BL Lac Candidates Research Articles

Identification of newly discovered radio-emitting X-ray sources: results from spectroscopy

We present optical identifications for a sample of 20 previously unknown X-ray/radio sources that are present both in the source catalogue of ROSAT PSPC pointed observations (ROSAT SRC) and in the NRAO VLA Sky Survey (NVSS). The optical spectroscopy was carried out with the 2.1-m telescope at San Pedro Martir (Mexico) during 1995 April and September. We have identified 15 active galactic nuclei [including 12 broad-emission-line (FWHM >1000 km s−1) objects, one bona fide BL Lac, one BL Lac candidate and one narrow-line (FWHM < 1000 km s−1) radio galaxy] and five radio galaxies. We derive the X-ray fluxes and luminosities by analysing the PSPC exposures, and show the radio morphology from the NVSS maps. We find that the correlation between the monochromatic X-ray luminosity at 2 keV and the core radio luminosity at 5 GHz for the radio galaxies in our sample follows that found for the 3CR radio galaxies, suggesting a possible nuclear origin for the X-ray emission in these sources. This correlation is weaker in the case of broad-line objects, indicating the presence of another (unbeamed) mechanism for the X-ray emission only weakly related to the radio emission.

The ROSAT Bright Survey: I. Identification of an AGN sample with hard ROSAT X‐ray spectra

AbstractThe ROSAT Bright Survey (RBS) aims to completely optically identify the more than 2000 brightest sources detected in the ROSAT all‐sky survey at galactic latitudes |b| &gt; 30° (excluding LMC, SMC, Virgo cluster). This paper presents a subsample of 66 bright point‐like ROSAT survey sources with almost hard PSPC spectra, the hardness ratio HR1 is &gt; 0.5 for most of the sources. Teh subsample could be nearly completely identified by low‐resolution optical spectroscopy with the following breakdown into object classes: 31 Seyfert galaxies, 22 BL Lac candidates, 5 clusters of galaxies, 1 cataclysmic variable, and 5 bright stars. Only one object remained unidentified and one X‐ray source was a spurious detection. The redshift distrbution peaks around 0.06 for the Seyferts and around 0.13 for the BL Lac candidates. Observations with medium spectral resolution were obtained for most of the new Seyfert galaxies. A large fraction (20 objects) are type 1 Seyfert galaxies, the other fraction includes Seyfert galaxies of type 1.5 – 1.8 (5 objects), two LINERs, and 4 possible narrow‐line Seyfert 1 galaxies (NLS1). About one third of the new Seyfert's have nearby companion galaxies displaying either emission or absorption lines at the same redshift. Among them are a couple of systems showing direct morphological evidence for interaction. The large fraction of interacting galaxies among our sample suggests a scenario where interaction is the main trigger of AGN activity.

The B3-VLA quasar sample

A new low frequency radio selected Sample of 125 Quasars complete down to 100 mJy at 408 MHz is pre- sented in this paper. The sample is a part of the B3-VLA sample: 1050 radiosources selected from the B3 catalogue at 408 MHz and observed at the VLA (1465 MHz, C and A congurations). Out of the 352 sources, identied on the POSS-I down to mr 20:0, 172 are quasar candidates. In this paper we give the nal assessment of the quasar sample from spectroscopic observations of the candidates. The nal complete quasar sample consists of 125 objects. Furthermore 3 Bl Lac objects have been identied and two Bl Lac candidates.

Discovery of X-ray selected AGNs, white dwarf and cataclysmic variable

In order to investigate selection criteria for a ROSAT X-ray selected sky survey of bright nearby quasars, test observations were carried out on the 2.16 m telescope of Beijing Observatory between 1995 April and 1996 October. In the process 73 quasars, 27 Seyferts. 1 BL Lac candidate, 1 white dwarf and 1 cataclysmic variable were discovered.

Interacting Galaxies — the X‐ray View

AbstractWe have selected a complete, flux‐limited sample of bright point‐like sources with absorbed X‐ray spectra from the ROSAT All‐Sky survey, the ROSAT Hard Survey (RHS). The sample is drawn from the high‐galactic latitude sky, |bII| &gt; ±30°, and avoids the general direction of the Magellanic Clouds and the Virgo Cluster. It comprises a total of 182 objects of which 118 were previosly catalogued objects and 64 were new AGN candidates. Through optical follow‐up studies at La Silla, Calar Alto and the 6m Zelentchuk telescope we could identify 56 of the new objects, thus achieving a spectrscopic completeness of 96% of the sample. The selection strategy turned out to be extremely useful. Out of the new identifications, 84% are indeed active galaxies or emission line galaxies. While 14 objects are new BL Lac candidates 33 of the new AGN are Seyfert galaxies with a redshift distribution in the range 0.021–0.63, peaking at around 0.06, i.e. relatively local. Surprisingly, about 40% of the X‐ray selected Seyfert galaxies seem to reside in interacting systems. With smaller selection uncertainties this fraction exceeds the number of paired galaxies in optically selected samples and therefore gives strong support to the idea that AGN activity is triggered by interaction.

Optical spectroscopy and polarization of a new sample of optically bright flat radio spectrum sources

A new sample of bright flat radio spectrum sources selected at 8.4 GHz and consisting of objects brighter than V= 17 is discussed. The sample was selected with three purposes in mind: (i) to find low-luminosity BL Lacertae (BL Lac) objects with radio luminosities comparable to those of BL Lacs selected at X-ray frequencies; (ii) to investigate the differences between BL Lacs and other flat radio spectrum sources; and (iii) to define a sample of nearby radio-loud objects, the host galaxies of which are easy to study. Using information on four observational parameters, radio polarization, optical percentage polarization, break contrast and equivalent width of the strongest emission line, we compare the properties of BL Lacs with those of other types of active galactic nuclei (AGN) found in the sample. We find that most of the objects have weak emission lines although some sources with Seyfert-type spectra were also found. With only a few exceptions, the two types of sources appear well separated in their observational properties. Among the objects studied we report 10 new BL Lacs and BL Lac candidates, and we define a ‘complete’ sample of bright flat radio spectrum sources that consists of those objects with redshift ≤ 0.1.

A Sample-oriented catalogue of BL Lacertae objects

We present a catalogue of 233 BL Lacertae objects compiled through an extensive bibliographic search updated to mid-1995. A large fraction of the sources listed in the catalogue belongs to well-defined samples and can be used for statistical purposes. A smaller fraction consists of miscellaneous (but confirmed) BL Lacs and of objects classified as BL Lac candidates. We discuss the selection criteria of the different samples, report the discovery of two previously unnoticed BL Lacs in the Palomar-Green survey, and comment on the possible association of some of the still unidentified high galactic latitude gamma-ray (EGRET) sources with BL Lacs. Some statistical properties of the catalogue are also briefly discussed.

The light curves for PG CV candidates: AAnS 34/2 (1993)225–232

In order to investigate selection criteria for a ROSAT X-ray selected sky survey of bright nearby quasars, test observations were carried out on the 2.16 m telescope of Beijing Observatory between 1995 April and 1996 October. In the process 73 quasars, 27 Seyferts. 1 BL Lac candidate, 1 white dwarf and 1 cataclysmic variable were discovered.

No evidence for radio-quiet BL Lacertae objects

Using a large, flux-limited sample of faint X-ray sources, a search has been conducted for radio-quiet BL Lacertae objects. None has been found. Thirty-two X-ray-selected BL Lac objects and BL Lac candidates have been found within the sources of the Einstein Medium Sensitivity Survey (EMSS). Thirty-one of these have been observed with the VLA and all have been detected at 5 GHz. While the optical magnitudes of the EMSS BL Lac objects range from 17 to 20.8, their radio-to-optical spectral indices occupy a very small range. The very bright X-ray-selected BL Lac objects like PKS 2155-304 and Markarian 501 have similar range values. Therefore, unlike the clear dichotomy between radio-loud quasars and radio-quiet QSOs, there is no evidence for two populations of Lacertids distinguished by radio loudness.

SPECTROSCOPY OF SIX X-RAY-SELECTED BL LACERTAE CANDIDATES.

ABSTRACT Results of a continuing program aimed at extending the small list of X-ray-selected BL Lac objects are reported. High-quality spectra have been obtained of six faint blue objects that lie within the positional error boxes of X-ray sources discovered serendipitously by the Einstein Observatory. Three of the objects are found to be previously uncataloged low-red-shift quasi-stellar objects, including one formerly suggested as BL Lac candidate. Two are faint galactic stars, while the final object has a featureless spectrum, and thus remains a candidate. Although X-ray selection may ultimately be an effective means of discovering faint BL Lac objects, such sources are evidently rare at the X-ray flux levels attainable by the Einstein Observatory.

An intense soft high-latitude X-ray source H2156-304 - A new BL Lacertae object

The discovery of an intense high-latitude soft X-ray source (designated H2156-304) detected with the low-energy detectors of the HEAO A-2 experiment is reported. The error box of the source includes a 14th mag starlike object that may be coincident with the radio source PKS 2155-304 and has been suggested as a BL Lac candidate. Intensity variations of H2156-304 on time scales of about 1 sec to 1 day are discussed, along with a probable flare from the source. The derived energy spectrum of the source is shown to be well fitted by either a two-temperature thermal model with temperatures of 16 million and 1.6 million K or a single power-law model with a spectral index of 2.4 + or - 0.3. Hydrogen column densities of approximately 2.5 x 10 to the 20th and 2.0 x 10 to the 20th H atoms/sq cm are obtained for the two-temperature and power-law models, respectively. The emission mechanism of H2156-304 is considered, and the characteristics of this source are compared with those of three known soft X-ray-emitting BL Lac objects.