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1.
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   

2.
Global VLBI images in intensity and linear polarization at     of the BL Lacertae objects 0235+164, 0300+470, 0735+178, 0954+658, 1803+784, 1823+568, and BL Lac and OVV quasar 3C 279 are presented and analysed. These are the highest resolution polarization images currently available for several of these sources. On the whole, the tendencies observed at longer centimetre wavelengths are also exhibited in these 1.3-cm images. When core polarization is detected, the corresponding polarization angles χ tend to lie either parallel or perpendicular to the direction of the inner jet. The core degrees of polarization are usually comparable to those typical at 6 cm, but somewhat lower in several cases, suggesting that the higher resolution data are better separating the contributions of intrinsically weakly polarized cores and highly polarized emerging knots. The observed χ vectors in the jets of 0735+178, 0954+658, 1823+568, BL Lac and 3C 279 are aligned with the jet, implying the presence of transverse magnetic fields. In 0300+470 and BL Lac, there are regions where the direction of χ seems to be neither parallel nor perpendicular to the jet; it is not clear whether this is due to genuine obliquity of the magnetic field structure or to inadequate knowledge of the local flow direction.  相似文献   

3.
We investigate the relationship between the optical and radio emission of active galactic nuclei (AGN) by analysing optical and 15+22+43 GHz Very Long Baseline Array (VLBA) polarization observations simultaneous to within a day for 11 BL Lacertae (BL Lac) objects and the blazar 3C279. We have determined and corrected for the Faraday rotation measures in the very long baseline interferometry (VLBI) cores, enabling us to compare the intrinsic (zero-wavelength) VLBI-core polarization angles and the optical polarization angles χopt. A clear alignment between these two angles emerges in the transition toward higher radio frequencies, and a prominent peak at 0° is visible in the distribution of |χopt−χ43 GHz|. This correlation implies that the magnetic-field orientations in the regions giving rise to the optical and radio polarization are the same, and can be easily understood if the radio and optical polarization are roughly cospatial. It is difficult to rule out the possibility that they arise in different regions in a straight jet with a uniform magnetic-field structure, but this seems less likely, since the VLBI jets of AGN are often bent on parsec-scales. This may suggest that much or all of the strong optical polarization in these sources arises in the inner radio jets, possibly associated with the formation and emergence of compact new VLBI components.  相似文献   

4.
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   

5.
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6.
VLBI total intensity and linear polarization images of the BL Lacertae object 0735+178 have been obtained at     using the     VLA as a ground array together with the HALCA orbiting antenna, and at     about a month later using the VLBA. The most surprising result to come from a comparison of these two images, which have nearly equivalent resolution, is that the path of the jet appears to be appreciably different at the two wavelengths. We have interpreted this as evidence for free–free absorption to the east of the core, near a bend of about 90° in the 2-cm image where the emission is very weak in the 6-cm image. Overall, the polarization structures seen in corresponding regions at the two wavelengths are in good agreement. We find tentative evidence for enhanced rotation measures near two sharp bends in the VLBI jet.  相似文献   

7.
We present the results of simultaneous VLBA polarization observations of the BL Lacertae object 0820+225 at 5, 8 and 15 GHz, together with earlier images at 5 GHz. This source has an unusually rich total intensity and polarization structure compared with other objects with comparable redshifts. The magnetic field in the inner part of the complex and highly twisted VLBI jet is transverse, as is typical of BL Lacertae objects, but becomes roughly longitudinal further from the core, possibly as a result of shear. Although the integrated rotation measure of 0820+225 is modest, the rotation-measure distribution on parsec scales is non-uniform, and clearly shows regions where the rotation measure is substantially higher than the integrated value.  相似文献   

8.
Total intensity and polarization λ =6 cm Very Large Array (VLA) and global very long baseline interferometry (VLBI) images of the quasar 0917+624 and the BL Lacertae object 0954+658 (both at epoch 1991.43) are analysed. Integrated measurements using the VLA during the VLBI observations indicated that, although there were no substantial total intensity variations, there were significant polarization variations for both sources during the 24-h VLBI experiment. The VLBI data were divided into 2–3 h segments in order to try to identify corresponding rapid variability in the VLBI structure. This analysis revealed intraday variability (IDV) in the VLBI core of 0917+624: both the polarized flux and the polarization position angle varied substantially on time-scales of ∼5–10 h. There is evidence that the VLA polarization variations for 0954+658 occurred in an inner VLBI jet component, where the polarized flux varied by ∼30–40 per cent on time-scales of ∼2 h. 0917+624 and 0954+658 were observed together with 0716+714, an object that also displayed IDV in the polarized flux density measured during our experiment (analysed in a separate paper). These three sources were targeted for the VLBI observations since they had been previously identified as intraday variables, but we had no way of knowing whether they would vary during our observations. The fact that all three exhibited IDV in polarization (but not in total intensity) during our experiment suggests that polarization IDV occurs frequently in at least some IDV sources.  相似文献   

9.
We investigate the distribution of mass M and orbital period P of extrasolar planets, taking account of selection effects caused by the limited velocity precision and duration of existing surveys. We fit the data on 72 planets to a power-law distribution of the form  d n = CM −α P −β(d M / M )(d P / P )  , and find  α= 0.11 ± 0.10  ,  β=−0.27 ± 0.06  for   M ≲ 10 M J  , where   M J  is the mass of Jupiter. The correlation coefficient between these two exponents is −0.31, indicating that uncertainties in the two distributions are coupled. We estimate that 4 per cent of solar-type stars have companions in the range  1 M J < M < 10 M J  ,  2d < P < 10 yr  .  相似文献   

10.
We present Chandra and XMM–Newton observations of 12 bright  [ f (2–10 keV) > 10−13 erg cm−2 s−1]  sources from the ASCA search for the High Energy Extragalactic Population (SHEEP) survey. Most of these have been either not observed or not detected previously with the ROSAT mission, and therefore they constitute a sample biased towards hard sources. The Chandra observations are important in locating the optical counterpart of the X-ray sources with accuracy. Optical spectroscopic observations show that our sample is associated with both narrow-line (NL) (six objects) and broad-line (BL) active galactic nuclei (AGN) (five objects), with one source remaining unidentified. Our sources cover the redshift range 0.04–1.29, spanning luminosities from 1042 to  1045 erg s−1  (2–10 keV). The NL sources have preferentially lower redshift (and luminosity) compared to the BL ones. This can be most easily explained in a model where the NL AGN are intrinsically less luminous than the BL ones in line with the results of Steffen et al. The X-ray spectral fittings show a roughly equal number of obscured  ( N H > 1022 cm−2)  and unobscured  ( N H < 1022 cm−2)  sources. There is a clear tendency for obscured sources to be associated with NL AGN and unobscured sources with BL ones. However, there is a marked exception with the highest obscuring column observed at a BL AGN at a redshift of z = 0.5.  相似文献   

11.
We present a catalogue of 147 serendipitous X-ray sources selected to have hard spectra ( α <0.5) from a survey of 188 ROSAT fields. Such sources must be the dominant contributors to the X-ray background at faint fluxes. We have used Monte Carlo simulations to verify that our technique is very efficient at selecting hard sources: the survey has 10 times as much effective area for hard sources as it has for soft sources above a 0.5–2 keV flux level of 10−14 erg cm−2 s−1. The distribution of best-fitting spectral slopes of the hard sources suggests that a typical ROSAT hard source in our survey has a spectral slope α ∼0. The hard sources have a steep number flux relation (d N /d S ∝ S − γ with a best-fitting value of γ =2.72±0.12) and make up about 15 per cent of all 0.5–2 keV sources with S >10−14 erg cm−2 s−1. If their N ( S ) continues to fainter fluxes, the hard sources will comprise ∼40 per cent of sources with 5×10−15< S <10−14 erg cm−2 s−1. The population of hard sources can therefore account for the harder average spectra of ROSAT sources with S <10−14 erg cm−2 s−1. They probably make a strong contribution to the X-ray background at faint fluxes and could be the solution to the X-ray background spectral paradox.  相似文献   

12.
We present the results of multifrequency (15 + 8 + 5 GHz) polarization Very Long Baseline Array (VLBA) observations of the three BL Lacertae objects 0745+241, 1418+546 and 1652+398 together with 5-GHz VLBI Space Observatory Programme (VSOP) observations of 1418+546 and 1.6- and 5-GHz VSOP observations of the blazar 1055+018. The jets of all these sources have polarization structure transverse to the jet axis, with the polarization E vectors aligned with the jet along the jet spine and 'sheaths' of orthogonal E vectors at one or both edges of the jet. The presence of polarization aligned with the jet near the 'spine' may indicate that the jets are associated with helical B fields that propogate outward with the jet flow; the presence of orthogonal polarization near the edges of the jet may likewise be a consequence of a helical jet B field, or may be owing to an interaction with the ambient medium on parsec scales. We have tentatively detected interknot polarization in 1055+018 with E aligned with the local jet direction, consistent with the possibility that the jet of this source is associated with a helical B field.  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.
Absorption lines of MgH and CaH N  = 1 − 0 transitions were searched for in foreground molecular clouds towards the continuum sources associated with Sgr B2 (M) and W49A (N). None of these lines was detected with our sensitivity level of ∼20 mK. Millimetric absorption lines of MgO, MgOH, CaO and CaOH were also searched for towards Sgr B2 (M) without success. The fractional abundances relative to molecular hydrogen are ≲ 1.0 × 10−11 for MgH, ≲ 7.9 × 10−13 for MgO, ≲ 1.6 × 10−10 for MgOH, ≲ 1.6 × 10−9 for CaH, ≲ 2.0 × 10−12 for CaO, and ≲ 2.5 × 10−10 for CaOH, respectively. The low abundances measured in absorption indicate that a significant fraction of interstellar magnesium and calcium cannot be tied up in their monohydrides, monoxides and monohydroxides. The low abundance of MgH also implies that grain-surface chemistry involving magnesium is not efficient and that magnesium is depleted on to grains to a factor of ≳ 102.5 in well-shielded molecular clouds.  相似文献   

15.
External Faraday rotation has been detected in both the core and the parsec-scale jet of BL Lac in a four-frequency very long baseline interferometry (VLBI) experiment. This unexpected result indicates the presence of significant amounts of thermal gas close to the nucleus of this object. The rotation measure (RM) in the jet components is constant, and differs from the currently accepted Galactic RM, indicating that this value (−205 rad m−2) is not applicable to the components in the parsec-scale jet. The similarity of the RM in these jet components leads us to suspect that the jet RM is caused by a foreground screen in our Galaxy, although we cannot rule out a combination of Galactic RM and RM local to the jet. If the jet RM is due solely to the Galaxy, this would mean that the currently accepted value of the foreground RM (−205 rad m−2 ) is not correct, either because the value changed between 1982 and 1997, or because the assumption of no intrinsic source rotation was incorrect, as it was at our later epoch of observation. Our observations suggest a value of     .
After correcting for the foreground RM, the core value is −427 rad m−2, which is unexpected since, owing to the weakness of their line emission, BL Lac objects are often assumed to be depleted in gas. The core RM appears to be variable, probably because of the presence of at least two polarized components close to the core the relative contributions of which vary with time.  相似文献   

16.
We present a stable procedure for defining and measuring the two point angular autocorrelation function,   w (θ) =[θ/θ0( V )]−Γ  , of faint  (25 < V < 29)  , barely resolved and unresolved sources in the Hubble Space Telescope Great Observatories Origins Deep Survey and Ultra Deep Field data sets. We construct catalogues that include close pairs and faint detections. We show, for the first time, that, on subarcsec scales, the correlation function exceeds unity. This correlation function is well fit by a power law with index  Γ≈ 2.5  and a  θ0= 10−0.1( V −25.8) arcsec  . This is very different from the values of  Γ≈ 0.7  and  θ0( r ) = 10−0.4( r −21.5) arcsec  associated with the gravitational clustering of brighter galaxies. This observed clustering probably reflects the presence of giant star-forming regions within galactic-scale potential wells. Its measurement enables a new approach to measuring the redshift distribution of the faintest sources in the sky.  相似文献   

17.
Central gravitational image detection is very important for the study of the mass distribution of the inner parts (∼100 pc) of lens galaxies. However, the detection of such images is extremely rare and difficult. We present a 1.7-GHz High Sensitivity Array (HSA) observation of the double-image radio lens system B1030+074. The data are combined with archive Very Long Baseline Array and global very long baseline interferometry (VLBI) observations, and careful consideration is given to the effects of noise, clean ing and self-calibration. An upper limit is derived for the strength of the central image of 180 μJy (90 per cent confidence level), considerably greater than would have been expected on the basis of a simple analysis. This gives a lower limit of ∼103 for the ratio of the brightest image to the central image. For cusped models of lens mass distributions, we have made use of this non-detection to constrain the relation between inner power-law slope β of the lensing galaxy mass profile, and its break radius r b. For   r b > 130 pc  the power-law slope is required to be close to isothermal  (β > 1.8)  . A flatter inner slope is allowed if a massive black hole is present at the centre of the lensing galaxy, but the effect of the black hole is small unless it is ∼10 times more massive than that implied by the relation between black hole mass and stellar velocity dispersion. By comparing four epochs of VLBI observations, we also detected possible superluminal motion in the jet in the brighter image A. The B jet remains unresolved, as expected from a simple lens model of the system.  相似文献   

18.
VLBI total intensity and linear polarization images of the BL Lacertae object OJ 287 have been obtained at     using a global ground array and the HALCA orbiting antenna, and at     two weeks earlier using the VLBA. In the ground-based 6-cm images, the source is dominated by a core–jet double structure the components of which are essentially unresolved. The baselines to the orbiting antenna resolve both of these compact components. In the VSOP images, the ground-based 'core' breaks up into several distinct components, demonstrating that this region is dominated by the contribution of bright, optically thin knots of jet emission. A very similar structure is observed in the 1.3-cm image. The magnetic field in the core is transverse, becomes longitudinal in the inner jet, then makes a sharp transition to a region of transverse field further from the core. This suggests that the field in the outer jet has become highly ordered in the transverse direction owing to the action of a shock; the physical nature of the extended region of longitudinal field closer to the core is not clear. The availability of nearly simultaneous observations with comparable resolution at widely spaced frequencies enabled detection of a ≃90° rotation in polarization position angle for the core, owing to the transition from the optically thick (6 cm) to the optically thin (1.3 cm) regime.  相似文献   

19.
Using eight dark matter haloes extracted from fully self-consistent cosmological N -body simulations, we perform microlensing experiments. A hypothetical observer is placed at a distance of 8.5 kpc from the centre of the halo measuring optical depths, event durations and event rates towards the direction of the Large Magellanic Cloud. We simulate 1600 microlensing experiments for each halo. Assuming that the whole halo consists of massive astronomical compact halo objects (MACHOs),   f = 1.0  , and a single MACHO mass is   m M= 1.0 M⊙  , the simulations yield mean values of  τ= 4.7+5.0−2.2× 10−7  and  Γ= 1.6+1.3−0.6× 10−6  events star−1 yr−1. We find that triaxiality and substructure can have major effects on the measured values so that τ and Γ values of up to three times the mean can be found. If we fit our values of τ and Γ to the MACHO collaboration observations, we find   f = 0.23+0.15−0.13  and   m M= 0.44+0.24−0.16  . Five out of the eight haloes under investigation produce f and m M values mainly concentrated within these bounds.  相似文献   

20.
We calculate the expected mid-infrared (MIR) molecular hydrogen line emission from the first objects in the Universe. As a result of their low masses, the stellar feedback from massive stars is able to blow away their gas content and collect it into a cooling shell where H2 rapidly forms and IR roto-vibrational (as for example the rest-frame 2.12 μm) lines carry away a large fraction (up to 10 per cent) of the explosion energy. The fluxes from these sources are in the range 10−21–10−17 erg s−1 cm−2 . The highest number counts are expected in the 20-μm band, where about 105 sources deg−2 are predicted at the limiting flux of 3×10−18 erg s−1 cm−2. Among the planned observational facilities, we find that the best detection perspectives are offered by the Next Generation Space Telescope ( NGST ), which should be able to reveal about 200 first objects in one hour observation time at its limiting flux in the above band. Therefore, mid-IR instruments appear to represent perfect tools to trace star formation and stellar feedback in the high ( z ≳5) redshift Universe.  相似文献   

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