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.     

Cosmological implications of BL Lac objects

It has become clear in recent years that relativistic beaming is a good explanation for the BL Lac phenomenon. Of studies based on the relativistic beaming model of BL Lac objects, we note that the orientation of jet's axis to the line-of-sight is very small and, therefore, the observed flux emitted from a rapidly moving source is orders of magnitude higher than the flux in its rest-frame:F _obs = ^{3 +} F _intr, where is the bulk relativistic Doppler factor. Then the observed apparent magnitudem _v must be corrected for this effect. For our 39 samples, the corrected apparent magnitudem _v ^corr and logZ have a good correlation.     

BL Lac objects and acceleration model

In this paper, the acceleration model is applied to data from RBLs (Radio Selected BL Lac objects). The corrected data from RBLs are found to be nearly the same as those from XBLs (X-Ray Selected BL Lac objects), the expected result, that points ofU - B, B - V of XBLs and RBLs occupy the same region in the (B - V)-(U - B) diagram has been obtained, and the Radio Doppler factors obtained by us are highly consistent with those found by others.     

2MASS observation of BL Lac objects II

In this paper, we have searched for the Two Micron All Sky Survey (2MASS) counterparts of 1434 BL Lacs. Eight hundred and thirty-three of 1434 BL Lacs (～58%) have spatially coincident 2MASS counterparts. Fermi-detected BL Lacs (FBLs) have a much higher 2MASS detection rate than non-Fermi-detected BL Lacs (non-FBLs) (～95% vs 49%). We compare the near-infrared (NIR) apparent magnitudes, monochromatic luminosities and spectral indices of different subclasses of BL Lacs: (i) FBLs are significantly brighter and more luminous than non-FBLs; (ii) low-energy peaked BL Lacs (LBLs) are significantly more luminous than high-energy peaked BL Lacs (HBLs); (iii) the NIR spectral index, α_IR, shows a sequence of decrease from LBLs to intermediate-energy peaked BL Lacs (IBLs) then to HBLs. The K_S band luminosity and NIR spectral index are both strongly anti-correlated with the synchrotron peak frequency (ν_peak). About 66% of BL Lacs in our sample are located close to the power law line in the J-H–H-K_S diagram, which implies predominantly non-thermal radiation in the NIR range. We label 218 BL Lacs whose NIR spectra show convex curvatures probably due to the significant contaminations of host-galaxy starlight. Finally we investigate the correlation between NIR and gamma-ray emission using the 2MASS observational data.     

Global VLBI polarisation observations of BL Lac objects

In May 1995, we obtained 5-GHz VLBI polarisation observations of 20 sources in the complete sample of BL Lac objects defined by H. Kühr and G. Schmidt. Ten antennas participated in the global VLBI observations. Total intensity and linear polarisation images of five sources have been made and analysed thus far. The images for 0003–066, 1538+149, and 2254+074 are presented, together with an analysis of the evolution of their polarisation structure and estimates of apparent velocities for components in their VLBI jets.     

BL Lac objects in the synchrotron proton blazar model

We calculate the spectral energy distribution (SED) of electromagnetic radiation and the spectrum of high-energy neutrinos from BL Lac objects in the context of the synchrotron proton blazar model. In this model, the high-energy hump of the SED is due to accelerated protons, while most of the low-energy hump is due to synchrotron radiation by co-accelerated electrons. To accelerate protons to sufficiently high energies to produce the high-energy hump, rather high magnetic fields are required. Assuming reasonable emission region volumes and Doppler factors, we then find that in low-frequency peaked BL Lacs (LBLs), which have higher luminosities than high-frequency peaked BL Lacs (HBLs), there is a significant contribution to the high-frequency hump of the SED from pion photoproduction and subsequent cascading, including synchrotron radiation by muons. In contrast, in HBLs we find that the high-frequency hump of the SED is dominated by proton synchrotron radiation. We are able to model the SED of typical LBLs and HBLs, and to model the famous 1997 flare of Markarian 501. We also calculate the expected neutrino output of typical BL Lac objects, and estimate the diffuse neutrino intensity due to all BL Lacs. Because pion photoproduction is inefficient in HBLs, as protons lose energy predominantly by synchrotron radiation, the contribution of LBLs dominates the diffuse neutrino intensity. We suggest that nearby LBLs may well be observable with future high-sensitivity TeV γ-ray telescopes.     

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.     

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.     

On the difference in short timescale light variation between the high and low states of BL Lac objects

Combining our data for three BL Lac objects obtained in 1993–1995 with other authors' we note that activity is stronger and short timescale variations more frequent during the high than the low state. We propose that the relativistic jet in the AGN may change its orientation with time, and the high state corresponds to smaller angles of inclination to the line of sight, and the low, to larger angles. This explains the difference in behaviour as well as the new phenomenon discovered in the monitoring by Takalo et al.     

GeV-TeV $\gamma $-ray energy spectral break of BL Lac objects

In this paper, we compile the very-high-energy and high-energy spectral indices of 43 BL Lac objects from the literature. Based on a simple math model, \(\Delta \Gamma_{obs}= \alpha {{{z}}}+\beta \), we present evidence for the origin of an observed spectral break that is denoted by the difference between the observed very-high-energy and high-energy spectral indices, \(\Delta \Gamma_{obs}\). We find by linear regression analysis that \(\alpha \ne 0\) and \(\beta \ne 0\). These results suggest that the extragalactic background light attenuation and the intrinsic curvature dominate on the GeV-TeV \(\gamma \)-ray energy spectral break of BL Lac objects. We argue that the extragalactic background light attenuation is an exclusive explanation for the redshift evolution of the observed spectral break.     

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.     

A study of optically featureless objects from the EC Survey: searching for new southern BL Lac objects

In an attempt to discover new southern BL Lac objects, 14 optically featureless objects from the Edinburgh–Cape Survey were selected. Optical polarimetry and spectroscopy, radio and IR observations were carried out in order to improve their classifications. The 14 objects were examined according to special criteria that are described. Their UBV and JHK colour–colour distributions, spectrograms, radio observations and visible-region polarimetry were utilized to conclude that the selected objects are not BL Lacs. Most of them are apparently not extragalactic objects. It is suggested that four of the 14 candidates are DC white dwarfs, one is a QSO, three are DA white dwarfs or sdB subdwarfs, one is a cataclysmic variable, one is a DAO/sdO and another is possibly a DA+dM binary (composite system with a 'primary' hot white dwarf and a 'secondary' M-type main-sequence dwarf) or a cataclysmic variable. One object is likely to be a subdwarf, while two remain unclassifiable.     

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.     

Constraining evolution of magnetic field strength in the dissipation region of two BL Lac objects

With the assumption that the optical variability timescale is dominated by the cooling time of the synchrotron process for BL Lac objects,we estimate time depen...     

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.     

On the relation between solar-flare gamma-ray emission and proton escape into interplanetary space

It is shown that escaping of solar flare energetic protons into interplanetary space as well as their relation to the flare gamma-ray emission depend on the parameter = 8p/B ₀ ² , where p is the pressure of hot plasma and energetic particles and B ₀ is the magnetic field in a flaring loop. If 1, the bulk of the energetic protons escape to the loss cone because of diffusion due to small-scale Alfvén-wave turbulence, and precipitate into the footpoints of the flaring loop. The flare then produces intense gamma-ray line emission and a weak flux of high energy protons in interplanetary space. If >_*0.3-1.0, then fast eruption of hot plasma and energetic particles out of the flaring loop occurs, this being due to the flute instability or magnetic-field-plasma nonequilibrium. The flare then produces a comparatively weak gamma-radiation and rather intense proton fluxes in interplanetary space. We predict a modulation of the solar flare gamma-ray line emission with a period 1 s during the impulsive phase that is due to the MHD-oscillations of the energy release volume. The time lag of the gamma-ray peaks with respect to the hard X-ray peaks during a simultaneous acceleration of electrons and protons can be understood in terms of strong diffusion.     

Unusual radio properties of the BL Lac object 0820+225

We present the results of simultaneous VLBA polarization observations of the BL Lacertae object 0820+225 at 5, 8 and 15 GHz, together with earlier images at 5 GHz. This source has an unusually rich total intensity and polarization structure compared with other objects with comparable redshifts. The magnetic field in the inner part of the complex and highly twisted VLBI jet is transverse, as is typical of BL Lacertae objects, but becomes roughly longitudinal further from the core, possibly as a result of shear. Although the integrated rotation measure of 0820+225 is modest, the rotation-measure distribution on parsec scales is non-uniform, and clearly shows regions where the rotation measure is substantially higher than the integrated value.