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1.
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   

2.
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.  相似文献   

3.
The aim of our observations is to investigate the intranight variability properties and the spectral variability of BL Lacertae. 799 optical multiband observations were intensively made with the Beijing–Arizona–Taiwan–Connecticut (BATC) 60/90 cm Schmidt telescope during the outburst composed of two subsequent flares in 2005. The second flare, whose rising phase lasted at least 44 d, was observed with amplitudes of more than 1.1 mag in three BATC optical bands. In general, on intranight time-scale the amplitude of variability and the variation rate are larger at the shorter wavelength, and the variation rate is comparable in the rising and decaying phases within each band. A possible time-lag between the light curves in bands e and m , around 11.6 min, was obtained. Based on the analysis of the colour index variation with the source brightness, the variability of BL Lacertae can be interpreted as having two components: a 'strongly chromatic' intranight component and a 'mildly chromatic' internight component, which may be the results of both intrinsic physical mechanism and geometric effects.  相似文献   

4.
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   

5.
It is generally believed that the high energy end of synchrotron emission, generated by the most energetic tail of relativistic electrons in the jets, account for the X‐ray emission of high‐energy peaked BL Lac objects (HBLs) and the optical emission of intermediate‐energy peaked BL Lac objects (IBLs). It is thus expected that both should show similar variability characteristics. One of the important variability parameters is the inter‐band time lag which probes the acceleration and cooling of relativistic particles responsible for the emission. The switches between soft and hard lags have been detected in the intra‐day X‐ray variability of a few HBLs, which is not the case for the intra‐day optical variability of IBLs yet. We present the results of our intra‐night optical observations for BL Lacertae, aiming at searching for hard lags of its optical variations, performed with the 80 cm telescope in fourteen nights of 2010 September‐November. Intra‐night changes of ∼0.2 mag were detected in most of nights. The intra‐night variability amplitude tends to become larger from red to blue wavelength, and the optical spectrum hardens with increasing brightness. The intra‐night variations correlate between different wavebands, but we did not find significant time lags, either soft or hard. Nevertheless, on November 2, the B band variations showed a sign of lagging the R band ones by 317±214 s. The claim of this hard lag is strongly limited by the photometric precision and time resolution. Therefore, the switches between soft and hard lags of IBLs in the optical bands needs further demonstration with more higher quality observations. (© 2016 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  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.
We present the results of photometric observations of BL Lacertae at 1 μm performed during the strong outburst that occurred in the summer of 1997. Measurements were carried out using the F99 filter of the Arizona system. Simultaneous observations in the B V bands were made with other telescopes. Large and fast variations were observed on the night of August 2 when the brightness of BL Lacertae changed by ∼ 0.5 mag in about 2 h: the variation amplitude is greater at higher frequency. Some implications of these results for the origin of the fast variations are discussed. We also report the first F99 calibration for three reference stars in the BL Lac field.  相似文献   

8.
We report monitoring observations of 20 high-luminosity active galactic nuclei (AGN), 12 of which are radio-quiet quasars (RQQs). Intranight optical variability (INOV) was detected for 13 of the 20 objects, including 5 RQQs. The variations are distinctly stronger and more frequent for blazars than for the other AGN classes. By combining these data with results obtained earlier in our programme, we have formed an enlarged sample consisting of 9 BL Lacs, 19 RQQs and 11 lobe-dominated radio-loud quasars (RLQs). The moderate level of rapid optical variability found for both RQQs and radio lobe-dominated quasars (LDQs) argues against a direct link between INOV and radio loudness. We supplemented the present observations of 3 BL Lacs with additional data from the literature. In this extended sample of 12 well observed BL Lacs, stronger INOV is found for the EGRET detected subset.  相似文献   

9.
The BL Lacertae object PKS 1413+135 is associated with a disc-dominated galaxy that heavily absorbs the BL Lac nucleus at optical and X-ray wavelengths. It has been argued whether this galaxy is actually the host galaxy of PKS 1413+135 or whether the BL Lac is a background QSO, gravitationally lensed by the apparent host galaxy. We have obtained deep high-resolution H -band images of this unusual BL Lac object using the UKIRT IRCAM3. Our observations show that the BL Lac nucleus is centred within 0.05 arcsec of the galaxy. Based on this result, we assess the probability for the lensing scenario and come to the conclusion that the disc galaxy is indeed the host of PKS 1413+135. The galaxy shows peanut-shaped isophotes, suggesting the presence of a central bar, which is a common feature of AGN.  相似文献   

10.
We present the results of a comparative study of the intranight optical variability (INOV) characteristics of radio-loud and radioquiet quasars, which involves a systematic intra-night optical monitoring of seven sets of high luminosity AGNs covering the redshift rangez ≃ 0.2 toz ≃ 2.2. The sample, matched in the optical luminosity—redshift(M B—z) plane, consists of seven radio-quiet quasars (RQQs), eight radio lobedominated quasars (LDQs), five radio core-dominated quasars (CDQs) and six BL Lac objects (BLs). Systematic CCD observations, aided by a careful data analysis procedure, have allowed us to detect INOV with amplitudes as low as about 1%. Present observations cover a total of 113 nights (720 hours) with only a single quasar monitored as continuously as possible on a given night. Considering the cases of only unambiguous detections of INOV we have estimated duty cycles (DCs) of 17%, 12%, 20% and 61% for RQQs, LDQs, CDQs, and BLs, respectively. The much lower amplitude and DC of ESfOV shown by RQQs compared to BLs may be understood in terms of their having optical synchrotron jets which are modestly misdirected from us. From our fairly extensive dataset, no general trend of a correlation between the INOV amplitude and the apparent optical brightness of the quasar is noticed. This suggests that the physical mechanisms of INOV and long term optical variability (LTOV) do not have a one-to-one relationship and different factors are involved. Also, the absence of a clear negative correlation between the INOV and LTOV characteristics of blazars of our sample points toward an inconspicuous contribution of accretion disk fluctuations to the observed INOV. The INOV duty cycle of the AGNs observed in this program suggests that INOV is associated predominantly with the highly polarized optical emission components. We also report new VLA imaging of two RQQs (1029 + 329 & 1252 + 020) in our sample which has yielded a 5 GHz detection in one of them (1252 + 020;S 5GHZ ≃ 1 mJy).  相似文献   

11.
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   

12.
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   

13.
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   

14.
The long baseline simultaneous multiband ( BVRI ) observations of blazar 3C 273 are presented. We have made 758 optical multiband observations with the Yunnan Astronomical Observatory (YAO) 103 cm and Shanghai Astronomical Observatory (SHAO) 156 cm from 2003 January to 2005 April. In our observational campaign, the average colour indices are   B − V = 0.21, V − R = 0.26  and   R − I = 0.31  . The mean magnitudes in B, V, R and I are 12.895, 12.698, 12.441 and 12.139, respectively. The source is in a steady state over observational campaign. The correlations between colour index and brightness are discussed. We find that the correlations of   B − V   versus B ,   V − R   versus V and   R − I   versus R have significant negative correlation, while   B − V   versus V ,   V − R   versus R and   R − I   versus I have positive correlation. These strong correlations imply that the spectrum becomes bluer (flatter) when the source becomes brighter, and redden (softer) when the source fades both in intraday and long-term variability. The spectral evolution trends of 3C 273 are consistent with those of BL Lac objects.  相似文献   

15.
Photometric monitoring of 12 BL Lacertae objects   总被引：1，自引：0，他引：1  
We present the results of our monitoring the flux variability of 12 BL Lac objects, which have variabilities on time-scales ranging from hours to months. Individual sources are discussed in detail. Three of them, OY091,     and     , show significant rapid variation (hours). Two of them, 3C 66A and Mrk 501, exhibit significant variability on time-scales of months. We find that 3C 66A has a variability period of     , supporting the 65-d period obtained by Lainela et al. The periodicity seems to be weak according to the Jurkevich     test. We also find a correlation between     colour index and B magnitude for Mrk 421. We discuss possible physical mechanisms on the basis of these observational phenomena.  相似文献   

16.
The results of a BeppoSAX target of opportunity (TOO) observation of the BL Lac object Mrk 421 during a high-intensity state are reported and compared with monitoring X-ray data collected with the BeppoSAX Wide Field Cameras (WFC) and the RXTE All Sky Monitor (ASM). The 0.1–100 keV spectrum of Mrk 421 shows continuous convex curvature that can be interpreted as the high-energy end of the synchrotron emission. The source shows significant short-term temporal and spectral variability, which can be interpreted in terms of synchrotron cooling. The comparison of our results with those of previous observations when the source was a factor 3–5 fainter shows evidence for strong spectral variability, with the maximum of the synchrotron power shifting to higher energy during high states. This behaviour suggests an increase in the number of energetic electrons during high states.  相似文献   

17.
We present a Chandra observation of the candidate BL Lac object EXO 0423.4−0840. The X-ray emission from EXO 0423.4−0840 is clearly extended, and is associated with an optical early-type galaxy, MCG-01-12-005, at the centre of cluster ClG 0422-09. We do not detect a point source that can be associated with a BL Lac, but we found a small radio source in the centre of MCG-01-12-005. The cluster gas temperature mapped by the Chandra observation drops continuously from 80 kpc towards the centre, and is locally single phase. We measure a metallicity profile that declines outwards with a value  0.8 Z⊙  in the centre, dropping to  0.35 Z⊙  at larger radius, which we interpret as a superposition of cluster gas and a dense interstellar medium (ISM) in the central galaxy. Although the temperature profile suggests that conduction is not efficient, the ISM and intra-cluster medium seem not to have mixed. The entropy profile declines continuously towards the centre, in agreement with recent results on groups and clusters. The radio source appears to have had some effect in terms of gas heating, as seen in the small-scale (∼10 kpc) entropy core, and the asymmetric hard emission on the same scale.  相似文献   

18.
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   

19.
We report on the progress of world array VLBI observations of the BL Lac object 1803+784 performed at λ=18 cm. A 16-station VLBI array, plus the phased VLA, and the 7-station MERLIN array simultaneously observed the source for 11 hours. These observations provide images of the transition region between the misaligned milliarcsecond- and arcsecond-jet. We show the MERLIN map and a combined full-array map at medium resolution. In addition we describe the present status of short-cm VLBI observations of this source.  相似文献   

20.
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   

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