[Show abstract][Hide abstract] ABSTRACT: Historically, 3C 66A has been considered a relative quiescent blazar. For that reason, 3C 66A was selected as a comparison source for OJ 287 in the OJ-94 project. However, after more detailed observation it turns out that the variability of 3C 66A itself is very interesting. We have analyzed the entire project data set of 3C 66A from fall of 1993 to spring of 1998 by using structure function analysis, Deeming periodograms, Scargle periodograms, and the folded light curves. Here we present the first preliminary evidence for the 65 day period in 3C 66A observed during the bright state. Our analysis indicates that this period is slowly slowing down. We will also discuss the possible physical mechanism producing the observed periodicity.
Full-text · Article · Jan 2009 · The Astrophysical Journal
[Show abstract][Hide abstract] ABSTRACT: We have made total flux density observations at high radio frequencies (90 and 230 GHz) of 12 southern AGNs that were classified as possible EGRET identifications in the Third EGRET Catalog. Our observations confirm the blazar nature of five of them. We have also studied sources that we considered good candidates for AGN counterparts of previously unidentified EGRET sources and that had not been observed in the millimeter domain before. Four of them showed millimeter range activity that may be related to their gamma-ray activity, making them good candidates for the EGRET source identification.
Preview · Article · Dec 2008 · The Astrophysical Journal
[Show abstract][Hide abstract] ABSTRACT: A large number of AGN have been monitored for nearly 30 years at 22, 37 and 87 GHz in Mets\"ahovi Radio Observatory. These data were combined with lower frequency 4.8, 8.0 and 14.5 GHz data from the University of Michigan Radio Astronomy Observatory, higher frequency data at 90 and 230 GHz from SEST, and supplementary higher frequency data from the literature to study the long-term variability of a large sample of AGN. Both the characteristics of individual flares from visual inspection and statistically-determined variability timescales as a function of frequency and optical class type were determined. Based on past behaviour, predictions of sources expected to exhibit large flares in 2008--2009 appropriate for study by GLAST and other instruments are made. The need for long-term data for properly understanding source behaviour is emphasised.
[Show abstract][Hide abstract] ABSTRACT: Context. Gigahertz-peaked spectrum (GPS) sources and high frequency peakers (HFPs) are among the smallest of active galactic nuclei currently believed to represent the earliest phases in the evolution of extragalactic radio sources. Recently there has been evidence of contamination by other types of radio sources among the GPS and HFP samples, but the confirmed GPS sources or HFPs also seem to form a very heterogeneous population.Aims. We study the statistical clustering of the GPS sources and the HFPs by taking as many source parameters as possible to find homogeneous groups among the sources. We expect the clustering to give us insight into the physical parameters that play a role in different source populations.Methods. We have collected a sample of 206 GPS sources and HFPs from the literature and gathered a massive database of various source properties, such as the redshift, the size, the polarization, the magnitudes, and the properties of the radio continuum. To visualize and to cluster these multidimensional data we used self-organising maps (SOM), which are neural networks trained by an unsupervised algorithm. We have classified the sources with an auxiliary classification to trace the locations of different types of radio continuum spectra on the map. Results. The sources form distinctive clusters on the map, which is supported by the accordant organisation of the non-numerical parameters not used in the analysis, such as the radio morphology and the optical identification. Our results confirm that the blazars contaminating the GPS and the HFP samples are physically different from the genuine GPS sources and HFPs, and they should be excluded from the samples. The genuine GPS sources form various clusters, which indicates the existence of different subpopulations, besides the expected galaxy-quasar dualism.
Full-text · Article · May 2008 · Astronomy and Astrophysics
[Show abstract][Hide abstract] ABSTRACT: We discuss the uses of total flux density monitoring for studies of blazars. By its very nature, monitoring requires large amounts of dedicated telescope time. Such efforts are, however, necessary, since the synchrotron continuum radiation provides only very limited amounts of information, and a single-epoch multifrequency snapshot spectrum is hardly ever sufficient to constrain possible theories and models for the emitting regions. In contrast, analysis of dedicated multifrequency monitoring data obtained over years can reveal the nature of variations, the physics of the synchrotron-emitting regions (shocked jets), and also provide information relevant to other processes such as the gamma-ray emission. Monitoring can also be used to derive fundamental parameters of the jets, their speeds, luminosities and viewing angles, fundamental data in our efforts to understand how active galactic nuclei are powered by supermassive black holes.
[Show abstract][Hide abstract] ABSTRACT: We have made long-term, high radio frequency observations of southern and equatorial active galactic nuclei (AGNs). After complementing these data with data from the literature, we have constructed the radio spectra of these sources and searched for sources with inverted spectra in the gigahertz range. We have identified 12 new sources with spectral shapes resembling those of the gigahertz-peaked spectrum (GPS) sources and eight other sources with inverted spectral parts in the gigahertz region. Several of these new GPS-source candidates have high (≥10 GHz) peak frequencies in the observer's frame, and they all exhibit strong long-term variability. We have also studied the variability behavior of known GPS sources, mainly quasar-type sources. All of the 14 sources included in our sample show moderate to extremely high radio variability, and at least 12 of them are also variable in the millimeter domain. Long-term monitoring of these sources shows that some of them have spectral shapes resembling those of classical variable flat-spectrum sources, indicating that some of these sources have been misidentified as GPS sources when only sparsely sampled data have been available. On the other hand, four of the variable sources show persistent GPS-type spectra at all stages of activity, suggesting that at least some of the GPS sources can be variable.
Preview · Article · Dec 2007 · The Astronomical Journal
[Show abstract][Hide abstract] ABSTRACT: We have complemented observations of the millimeter spectra (90 and 230 GHz) of a complete sample of Southern flat-spectrum active galactic nuclei. We discuss the overall shape of the radio spectrum of these sources and identify two new gigahertz-peaked spectrum sources with unusually high peak frequencies (10–20 GHz). We also discuss the variability behavior in the millimeter domain.
Preview · Article · Dec 2007 · The Astronomical Journal
[Show abstract][Hide abstract] ABSTRACT: We present a study of variability time scales in a large sample of Active Galactic Nuclei at several frequencies between 4.8 and 230 GHz. We investigate the differences of various AGN types and frequencies and correlate the measured time scales with physical parameters such as the luminosity and the Lorentz factor. Our sample consists of both high and low polarization quasars, BL Lacertae objects and radio galaxies. The basis of this work is the 22 GHz, 37 GHz and 87 GHz monitoring data from the Metsahovi Radio Observatory spanning over 25 years. In addition,we used higher 90 GHz and 230 GHz frequency data obtained with the SEST-telescope between 1987 and 2003. Further lower frequency data at 4.8 GHz, 8 GHz and 14.5 GHz from the University of Michigan monitoring programme have been used. We have applied three different statistical methods to study the time scales: The structure function, the discrete correlation function and the Lomb-Scargle periodogram. We discuss also the differences and relative merits of these three methods. Our study reveals that smaller flux density variations occur in these sources on short time scales of 1-2 years, but larger outbursts happen quite rarely, on the average only once in every 6 years. We do not find any significant differences in the time scales between the source classes. The time scales are also only weakly related to the luminosity suggesting that the shock formation is caused by jet instabilities rather than the central black hole. Comment: 19 pages, 12 figures, Accepted for publication in A&A
Preview · Article · May 2007 · Astronomy and Astrophysics
[Show abstract][Hide abstract] ABSTRACT: We have been studying the long term variability time scales of a large
sample of Active Galactic Nuclei (AGNs), which have been monitored in
the Metsähovi radio observatory at 22, 37 and 87 GHz. In the sample
we have 80 sources from different AGN classes including BL lacertae
objects (BLOs), highly polarized quasars (HPQs), low polarisation
quasars (LPQs) and radio galaxies (GALs). In our study we used several
frequency bands between 4.8 and 230 GHz. The lower frequency data at
4.8-14.5 GHz are from the University of Michigan Radio Observatory
(UMRAO) and the higher frequency data at 90 and 230 GHz mostly from the
SEST telescope. We used the structure function, discrete autocorrelation
function and the Lomb-Scargle periodogram to derive the time scales.
Motivation for using many methods, was to study the differences between
the methods and the reliability of the results. Our results reveal that
time scales shorten with increasing frequency. When considering the
observational time scales we did not find significant differences
between the AGN types. After redshift correcting the results, the time
scales between the flares showed significant difference between the BLOs
and the quasars. This means that the rate at which shocks are ejected
into the jet is different for BLOs and quasars.
[Show abstract][Hide abstract] ABSTRACT: The properties of compact extragalactic radio sources are investigated using a sample of 27 sources extensively observed at millimeter frequencies. It is suggested that the radio variability of all AGN, independent of their classification, can be understood in terms of growing and decaying shocks in a jet.
[Show abstract][Hide abstract] ABSTRACT: Long term monitoring results from mid 1995 to the end of 2000 of quasar observations at 22, 37 and 87 GHz done at the Metsähovi radio observatory are presented. Approximately 15 700 observations are published here.
Full-text · Article · Nov 2004 · Astronomy and Astrophysics
[Show abstract][Hide abstract] ABSTRACT: Long term monitoring results from mid 1995 to the end of 2000 of quasar
observations at 22, 37 and 87GHz done at the Metsahovi radio observatory
are presented. Approximately 15700 observations are published here.
(2 data files).
[Show abstract][Hide abstract] ABSTRACT: BL Lacertae has been the target of four observing campaigns by the Whole Earth Blazar Telescope (WEBT) collaboration. In this paper we present UBVRI light curves obtained by theWEBT from 1994 to 2002, including the last, extended BL Lac 2001 campaign. A total of about 7500 optical observations performed by 31 telescopes from Japan to Mexico have been collected, to be added to the ∼15 600 observations of the BL Lac Campaign 2000. All these data allow one to follow the source optical emission behaviour with unprecedented detail. The analysis of the colour indices reveals that the flux variability can be interpreted in terms of two components: longer-term variations occurring on a fewday time scale appear as mildly-chromatic events, while a strong bluer-when-brighter chromatism characterizes very fast (intraday) flares. By decoupling the two components, we quantify the degree of chromatism inferring that longer-term flux changes imply moving along a ∼0.1 bluerwhen- brighter slope in the B − R versus R plane; a steeper slope of ∼0.4 would distinguish the shorter-term variations. This means that, when considering the long-term trend, the B-band flux level is related to the R-band one according to a power law of index ∼1.1. Doppler factor variations on a “convex” spectrum could be the mechanism accounting for both the long-term variations and their slight chromatism. Reig Torres, Pablo, Pablo.Reig@uv.es
Full-text · Article · Jul 2004 · Astronomy and Astrophysics
[Show abstract][Hide abstract] ABSTRACT: BL Lacertae (BL Lac) was the target of an extensive multiwavelength monitoring campaign in the second half of 2000. Simultaneous or quasi-simultaneous observations were taken at radio (University of Michigan Radio Astronomy Observatory and Metsähovi Radio Telescope) and optical (Whole Earth Blazar Telescope [WEBT] collaboration) frequencies, in X-rays (BeppoSAX and RXTE), and at very high energy gamma rays (HEGRA). The WEBT optical campaign achieved an unprecedented time coverage, virtually continuous over several 10–20 hr segments. It revealed intraday variability on timescales of ∼1.5 hr and evidence for spectral hardening associated with increasing optical flux. During the campaign, BL Lac underwent a major transition from a rather quiescent state prior to 2000 September, to a flaring state for the rest of the year. This was also evident in the X-ray activity of the source. BeppoSAX observations on July 26–27 revealed a rather low X-ray flux and a hard spectrum, while a BeppoSAX pointing on 2000 October 31–November 2 indicated significant variability on timescales of ≲a few hours and provided evidence for the synchrotron spectrum extending out to ∼10 keV during that time. During the July 26–27 observation, there is a tantalizing, although not statistically significant, indication of a time delay of ∼4–5 hr between the BeppoSAX and the R-band light curves. Also, a low-significance detection of a time delay of 15 days between the 14.5 and 22 GHz radio light curves is reported. Several independent methods to estimate the comoving magnetic field in the source are presented, suggesting a value of ∼2e G, where eB is the magnetic field equipartition factor with respect to the electron energy density in the jet.
Full-text · Article · Oct 2003 · The Astrophysical Journal
[Show abstract][Hide abstract] ABSTRACT: The Whole Earth Blazar Telescope (WEBT) is an international consortium
of about 30 optical observatories and 2 radio observatories devoted to
blazar monitoring during optical and multifrequency campaigns. The
dispersion in longitude of the WEBT members allows them to obtain dense
and quasi-continuous light curves, minimizing gaps due to Earth
rotation. A few results from the BL Lac 2000 WEBT campaign are briefly
[Show abstract][Hide abstract] ABSTRACT: We discuss the radio continuum spectra and variability of EGRET-detected
blazars and present centimeter-to-millimeter data for a set of AGNs that
are probable or possible counterparts of the EGRET detections in the
Third EGRET Catalog. We discuss the typical radio-to-mm properties of
[Show abstract][Hide abstract] ABSTRACT: We present $UBVRI$ light curves of BL Lacertae from May 2000 to January 2001, obtained by 24 telescopes in 11 countries. More than 15 000 observations were performed in that period, which was the extension of the Whole Earth Blazar Telescope (WEBT) campaign originally planned for July–August 2000. The exceptional sampling reached allows one to follow the flux behaviour in fine detail. Two different phases can be distinguished in the light curves: a first, relatively low-brightness phase is followed by an outburst phase, after a more than $1\rm\,mag$ brightening in a few weeks. Both the time duration (about $100\rm\,d$) and the variation amplitude (roughly $0.9\rm\,mag$) are similar in the two phases. Rapid flux oscillations are present all the time, involving variations up to a few tenths of mag on hour time scales, and witnessing an intense intraday activity of this source. In particular, a half-mag brightness decrease in about $7\rm\,h$ was detected on August 8–9, 2000, immediately followed by a ~$0.4 \rm \, mag$ brightening in $1.7\rm\,h$. Colour indexes have been derived by coupling the highest precision $B$ and $R$ data taken by the same instrument within $20\rm\,min$ and after subtracting the host galaxy contribution from the fluxes. The 620 indexes obtained show that the optical spectrum is weakly sensitive to the long-term trend, while it strictly follows the short-term flux behaviour, becoming bluer when the brightness increases. Thus, spectral changes are not related to the host galaxy contribution, but they are an intrinsic feature of fast flares. We suggest that the achromatic mechanism causing the long-term flux base-level modulation can be envisaged in a variation of the relativistic Doppler beaming factor, and that this variation is likely due to a change of the viewing angle. Discrete correlation function (DCF) analysis reveals the existence of a characteristic time scale of variability of ~$7\rm\,h$ in the light curve of the core WEBT campaign, while no measurable time delay between variations in the $B$ and $R$ bands is found.
Full-text · Article · Aug 2002 · Astronomy and Astrophysics