Investigating properties of a set of variable AGN with cluster analysis

Nair, A. D.

doi:10.1093/mnras/287.3.641

Cited by 7 publications

(4 citation statements)

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“…This cluster analysis indicates that the variability indices at 4.8 and 14.5 GHz are most strongly related and also identifies a weaker relation between S av (14.5) and α av . In contrast with the result of Nair (1997), based on long-term Rosemary Hill optical data for a sample of 23 BL Lacs (of which 20 are contained in our group) our cluster analysis results quantify that the amplitude of variability in the radio band does not increase with redshift.…”

Section: Although Our Study Finds Variability Differences Between Bl contrasting

confidence: 99%

Centimeter‐Wavelength Total Flux and Linear Polarization Properties of Radio‐loud BL Lacertae Objects

Aller

Hughes

et al. 1999

ApJ

108

110

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We present results from a long-term program to quantify the range of behavior of the cm-wavelength total flux and linear polarization variability properties of a sample of 41 radio-loud BL Lac objects using weekly to tri-monthly observations with the University of Michigan 26-meter telescope operating at 14.5, 8.0, and 4.8 GHz; these observations are used to identify class-dependent differences between these BL Lacs and QSOs in the Pearson-Readhead sample. As a group, the BL Lacs are found to be more highly variable in total flux density than the QSOs. These changes are often nearly-simultaneous and of comparable amplitude at 14.5 and 4.8 GHz, which contrasts with the behavior in the QSOs and supports the existence of class-dependent differences in opacity within the parsec-scale jet flows. Structure function analyses of the flux observations quantify that a characteristic timescale is identifiable in only 1/3 of the BL Lacs and that in the majority of the program sources the activity is uncorrelated within the timescales probed. The time-averaged fractional linear polarizations are only on the order of a few percent and are consistent with the presence of tangled magnetic fields within the emitting regions. In many sources a preferred long-term orientation of the electric vector position angle is present. When compared with the VLBI structural axis, no preferred position angle difference is identified; the derived distribution resembles that known for core components from VLBP measurements. The polarized flux typically exhibits variability with timescales of months to a few years and exhibits the signature of a propagating shock during several resolved outbursts. The flux and polarization variability indicate that the source emission is predominately due to evolving source components and supports the occurrence of more frequent shock formation in BL Lac parsec-scale flows than in QSO jets where the magnetic field topology -3even during outbursts is similar to that of the underlying quiescent flow. The differences in variability behavior and polarization between BL Lacs and QSOs which we find can be explained by differences in stability between the jet flows found by recent studies of relativistic hydrodynamic flows.

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Section: Although Our Study Finds Variability Differences Between Bl contrasting

confidence: 99%

Centimeter‐Wavelength Total Flux and Linear Polarization Properties of Radio‐loud BL Lacertae Objects

Aller

Hughes

et al. 1999

ApJ

108

110

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“…1) graphically displays the relationships between the various properties of the FS-PR sources. In order to investigate possible groupings in this diagram, we have applied a cluster analysis to our data (e.g., Nair 1997). This method searches for natural groupings among variables that are most correlated.…”

Section: Cluster Analysismentioning

confidence: 99%

The Pearson‐Readhead Survey of Compact Extragalactic Radio Sources from Space. II. Analysis of Source Properties

Lister

Tingay

Preston

2001

ApJ

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We have performed a multidimensional correlation analysis on the observed properties of a statistically complete core-selected sample of compact radio-loud active galactic nuclei based on data from the VLBI Space Observing Programme (Paper I) and previously published studies. Our sample is drawn from the well-studied Pearson-Readhead (PR) survey and is ideally suited for investigating the general e †ects of relativistic beaming in compact radio sources. In addition to conÐrming many previously known correlations, we have discovered several new trends that lend additional support to the beaming model. These trends suggest that the most highly beamed sources in core-selected samples tend to have (1) high optical polarizations ; (2) large parsec-kiloparsec-scale jet misalignments ; (3) prominent VLBI core components ; (4) one-sided, core, or halo radio morphology on kiloparsec scales ; (5) narrow emission line equivalent widths ; and (6) a strong tendency for intraday variability at radio wavelengths. We have used higher resolution space and ground-based VLBI maps to conÐrm the bimodality of the jet misalignment distribution for the PR survey and Ðnd that the sources with aligned parsec-and kiloparsec-scale jets generally have arcsecond-scale radio emission on both sides of the core. The aligned sources also have broader emission line widths. We Ðnd evidence that the BL Lacertae objects in the PR survey are all highly beamed and have very similar properties to the high optically polarized quasars, with the exception of smaller redshifts. A cluster analysis on our data shows that after partialing out the e †ects of redshift, the luminosities of our sample objects in various wave bands are generally well correlated with each other but not with other source properties.

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“…If this is so, then the reason some HPQs are not seen in γ-rays is probably due to intrinsic source differences or time variability. Nair (1997) has stated that cluster analysis shows that γ-ray sources have larger amplitudes of optical variability and are bluer at larger redshifts. Punsly (1996) has found that γ-ray quasars have larger mm spectral peaks relative to their cm spectral peaks than other core-dominated quasars.…”

Section: Comparison Of Egret and Non-egret Sourcesmentioning

confidence: 99%

Geodetic VLBI Observations of EGRET Blazars

Piner

Kingham

1998

ApJ

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ABSTRACT, and we derive lower limits on the ejection times of any such components. The misalignment angle distribution of the EGRET sources is compared to the distribution for blazars as a whole, and we Ðnd that EGRET sources belong preferentially to neither the aligned nor the misaligned population. We also compare the average values for the apparent velocities and Doppler beaming factors for the EGRET and non-EGRET blazars, and Ðnd no signiÐcant di †erences. We thus Ðnd no indication, within the measurement errors, that EGRET blazars are any more strongly beamed than their counterparts that have not been detected in c-rays.

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Investigating properties of a set of variable AGN with cluster analysis

Cited by 7 publications

References 0 publications

Centimeter‐Wavelength Total Flux and Linear Polarization Properties of Radio‐loud BL Lacertae Objects

Centimeter‐Wavelength Total Flux and Linear Polarization Properties of Radio‐loud BL Lacertae Objects

The Pearson‐Readhead Survey of Compact Extragalactic Radio Sources from Space. II. Analysis of Source Properties

Geodetic VLBI Observations of EGRET Blazars

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