184 research outputs found
Dual-Frequency Observations of 140 Compact, Flat-Spectrum Active Galactic Nuclei for Scintillation-Induced Variability
The 4.9 GHz Micro-Arcsecond Scintillation-Induced Variability (MASIV) Survey
detected a drop in Interstellar Scintillation (ISS) for sources at redshifts z
> 2, indicating an apparent increase in angular diameter or a decrease in flux
density of the most compact components of these sources, relative to their
extended emission. This can result from intrinsic source size effects or
scatter broadening in the Intergalactic Medium (IGM), in excess of the expected
(1+z)^0.5 angular diameter scaling of brightness temperature limited sources
due to cosmological expansion. We report here 4.9 GHz and 8.4 GHz observations
and data analysis for a sample of 140 compact, flat-spectrum sources which may
allow us to determine the origin of this angular diameter-redshift relation by
exploiting their different wavelength dependences. In addition to using ISS as
a cosmological probe, the observations provide additional insight into source
morphologies and the characteristics of ISS. As in the MASIV Survey, the
variability of the sources is found to be significantly correlated with
line-of-sight H-alpha intensities, confirming its link with ISS. For 25
sources, time delays of about 0.15 to 3 days are observed between the
scintillation patterns at both frequencies, interpreted as being caused by a
shift in core positions when probed at different optical depths. Significant
correlation is found between ISS amplitudes and source spectral index; in
particular, a large drop in ISS amplitudes is observed at spectral indices of <
-0.4 confirming that steep spectrum sources scintillate less. We detect a
weakened redshift dependence of ISS at 8.4 GHz over that at 4.9 GHz, with the
mean variance at 4-day timescales reduced by a factor of 1.8 in the z > 2
sources relative to the z < 2 sources, as opposed to the factor of 3 decrease
observed at 4.9 GHz. This suggests scatter broadening in the IGM.Comment: 30 pages, 14 figures, accepted for publication in the Astronomical
Journa
First Results from MASIV: The Micro-Arcsecond Scintillation-Induced Variability Survey
We are undertaking a large-scale, Micro-Arcsecond Scintillation-Induced
Variability (MASIV) survey of the northern sky, Dec > 0 deg, at 4.9 GHz with
the VLA. Our objective is to construct a sample of 100 to 150 scintillating
extragalactic sources with which to examine both the microarcsecond structure
and the parent populations of these sources, and to probe the turbulent
interstellar medium responsible for the scintillation. We report on our first
epoch of observations which revealed variability on timescales ranging from
hours to days in 85 of 710 compact flat-spectrum sources. The number of highly
variable sources, those with RMS flux density variations greater than 4% of the
mean, increases with decreasing source flux density but rapid, large amplitude
variables such as J1819+3845 are very rare. When compared with a model for the
scintillation due to irregularities in a 500 pc thick electron layer, our
preliminary results indicate maximum brightness temperatures ~10E+12 K, similar
to those obtained from VLBI surveys even though interstellar scintillation is
not subject to the same angular resolution limit.Comment: 18 pages, 5 figures. To appear in the Astronomical Journa
Why Do Compact Active Galactic Nuclei at High Redshift Scintillate Less?
The fraction of compact active galactic nuclei (AGNs) that exhibit
interstellar scintillation (ISS) at radio wavelengths, as well as their
scintillation amplitudes, have been found to decrease significantly for sources
at redshifts z > 2. This can be attributed to an increase in the angular sizes
of the \muas-scale cores or a decrease in the flux densities of the compact
\muas cores relative to that of the mas-scale components with increasing
redshift, possibly arising from (1) the space-time curvature of an expanding
Universe, (2) AGN evolution, (3) source selection biases, (4) scatter
broadening in the ionized intergalactic medium (IGM) and intervening galaxies,
or (5) gravitational lensing. We examine the frequency scaling of this redshift
dependence of ISS to determine its origin, using data from a dual-frequency
survey of ISS of 128 sources at 0 < z < 4. We present a novel method of
analysis which accounts for selection effects in the source sample. We
determine that the redshift dependence of ISS is partially linked to the
steepening of source spectral indices ({\alpha}^8.4_4.9) with redshift, caused
either by selection biases or AGN evolution, coupled with weaker ISS in the
{\alpha}^8.4_4.9 < -0.4 sources. Selecting only the -0.4 < {\alpha}^8.4_4.9 <
0.4 sources, we find that the redshift dependence of ISS is still significant,
but is not significantly steeper than the expected (1+z)^0.5 scaling of source
angular sizes due to cosmological expansion for a brightness temperature and
flux-limited sample of sources. We find no significant evidence for scatter
broadening in the IGM, ruling it out as the main cause of the redshift
dependence of ISS. We obtain an upper limit to IGM scatter broadening of <
110\muas at 4.9 GHz with 99% confidence for all lines of sight, and as low as <
8\muas for sight-lines to the most compact, \sim 10\muas sources.Comment: 38 pages, 13 figures, accepted for publication in The Astrophysical
Journa
Scintillation in the Circinus Galaxy water megamasers
We present observations of the 22 GHz water vapor megamasers in the Circinus
galaxy made with the Tidbinbilla 70m telescope. These observations confirm the
rapid variability seen earlier by Greenhill et al (1997). We show that this
rapid variability can be explained by interstellar scintillation, based on what
is now known of the interstellar scintillation seen in a significant number of
flat spectrum AGN. The observed variability cannot be fully described by a
simple model of either weak or diffractive scintillation.Comment: 10 pages, 5 figures. AJ accepte
Interferometric Phase Calibration Sources in the Declination Range 0deg to -30deg
We present a catalog of 321 compact radio sources in the declination range
0deg > delta > -30deg. The positions of these sources have been measured with a
two-dimensional rms accuracy of 35 milliarcseconds using the NRAO Very Large
Array. Each source has a peak flux density >50 mJy at 8.4 GHz. We intend for
this catalog to be used mainly for selection of phase calibration sources for
radio interferometers, although compact radio sources have other scientific
uses.Comment: 9 pages. To appear in ApJS. Catalog (Table 3) is abbreviated in
printed version. Complete catalog available at
ftp://ftp.aoc.nrao.edu/pub/staff/jwrobel/WPW2003_ApJS.tx
The Micro-Arcsecond Scintillation-Induced Variability (MASIV) Survey II: The First Four Epochs
We report on the variability of 443 flat spectrum, compact radio sources
monitored using the VLA for 3 days in 4 epochs at ~ 4 month intervals at 5 GHz
as part of the Micro-Arcsecond Scintillation-Induced Variability (MASIV)
survey. Over half of these sources exhibited 2-10% rms variations on timescales
over 2 days. We analyzed the variations by two independent methods, and find
that the rms variability amplitudes of the sources correlate with the emission
measure in the ionized Interstellar Medium along their respective lines of
sight. We thus link the variations with interstellar scintillation of
components of these sources, with some (unknown) fraction of the total flux
density contained within a compact region of angular diameter in the range
10-50 micro-arcseconds. We also find that the variations decrease for high mean
flux density sources and, most importantly, for high redshift sources. The
decrease in variability is probably due either to an increase in the apparent
diameter of the source, or a decrease in the flux density of the compact
fraction beyond z ~ 2. Here we present a statistical analysis of these results,
and a future paper will the discuss the cosmological implications in detail.Comment: 62 pages, 13 figures. Accepted for publication in the Astrophysical
Journa
Angular Broadening of Intraday Variable AGN. II. Interstellar and Intergalactic Scattering
We analyze a sample of 58 multi-wavelength, Very Long Baseline Array
observations of active galactic nuclei (AGN) to determine their scattering
properties. Approximately 75% of the sample consists of AGN that exhibit
centimeter-wavelength intraday variability (interstellar scintillation) while
the other 25% do not show intraday variability. We find that interstellar
scattering is measurable for most of these AGN, and the typical broadening
diameter is 2 mas at 1 GHz. We find that the scintillating AGN are typically at
lower Galactic latitudes than the non-scintillating AGN, consistent with the
scenario that intraday variability is a propagation effect from the Galactic
interstellar medium. The magnitude of the inferred interstellar broadening
measured toward the scintillating AGN, when scaled to higher frequencies, is
comparable to the diameters inferred from analyses of the light curves for the
more well-known intraday variable sources. However, we find no difference in
the amount of scattering measured toward the scintillating versus
non-scintillating AGN. A consistent picture is one in which the scintillation
results from localized regions ("clumps") distributed throughout the Galactic
disk, but which individually make little contribution to the angular
broadening. Of the 58 AGN observed, 37 (64%) have measured redshifts. At best,
a marginal trend is found for scintillating (non-scintillating) AGN to have
smaller (larger) angular diameters at higher redshifts. We also use our
observations to try to constrain the possibility of intergalactic scattering.
While broadly consistent with the scenario of a highly turbulent intergalactic
medium, our observations do not place significant constraints on its
properties.Comment: 13 pages, 4 figures; AASTeX format; ApJ in pres
Rapid Variability and Annual Cycles in the Characteristic Time-scale of the Scintillating Source PKS 1257-326
Rapid radio intra-day variability (IDV) has been discovered in the southern
quasar PKS 1257-326. Flux density changes of up to 40% in as little as 45
minutes have been observed in this source, making it, along with PKS 0405-385
and J1819+3845, one of the three most rapid IDV sources known. We have
monitored the IDV in this source with the Australia Telescope Compact Array
(ATCA) at 4.8 and 8.6 GHz over the course of the last year, and find a clear
annual cycle in the characteristic time-scale of variability. This annual cycle
demonstrates unequivocally that interstellar scintillation is the cause of the
rapid IDV at radio wavelengths observed in this source. We use the observed
annual cycle to constrain the velocity of the scattering material, and the
angular size of the scintillating component of PKS 1257-326. We observe a time
delay, which also shows an annual cycle, between the similar variability
patterns at the two frequencies. We suggest that this is caused by a small (~10
microarcsecond) offset between the centroids of the 4.8 and 8.6 GHz components,
and may be due to opacity effects in the source. The statistical properties of
the observed scintillation thus enable us to resolve source structure on a
scale of ~10 microarcseconds, resolution orders of magnitude higher than
current VLBI techniques allow. General implications of scintillation for the
physical properties of sources and the turbulent ISM are discussed.Comment: 28 pages, 7 figures. Accepted for publication in the Astrophysical
Journa
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