Sample Of BL Lac Objects Research Articles

The prediction of the spectral properties of BL Lac samples

The distribution of infrared colours (J-K) of radio- and X-ray-selected BL Lacs is predicted and compared with the observations of two existing samples of BL Lac objects. We make use of the ‘recognition analysis’ formalism introduced in previous papers and show how the infrared spectral properties for BL Lacs are well predicted by adding a galaxy and BL Lac component in different proportions. We also calculate the distributions of the ratio of galaxy light to BL Lac light for the two samples of BL Lac objects and find that the criteria traditionally used for the selection of BL Lac samples miss objects, many of which could be found if smaller observing apertures were used in the identification process.

Optical Identifications for a Sample of REXs: Search for BL Lac Objects

Twenty six Radio Emitting X-ray Sources (REXs) have been observed in two observing runs as part of a project aimed at selecting a new and large sample of BL Lac objects (see Wolter et al. this conference). Sources were selected by cross-correlating ROSAT-PSPC public images against VLA radio surveys. Here we present results from the first observations of 14 sources conducted in April 1995 with a B&C spectrograph in longslit mode using a CCD-Tektronix TK-1024 AB detector at the 2.1m San Pedro Martir telescope (Mexico). Table 1 shows the data derived for the 8 confirmed REXs. X-ray fluxes are in units of erg cm–2 sec–1 in the 0.1-2.4 keV band, radio fluxes are in mJy at 5 GHz, X-ray luminosities are in erg sec–1 in the energy range 0.1-2.4 keV and radio luminosities are in erg sec–1 at 5 GHz. We assumed Ho = 50 km sec–1 Mpc–1 and qo = 0. A more detailed description of individual sources will be published elsewhere.

Extended Emission in BL Lac Objects

The study of the extended emission and polarization properties of BL Lacs is an important step for the identification of their parent population. FRI radio sources, the supposed parent population of BL Lacs, have weaker extended radio luminosity and a dominant inferred magnetic field perpendicular to the jet, while FRII radio sources, the supposed parent population of quasars, have stronger extended radio power and an inferred magnetic field parallel to the jet. The only complete sample of radio selected BL Lacs (1 Jy sample, Stickel et al. 1991, ApJ, 374, 431) contains 34 objects. Unfortunately, about half of 1 Jy BL Lacs do not have very high dynamic range images, necessary to detect the low emissivity radio emission surrounding the bright compact source, either because the object was never observed, or because the observation was carried out at the beginning of 1980s with low sensitivity. In 1994 we started a programme using the VLA (A, B, and D configuration, see Table 1) and the WSRT (W in Table 1) to complete the high sensitivity radio imaging of the 1 Jy sample. We aim to investigate morphology and polarization properties, as well as the luminosity of the extended emission. This contribution presents the L band observations. The results are very preliminary, some of the data reduction is still in progress as well as the statistical analysis. The sources in Table 1 have been roughly classified as extended (E), or point-like (P) if no extended feature was detected. Among the 15 sources observed at the highest resolution 13 were classified as extended. In many sources we detect significantly much more extended flux than previously reported from earlier observations. Almost all the BL Lac objects we observed at the highest resolution show some extended features; furthermore, in a few cases, we detected emission on the arcminute scale. The power of the extended luminosity covers 3 orders of magnitude, 3 objects (0537–441, 0820+225, and 2240–260) have values typical of a FRII radio source. These new data will be used for an updated statistical analysis of the properties of the extended emission in the 1 Jy sample of BL Lac objects.

Luminosity functions of BL Lacertae objects

view Abstract Citations (48) References (41) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Luminosity Functions of BL Lacertae Objects Wolter, Anna ; Caccianiga, Alessandro ; della Ceca, Roberto ; Maccacaro, Tommaso Abstract We derive here the luminosity functions in the radio, X-ray, and, for the first time, optical band, of two complete samples of BL Lac objects. The samples analyzed arc the only sizable samples currently available with a reasonably complete flux and redshift information and are derived from X-ray selection (XBL; 30 objects from the Extended Medium- Sensitivity Survey (EMSS) sample) and radio selection (RBL; 34 objects from the 1 Jy sample). We study the evolution properties of the XBL sample and find a negative evolution of the X-ray luminosity function [L_x_(z)~L_x_(0)e^- 7.0t_] consistent, albeit less dramatic, with previous findings from a smaller EMSS-derived sample (Morris et al. 1991). The RBL sample, instead, shows a marginal positive evolution in the radio band (Stickel et al. 1991). We then derive the evolution parameters and compute the de- evolved luminosity function for the two samples in the three bands (radio, optical, and X-ray). From the comparison of luminosity functions we find that XBLs outnumber RBLs in the optical and X-ray band, but they are found in consistent numbers, but at lower luminosities, in the radio band. These results are in agreement with models like the wide jet model (Celotti et al. 1993); however, the different evolution parameter and the sign of evolution for the two samples have yet to be understood and explained theoretically. A density of ~20 BL Lac objects per steradian at m_v_ = 17 is derived from the integration of the XBL optical luminosity function. We find that the negative results obtained by current polarization and color surveys are consistent with our results and with the properties of BL Lac objects. New search techniques have to be devised in order to create sizable samples of optically selected BL Lac objects. Publication: The Astrophysical Journal Pub Date: September 1994 DOI: 10.1086/174622 Bibcode: 1994ApJ...433...29W Keywords: Astronomical Models; Bl Lacertae Objects; Luminosity; Radio Galaxies; Radio Sources (Astronomy); Spectral Bands; Visible Spectrum; X Ray Sources; Continuums; Mathematical Models; Red Shift; Astrophysics; GALAXIES: BL LACERTAE OBJECTS: GENERAL; RADIO CONTINUUM: GALAXIES; X-RAYS: GALAXIES full text sources ADS | data products NED (64) SIMBAD (63)

The near-infrared-optical-ultraviolet emission of BL Lacertae objects

view Abstract Citations (31) References (44) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Near-Infrared--Optical--Ultraviolet Emission of BL Lacertae Objects Pian, E. ; Falomo, R. ; Scarpa, R. ; Treves, A. Abstract Near-infrared, optical, and ultraviolet quasi-simultaneous observations of 11 BL Lacertae objects are reported. For all but one source the dereddened spectral flux distribution in the (8 x 10^13^)-(2 x 10^15^) Hz frequency range can be described by a single power law f_v_ is proportional to ν^-α^ with average spectral index <a> = 0.88 +/- 0.42 (standard deviation) plus, where relevant, the contribution of the host galaxy. In most cases the nonsimultaneous soft X-ray fluxes obtained by the Einstein Observatory lie on or below the extrapolation of the power law. The results are compared with the average spectral properties of other samples of BL Lac objects studied separately in the IR-optical and in the UV bands. The implications for existing models of the objects are briefly discussed. Publication: The Astrophysical Journal Pub Date: September 1994 DOI: 10.1086/174592 arXiv: arXiv:astro-ph/9403014 Bibcode: 1994ApJ...432..547P Keywords: Bl Lacertae Objects; Infrared Radiation; Light Emission; Power Series; Spectral Energy Distribution; Ultraviolet Emission; X Ray Sources; Astronomical Models; Heao 2; Infrared Astronomy; Light (Visible Radiation); Stellar Spectrophotometry; Synchrotron Radiation; Ultraviolet Astronomy; Astrophysics; GALAXIES: BL LACERTAE OBJECTS: GENERAL; INFRARED: GALAXIES; ULTRAVIOLET: GALAXIES; X-RAYS: GALAXIES; Astrophysics E-Print: 23 pages, latex file, 2 figures available as postscript files appended at the end of the latex text file, Ref. S.I.S.S.A. 31/94/A full text sources arXiv | ADS | data products SIMBAD (11) NED (11) MAST (1)

Optical Microvariability in Complete Samples of BL Lac Objects

A radio-selected (1 Jy, Stickel et al., 1991) and a x-ray selected (EXOSAT, Giommi et al., 1991) sample of BL Lac objects was monitored by performing relative CCD photometry in order to examine the duty cycle, the dominant time-scales and the typical amplitudes of the variability. The samples consist of 34 and 11 objects, respectively.

Optical polarimetry and photometry of X-ray selected BL Lacertae objects

We present the data from 3 years of monitoring the optical polarization and apparent brightness of 37 X-ray-selected BL Lacertae objects. The monitored objects include a complete sample drawn from the Einstein Extended Medium Sensitivity Survey. We confirm the BL Lac identifications for 15 of these 22 objects. We include descriptions of the objects and samples in our monitoring program and of the existing complete samples of BL Lac objects, highly polarized quasars, optically violent variable quasars, and blazars.

Homogeneous J, H, K, L photometry of a sample of BL Lac objects

Homogeneous J, H, K, L, photometry of 42 BL Lacertae objects is presented. The observations cover a period of 3 yr with typical time interval of 6 months. We show that the near-IR nonthermal emission of all the objects but two is well described by a single power law (f ν ∞ν −α ) after accounting for host galaxy contribution. In spite of large flux variability, changes of spectral index are small and rare. In the few cases where significant variations of α are observed, no evidence of correlation with the flux level is found.

The luminosity function and cosmological evolution of X-ray-selected BL Lacertae objects

Redshifts are presented for an X-ray-selected, flux-limited sample of BL Lac objects. The median X-ray luminosity is derived. A correlation between the optical and radio luminosity of these objects is found. The complete redshift information allows measurement of the objects' evolution with redshift and calculation of a luminosity function at zero redshift. Evidence for strong negative evolution (BL Lac objects were either less common or less luminous in the recent past) is found. The differences between radio- and X-ray-selected samples of BL Lac objects are discussed

The complete sample of 1 Jansky BL Lacertae objects. I - Summary properties

This paper describes the first homogeneous, flux-limited sample of radio-selected BL Lac objects, taken from the 1 Jy survey of Kuhr et al. These 34 BL Lac objects comprise the only well-defined sample of BL Lac objects outside the X-ray band. The selection criteria include flat radio spectra, optical counterparts visible on Sky Survey Plates, and optical emission lines weaker than 5 A equivalent width in the rest frame. With two exceptions, all of the BL Lac objects also have high optical polarization, P_opt_ &gt; 3%. Redshifts from weak emission lines, usually narrow [O II] λ3727, [O III] λ5007, or broad Mg II λ2798, are available for ~ 3/4 of the sample. The nearby (z &lt; 0.2) BL Lac objects, at least, sit in elliptical galaxies and may occur in groups or small clusters of galaxies. Gravitational lensing appears to affect one of the high-redshift (z &gt;~ 0.5) BL Lac objects and could be important in others as well. The radio number counts derived from the 1 Jy BL Lac sample are roughly Euclidean over the range between 1 and ~ 5 Jy, and there is evidence for evolution at the 2 σ level. The local radio luminosity function of BL Lac objects is derived and the total number density for P_5 GHz_ &gt;~ 6 x 10^31^ ergs s^-1^ Hz^-1^ is ~ 40 Gpc^-3^.

Complete samples of radio-selected BL Lac objects

A uniform sample of 43 BL Lacertae objects selected from two flux-limited radio surveys is presented. From confirming optical polarimetric observations, it is clear that the combination of a flat high-frequency radio spectrum (11-6 cm) and featureless optical continuum are very effective at isolating classical BL Lacs, with 95 percent of sources satisfying these criteria also proving to be strongly polarized on at least one occasion. Variability is also a general characteristic of the sample, with a typical radio-selected BL Lac spending 60 percent of the time in a state of high polarization (P greater than 3 percent). The completeness of this sample of BL Lac objects and the relationship between the radio-selected nuclei and their more docile X-ray-selected counterparts are discussed. 26 refs.

Einstein spectra of quasars

The results of the initial stage of the CfA survey of Quasar Energy Distributions (QEDs) are reviewed. Einstein Imaging Proportional Counter (IPC) spectra of 33 quasars have been studied by fitting a single power law slope and absorption by an equivalent column density of neutral hydrodgen, N H . A wide range of energy index, α E , is seen and α E increases as the radio loudness of a quasar decreases. Comparison with the higher energy HEAO-A2 data leads us to a two-component model for th x-ray spectrum. The x-ray column density is systematically lower than the 21 cm measured Galactic column density along the same line of sight. This discrepancy is not present in a comparison sample of Bl Lac objects and so the cause must be intrinsic to the quasar. A third component, an ultra-soft excess (<0.28keV) above a single power law in the quasar spectra, provides a plausible explanation. This excess could be the high energy tail of the blue bump in the optical and ultra-violet (UV).

Quasar energy distributions. I - Soft X-ray spectra of quasars

As the initial stage of a study of quasar energy distributions (QEDs), Einstein IPC spectra of 24 quasars are presented. These are combined with previously reported IPC spectra to form a sample of 33 quasars with well-determined soft X-ray slopes. A correlation analysis shows that radio loudness, rather than redshift or luminosity, is fundamentally related to the X-ray slope. This correlation is not followed by higher energy spectra of active galaxies. Two components are required to explain both sets of results. The best-fit column densities are systematically smaller than the Galactic values. The same effect is not present in a sample of BL Lac objects, implying that the effect is intrinsic to the quasars and is caused by a low-energy turnup in the quasar spectra.

Continuum distributions of an X-ray observed sample of BL Lac objects

view Abstract Citations (89) References (68) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Continuum distributions of an X-ray observed sample of BL Lac objects. Cruz-Gonzalez, I. ; Huchra, J. P. Abstract The authors have studied the broadband spectral energy distributions of 25 BL Lac objects from radio to X-ray frequencies. After a brief description of the data base, they discuss the qualitative characteristics of the spectral continuum distributions. This is followed by a quantitative analysis of the spectral indices in the various regimes and an estimate of the stellar contribution. The Synchrotron Self-Compton model is discussed, along with the question of the high-frequency turnover of the synchrotron spectrum. A summary of the conclusions is presented. Publication: The Astronomical Journal Pub Date: April 1984 DOI: 10.1086/113535 Bibcode: 1984AJ.....89..441C Keywords: Astronomical Spectroscopy; Bl Lacertae Objects; Continuous Spectra; Radio Spectra; Spectral Energy Distribution; X Ray Spectra; Astronomical Catalogs; Broadband; Infrared Spectra; Radiant Flux Density; Spectrum Analysis; Synchrotron Radiation; Visible Spectrum; Astrophysics full text sources ADS | data products SIMBAD (25) NED (25)