Sample Of BL Lacertae Objects Research Articles

Relativistic Beaming and Orientation Effects in BL Lacertae Objects

We use the correlation between the core-to-lobe radio luminosity ratio (R) and the linear size (D) of a sample of BL Lacertae objects to investigate the relativistic beaming and radio source orientation paradigm for high peaked and low-peaked BL Lacs (X-ray and radio selected BL Lacs respectively) and to constrain relativistic beaming model for this extreme class of active galactic nuclei. We show that the R − D distributions of the BL Lac populations contradict blazar orientation sequence, with the X-ray selected BL Lacs (XBLs) being more consistent with the beaming and orientation model. On the premise that Fanaroff-Riley Type I radio galaxies are the unbeamed parent population of these objects, we derive the bulk Lorentz factor of the jets, γ~7 − 20 corresponding to a critical cone angle for optimum boosting, φc of ~ 1° − 4°, while on average, these objects are inclined at 5° − 12° to the line-of-sight. The implications of these results for the blazar unification sequence are discussed.

On the relationship between BL Lacertae objects and radio galaxies

We present deep radio images at 1.4 GHz of a large and complete sample of BL Lacertae objects (BL Lacs) selected from the Deep X-ray Radio Blazar Survey (DXRBS). We have observed 24 northern sources with the Very Large Array (VLA) in both its A and C configurations and 15 southern sources with the Australia Telescope Compact Array (ATCA) in its largest configuration. We find that in the DXRBS, as in the 1-Jy survey, which has a radio flux limit roughly ten times higher than the DXRBS, a considerable number (about a third) of BL Lacs can be identified with the relativistically beamed counterparts of Fanaroff-Riley type II (FR II) radio galaxies. We attribute the existence of FR II-BL Lacs, which is not accounted for by current unified schemes, to an inconsistency in our classification scheme for radio-loud active galactic nuclei (AGN). Taking the extended radio power as a suitable measure of intrinsic jet power, we find similar average values for low- (LBL) and high-energy peaked BL Lacs (HBL), contrary to the predictions of the blazar sequence.

Parsec-scale magnetic field structures in HEAO-1 BL Lacs

We present very long baseline interferometry polarization images of an X-ray selected sample of BL Lacertae objects belonging to the first High Energy Astronomy Observatory (HEAO-1) and the ROSAT–Green Bank (RGB) surveys. These are primarily high-energy-peaked BL Lacs (HBLs) and exhibit core-jet radio morphologies on pc-scales. They show moderately polarized jet components, similar to those of low-energy-peaked BL Lacs (LBLs). The fractional polarization in the unresolved cores of the HBLs is, on average, lower than in the LBLs, while the fractional polarizations in the pc-scale jets of HBLs and LBLs are comparable. However a difference is observed in the orientation of the inferred jet magnetic fields – while LBL jets are wellknown to preferentially exhibit transverse magnetic fields, the HBL jets tend to display longitudinal magnetic fields. Although a ‘spine-sheath’ jet velocity structure, along with larger viewing angles for HBLs could produce the observed magnetic field configuration, differences in other properties of LBLs and HBLs, such as their total radio power, cannot be fully reconciled with the different-angle scenario alone. Instead it appears that LBLs and HBLs differ intrinsically, perhaps in the spin rates of their central black holes.

37 GHz Observations of a Large Sample of BL Lacertae Objects

We present 37 GHz data obtained at Metsahovi Radio Observatory in 2001 December - 2005 April for a large sample of BL Lacertae objects. We also report the mean variability indices and radio spectral indices in frequency intervals 5 - 37 GHz and 37 - 90 GHz. Approximately 34 % of the sample was detected at 37 GHz, 136 BL Lacertae objects in all. A large majority of the detected sources were low-energy BL Lacs (LBLs). The variability index values of the sample were diverse, the mean fractional variability of the sample being \Delta S_2 = 0.31. The spectral indices also varied widely, but the average radio spectrum of the sample sources is flat. Our observations show that many of the high-energy BL Lacs (HBL), which are usually considered radio-quiet, can at times be detected at 37 GHz.

Spectral energy distributions of a large sample of BL Lacertae objects

We have collected a large amount of multifrequency data for the objects in the Metsahovi Radio Observatory BL Lacertae sample and computed their spectral energy distributions (SED). This is the first time the SEDs of BL Lacs have been studied with a sample of over 300 objects. The synchrotron components of the SEDs were fitted with a parabolic function to determine the synchrotron peak frequency. We checked the dependence between luminosities at several frequency bands and synchrotron peak frequency to test the blazar sequence scenario, which states that the source luminosity depends on the location of the synchrotron peak. We also calculated broad band spectral indices and plotted them against each other and the peak frequency. The range of peak frequencies in our study was considerably extended compared to previous studies. There were 22 objects for which log\nu_{peak}>19. The data shows that at 5 GHz, 37 GHz and 5500 A there is negative correlation between luminosity and nu_{peak}. There is no significant correlation between source luminosity at synchrotron peak and peak frequency. Several low radio luminosity-low energy peaked BL Lacs were found. The negative correlation between broad band spectral indices and nu_{peak} is also significant, although there is substantial scatter. Therefore we find that neither alpha_{rx} nor alpha_{ro} can be used to determine the synchrotron peak of BL Lacs. On the grounds of our results we conclude that the blazar sequence scenario is not valid. In all our results the BL Lac population is continuous with no hint of the bimodality of the first BL Lac samples.

On the Cross-Correlation between the Arrival Direction of Ultra-High-Energy Cosmic Rays, BL Lacertae Objects, and EGRET Detections: A New Way to Identify EGRET Sources?

With the aim of testing recent claims for a particularly strong correlation between ultra-high-energy cosmic rays (UHECRs), observed with the AGASA and Yakutsk experiments, and a sample of BL Lacertae objects, we conduct a blind statistical assessment. We search for associations between the same set of BL Lac objects and the arrival directions of 33 relevant UHECRs observed with the Haverah Park and Volcano Ranch experiments. Within the accuracy of the angle determination, there are no positional coincidences. The probability that this null result arises as a statistical fluctuation from the strongly correlated case is less than 5%. This implies that the possible correlation between the arrival directions of UHECRs and BL Lac objects is not statistically sustained. We discuss the impact of our findings on the proposed additional connection among UHECRs, BL Lac objects, and EGRET γ-ray blazars. Recently, such an association was used as a classification technique for EGRET sources. Here we show that its main underlying hypothesis (i.e., the EGRET angular uncertainty is twice that quoted in the third EGRET catalog) grossly underestimates the goodness of existing γ-ray data.

The Properties of the Radio-Selected 1 J[CLC]y[/CLC] Sample of BL Lacertae Objects

We present new optical and near-IR spectroscopy, as well as new high dynamic range, arcsecond-resolution VLA radio maps, of BL Lacertae objects from the complete radio-selected jansky (1 Jy) sample (RBLs) for which such data were not previously available. Redshift information is now available for all but six of the 37 BL Lac objects in the 1 Jy sample. Of the 31 with redshift information, four redshifts are only minimum values based on absorption lines, and four other objects have uncertain redshifts based on the detection of only a single emission line. Unlike BL Lac objects from the complete X-ray–selected Einstein Medium Sensitivity Survey (EMSS) sample (XBLs), most RBLs possess weak but moderately luminous emission lines. The emission-line luminosities of RBLs are several orders of magnitude lower than flat-spectrum radio quasars (FSRQs); however, there is significant overlap in the luminosity distributions of the two classes. All but one object in the 1 Jy sample has now been observed with the VLA, and extended flux was detected for all but three of the observed objects. Whereas nearly all XBLs have extended power levels consistent with FR 1s, more than half of the RBLs have extended radio power levels too luminous to be beamed FR 1 radio galaxies. In fact, we find evidence for and examples of three distinct mechanisms for creating the BL Lac phenomenon in the 1 Jy sample: beamed FR 1s, beamed FR 2s, and possibly a few gravitationally lensed quasars. The V/Vmax value determined for the 1 Jy sample is 0.614 ± 0.047, which is markedly different from the negative evolution seen in the EMSS and other XBL samples. A correlation between logarithmic X-ray–to–radio flux ratio and V/Vmax value is observed across the EMSS and 1 Jy samples from negative evolution in the more extreme XBLs to positive evolution in the more extreme RBLs. There is evidence that the selection criteria chosen by Stickel et al. eliminates some BL Lac objects from the 1 Jy sample, although how many is unknown. In addition, several objects currently in the sample have exhibited strong emission lines in one or more epochs, suggesting that they should be reclassified as FSRQs. However, these selection effects cannot account for the observed discrepancy in XBL and RBL properties. From these observational properties, we conclude that RBLs and XBLs cannot be related by viewing angle alone, and that RBLs are more closely related to FSRQs.

The Properties of the X-Ray-selected EMSS Sample of BL Lacertae Objects

We present updated and complete radio, optical, and X-ray data for BL Lac objects in the Einstein Medium Sensitivity Survey (EMSS). The complete sample first presented in Morris et al. is updated to include 26 BL Lac objects in total, and we define a new, virtually complete sample consisting of 41 EMSS BL Lac objects (the sample). New high signal-to-noise ratio, arcsecond-resolution VLA observations are also presented for 11 EMSS BL Lac objects, completing VLA observations of the M91 sample. The addition of four new objects, as well as updated X-ray flux and redshift information, has increased the Ve/Va value for the M91 sample to 0.399 ± 0.057 and Ve/Va = 0.427 ± 0.045 for the newly defined D40 sample. In conjunction with other studies of X-ray–selected BL Lac (XBL) samples, these results solidify negative evolution for XBLs, especially for more extreme high-energy–peaked BL Lac objects, for which we find Ve/Va = 0.271 ± 0.077. The observed Ve/Va, spectral, and radio properties of XBLs are completely consistent with being the beamed population of low-luminosity, FR-1 radio galaxies. However, our VLA observations do confirm that XBLs are too core dominated to be consistent with a beamed population of FR-1's seen at intermediate angles, as suggested by the unified model, if XBLs have moderate outflow velocities (γ ~ 5).

Polarization VLBI Observations of a Complete Sample of Northern BL Lacertae Objects

AbstractThe polarization electric vectors in the VLBI jets of BL Lacertae objects are typically aligned with the jet structure. If the jet radio emission is optically thin synchrotron emission, this implies that the magnetic field is perpendicular to the jet, usually interpreted as a signature of shocks. The distribution of polarization position angles in the VLBI core components appears to be bimodal, with the polarization angles either aligned with or perpendicular to the jet direction. In order to study the origin of this characteristic polarization structure, we have made VLBI polarization observations of all 34 sources in the Kühr and Schmidt sample of BL Lacertae objects.

Fanaroff-Riley I galaxies as the parent population of BL Lacertae objects. II - Optical constraints

view Abstract Citations (77) References (32) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Fanaroff-Riley I Galaxies as the Parent Population of BL Lacertae Objects. II. Optical Constraints Padovani, Paolo ; Urry, C. Megan Abstract As part of a continuing investigation of the hypothesis that BL Lacertae objects are low-luminosity radio galaxies with relativistic jets pointing along the line of sight, we consider the number density of these objects at optical wavelengths. There are no well-defined optical samples of BL Lacertae objects, but, using existing catalogs of active galactic nuclei, we obtain new estimates of the optical counts and luminosity function of BL Lacertae objects that are not inconsistent with the beaming hypothesis. Our calculations depend primarily on observed quantities, including the optical luminosity function of the assumed parent population, Fanaroff-Riley type I (FR I) galaxies. Although FR I galaxies have relatively low radio luminosities, they are bright optical galaxies and their luminosity function is very steep. We extend the formalism of Urry and Shafer for comparing beamed and parent populations to cases, such as the optical one, where galaxy light represents a substantial fraction of the total luminosity. The Lorentz factor of the optical jet is estimated to be roughly in the range 8 < γ_0_ < 20, larger than the X-ray value, γ_x_ ~ 3, found by Padovani and Urry. Since currently observed BL Lacertae objects already have luminosities below the predicted break in the luminosity function, itself a signature of beaming, a definite test of the beaming hypothesis does not depend on more sensitive surveys (in contrast to the X-ray case); it simply requires a sizable optically selected sample of BL Lacertae objects. Publication: The Astrophysical Journal Pub Date: February 1991 DOI: 10.1086/169701 Bibcode: 1991ApJ...368..373P Keywords: Bl Lacertae Objects; Galactic Structure; Radio Galaxies; Active Galactic Nuclei; Astronomical Catalogs; Astronomical Models; Luminosity; Astrophysics; BL LACERTAE OBJECTS; GALAXIES: JETS; GALAXIES: NUCLEI; RADIO SOURCES: GALAXIES full text sources ADS | Related Materials (2) Part 1: 1990ApJ...356...75P Part 3: 1991ApJ...382..501U