首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 15 毫秒
1.
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.  相似文献   

2.
A unifying view of the spectral energy distributions of blazars   总被引：3，自引：0，他引：3  
We collect data at well-sampled frequencies from the radio to the γ-ray range for the following three complete samples of blazars: the Slew survey, the 1-Jy samples of BL Lacs and the 2-Jy sample of flat-spectrum radio-loud quasars (FSRQs). The fraction of objects detected in γ-rays ( E  ≳ 100 MeV) is ∼ 17, 26 and 40 per cent in the three samples respectively. Except for the Slew survey sample, γ-ray detected sources do not differ either from other sources in each sample, or from all the γ-ray detected sources, in terms of the distributions of redshift, radio and X-ray luminosities or of the broad-band spectral indices (radio to optical and radio to X-ray). We compute average spectral energy distributions (SEDs) from radio to γ-rays for each complete sample and for groups of blazars binned according to radio luminosity, irrespective of the original classification as BL Lac or FSRQ. The resulting SEDs show a remarkable continuity in that (i) the first peak occurs in different frequency ranges for different samples/luminosity classes, with most luminous sources peaking at lower frequencies; (ii) the peak frequency of the γ-ray component correlates with the peak frequency of the lower energy one; (iii) the luminosity ratio between the high and low frequency components increases with bolometric luminosity. The continuity of properties among different classes of sources and the systematic trends of the SEDs as a function of luminosity favour a unified view of the blazar phenomenon: a single parameter, related to luminosity, seems to govern the physical properties and radiation mechanisms in the relativistic jets present in BL Lac objects as well as in FSRQs. The general implications of this unified scheme are discussed while a detailed theoretical analysis, based on fitting continuum models to the individual spectra of most γ-ray blazars, is presented in a separate paper.  相似文献   

3.
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.  相似文献   

4.
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.  相似文献   

5.
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.  相似文献   

6.
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.  相似文献   

7.
Although radio-quiet quasars (RQQs) constitute ≳ 90 per cent of optically identified quasar samples, their radio properties are only poorly understood. In this paper we present the results of a multi-frequency VLA study of 27 low-redshift RQQs. We detect radio emission from 20 objects, half of which are unresolved (≤ 0.24 arcsec). In cases where significant structure can be resolved, double, triple and linear radio sources on scales of a few kpc are found. The radio emission (typically) has a steep spectrum (α ∼ 0.7, where S  ∝ ν−α), and high brightness temperatures ( T B ≥ 105 K) are measured in some of the radio components. The RQQs form a natural extension to the radio luminosity–absolute magnitude distribution of nearby Seyfert 1s. We conclude that a significant fraction of the radio emission in RQQs originates in a compact nuclear source directly associated with the quasar. There are no significant differences between the radio properties of RQQs with elliptical hosts and those in disc galaxies within the current sample.  相似文献   

8.
We present spectroscopic observations of a sample of faint gigahertz peaked‐spectrum (GPS) radio sources drawn from the Westerbork Northern Sky Survey (WENSS). Redshifts have been determined for 19 (40 per cent) of the objects. The optical spectra of the GPS sources identified with low‐redshift galaxies show deep stellar absorption features. This confirms previous suggestions that their optical light is not significantly contaminated by active galactic nucleus-related emission, but is dominated by a population of old (>9 Gyr) and metal-rich (>0.2 [Fe/H]) stars, justifying the use of these (probably) young radio sources as probes of galaxy evolution. The optical spectra of GPS sources identified with quasars are indistinguishable from those of flat-spectrum quasars, and clearly different from the spectra of compact steep‐spectrum (CSS) quasars. The redshift distribution of the GPS quasars in our radio-faint sample is comparable to that of the bright samples presented in the literature, peaking at z ∼2–3. It is unlikely that a significant population of low-redshift GPS quasars is missed as a result of selection effects in our sample. We therefore claim that there is a genuine difference between the redshift distributions of GPS galaxies and quasars, which, because it is present in both the radio-faint and bright samples, cannot be caused by a redshift–luminosity degeneracy. It is therefore unlikely that the GPS quasars and galaxies are unified by orientation, unless the quasar opening angle is a strong function of redshift. We suggest that the GPS quasars and galaxies are unrelated populations and just happen to have identical observed radio spectral properties, and hypothesize that GPS quasars are a subclass of flat-spectrum quasars.  相似文献   

9.
We present and discuss the results of very-long baseline interferometry (VLBI, EVN) observations of three low-luminosity     broad emission line active galactic nuclei (AGNs) carefully selected from a sample of flat-spectrum radio sources (CLASS). Based on the total and the extended radio power at 5 and at 1.4 GHz respectively, these objects should be technically classified as radio-quiet AGN and thus the origin of their radio emission is not clearly understood. The VLBI observations presented in this paper have revealed compact radio cores which imply a lower limit on the brightness temperature of about     . This result rules out a thermal origin for the radio emission and strongly suggests an emission mechanism similar to that observed in more powerful radio-loud AGNs. Since, by definition, the three objects show a flat (or inverted) radio spectrum between 1.4 and 8.4 GHz, the observed radio emission could be relativistically beamed. Multi-epoch VLBI observations can confirm this possibility in two years' time.  相似文献   

10.
We examine the optical emission-line properties of brightest cluster galaxies (BCGs) selected from two large, homogeneous data sets. The first is the X-ray selected National Optical Astronomy Observatory Fundamental Plane Survey (NFPS), and the second is the C4 catalogue of optically selected clusters built from the Sloan Digital Sky Survey Data Release 3 (SDSS DR3). Our goal is to better understand the optical line emission in BCGs with respect to properties of the galaxy and the host cluster. Throughout the analysis we compare the line emission of the BCGs to that of a control sample made of the other bright galaxies near the cluster centre. Overall, both the NFPS and SDSS show a modest fraction of BCGs with emission lines (∼15 per cent). No trend in the fraction of emitting BCGs as a function of galaxy mass or cluster velocity dispersion is found. However, we find that, for those BCGs found in cooling flow clusters,  71+9−14  have optical emission. Furthermore, if we consider only BCGs within 50 kpc of the X-ray centre of a cooling flow cluster, the emission-line fraction rises further to  100+0−15  per cent. Excluding the cooling flow clusters, only ∼10 per cent of BCGs are line emitting, comparable to the control sample of galaxies. We show that the physical origin of the emission-line activity varies: in some cases it has LINER-like line ratios, whereas in others it is a composite of star-formation and LINER-like activity. We conclude that the presence of emission lines in BCGs is directly related to the cooling of X-ray gas at the cluster centre.  相似文献   

11.
We present a calculation of the blazar contribution to the extragalactic diffuse γ -ray background (EGRB) in the EGRET energy range. Our model is based on inverse-Compton scattering as the dominant γ -ray production process in the jets of flat spectrum radio quasars (FSRQs) and BL Lac objects, and on the unification scheme of radio-loud AGN. According to this picture, blazars represent the beamed fraction of the Fanaroff–Riley radio galaxies (FR galaxies).
The observed log  N –log  S distribution and redshift distribution of both FSRQs and BL Lacs constrain our model. Depending slightly on the evolutionary behaviour of blazars, we find that unresolved AGN underproduce the intensity of the extragalactic background radiation. With our model only 20–40 per cent of the extragalactic background emission can be explained by unresolved blazars if we integrate to a maximum redshift of Z max=3. For Z max=5, blazars could account for 40–80 per cent of the EGRB. Roughly 70–90 per cent of the AGN contribution to the EGRB would result from BL Lacs. While the systematic uncertainties in our estimate for the FSRQ contribution appear small, in the case of BL Lacs our model parameters are not consistent with the results from studies in other wavelength regimes, and therefore may have larger systematic uncertainties. Thus we end up with two possibilities, depending on whether we underpredict or overpredict the BL Lac contribution: either unresolved AGN cannot account for the entire EGRB, or unresolved BL Lacs produce the observed background.
We predict a significant flattening of the γ -ray log  N –log  S function in the next two decades of flux below the EGRET threshold.  相似文献   

12.
We present results from a 3-yr study of the 15-GHz variability of 51 9C sources. 48 of these sources make up a subsample of a larger one complete to 25 mJy in 9C, and as the sources are selected pseudo-randomly the results should be representative of the complete sample. 29 per cent of this subsample are found to be variable above the flux calibration uncertainties of ∼6 per cent. 50 per cent of the flat-spectrum objects are variable whilst none of the steep-spectrum objects or the objects with convex spectra peaking below 5 GHz are variable. Nine of the objects studied have convex spectra and peak frequencies above 5 GHz; eight of these were found to vary at 15 GHz, suggesting that the high-frequency peaking class in this sample is largely populated by objects with jets aligned close to the line of sight whose emission is dominated by beamed components.  相似文献   

13.
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.  相似文献   

14.
Intermediate BL Lac objects   总被引：1，自引：0，他引：1  
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.  相似文献   

15.
We present JHKL ' photometry of a complete sample of steep-spectrum radio-loud quasars from the revised 3CR catalogue in the redshift range 0.65 z <1.20. After correcting for contributions from emission lines and the host galaxies, we investigate their spectral energy distributions (SEDs) around 1 μm. About 75 per cent of the quasars are tightly grouped in the plane of optical spectral index, α opt, versus near-infrared spectral index, α IR, with the median value of α opt close to the canonical value, and the median α IR slightly flatter. We conclude that the fraction of moderately obscured, red quasars decreases with increasing radio power, in accordance with the 'receding torus' model which can also explain the relatively flat median near-infrared spectra of the 3CR quasars. Two of the red quasars have inverted infrared spectral indices, and we suggest that their unusual SEDs might result from a combination of dust-scattered and transmitted quasar light.  相似文献   

16.
We present new spectroscopic observations of the southern radio galaxy         . Despite the flat-spectrum character of the radio emission from this source, our optical spectra show no sign of the broad permitted lines and non-stellar continuum characteristic of quasar nuclei and broad-line radio galaxies. However, the high-ionization forbidden lines, including [O  iii ] λλ 5007, 4959, are unusually broad for a narrow-line radio galaxy     , and are blueshifted by 600 km s−1 relative to the low-ionization lines such as [O  ii ] λλ 3726,3729. The [O  ii ] lines are also considerably narrower     than the [O  iii ] lines, and have a redshift consistent with that of the recently detected H  i 21-cm absorption-line system. Whereas the kinematics of the [O  iii ] emission lines are consistent with outflow in an inner narrow-line region, the properties of the [O  ii ] emission lines suggest that they are emitted by a more extended and quiescent gaseous component. We argue that, given the radio properties of the source, our line of sight is likely to be lying close to the direction of bulk outflow of the radio jets. In this case it is probable that the quasar nucleus is entirely obscured at optical wavelengths by the material responsible for the H  i absorption-line system. The unusually broad [O  iii ] emission lines suggest that the radio source is intrinsically compact. Overall, our data are consistent the idea that     is a radio source in an early stage of evolution.  相似文献   

17.
We study the 37 brightest radio sources in the Subaru/ XMM–Newton Deep Field. We have spectroscopic redshifts for 24 of 37 objects and photometric redshifts for the remainder, yielding a median redshift z med for the whole sample of   z med≃ 1.1  and a median radio luminosity close to the 'Fanaroff–Riley type I/type II (FR I/FR II)' luminosity divide. Using mid-infrared (mid-IR) ( Spitzer MIPS 24 μm) data we expect to trace nuclear accretion activity, even if it is obscured at optical wavelengths, unless the obscuring column is extreme. Our results suggest that above the FR I/FR II radio luminosity break most of the radio sources are associated with objects that have excess mid-IR emission, only some of which are broad-line objects, although there is one clear low-accretion-rate object with an FR I radio structure. For extended steep-spectrum radio sources, the fraction of objects with mid-IR excess drops dramatically below the FR I/FR II luminosity break, although there exists at least one high-accretion-rate 'radio-quiet' QSO. We have therefore shown that the strong link between radio luminosity (or radio structure) and accretion properties, well known at z ∼ 0.1, persists to z ∼ 1. Investigation of mid-IR and blue excesses shows that they are correlated as predicted by a model in which, when significant accretion exists, a torus of dust absorbs ∼30 per cent of the light, and the dust above and below the torus scatters ≳1 per cent of the light.  相似文献   

18.
The results of very long baseline interferometry (VLBI) total intensity ( I ) and linear polarization ( P ) observations at     are presented for 10 radio bright BL Lacertae objects. These images complete first-epoch polarization observations for the 1-Jy sample of northern BL Lacertae objects defined by Kühr & Schmidt. Estimates of superluminal speeds are presented for several sources, bringing the total number of sources in the sample for which such estimates are available to 16. Second-epoch observations currently being reduced should yield speed estimates for VLBI features in essentially all the sources in the sample. The jet magnetic fields of these BL Lacertae objects are usually transverse to the local jet direction, but a sizeable minority (about 30 per cent) have VLBI jet components with longitudinal magnetic fields. This may suggest that the conditions in the VLBI jets of BL Lacertae objects are favourable for the formation of relativistic shocks; alternatively, it may be that the toroidal component of the intrinsic jet magnetic field is characteristically dominant in these sources.  相似文献   

19.
Little is known about the statistics of gravitationally lensed quasars at large (7–30 arcsec) image separations, which probe masses on the scale of galaxy clusters. We have carried out a survey for gravitationally lensed objects, among sources in the FIRST 20-cm radio survey that have unresolved optical counterparts in the digitizations of the Palomar Observatory Sky Survey. From the statistics of ongoing surveys that search for quasars among FIRST sources, we estimate that there are about 9100 quasars in this source sample, making this one of the largest lensing surveys to date. Using broad-band imaging, we have isolated all objects with double radio components separated by 5–30 arcsec that have unresolved optical counterparts with similar BVI colours. Our criteria for similar colours conservatively allow for observational error and for colour variations due to time delays between lensed images. Spectroscopy of these candidates shows that none of the pairs are lensed quasars. This sets an upper limit (95 per cent confidence) on the lensing fraction in this survey of 3.3×10−4, assuming 9100 quasars. Although the source redshift distribution is poorly known, a rough calculation of the expected lensing frequency and the detection efficiencies and biases suggests that simple theoretical expectations are of the same order of magnitude as our observational upper limit. Our procedure is novel in that our exhaustive search for lensed objects does not require prior identification of the quasars in the sample as such. Characterization of the FIRST-selected quasar population will enable use of our result to constrain quantitatively the mass properties of clusters.  相似文献   

20.
We report on the first SCUBA detection of a type 2 QSO at   z = 3.660  in the Chandra Deep Field South. This source is X-ray-absorbed, shows only narrow emission lines in the optical spectrum and is detected in the submillimetre: it is the ideal candidate in an evolution scheme for active galactic nuclei (AGN) (e.g. Fabian 1999 ; Page et al. 2004 ) of an early phase corresponding to the main growth of the host galaxy and formation of the central black hole. The overall photometry (from the radio to the X-ray energy band) of this source is well reproduced by the spectral energy distribution (SED) of NGC 6240, while it is incompatible with the spectrum of a type 1 QSO (3C 273) or a starburst galaxy (Arp 220). Its submillimetre (850 μm) to X-ray (2 keV) spectral slope  (αSX)  is close to the predicted value for a Compton-thick AGN in which only 1 per cent of the nuclear emission emerges through scattering. Using the observed flux at 850 μm we have derived a star formation rate of  550–680 M⊙ yr−1  and an estimate of the dust mass   M dust= 4.2 × 108 M⊙  .  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号