Interacting Galaxies — the X-ray View

We 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, |b^II| > ±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.     

A survey of hard spectrum ROSAT sources – II. Optical identification of hard sources

We have surveyed 188 ROSAT Position Sensitive Proportional Counter (PSPC) fields for X-ray sources with hard spectra ( α <0.5); such sources must be major contributors to the X-ray background at faint fluxes. In this paper we present optical identifications for 62 of these sources: 28 active galactic nuclei (AGN) which show broad lines in their optical spectra (BLAGN), 13 narrow emission line galaxies (NELGs), five galaxies with no visible emission lines, eight clusters and eight Galactic stars.
The BLAGN, NELGs and galaxies have similar distributions of X-ray flux and spectra. Their ROSAT spectra are consistent with their being AGN obscured by columns of 20.5< log( N _H/cm⁻²)<23 . The hard spectrum BLAGN have a distribution of X-ray to optical ratios which is similar to that found for AGN from soft X-ray surveys (1< α _OX<2) . However, a relatively large proportion (15 per cent) of the BLAGN, NELGs and galaxies are radio loud. This could be because the radio jets in these objects produce intrinsically hard X-ray emission, or if their hardness is caused by absorption, it could be because radio-loud objects are more X-ray luminous than radio-quiet objects. The eight hard sources identified as clusters of galaxies are the brightest, and softest group of sources and hence clusters are unlikely to be an important component of the hard, faint population.
We propose that BLAGN are likely to constitute a significant fraction of the faint, hard, 0.5–2 keV population and could be important to reproducing the shape of the X-ray background, because they are the most numerous type of object in our sample (comprising almost half the identified sources), and because all our high redshift ( z >1) identified hard sources have broad lines.     

ROSAT public PSPC observations in the Las Campanas Redshift Survey

The Las Campanas Redshift Survey, an optically selected survey which contains 26 418 galaxy redshifts, has been correlated with “The First ROSAT Source Catalogue of Pointed Observations with the PSPC,” which contains 50 408 sources from 2876 ROSAT pointed observations. Ten matches were found. The optical spectra of most of the ten matches show weak narrow emission lines. Due to their high x-ray luminosities, their high x-ray-to-optical flux ratios, and the evidence of rapid x-ray variability in the two brightest matches, we interpret the majority of these objects to be narrow-line Seyfert galaxies or “hidden” active galactic nuclei. Of the ten matches, only one galaxy shows the characteristics of a bona fide starburst.     

ROSAT results on narrow-line Seyfert 1 galaxies

The excellent soft X-ray sensitivity of the PSPC detector onboard the ROSAT satellite provided the first chance to study precisely the spectral and timing properties of Narrow-Line Seyfert 1 galaxies. ROSAT observations of Narrow-Line Seyfert 1 galaxies have revealed (1) the existence of a giant soft X-ray excess, (2) a striking, clear correlation between the strength of the soft X-ray excess emission and the FWHM of the Hβ line, (3) the general absence of significant soft X-ray absorption by neutral hydrogen above the Galactic column, (4) short doubling time scales down to about 1000 s, (5) the existence of persistent giant (above a factor of 10), and rapid (less than 1 day) X-ray variability in extragalactic sources. The soft X-ray results on Narrow-Line Seyfert 1 galaxies indicate that their black hole regions are directly visible, further supporting the Seyfert 1 nature of these objects. The extreme X-ray properties of Narrow-Line Seyfert 1 galaxies make them ideal objects for understanding many of the problems raised generally by the Seyfert phenomenon.     

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.     

Discovery of Five Narrow-Line Seyfert 1 Galaxies and Im- plications on the NLS1 Model

We report the discovery of five Narrow-Line Seyfert 1 galaxies (NLSls) identified from the ROSAT All-Sky Survey bright sources. One of them has a quasar-like luminosity and two, including the quasar-like one, have close companions and/or show interacting features. We calculate the central black hole masses and Eddington ratios for the five NLSls. In combination with the objects of Kaspi et al., we find that NLSls have smaller central black hole masses and higher accretion rate than normal Seyfert 1s.     

Discovery of Five Narrow－Line Seyfert 1 Galaxies and Implications on the NLS1 Model

We report the discovery of five Narrow-Line Seyfert 1 galaxies (NLSls) identified from the ROSAT All-Sky Survey bright sources. One of them has a quasarlike luminosity and two, including the quasar-like one, have close companions and/or show interacting features. We calculate the central black hole masses and Eddington ratios for the five NLSls. In combination with the objects of Kaspi et al., we find that NLSls have smaller central black hole masses and higher accretion rate than normal Seyfert ls.     

Optical spectra and redshifts of radio sources from the Zelenchuk survey

We performed spectroscopic observations of 22 radio sources from the Zelenchuk survey (Sternberg Astronomical Institute) using the 6-m and 1-m Special Astrophysical Observatory telescopes. For 18 objects, we determined the redshifts. Ten, seven, and one of these objects were identified with quasars, elliptical galaxies, and a Seyfert galaxy, respectively. Four radio sources have a continuum spectrum, and three of them are BL Lac objects. We failed to classify one object.     

The ROSAT Brightest Cluster Sample – IV. The extended sample

We present a low-flux extension of the X-ray-selected ROSAT Brightest Cluster Sample (BCS) published in Paper I of this series. Like the original BCS and employing an identical selection procedure, the BCS extension is compiled from ROSAT All-Sky Survey (RASS) data in the northern hemisphere ( δ ≥0°) and at high Galactic latitudes (| b |≥20°). It comprises 99 X-ray-selected clusters of galaxies with measured redshifts z ≤0.3 (as well as eight more at z >0.3) and total fluxes between 2.8×10⁻¹² and 4.4×10⁻¹² erg cm⁻² s⁻¹ in the 0.1–2.4 keV band (the latter value being the flux limit of the original BCS). The extension can be combined with the main sample published in 1998 to form the homogeneously selected extended BCS (eBCS), the largest and statistically best understood cluster sample to emerge from the RASS to date.
The nominal completeness of the combined sample (defined with respect to a power-law fit to the bright end of the BCS log  N –log  S distribution) is relatively low at 75 per cent (compared with 90 per cent for the high-flux sample of Paper I). However, just as for the original BCS, this incompleteness can be accurately quantified, and thus statistically corrected for, as a function of X-ray luminosity and redshift.
In addition to its importance for improved statistical studies of the properties of clusters in the local Universe, the low-flux extension of the BCS is also intended to serve as a finding list for X-ray-bright clusters in the northern hemisphere which we hope will prove useful in the preparation of cluster observations with the next generation of X-ray telescopes such as Chandra and XMM-Newton .
An electronic version of the eBCS can be obtained from the following URL: http://www.ifa.hawaii.edu/~ebeling/clusters/BCS.html.     

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.     

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.     

Optical and near-infrared imaging of faint Gigahertz Peaked Spectrum sources

A sample of 47 faint Gigahertz Peaked Spectrum (GPS) radio sources selected from the Westerbork Northern Sky Survey (WENSS) has been imaged in the optical and near-infrared, resulting in an identification fraction of 87 per cent. The R  −  I R  −  K colours of the faint optical counterparts are as expected for passively evolving elliptical galaxies, assuming that they follow the R -band Hubble diagram as determined for radio-bright GPS galaxies. We find evidence that the radio spectral properties of the GPS quasars are different from those of GPS galaxies. The observed distribution of radio spectral peak frequencies for GPS sources optically identified with bright stellar objects (presumably quasars) is shifted compared with GPS sources identified with faint or extended optical objects (presumably galaxies), in the sense that a GPS quasar is likely to have a higher peak frequency than a GPS galaxy. This means that the true peak frequency distribution is different for the GPS galaxies and quasars, because the sample selection effects are independent of optical identification. The correlation between peak frequency and redshift that has been suggested for bright sources has not been found in this sample; no correlation exists between R magnitude (and therefore redshift) and peak frequency for the GPS galaxies. We therefore believe that the claimed correlation is actually caused by the dependence of the peak frequency on optical host, because the GPS galaxies are generally at lower redshifts than the quasars. The difference in the peak frequency distributions of the GPS galaxies and quasars is further evidence against the hypothesis that they form a single class of object.     

活动星系核的演化与宇宙学红移

通过搜集了457个活动星系核样本,根据活动星系核的演化实质是指宇宙时标上的变化,讨论了红移量与活动星系核演化的关系,最终证明了活动星系核的演化分为两个序列:(1)从类星体到Seyfert星系之间的演化;(2)平谱射电类星体(FSRQ)—BL Lac天体—射电星系(RG)的演化。     

The Deep X-Ray Radio Blazar Survey (DXRBS) – II. New identifications

We have searched the archived, pointed ROSAT Position Sensitive Proportional Counter data for blazars by correlating the WGACAT X-ray data base with several publicly available radio catalogues, restricting our candidate list to serendipitous X-ray sources with a flat radio spectrum ( α _r≤0.70, where S _ν ∝ ν ^{− α} ). This makes up the Deep X-ray Radio Blazar Survey (DXRBS). Here we present new identifications and spectra for 106 sources, including 86 radio-loud quasars, 11 BL Lacertae objects, and nine narrow-line radio galaxies. Together with our previously published objects and already-known sources, our sample now contains 298 identified objects: 234 radio-loud quasars [181 flat-spectrum quasars: FSRQ ( α _r≤0.50) and 53 steep-spectrum quasars: SSRQ], 36 BL Lacs and 28 narrow-line radio galaxies. Redshift information is available for 96 per cent of these. Thus our selection technique is ∼90 per cent efficient at finding radio-loud quasars and BL Lacs. Reaching 5-GHz radio fluxes ∼50 mJy and 0.1–2.0 keV X-ray fluxes a few ×10⁻¹⁴ erg cm⁻² s⁻¹, DXRBS is the faintest and largest flat-spectrum radio sample with nearly complete (∼85 per cent) identification. We review the properties of the DXRBS blazar sample, including redshift distribution and coverage of the X-ray-radio–power plane for quasars and BL Lacs. Additionally, we touch upon the expanded multiwavelength view of blazars provided by DXRBS. By sampling for the first time the faint end of the radio and X-ray luminosity functions, this sample will allow us to investigate the blazar phenomenon and the validity of unified schemes down to relatively low powers.     

新发现的X射线选活动星系核、白矮星和激变变星

为了研究ROSATX射线选亮近类星体巡天的选择判据 ,从 1 995年 4月至 1 996年 1 0月期间 ,利用北京天文台 2 .1 6米望远镜进行了试验观测 ,发现了 73个类星体、2 7个赛弗特星系、1个BLLac候选体、1个白矮星和 1个激变变星     

Host galaxies and the unification of radio-loud AGN

Our HST WFPC2 survey of 110 BL Lac objects, from six complete X-ray-, radio-, and optically-selected catalogs, probes the host galaxies of low-luminosity radio sources in the redshift range 0<z<1.35. The host galaxies are luminous ellipticals, well matched in radio power and galaxy magnitude to FR I radio galaxies. Similarly, the host galaxies of high luminosity quasars occupy the same region of this plane as FR II radio galaxies (matched in redshift). This strongly supports the unification of radio-loud AGN, and suggests that studying blazars at high redshift is a proxy for investigating less luminous (to us) but intrinsically identical radio galaxies, which are harder to find at high z. Accordingly, the difference between low-power jets in BL Lac objects and high-power jets in quasars can then be related to the FR I/FR II dichotomy; and the evolution of blazar host galaxies or their nuclei (jets) should correspond to the evolution of radio galaxies.     

The redshift distribution of FIRST radio sources at 1 mJy

We present spectra for a sample of radio sources from the FIRST survey, and use them to define the form of the redshift distribution of radio sources at mJy levels. We targeted 365 sources and obtained 46 redshifts (13 per cent of the sample). We find that our sample is complete in redshift measurement to R ∼18.6, corresponding to z ∼0.2. Galaxies were assigned spectral types based on emission-line strengths. Early-type galaxies represent the largest subset (45 per cent) of the sample and have redshifts 0.15≲ z ≲0.5; late-type galaxies make up 15 per cent of the sample and have redshifts 0.05≲ z ≲0.2; starbursting galaxies are a small fraction (∼6 per cent), and are very nearby ( z ≲0.05). Some 9 per cent of the population have Seyfert 1/quasar-type spectra, all at z ≳0.8, and 4 per cent are Seyfert 2 type galaxies at intermediate redshifts ( z ∼0.2).
Using our measurements and data from the Phoenix survey (Hopkins et al.), we obtain an estimate for N ( z ) at S _1.4 GHz≥1 mJy and compare this with model predictions. At variance with previous conclusions, we find that the population of starbursting objects makes up ≲5 per cent of the radio population at S ∼1 mJy.     

Re-Identification of the ''''Enigmatic'''' X-ray Source 1RXS J114003.0+124112

The ROSAT X-ray source 1RXS J114003.0 124112 was identified as a starburst galaxy at redshift 0.177 by He et al. The authors also noted that the source is almost two orders of magnitude brighter in X-ray than the X-ray-brightest starburst galaxy and it seems to be in a merging system, making this source an enigmatic system demanding further observations. Here we report a re-identification of 1RXS J114003.0 124112 using observations on the 2.6m telescope at Byurakan Astrophysical Observatory, Armenia and the SDSS data. The results indicate that the starburst activity is associated with the brighter object of the system, while the fainter object is a typical Seyfert 1 galaxy at a different redshift (0.282). Therefore, the two objects are not in a merging system, and the Seyfert 1 galaxy naturally accounts for the high X-ray flux. Three more objects reside in the vicinity, but they are all too faint to be responsible for the high X-ray flux.     

Variations in the broad-band spectra of BL Lac objects: millimetre observations of an X-ray-selected sample

Observations at millimetre wavelengths are presented for a representative sample of 22 X-ray-selected BL Lac objects (XBLs). This sample comprises 19 high-energy cut-off BL Lac objects (HBLs), 1 low-energy cut-off BL Lac object (LBL) and 2 'intermediate' sources. Data for LBLs, which are mostly radio-selected BL Lac objects (RBLs), are taken from the literature. It is shown that the radio–millimetre spectral indices of HBLs     are slightly steeper than those of the LBLs     . A correlation exists between α _5–230 and 230 GHz luminosity. While this correlation could be an artefact of comparing two populations of BL Lac objects with intrinsically different radio properties, it is also consistent with the predictions of existing unified schemes that relate BL Lac objects to Fanaroff–Riley class I radio galaxies.
The HBLs have significantly flatter submillimetre–X-ray spectral indices     than the LBLs     although the two intermediate sources also have intermediate values of α _230–X∼−0.9. It is argued that this difference cannot be explained entirely by the viewing-angle hypothesis and requires a difference in physical-source parameters. The α _230–X values for the HBLs are close to the canonical value found for large samples of radio sources and thus suggest that synchrotron radiation is the mechanism that produces the X-ray emission. As suggested by Padovani & Giommi, the inverse-Compton mechanism is likely to dominate in the LBLs requiring the synchrotron spectra of these sources to steepen or cut off at lower frequencies than those of the HBLs.     

BeppoSAX observations of 1-Jy BL Lacertae objects – I

We present new BeppoSAX observations of seven BL Lacertae objects selected from the 1-Jy sample plus one additional source. The collected data cover the energy range     (observer's frame), reaching ∼50 keV for one source (BL Lac). All sources characterized by a peak in their multifrequency spectra at infrared/optical energies (i.e., of the low-energy peaked BL Lac type, LBL) display a relatively flat     X-ray spectrum, which we interpret as inverse Compton emission. Four objects (two-thirds of the LBLs) show some evidence for a low-energy steepening, which is probably due to the synchrotron tail merging into the inverse Compton component around ∼     . If this were generally the case with LBLs, it would explain why the     ROSAT spectra of our sources are systematically steeper than the BeppoSAX ones     . The broad-band spectral energy distributions fully confirm this picture, and a synchrotron inverse Compton model allows us to derive the physical parameters (intrinsic power, magnetic field, etc.) of our sources. Combining our results with those obtained by BeppoSAX on BL Lacs covering a wide range of synchrotron peak frequency, ν _peak, we confirm and clarify the dependence of the X-ray spectral index on ν _peak originally found in ROSAT data.