106 research outputs found
Annual modulation in the scattering of J1819+3845: peculiar plasma velocity and anisotropy
We present two years of monitoring observations of the extremely variable quasar J1819+3845. We observe large yearly changes in the timescale of the variations (from ~ 1 hour to ~ 10 hours at 5GHz). This annual effect can only be explained if the variations are caused by a propagation effect, and thus affected by the Earth's relative speed through the projected intensity pattern. To account for this effect, the scattering plasma must have a transverse velocity with respect to the local standard of rest. The velocity calculated from these observations is in good agreement with that obtained from a two telescope delay experiment (Dennett-Thorpe & de Bruyn 2001). We also show that either the source itself is elongated, or that the scattering plasma is anisotropic, with an axial ratio of >6:1. As the source is extended on scales relevant to the scattering phenomenon, it seems plausible that the anisotropy is due to the source itself, but this remains to be investigated. From the scintillation characteristics we find that the scattering material is a very strong, thin scatterer within ~ten parsecs. We determine a source size at 5GHz of 100 to 900microarcsecs, and associated brightness temperatures of 10^{10} to 10^{12}K
Asymmetry of jets, lobe size and spectral index in radio galaxies and quasars
We investigate the correlations between spectral index, jet side and extent
of the radio lobes for a sample of nearby FRII radio galaxies. In
Dennett-Thorpe et al. (1997) we studied a sample of quasars and found that the
high surface brightness regions had flatter spectra on the jet side (explicable
as a result of Doppler beaming) whilst the extended regions had spectral
asymmetries dependent on lobe length. Unified schemes predict that asymmetries
due to beaming will be much smaller in narrow-line radio galaxies than in
quasars: we therefore investigate in a similar manner, a sample of radio
galaxies with detected jets. We find that spectral asymmetries in these objects
are uncorrelated with jet sidedness at all brightness levels, but depend on
relative lobe volume. Our results are not in conflict with unified schemes, but
suggest that the differences between the two samples are due primarily to power
or redshift, rather than to orientation. We also show directly that hotspot
spectra steepen as a function of radio power or redshift. Whilst a shift in
observed frequency due to the redshift may account for some of the steepening,
it cannot account for all of it, and a dependence on radio power is required.Comment: accepted for publication in MNRAS, 10 pages; typos/minor correctio
Relativistic and slowing down: the flow in the hotspots of powerful radio galaxies and quasars
Pairs of radio emitting jets with lengths up to several hundred kiloparsecs
emanate from the central region (the `core') of radio loud active galaxies.
In the most powerful of them, these jets terminate in the `hotspots', compact
high brightness regions, where the jet flow collides with the intergalactic
medium (IGM). Although it has long been established that in their inner
(parsec) regions these jet flows are relativistic, it is still not clear
if they remain so at their largest (hundreds of kiloparsec) scales. We argue
that the X-ray, optical and radio data of the hotspots, despite their
at-first-sight disparate properties, can be unified in a scheme involving a
relativistic flow upstream of the hotspot that decelerates to the
sub-relativistic speed of its inferred advance through the IGM and viewed at
different angles to its direction of motion. This scheme, besides providing an
account of the hotspot spectral properties with jet orientation, it also
suggests that the large-scale jets remain relativistic all the way to the
hotspots.Comment: to appear in ApJ
Detection of Six Rapidly Scintillating AGNs and the Diminished Variability of J1819+3845
The extreme, intra-hour and > 10% rms flux density scintillation observed in
AGNs such as PKS 0405-385, J1819+3845 and PKS 1257-326 at cm wavelengths has
been attributed to scattering in highly turbulent, nearby regions in the
interstellar medium. Such behavior has been found to be rare. We searched for
rapid scintillators among 128 flat spectrum AGNs and analyzed their properties
to determine the origin of such rapid and large amplitude radio scintillation.
The sources were observed at the VLA at 4.9 and 8.4 GHz simultaneously at two
hour intervals over 11 days. We detected six rapid scintillators with
characteristic time-scales of
10%. We found strong lines of evidence linking rapid scintillation to the
presence of nearby scattering regions, estimated to be < 12 pc away for ~ 200
muas sources and < 250 pc away for ~ 10 muas sources. We attribute the scarcity
of rapid and large amplitude scintillators to the requirement of additional
constraints, including large source compact fractions. J1819+3845 was found to
display ~ 2% rms variations at ~ 6 hour time-scales superposed on longer > 11
day variations, suggesting that the highly turbulent cloud responsible for its
extreme scintillation has moved away, with its scintillation now caused by a
more distant screen ~ 50 to 150 pc away.Comment: 5 pages, 3 figures, accepted for publication in Astronomy and
Astrophysic
Asymmetry of jets, lobe length and spectral index in quasars
The less depolarized lobe of a radio source is generally the lobe containing
the jet (Laing-Garrington correlation) but the less depolarized lobe is also
generally that with the flatter radio spectrum (Liu-Pooley correlation). Both
effects are strong; taken together they would imply a correlation between jet
side and lobe spectral index, i.e. between an orientation-dependent feature and
one which is intrinsic. We test this prediction using detailed spectral imaging
of a sample of quasars with well-defined jets and investigate whether the
result can be reconciled with the standard interpretation of one-sided jets in
terms of relativistic aberration. Our central finding is that the spectrum of
high surface brightness regions is indeed flatter on the jet side, but that the
spectrum of low surface brightness regions is flatter on the side with the
longer lobe. We discuss possible causes for these correlations and favour
explanations in terms of relativistic bulk motion in the high surface
brightness regions and differential synchrotron ageing in the extended lobe
material.Comment: to be published in MNRAS. 12 pages, uses mn.sty. Replacement corrects
for error in postscript (fig. 1
On the influence of the Sun on the rapid variability of compact extragalactic sources
Starting from December 2004, a program for the monitoring of intraday
variable sources at a frequency of 5 GHz was performed at the Urumqi
Observatory. The analysis of the variability characteristics of the
flat-spectrum radio source AO 0235+164 revealed the existence of an annual
cycle in the variability amplitude. This appears to correlate with the solar
elongation of the source. A thorough analysis of the results of the MASIV IDV
survey --- which provides the variability characteristics of a large sample of
compact radio sources --- confirms that there is a small but detectable
component of the observed fractional modulation which increases with decreasing
solar elongation. We discuss the hypothesis that the phenomenon is related to
interplanetary scintillation.Comment: 10 pages, 12 figures and 2 tables. Accepted for publication in
Astronomy and Astrophysic
Diffractive Interstellar Scintillation of the Quasar J1819+3845 at 21cm
We report the discovery of fast, frequency-dependent intensity variations
from the scintillating intra-day variable quasar J1819+3845 at a wavelength of
21cm which resemble diffractive interstellar scintillations observed in
pulsars. The timescale (down to 20 min) and the bandwidth (frequency
decorrelation bandwidth of 160 MHz) of the observed variations jointly imply
that the component of the source exhibiting this scintillation must possess a
brightness temperature well in excess of the inverse Compton limit. A specific
model in which both the source and scintillation pattern are isotropic implies
a brightness temperature 0.5 x 10^13 z_{pc} K, where previous estimates place
the distance to the scattering medium in the range z_{pc}=4-12pc, yielding a
minimum brightness temperature >20 times the inverse Compton limit. An
independent estimate of the screen distance using the 21cm scintillation
properties alone indicates a minimum screen distance of z approx 40pc and a
brightness temperature above 2 x 10^14 K. There is no evidence for anisotropy
in the scattering medium or source from the scintillation characteristics, but
these estimates may be reduced by a factor comparable to the axial ratio if the
source is indeed elongated.Comment: A&A in press, 18 pages, 9 fig
Scintillation-induced Variability in Radio Absorption Spectra against Extragalactic Sources
Spectral features absorbed against some radio quasars exhibit ~50mJy
variations, with the lines varying both relative to the continuum and, when
several lines are present, even relative to one another. We point out that such
variability can be expected as a consequence of refractive scintillation caused
by the interstellar medium of our Galaxy. Scintillation can cause independent
variations between closely-spaced spectral lines, and can even alter the line
profile. The background source need not be compact to exhibit spectral
variability. The variability can be used to infer the parsec to sub-parsec
scale structure of the intervening absorbing material. We discuss the
importance of scintillation relative to other possible origins of spectral
variability. The present theory is applied to account for the variations
observed in the HI-absorbed quasar PKS 1127-145.Comment: A&A in press, 17 pages, 3 fig
Intra-day variability observations of S5 0716+714 over 4.5 years at 4.8 GHz
We aim to search for evidence of annual modulation in the time scales of the
BL Lac object S5 0716+714. The intra-day variability (IDV) observations were
carried out monthly from 2005 to 2009, with the Urumqi 25m radio telescope at
4.8 GHz. The source has shown prominent IDV as well as long-term flux
variations. The IDV time scale does show evidence in favor of an annual
modulation, suggesting that the IDV of 0716+714 is dominated by interstellar
scintillation. The source underwent a strong outburst phase between mid-2008
and mid-2009; a second intense flare was observed in late 2009, but no
correlation between the total flux density and the IDV time scale is found,
implying that the flaring state of the source does not have serious
implications for the general characteristics of its intra-day variability.
However, we find that the inner-jet position angle is changing throughout the
years, which could result in an annual modulation noise in the anisotropic ISS
model fit. There is also an indication that the lowest IDV amplitudes (rms flux
density) correspond to the slowest time scales of IDV, which would be
consistent with an ISS origin of the IDV of 0716+714.Comment: 6 pages, 7 figures, accepted for publication in A&A; corrected typos
in Table
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