BL Lac Candidates Research Articles

Polarimetry of optically selected BL Lacertae candidates from the SDSS

We present and discuss polarimetric observations of 182 targets drawn from an optically selected sample of 240 probable BL Lac candidates out of the SDSS compiled by Collinge et al. (2005). In contrast to most other BL Lac candidate samples extracted from the SDSS, its radio- and/or X-ray properties have not been taken into account for its derivation. Thus, because its selection is based on optical properties alone, it may be less prone to selection effects inherent in other samples derived at different frequencies, so it offers a unique opportunity to extract the first unbiased BL Lac luminosity function that is suitably large in size. We found 124 out of 182 targets (68%) to be polarized, 95 of the polarized targets (77%) to be highly polarized (> 4%). The low-frequency peaked BL Lac candidates in the sample are on average only slightly more polarized than the high-frequency peaked ones. Compared to earlier studies, we found a high duty cycle in high polarization (~66 +2/-14% to be > 4% polarized) in high-frequency peaked BL Lac candidates. This may come from our polarization analysis, which minimizes the contamination by host galaxy light. No evidence of radio-quiet BL Lac objects in the sample was found. Our observations show that the probable sample of BL Lac candidates of Collinge et al. (2005) indeed contains a large number of bona fide BL Lac objects. High S/N spectroscopy and deep X-ray observations are required to construct the first luminosity function of optically selected BL Lac objects and to test more stringently for any radio-quiet BL Lac objects in the sample.

MULTIWAVELENGTH OBSERVATIONS OF RADIO-QUIET QUASARS WITH WEAK EMISSION LINES

We present radio and X-ray observations, as well as optical light curves, for a subset of 26 BL Lac candidates from the Sloan Digital Sky Survey (SDSS) lacking strong radio emission and with z < 2.2. Half of these 26 objects are shown to be stars, galaxies, or absorbed quasars. We conclude that the other 13 objects are active galactic nuclei (AGNs) with abnormally weak emission features; 10 of those 13 are definitively radio quiet, and, for those with available optical light curves, their level of optical flux variability is consistent with radio-quiet quasars. We cannot exclude the possibility that some of these 13 AGNs lie on the extremely radio-faint tail of the BL Lac distribution, but our study generally supports the notion that all BL Lac objects are radio-loud. These radio-quiet AGNs appear to have intrinsically weak or absent broad emission line regions (BELRs), and, based on their X-ray properties, we argue that some are low-redshift analogs to weak line quasars (WLQs). SDSS BL Lac searches are so far the only systematic surveys of the SDSS database capable of recovering such exotic low-redshift WLQs. There are 71 more z < 2.2 radio-quiet BL Lac candidates already identified in the SDSS, but not considered here, and many of those might be best unified with WLQs as well. Future studies combining low- and high-redshift WLQ samples will yield new insight on our understanding of the structure and formation of AGN BELRs.

Correlation of Fermi Large Area Telescope sources with the 20-GHz Australia Telescope Compact Array radio survey

We cross correlate the Fermi 11 months survey catalogue (1FGL) with the 20 GHz Australia Telescope Compact Array radio survey catalogue (AT20G) composed by 5890 sources at declination <0 deg. Among the 738 Fermi sources distributed in the southern sky we find 230 highly probable candidate counterparts in the AT20G survey. Of these, 222 are already classified as blazars (176 of known type and 46 of unknown optical class) in the Fermi 1-year LAT AGN Catalogue (1LAC) and 8 are new associations. By studying the gamma-ray and radio properties of these associations we find a strong correlation between the gamma-ray flux (above 100 MeV) and the 20 GHz flux density. This correlation is more than 3 sigma statistically significant both for the population of BL Lacs and of FSRQ considered separately. We also find that the radio counterparts associated to the Fermi sources have on average flat radio spectra between 5 and 20 GHz and that Fermi gamma-ray sources are not preferentially associated with "ultra inverted spectrum" radio sources. For 2 of the 8 new associations we build the broad band spectral energy distribution combining Fermi, Swift and radio observations. One of these two sources is identified with the high redshift FSRQ Swift J1656.3-3302 (z=2.4) and we classify the other source as a candidate new FSRQ. We also study the brightest radio source of the 46 associations without an optical classification and classify it as a new BL Lac candidate "twin" of the prototypical BL Lac OJ 287 if its redshift is somewhat larger, z~0.4.

OPTICALLY SELECTED BL LACERTAE CANDIDATES FROM THE SLOAN DIGITAL SKY SURVEY DATA RELEASE SEVEN

We present a sample of 723 optically selected BL Lac candidates from the SDSS DR7 spectroscopic database encompassing 8250 deg^2 of sky; our sample constitutes one of the largest uniform BL Lac samples yet derived. Each BL Lac candidate has a high-quality SDSS spectrum from which we determine spectroscopic redshifts for ~60% of the objects. Redshift lower limits are estimated for the remaining objects utilizing the lack of host galaxy flux contamination in their optical spectra; we find that objects lacking spectroscopic redshifts are likely at systematically higher redshifts. Approximately 80% of our BL Lac candidates match to a radio source in FIRST/NVSS, and ~40% match to a ROSAT X-ray source. The homogeneous multiwavelength coverage allows subdivision of the sample into 637 radio-loud BL Lac candidates and 86 weak-featured radio-quiet objects. The radio-loud objects broadly support the standard paradigm unifying BL Lac objects with beamed radio galaxies. We propose that the majority of the radio-quiet objects may be lower-redshift (z<2.2) analogs to high-redshift weak line quasars (i.e., AGN with unusually anemic broad emission line regions). These would constitute the largest sample of such objects, being of similar size and complementary in redshift to the samples of high-redshift weak line quasars previously discovered by the SDSS. However, some fraction of the weak-featured radio-quiet objects may instead populate a rare and extreme radio-weak tail of the much larger radio-loud BL Lac population. Serendipitous discoveries of unusual white dwarfs, high-redshift weak line quasars, and broad absorption line quasars with extreme continuum dropoffs blueward of rest-frame 2800 Angstroms are also briefly described.

A LARGE SAMPLE OF BL LAC OBJECTS FROM THE SDSS AND FIRST

We present a large sample of 501 radio-selected BL Lac candidates from a combination of Sloan Digital Sky Survey (SDSS) Data Release 5 optical spectroscopy and the Faint Images of the Radio Sky at Twenty cm (FIRST) radio survey; this is one of the largest BL Lac samples yet assembled, and each object emerges with homogeneous data coverage. Each candidate is detected in the radio from FIRST and confirmed in SDSS optical spectroscopy to have (1) no emission feature with measured rest-equivalent width larger than 5 A and (2) no measured Ca II H/K depression larger than 40%. We subdivide our sample into 426 higher-confidence candidates and 75 lower-confidence candidates. We argue that contamination from other classes of objects that formally pass our selection criteria is small, and we identify a few very-rare radio active galactic nuclei (AGNs) with unusual spectra that are probably related to broad absorption line quasars. About one-fifth of our sample were known BL Lac objects prior to the SDSS. A preliminary analysis of the sample generally supports the standard beaming paradigm. While we recover sizable numbers of low-energy and intermediate-energy cutoff BL Lac objects (LBLs and IBLs, respectively), there are indications of a potential bias toward recovering high-energy cutoff BL Lac objects (HBLs) from SDSS spectroscopy. Such a large sample may eventually provide new constraints on BL Lac unification models and their potentially peculiar cosmic evolution; in particular, our sample contains a significant number of higher redshift objects, a sub-population for which the standard paradigm has yet to be rigorously constrained.

Highly Polarized Optically Selected BL Lacertae Objects

Observations of candidate BL Lacertae objects spectroscopically selected from the Sloan Digital Sky Survey (SDSS) reveal a large fraction with high polarization (P > 3%). This result confirms that synchrotron radiation makes an important contribution to the observed optical continuum for most objects in the sample. The SDSS sample can be divided into separate categories, with objects of undetermined redshift generally having the highest optical polarization. Polarization as high as 23% and the lack of spectral features suggests that the synchrotron continuum completely dominates the spectra of these sources. The mean polarization levels observed for objects having measured redshifts is much lower, with the maximum observed polarization for this group being ~10%. The lower polarizations of these objects are reminiscent of the less spectacular polarization levels shown by BL Lac objects discovered in X-ray surveys. We find no SDSS BL Lac candidates at z > 1 with P > 3%, calling their classification as BL Lac objects into question. In addition, the existence of radio-quiet BL Lac objects is not verified since none of 10 potentially radio-weak BL Lac candidates observed are highly polarized. Regardless of whether the high-redshift and radio-weak objects are included in this optical sample, the overall levels of polarization observed are intermediate between those seen for X-ray and radio-selected BL Lac objects.

The 2QZ BL Lac survey - II

We report on our further analysis of the expanded and revised sample of potential BL Lac objects (the 2BL) optically identified from two catalogues of blue-selected (UV excess) point sources, the 2dF and 6dF QSO Redshift Surveys (2QZ and 6QZ). The 2BL comprises 52 objects with no apparent proper motion, over the magnitude range 16.0 < bj< 20.0. Follow-up high signal-to-noise spectra of 36 2BL objects and NIR imaging of 18 objects, together with data for 19 2BL objects found in the Sloan Digital Sky survey (SDSS), show 17 objects to be stellar, while a further 16 objects have evidence of weak, broad emission features, although for at least one of these the continuum level has clearly varied. Classification of three objects remains uncertain,with NIR results indicating a marked reduction in flux as compared to SDSS optical magnitudes. Seven objects have neither high signal-to-noise spectra nor NIR imaging. Deep radio observations of 26 2BL objects at the VLA resulted in only three further radio-detections, however none of the three is classed as a featureless continuum object. Seven 2BL objects with a radio detection are confirmed as candidate BL Lac objects while one extragalactic (z=0.494) continuum object is undetected at radio frequencies. One further radio-undetected object is also a potential BL Lac candidate. However it would appear that there is no significant population of radio-quiet BL Lac objects.

A Synoptic Multiwavelength Analysis of a Large Quasar Sample

We present variability and multiwavelength photometric information for the 933 known quasars in the QUEST Variability Survey. These quasars are grouped into variable and nonvariable populations based on measured variability confidence levels. In a time-limited synoptic survey, we detect an anticorrelation between redshift and the likelihood of variability. Our comparison of variability likelihood to radio, IR, and X-ray data is consistent with earlier quasar studies. Using already known quasars as a template, we introduce a light-curve morphology algorithm that provides an efficient method for discriminating variable quasars from periodic variable objects in the absence of spectroscopic information. The establishment of statistically robust trends and efficient nonspectroscopic selection algorithms will aid in quasar identification and categorization in upcoming massive synoptic surveys. Finally, we report on three interesting variable quasars, including variability confirmation of the BL Lac candidate PKS 1222+037.

Optically Identified BL Lacertae Objects from the Sloan Digital Sky Survey

We present a sample of 386 BL Lacertae candidates identified from 2860 deg2 of the Sloan Digital Sky Survey (SDSS) spectroscopic database. The candidates are primarily selected to have quasi-featureless optical spectra and low proper motions as measured from SDSS and USNO-B positions; however, our ability to separate Galactic from extragalactic quasi-featureless objects (QFOs) on the basis of proper motion alone is limited by the lack of reliable proper motion measurements for faint objects. Fortunately, high proper motion QFOs, mostly DC white dwarfs, populate a well-defined region of color space, approximately corresponding to blackbodies with temperatures in the range 7000–12,000 K. QFOs with measurable redshifts or X-ray or radio counterparts (i.e., evidence of an extragalactic/active galactic nucleus nature) loosely follow a track in color space that corresponds to power-law continua plus host galaxy starlight, with typical power-law slopes in the range 1 < αopt < 2 (fν ∝ ν-α). Based largely on this remarkably clean color separation, we subdivide the sample into 240 probable candidates and 146 additional less probable (likely stellar) candidates. The probable BL Lac candidates have multiwavelength properties consistent with the range of previously known BL Lac objects, with an apparent preponderance of objects with synchrotron peaks at relatively high energies (HBL/XBL type). The majority of the 154 objects with measurable redshifts have z < 1, with a median of 0.45; there are also a handful of high-redshift objects extending up to z = 5.03. We identify a small number of potential radio-quiet BL Lac candidates, although more sensitive radio observations are needed to confirm their radio-quiet nature.

Optical follow-up of BL Lac candidates in the 2BL sample

We investigate the nature of the BL Lac candidates in the northern part of the 2BL sample (Londish et al. 2002, MNRAS, 334, 941) searching for optical variability by means of R C band photometry with the Asiago 1.8 and Loiano 1.5 m telescopes during years 2002, 2003 and 2004. We also made aperture photometry of the candidates on the plates available on the web from the Palomar and UK Schmidt telescopes. No significant variability was detected for the majority of the objects without detected radio and/or X-ray emissions, while clear variability was found for the few sources of the sample with strong radio and/or X-ray emission, which is a constant feature of the classical BL Lacs. Some considerations on the nature of the 2BL sources are made, taking into account their most recent data available in proper motions catalogues (SuperCOSMOS, USNO-B 1) and in the Sloan Digital Sky Survey.

New optical polarization measurements of quasi-stellar objects. The data

New linear polarization measurements (mainly in the V band) are presented for 203 quasi-stellar objects (QSOs). The sample is made up of 94 QSOs located in the North Galactic Pole (NGP) region and of 109 QSOs in the South Galactic Pole (SGP) region. First time measurements have been obtained for 184 QSOs. Among them, 109 known radio-emitters, 42 known Broad Absorption Line (BAL) QSOs, and 1 gravitationally lensed quasi-stellar object. We found high polarization levels (p > 3%) for 12 QSOs, including the BAL QSO SDSS J1409+0048. For 10 objects, measurements obtained at different epochs do exist. Two of them show evidence for variability: the highly polarized BL Lac candidate PKS 1216-010 and the radio source PKS 1222+037

A Large, Uniform Sample of X-Ray-emitting AGNs: Selection Approach and an Initial Catalog from theROSATAll-Sky and Sloan Digital Sky Surveys

Many open questions in X-ray astronomy are limited by the relatively small number of objects in uniform optically identified and observed samples, especially when rare subclasses are considered or when subsets are isolated to search for evolution or correlations between wavebands. We describe the initial results of a new program aimed to ultimately yield ~104 fully characterized X-ray source identifications?a sample about an order of magnitude larger than earlier efforts. The technique is detailed and employs X-ray data from the ROSAT All-Sky Survey (RASS) and optical imaging and spectroscopic follow-up from the Sloan Digital Sky Survey (SDSS); these two surveys prove to be serendipitously very well matched in sensitivity. As part of the SDSS software pipelines, optical objects in the SDSS photometric catalogs are automatically positionally cross-correlated with RASS X-ray sources. Then priorities for follow-on SDSS optical spectra of candidate counterparts are automatically assigned using an algorithm based on the known ratios of fx/fopt for various classes of X-ray emitters at typical RASS fluxes of ~10-13 ergs cm-2 s-1. SDSS photometric parameters for optical morphology, magnitude, and colors, plus FIRST radio information, serve as proxies for object class. Initial application of this approach to RASS/SDSS data from 1400 deg2 of sky provides a catalog of more than 1200 spectroscopically confirmed quasars and other AGNs that are probable RASS identifications. Most of these are new identifications, and only a few percent of the AGN counterparts are likely to be random superpositions. The magnitude and redshift ranges of the counterparts are very broad, extending over 15 < m < 21 and 0.03 < z < 3.6, respectively. Although most identifications are quasars and Seyfert 1 galaxies, a variety of other AGN subclasses are also sampled. Substantial numbers of rare AGN types are found, including more than 130 narrow-line Seyfert 1 galaxies and 45 BL Lac candidates. These early results already provide a very sizable set of source identifications, demonstrate the utility of the sample in multiwaveband investigations, and show the capability of the joint RASS/SDSS approach to efficiently proceed toward the largest homogeneously selected/observed sample of X-ray?emitting quasars and other kinds of AGNs.

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.

Properties of BL Lac Objects with Redshift ⩽ 0.2

We present a large sample which includes 82 BL Lac objects with redshifts below 0.2 from recent literature. We find strong correlations ill both flux and luminosity between the radio (5 GHz) and optical bands (5500 Angstrom). The correlations in other bands are very weak. Five TeV BL Lacs and two suspect sources are found to have similar properties as high-frequency-peaked BL Lacs (HBLs). Our results suggest that both the radio and optical emissions are from the same radiation mechanism in the SSC model. The TeV BL Lac candidates should be HBLs or HBL-like objects with small redshifts.

The New BL Lac Candidates from the FBS

AbstractSix new possible BL Lac candidates from the FBS are presented.

Intermediate BL Lac objects

The 200-mJy sample, defined by Marcha 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 αXOX as expected in the spectral energy distribution unification scenario. Finally, we briefly discuss the role of the WLRGs.

ROSAT/ASCAObservations of a Serendipitous BL Lacertae Object PKS 2316−423: The Variable High‐Energy Tail of Synchrotron Radiation

We present the analysis of archival data from ROSAT and ASCA of a serendipitous BL Lac object PKS 2316-423. Because of its featureless nonthermal radio/optical continuum, PKS 2316-423 has been called a BL Lac candidate in the literature. PKS 2316-423 was evidently variable over the multiple X-ray observations; in particular, a variable high-energy tail of the synchrotron radiation is revealed. The X-ray spectral analysis provides further evidence of the synchrotron nature of its broadband spectrum: a steep and downward-curving spectrum between 0.1 and 10 keV, typical of high-energy peaked BL Lac objects (HBLs). The spectral energy distribution (SED) through radio to X-ray yields the synchrotron radiation peak at frequency νp = 7.3 × 1015 Hz, with integrated luminosity of Lsyn = 2.1 × 1044 ergs s-1. The averaged SED properties of PKS 2316-423 are very similar to those intermediate BL Lac objects (IBLs) found recently in several deep surveys, such as Deep X-ray Radio Blazar, Radio-Emitting X-ray, and ROSAT-Green Bank surveys. We suggest that PKS 2316-423 is an IBL though it also shows some general features of an HBL. Actually, this double attribute of PKS 2316-423 provides a good test of the prediction that an IBL object can show either synchrotron or inverse Compton characteristics in different variability states.

Identification of active galaxies behind the Coma cluster of galaxies

We describe the results of an efficient survey to identify bright B < ∼ 18 active galactic nuclei (AGN) within ≈ 4 • (4.7 h −1 Mpc projected radius) of the centre of the Coma cluster of galaxies. The candidates have been carefully selected either as unresolved UV-excess objects with no detected proper motion, or from identifications on objective-prism plates including a few ROSAT sources. A global success rate larger than 30% is achieved. Low-resolution spectra are presented for the extragalactic objects identified in the course of the survey: the sample contains 16 emission-line objects, a BL Lac candidate and an elliptical galaxy. The AGNs of this list are unique targets to study the association of the Lyman-α forest with large-scale structures and galactic haloes at low redshift, to search for warm gas within the Coma cluster and nearby filaments, and for optical monitoring to detect the micro-lensing variability produced by the baryonic dark matter in the cluster.

High X-ray-to-optical flux ratio RASS-BSC sources

We have used (where c is count rate, and E stands for E magnitude from POSS) as an alternative expression for to study the X-ray-to-optical flux ratio distribution for known sources in ROSAT All Sky Survey Bright Source Catalog (RASS-BSC). Attention focuses on which classes of X-ray emitters have the highest X-ray-to-optical flux ratios. Then a high X-ray-to-optical flux ratio criterion is set and used to select a sample with 34 unidentified X-ray sources from the RASS-BSC for optical spectroscopy. The optical identifications of the sample are presented. Those 34 X-ray sources have been identified in the following classes: 10 emission line active galactic nuclei, 7 BL Lacertae objects and 3 BL Lac candidates, 7 clusters of galaxies, 4 Galactic objects and 3 objects remain unidentified due to low signal to noise ratio of the spectra. Identification plausibility is based upon the optical spectroscopy, morphology in the DSS image and the X-ray properties of the sources. We find that preliminary classification can be achieved while extent and hardness ratio in X-ray band, optical morphology and X-ray-to-optical flux ratio are concerned. All of the X-ray sources in the sample are identified with previously known classes of X-ray emitters, and the result is in accord with the E magnitude vs. () diagram. The proportions of WD and CVs are much higher than that in the EMSS sample. We consider it is due to the lower bandpass of the ROSAT PSPC and the softness of the energy spectrum of these objects. AGN detected in this sample have extremely soft X-ray emission, which infers the presence of a soft X-ray excess in the AGN of this sample. The optical identification results also prove that to find BL Lacertae objects by high X-ray-to-optical flux ratio is an efficient way.

The REX Survey: A Search for Radio‐emitting X‐Ray Sources

We present the scientific goals, strategy, and first results of the REX project, an effort aimed at creating a sizable and statistically complete sample of radio-emitting X-ray sources (REXs) using the available data from a VLA survey (NVSS) and the ROSAT PSPC archive. Through a positional cross-correlation of the two data sets, we have derived a sample of about 1600 REXs. Among the 393 REXs identified so far (either from literature or from our own spectroscopic observations) a high fraction are active galactic nuclei (AGNs; about 60%-80%), typically radio-loud QSOs, and BL Lac objects. The remaining sources are galaxies, typically radio galaxies isolated or in clusters. Thanks to the low flux limits in the radio (5 mJy at 1.4 GHz) and in the X-ray band (~5×10−14 ergs s-1 cm-2, 0.5-2.0 keV) and the large area of sky covered by the survey (2183 deg2), we intend to derive a new complete and unbiased sample of BL Lac objects that will contain both radio-selected (RBL) and X-ray-selected (XBL) objects. In this way, the apparent dichotomy resulting from the current samples of BL Lac objects can be directly analyzed in a unique sample. Moreover, the high number of BL Lac objects expected in the REX sample (~200) will allow an accurate estimate of their statistical properties, such as the X-ray, radio, and optical luminosity functions and the cosmological evolution. For these reasons, the REX sample will be a powerful tool to accurately test the current theoretical models proposed for BL Lac objects. To date, we have discovered 15 new BL Lac objects and 11 BL Lac candidates with optical properties intermediate between those of a typical elliptical galaxy and those of a typical BL Lac object. These objects could harbor sources of nonthermal continuum in their nuclei, and if confirmed they could represent the faint tail of the BL Lac population. The existence of such weak BL Lac objects has been matter of discussion in recent literature and could lead to a reassessment of the defining criteria of a BL Lac object, and consequently to a revision of their cosmological and statistical properties. Finally, the sample of ~800 emission-line AGNs generated by the REX survey will be useful in addressing many of the open questions regarding AGN phenomenology, such as the relationship between radio-loud and radio-quiet AGNs.