42 research outputs found
A multifrequency analysis of radio variability of blazars
We have carried out a multifrequency analysis of the radio variability of
blazars, exploiting the data obtained during the extensive monitoring programs
carried out at the University of Michigan Radio Astronomy Observatory (UMRAO,
at 4.8, 8, and 14.5 GHz) and at the Metsahovi Radio Observatory (22 and 37
GHz). Two different techniques detect, in the Metsahovi light curves, evidences
of periodicity at both frequencies for 5 sources (0224+671, 0945+408, 1226+023,
2200+420, and 2251+158). For the last three sources consistent periods are
found also at the three UMRAO frequencies and the Scargle (1982) method yields
an extremely low false-alarm probability. On the other hand, the 22 and 37 GHz
periodicities of 0224+671 and 0945+408 (which were less extensively monitored
at Metsahovi and for which we get a significant false-alarm probability) are
not confirmed by the UMRAO database, where some indications of ill-defined
periods about a factor of two longer are retrieved. We have also investigated
the variability index, the structure function, and the distribution of
intensity variations of the most extensively monitored sources. We find a
statistically significant difference in the distribution of the variability
index for BL Lac objects compared to flat-spectrum radio quasars (FSRQs), in
the sense that the former objects are more variable. For both populations the
variability index steadily increases with increasing frequency. The
distribution of intensity variations also broadens with increasing frequency,
and approaches a log-normal shape at the highest frequencies. We find that
variability enhances by 20-30% the high frequency counts of extragalactic
radio-sources at bright flux densities, such as those of the WMAP and Planck
surveys.Comment: A&A accepted. 12 pages, 16 figure
30 years of multi-wavelength observations of 3C 273
We present a wide multi-wavelength database of most observations of the
quasar 3C 273 obtained during the last 30 years. This database is the most
complete set of observations available for an active galactic nucleus (AGN). It
contains nearly 20'000 observations grouped together into 70 light curves
covering 16 orders of magnitude in frequency from the radio to the gamma-ray
domain.
The database is constituted of many previously unpublished observations and
of most publicly available data gathered in the literature and on the World
Wide Web (WWW). It is complete to the best of our knowledge, except in the
optical (UBV) domain where we chose not to add all observations from the
literature. In addition to the photometric data, we present the spectra of 3C
273 obtained by the International Ultraviolet Explorer (IUE) satellite. In the
X-ray domain, we used the spectral fit parameters from the literature to
construct the light curves.
Apart from describing the data, we show the most representative light curves
and the average spectrum of 3C 273. The database is available on the WWW in a
homogeneous and clear form and we wish to update it regularly by adding new
observations.Comment: 12 pages, 6 figures, to be published in A&AS, data available at:
http://obswww.unige.ch/3c273
Helical motion and the origin of QPO in blazar-type sources
Recent observations and analysis of blazar sources provide strong evidence
for (i) the presence of significant periodicities in their lightcurves and (ii)
the occurrence of helical trajectories in their radio jets. In scenarios, where
the periodicity is caused by differential Doppler boosting effects along a
helical jet path, both of these facts may be naturally tied together. Here we
discuss four possible driving mechanisms for the occurrence of helical
trajectories: orbital motion in a binary system, Newtonian-driven jet
precession, internal jet rotation and motion along a global helical magnetic
field. We point out that for non-ballistic helical motion the observed period
may appear strongly shortened due to classical travel time effects. Finally,
the possible relevance of the above mentioned driving mechanisms is discussed
for Mkn~501, OJ 287 and AO 0235+16.Comment: 6 pages, 1 figure; presented at the 5th Microquasar Workshop,
Beijing, June 2004. Accepted for publication in the Chinese Journal of
Astronomy and Astrophysic
Optical Properties of faint FIRST Variable Radio Sources
A sample of 123 radio sources that exhibit significant variations at 1.4 GHz
on a seven year base-line has been created using FIRST VLA B-array data from
1995 and 2002 on a strip at dec=0 near the south Galactic cap. This sample
spans the range of radio flux densities from ~2 to 1000 mJy. It presents both
in size and radio flux density range a unique starting point for variability
studies of galaxies and quasars harboring lower luminosity Active Galactic
Nuclei (AGN). We find, by comparing our variable and non-variable control
samples to the Sloan Digital Sky Survey the following: 1) The quasar fraction
of both the variable and non-variable samples declines as a function of
declining radio flux density levels; 2) our variable sample contains a
consistently higher fraction of quasars than the non-variable control sample,
irrespective of radio flux; 3) the variable sources are almost twice as likely
to be retrieved from the optical SDSS data than the non-variable ones; 4) based
on relative numbers, we estimate that quasars are about five times more likely
to harbor a variable radio source than are galaxies; and 5) there does not
appear to be any significant optical color offset between the two samples, even
though the suggestive trend for sources to be bluer when variable has been
detected before and may be real. This leads us to conclude that both radio
variability and radio flux density levels, in combination with accurate optical
information, are important discriminators in the study of (radio) variability
of galaxies. The latter start to dominate the source counts below about 20 mJy.
In any case, variability appears to be an intrinsic property of radio sources,
and is not limited to quasars. Radio variability at low flux density levels may
offer a unique tool in AGN unification studies.Comment: 22 pages, 10 color figures. Accepted for publication in AJ, May issu
A search for periodicity in the light curves of selected blazars
We present an analysis of multifrequency light curves of the sources 2223-052
(3C 446), 2230+114 (CTA 102), and 2251+158 (3C 454.3), which had shown evidence
of quasi-periodic activity. The analysis made use of data from the University
of Michican Radio Astronomy Observatory (USA) at 4.8, 8, and 14.5 GHz, as well
as the Metsahovi Radio Astronomy Observatory (Finland) at 22 and 37 GHz.
Application of two different methods (the discrete autocorrelation function and
the method of Jurkevich) both revealed evidence for periodicity in the flux
variations of these sources at essentially all frequencies. The periods derived
for at least two of the sources -- 2223-052 and 2251+158-- are in good
agreement with the time interval between the appearance of successive VLBI
components. The derived periods for 2251+158 (P = 12.4 yr and 2223-052 (P = 5.8
yr) coincide with the periods found earlier by other authors based on optical
light curves.Comment: 27 pages, 11 figures, accepted for publication in Astronomy Report
Radio variability properties for radio sources
In this paper, we used the database of the university of Michigan Radio
Astronomy Observatory (UMRAO) at three (4.8 GHz, 8.0 GHZ, and 14.5 GHz) radio
frequency to analyze the radio light curves by the power spectral analysis
method in search of possible periodicity. The analysis results showed that the
radio sources display astrophysically meaningful periodicity ranging from 2.2
to 20.8 years in their light curves at the three frequencies. We also
calculated the variability parameters and investigated the correlations between
the variability parameter and the flux density. For the variability parameters,
we found that the parameters at higher frequency are higher than those in the
lower frequency. In addition, the variability parameters of BL Lacertae objects
are larger than those of flat-spectrum radio quasars. suggesting that they are
more variable than flat spectrum radio quasars.Comment: 28 pages, 6 figures, 3 tables, A&A in pres
The WEBT BL Lacertae Campaign 2001 and its extension : Optical light curves and colour analysis 1994–2002
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, [email protected]
The WEBT BL Lac Campaign 2000
We present UBVRI light curves of BL Lacertae from May 2000 to January 2001,
obtained by 24 telescopes in 11 countries. More than 15000 observations were
performed in that period, which was the extension of the Whole Earth Blazar
Telescope (WEBT) campaign originally planned for July-August 2000. 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. Colour indexes have been derived by coupling the highest precision B
and R data taken by the same instrument within 20 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 about 7 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.Comment: 14 pages, 8 PostScript figures, 5 JPEG figures, in press for A&
Coordinated Multiwavelength Observations of BL Lacertae in 2000
BL Lacertae was the target of an extensive multiwavelength monitoring
campaign in the second half of 2000. Simultaneous or quasi-simultaneous
observations were taken at radio (UMRAO and Metsaehovi) and optical(WEBT
collaboration) frequencies, in X-rays (BeppoSAX and RXTE), and at VHE
gamma-rays (HEGRA). The WEBT optical campaign achieved an unprecedented time
coverage, virtually continuous over several 10 - 20 hour segments. It revealed
intraday variability on time scales of ~ 1.5 hours and evidence for spectral
hardening associated with increasing optical flux. During the campaign, BL
Lacertae underwent a major transition from a rather quiescent state prior to
September 2000, 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 Oct. 31 - Nov. 2, 2000, indicated significant variability on time
scales 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, though not statistically significant, indication of a
time delay of ~ 4 - 5 hr between the BeppoSAX and the R-band light curve. Also,
a low-significance detection of a time delay of 15 d between the 14.5 GHz and
the 22 GHz radio light curves is reported. Several independent methods to
estimate the co-moving magnetic field in the source are presented, suggesting a
value of ~ 2 e_B^{2/7} G, where e_B is the magnetic-field equipartition factor
w.r.t. the electron energy density in the jet.Comment: Accepted for publication in Ap
The WEBT Campaign on the Intermediate BL Lac Object 3C66A in 2007-2008
Prompted by a high optical state in September 2007, the Whole Earth Blazar
Telescope (WEBT) consortium organized an intensive optical, near-IR (JHK) and
radio observing campaign on the intermediate BL Lac object 3C 66A throughout
the fall and winter of 2007 -- 2008. The source remained in a high optical
state throughout the observing period and exhibited several bright flares on
time scales of ~ 10 days. This included an exceptional outburst around
September 15 - 20, 2007, reaching a peak brightness at R ~ 13.4. Our campaign
revealed microvariability with flux changes up to |dR/dt| ~ 0.02 mag/hr. Our
observations do not reveal evidence for systematic spectral variability or
spectral lags. We infer a value of the magnetic field in the emission region of
B ~ 19 e_B^{2/7} \tau_h^{-6/7} D_1^{13/7} G. From the lack of systematic
spectral variability, we can derive an upper limit on the Doppler factor, D <=
28 \tau_h^{-1/8} e_B^{3/16}. This is in agreement with superluminal motion
measurements of \beta_{app} \le 27 and argues against models with very high
Lorentz factors of \Gamma > 50, required for a one-zone SSC interpretation of
some high-frequency-peaked BL Lac objects detected at TeV gamma-ray energies.Comment: Accepted for publication in The Astrophysical Journa