 than that in the interstellar medium in our Galaxy. The small ratio is consistent with the idea that the obscuration towards the nucleus of NGC 5506 is caused mainly by dust grains in its host galaxy (type S0/a) and that the contribution of carbonaceous dust grains to the visual extinction there is smaller than that in our Galaxy (type Sb/bc).     

BL Lac天体处于高态与低态时的短时标光变的差异及机制研究

分析了１９９３－１９９５年ＢＬＬａｃ、ＯＪ２８７和ＰＫＳ０７３５＋１７８三颗ＢＬＬａｃ天体光变的监测结果,并结合文献上他人发表的资料,发现一些ＢＬＬａｃ天体处于高态时活动性较强,短时标光变频繁发生,而处于低态时活动性较弱,短时标光变很少发生．文中提出活动星系核的相对论性喷流方向会改变,从而导致长期光变;高态是喷流方向与观测者视线夹角较小的表现,而低态则是夹角较大的表现。利用该模型能够解释ＢＬＬａｃ天体短时标光变行为在高态和低态下的差别,并能很好地解释Ｔａｋａｌｏ等人监测ＯＪ２８７时发现的新现象．