119 research outputs found
Statistical analyses of long-term variability of AGN at high radio frequencies
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&
Two classes of radio flares in the blazar PKS 0420014
The two 5GHz VLBI (Very Long Baseline Interferometry) observations (1996 June
and 1997 November) presented in this paper, combined with several formerb VLBI
observations at 8.4GHz and 5GHz, suggest that the radio flares of the blazar
PKS 0420014 can be divided into two classes according to their geometric
origins in 5 or 8.4GHz VLBI maps and the properties of light curves. One class
of flares, which we call {\it core flares}, originate from the core. Core
flares have large lags between the light curves at different frequencies, and
will probably lead to the ejection of new jet components. The other class of
flares, which we call {\it jet flares}, come from jet components. Jet flares
vary simultaneously at different wavelengths, and may due to the Doppler
boosting effect of rotating knots moving along a helical jet. The radio flare
in 1991, accompanied by a simultaneous gamma-ray flare, was identified as a
core flare.Comment: 9 pages, 3 figure
Long-term radio variability of AGN: flare characteristics
We have studied the flare characteristics of 55 AGN at 8 different frequency
bands between 4.8 and 230 GHz. Our extensive database enables us to study the
various observational properties of flares in these sources and compare our
results with theoretical models. We visually extracted 159 individual flares
from the flux density curves and calculated different parameters, such as the
peak flux density and duration, in all the frequency bands. The selection of
flares is based on the 22 and 37 GHz data from Mets\"ahovi Radio Observatory
and 90 and 230 GHz data from the SEST telescope. Additional lower frequency
4.8, 8, and 14.5 GHz data are from the University of Michigan Radio
Observatory. We also calculated variability indices and compared them with
earlier studies. The observations seem to adhere well to the shock model, but
there is still large scatter in the data. Especially the time delays between
different frequency bands are difficult to study due to the incomplete sampling
of the higher frequencies. The average duration of the flares is 2.5 years at
22 and 37 GHz, which shows that long-term monitoring is essential for
understanding the typical behaviour in these sources. It also seems that the
energy release in a flare is independent of the duration of the flare.Comment: 11 pages, 9 figures, 2 tables, accepted for publication in A&
The extragalactic radio-source population at 95 GHz
We have used the Australia Telescope Compact Array (ATCA) at 95GHz to carry
out continuum observations of 130 extragalactic radio sources selected from the
Australia Telescope 20GHz (AT20G) survey. Over 90% of these sources are
detected at 95 GHz, and we use a triple-correlation method to measure
simultaneous 20 and 95 GHz flux densities. We show that the ATCA can measure
95GHz flux densities to ~10% accuracy in a few minutes for sources above
~50mJy.
The median 20-95GHz spectral index does not vary significantly with flux
density for extragalactic sources with S20>150 mJy. This allows us to estimate
the extragalactic radio source counts at 95GHz by combining our observed
20-95GHz spectral-index distribution with the accurate 20GHz source counts
measured in the AT20G survey. The resulting 95GHz source counts down to 80 mJy
are significantly lower than those found by several previous studies. The main
reason is that most radio sources with flat or rising spectra in the frequency
range 5-20GHz show a spectral turnover between 20 and 95 GHz. As a result,
there are fewer 95GHz sources (by almost a factor of two at 0.1 Jy) than would
be predicted on the basis of extrapolation from the source populations seen in
lower-frequency surveys. We also derive the predicted confusion noise in CMB
surveys at 95GHz and find a value 20-30% lower than previous estimates.
The 95GHz source population at the flux levels probed by this study is
dominated by QSOs with a median redshift z~1. We find a correlation between
optical magnitude and 95GHz flux density which suggests that many of the
brightest 95 GHz sources are relativistically beamed, with both the optical and
millimetre continuum significantly brightened by Doppler boosting.Comment: Replaced with final version (MNRAS, in press), 15 pages plus two
landscape data table
Optical and radio behaviour of the BL Lacertae object 0716+714
Eight optical and four radio observatories have been intensively monitoring
the BL Lac object 0716+714 in the last years: 4854 data points have been
collected in the UBVRI bands since 1994, while radio light curves extend back
to 1978. Many of these data are presented here for the first time. The
long-term trend shown by the optical light curves seems to vary with a
characteristic time scale of about 3.3 years, while a longer period of 5.5-6
years seems to characterize the radio long-term variations. In general, optical
colour indices are only weakly correlated with brightness. The radio flux
behaviour at different frequencies is similar, but the flux variation amplitude
decreases with increasing wavelength. The radio spectral index varies with
brightness (harder when brighter), but the radio fluxes seem to be the sum of
two different-spectrum contributions: a steady base level and a harder-spectrum
variable component. Once the base level is removed, the radio variations appear
as essentially achromatic, similarly to the optical behaviour. Flux variations
at the higher radio frequencies lead the lower-frequency ones with week-month
time scales. The behaviour of the optical and radio light curves is quite
different, the broad radio outbursts not corresponding in time to the faster
optical ones and the cross-correlation analysis indicating only weak
correlation with long time lags. However, minor radio flux enhancements
simultaneous with the major optical flares can be recognized, which may imply
that the mechanism producing the strong flux increases in the optical band also
marginally affects the radio one.Comment: 18 pages, 15 Postscript figures, 5 JPEG figures, accepted for
publication in A&
Towards a mechanistic understanding of linguistic diversity
Our species displays remarkable linguistic diversity. While the uneven distribution of this diversity demands explanation, the drivers of these patterns have not been conclusively determined. We address this issue in two steps. First, we review previous empirical studies that have suggested environmental, geographical, and socio-cultural drivers of linguistic diversification. However, contradictory results and methodological variation make it difficult to draw general conclusions. Second, we outline a program for future research. We suggest that future analyses should account for interactions among causal factors, lack of spatial and phylogenetic independence of data, and transitory patterns. Recent analytical advances in biogeography and evolutionary biology, such as simulation modeling of diversity patterns, hold promise for testing four key mechanisms of language diversification proposed here: neutral change, population movement, contact, and selection. Future modeling approaches should also evaluate how the outcomes of these processes are influenced by demography, environmental heterogeneity, and time
Structure of the Accretion Flow in Broad-Line Radio Galaxies: The Case of 3C390.3
We present XMM and Suzaku observations of the Broad-Line Radio Galaxy (BLRG)
3C390.3. The Fe Ka line has a width FWHM ~ 8,800 km/s, consistent within a
factor two with the width of the double-peaked H_alpha line, suggesting an
origin from the Broad Line Region. The data show for the first time a weak,
broad bump extending from 5 to 7 keV. When fitted with a Gaussian, its centroid
energy is 6.6 keV in the source's rest-frame with FWHM of 43,000 km/s and EW of
50 eV; its most likely interpretation is emission from He-like Fe (Fe XXV),
suggesting the presence of an ionized medium in the inner regions of 3C390.3.
The broad-band 0.5-100 keV continuum is well described by a single power law
with photon index Gamma=1.6 and cutoff energy 157 keV, plus cold reflection
with strength R=0.5. In addition, ionized reflection is required to account for
the 6.6 keV bump in the broad-band continuum, yielding an ionization parameter
xi ~ 2700 ergs cm s^-1; the inner radius of the ionized reflector is
constrained to be larger than 20 r_G, although this result depends on the
assumed emissivity profile of the disk. If true, we argue that the lack of
broad Fe K emission from within 20 r_G indicates that the innermost regions of
the disk in 3C390.3 are obscured and/or poorly illuminated. While the SED of
3C390.3 is generally dominated by accretion-related continuum, during accretion
low states the jet can significantly contribute in the optical to X-ray bands
via synchrotron self-Compton emission. (Abridged)Comment: 7 figures, 5 tables, accepted for publication in Ap
An Intense Gamma-Ray Flare of PKS1622-297
We report the observation by the Compton Gamma Ray Observatory of a
spectacular flare of radio source PKS 1622-297. A peak flux of 17E-6 cm^-2 s^-1
(E > 100 MeV) was observed. The corresponding isotropic luminosity is 2.9E49
erg/s. We find that PKS 1622-297 exhibits gamma-ray intra-day variability. A
flux increase by a factor of at least 3.6 was observed to occur in less than
7.1 hours (with 99% confidence). Assuming an exponential rise, the
corresponding doubling time is less than 3.8 hours. A significant flux decrease
by a factor of ~2 in 9.7 hours was also observed. Without beaming, the rapid
flux change and large isotropic luminosity are inconsistent with the
Elliot-Shapiro condition (assuming that gas accretion is the immediate source
of power for the gamma-rays). This inconsistency suggests that the gamma-ray
emission is beamed. A minimum Doppler factor of 8.1 is implied by the observed
lack of pair-production opacity (assuming x-rays are emitted co-spatially with
the gamma-rays). Simultaneous observation by EGRET and OSSE finds a spectrum
adequately fit by a power law with photon index of -1.9. Although the
significance is not sufficient to establish this beyond doubt, the high-energy
gamma-ray spectrum appears to evolve from hard to soft as a flare progresses.Comment: 14 pages, 4 figures, 1 tabl
Evolution of Cold Circumstellar Dust Around Solar-Type Stars
We present submillimeter (CSO 350um) and millimeter (SEST 1.2 mm, OVRO 3 mm)
photometry for 125 solar-type stars from the FEPS Spitzer Legacy program that
have masses between ~0.5 and 2.0 Msun and ages from 3 Myr to 3 Gyr. Continuum
emission was detected toward four stars with a signal to noise ratio >= 3$: the
classical T Tauri stars RX J1842.9-3532, RX J1852.3-3700, and PDS 66 with SEST,
and the debris disk system HD 107146 with OVRO. RXJ1842.9-3532 and
RXJ1852.3-3700 are located in projection nearby the R CrA molecular cloud with
estimated ages of ~10 Myr, while PDS66 is a probable member of the 20 Myr old
Lower Centaurus-Crux subgroup of the Sco-Cen OB association. The continuum
emission toward these three sources is unresolved at the 24'' SEST resolution
and likely originates from circumstellar accretion disks, each with estimated
dust masses of ~5x10**-5 Msun. Analysis of the visibility data toward HD107146
(age 80-200 Myr) indicates that the 3 mm continuum emission is centered on the
star within the astrometric uncertainties and resolved with a gaussian-fit FWHM
size of (6.5'' +/- 1.4'') x (4.2''+/-1.3''), or 185 AUx120 AU. The results from
our continuum survey are combined with published observations to quantify the
evolution of dust mass with time by comparing the mass distributions for
samples with different stellar ages. The frequency distribution of
circumstellar dust masses around solar-type stars in the Taurus molecular cloud
(age ~2 Myr) is distinguished from that around 3-10 Myr and 10-30 Myr old stars
at a significance level of ~1,5sigma and 3sigma respectively. These results
suggest a decrease in the mass of dust contained in small dust grains and/or
changes in the grain properties by stellar ages of 10-30 Myr, consistent with
previous conclusions. (abridged)Comment: 37 pages, 8 figures, accepted for publication in the Astronomical
Journa
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