175 research outputs found
Extremely Anisotropic Scintillations
A small number of quasars exhibit interstellar scintillation on time-scales
less than an hour; their scintillation patterns are all known to be
anisotropic. Here we consider a totally anisotropic model in which the
scintillation pattern is effectively one-dimensional. For the persistent rapid
scintillators J1819+3845 and PKS1257-326 we show that this model offers a good
description of the two-station time-delay measurements and the annual cycle in
the scintillation time-scale. Generalising the model to finite anisotropy
yields a better match to the data but the improvement is not significant and
the two additional parameters which are required to describe this model are not
justified by the existing data. The extreme anisotropy we infer for the
scintillation patterns must be attributed to the scattering medium rather than
a highly elongated source. For J1819+3845 the totally anisotropic model
predicts that the particular radio flux variations seen between mid July and
late August should repeat between late August and mid November, and then again
between mid November and late December as the Earth twice changes its direction
of motion across the scintillation pattern. If this effect can be observed then
the minor-axis velocity component of the screen and the orientation of that
axis can both be precisely determined. In reality the axis ratio is finite,
albeit large, and spatial decorrelation of the flux pattern along the major
axis may be observable via differences in the pairwise fluxes within this
overlap region; in this case we can also constrain both the major-axis velocity
component of the screen and the magnitude of the anisotropy.Comment: 5 pages, 4 figures, MNRAS submitte
The microarcsecond structure of an active galactic nucleus jet via interstellar scintillation
We describe a new tool for studying the structure and physical
characteristics of ultracompact AGN jets and their surroundings with
microarcsecond precision. This tool is based on the frequency dependence of the
light curves observed for intra-day variable radio sources, where the
variability is caused by interstellar scintillation. We apply this method to
PKS1257-326 to resolve the core-shift as a function of frequency on scales well
below ~12 microarcseconds. We find that the frequency dependence of the
position of the scintillating component is r \propto \nu^{-0.1 \pm 0.24} (99%
confidence interval) and the frequency dependence of the size of the
scintillating component is d \propto \nu^{-0.64 \pm 0.006}. Together, these
results imply that the jet opening angle increases with distance along the jet:
d \propto r^{n_d}$ with n_d > 1.8. We show that the flaring of the jet, and
flat frequency dependence of the core position is broadly consistent with a
model in which the jet is hydrostatically confined and traversing a steep
pressure gradient in the confining medium with p \propto r^{-n_p} and n_p > 7.
Such steep pressure gradients have previously been suggested based on VLBI
studies of the frequency dependent core shifts in AGN.Comment: accepted for publication in Ap
The Deep X-ray Radio Blazar Survey (DXRBS). II. New Identifications
We have searched the archived, pointed ROSAT Position Sensitive Proportional
Counter data for blazars by correlating the WGACAT X-ray database with several
publicly available radio catalogs, restricting our candidate list to
serendipitous X-ray sources with a flat radio spectrum (alpha_r <= 0.70). This
makes up the Deep X-ray Radio Blazar Survey (DXRBS). Here we present new
identifications and spectra for 106 sources, including 86 radio-loud quasars,
11 BL Lacertae objects, and 9 narrow-line radio galaxies. Together with our
previously published objects and already known sources, our sample now contains
298 identified objects: 234 radio-loud quasars (181 flat-spectrum quasars: FSRQ
[alpha_r <= 0.50] and 53 steep-spectrum quasars: SSRQ), 36 BL Lacs, and 28
narrow-line radio galaxies. Redshift information is available for 96% of these.
Thus our selection technique is ~ 90% efficient at finding radio-loud quasars
and BL Lacs. Reaching 5 GHz radio fluxes ~ 50 mJy and 0.1-2.0 keV X-ray fluxes
a few x 10^-14 erg/cm^2/s, DXRBS is the faintest and largest flat-spectrum
radio sample with nearly complete (~ 85%) identification. We review the
properties of the DXRBS blazar sample, including redshift distribution and
coverage of the X-ray-radio power plane for quasars and BL Lacs. Additionally,
we touch upon the expanded multiwavelength view of blazars provided by DXRBS.
By sampling for the first time the faint end of the radio and X-ray luminosity
functions, this sample will allow us to investigate the blazar phenomenon and
the validity of unified schemes down to relatively low powers.Comment: 33 pages, 5 figures. Accepted for publication in MNRAS. Postscript
file also available at http://www.stsci.edu/~padovani/survey.htm
Parsec-scale radio morphology and variability of a changing-look AGN: the case of Mrk 590
We investigate the origin of the parsec-scale radio emission from the
changing-look active galactic nucleus (AGN) of Mrk 590, and examine whether the
radio power has faded concurrently with the dramatic decrease in accretion
rates observed between the 1990s and the present. We detect a compact core at
1.6 GHz and 8.4 GHz using new Very Long Baseline Array observations, finding no
significant extended, jet-like features down to 1 pc scales. The flat
spectral index () and high brightness temperature
() indicate self-absorbed synchrotron emission
from the AGN. The radio to X-ray luminosity ratio of , similar to that in coronally active stars, suggests
emission from magnetized coronal winds, although unresolved radio jets are also
consistent with the data. Comparing new Karl G. Jansky Very Large Array
measurements with archival and published radio flux densities, we find ,
, and (insignificantly) flux density decreases between the 1990s
and the year 2015 at 1.4 GHz, 5 GHz and 8.4 GHz respectively. This trend,
possibly due to the expansion and fading of internal shocks within the
radio-emitting outflow after a recent outburst, is consistent with the decline
of the optical-UV and X-ray luminosities over the same period. Such correlated
variability demonstrates the AGN accretion-outflow connection, confirming that
the changing-look behaviour in Mrk 590 originates from variable accretion rates
rather than dust obscuration. The present radio and X-ray luminosity
correlation, consistent with low/hard state accretion, suggests that the black
hole may now be accreting in a radiatively inefficient mode.Comment: 14 pages, 5 tables, 5 figures, accepted for publication in MNRA
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
EVN & MERLIN studies of a new sample of BL Lac objects
The recent Deep X-ray Radio Blazar Survey (DXRBS) has identified a sample of BL Lac objects spanning the intermediate range of spectral energy distributions between “classical” X-ray selected and radio-selected samples of BL Lacs. Detailed studies of such samples are needed to answer some of the currently open questions regarding the nature of BL Lacs and their place in a unified model of AGN. High-resolution radio imaging provides direct information on jet evolution and beaming parameters. We present some preliminary results from EVN & MERLIN observations of sources in the DXRBS BL Lac sample for which little or no high-resolution radio data were previously available
Observations of Intrahour Variable Quasars: Scattering in our Galactic Neighbourhood
Interstellar scintillation (ISS) has been established as the cause of the
random variations seen at centimetre wavelengths in many compact radio sources
on timescales of a day or less. Observations of ISS can be used to probe
structure both in the ionized insterstellar medium of the Galaxy, and in the
extragalactic sources themselves, down to microarcsecond scales. A few quasars
have been found to show large amplitude scintillations on unusually rapid,
intrahour timescales. This has been shown to be due to weak scattering in very
local Galactic ``screens'', within a few tens of parsec of the Sun. The short
variability timescales allow detailed study of the scintillation properties in
relatively short observing periods with compact interferometric arrays. The
three best-studied ``intrahour variable'' quasars, PKS 0405-385, J1819+3845 and
PKS 1257-326, have been instrumental in establishing ISS as the principal cause
of intraday variability at centimetre wavelengths. Here we review the relevant
results from observations of these three sources.Comment: 10 pages, 4 figures, to appear in Astronomical and Astrophysical
Transaction
The connection between radio and high energy emission in black hole powered systems in the SKA era
Strong evidence exists for a highly significant correlation between the radio
flux density and gamma-ray energy flux in blazars revealed by Fermi. However,
there are central issues that need to be clarified in this field: what are the
counterparts of the about 30% of gamma-ray sources that are as yet
unidentified? Are they just blazars in disguise or they are something more
exotic, possibly associated with dark matter? How would they fit in the
radio-gamma ray connection studied so far?
With their superb sensitivity, SKA1-MID and SKA1-SUR will help to resolve all
of these questions. Even more, while the radio-MeV/GeV connection has been
firmly established, a radio-VHE connection has been entirely elusive so far.
The advent of CTA in the next few years and the expected CTA-SKA1 synergy will
offer the chance to explore this connection, even more intriguing as it
involves the opposite ends of the electromagnetic spectrum and the acceleration
of particles up to the highest energies.
We are already preparing to address these questions by exploiting data from
the various SKA pathfinders and precursors. We have obtained 18 cm European
VLBI Network observations of E>10 GeV sources, with a detection rate of 83%.
Moreover, we are cross correlating the Fermi catalogs with the MWA
commissioning survey: when faint gamma-ray sources are considered, pure
positional coincidence is not significant enough for selecting counterparts and
we need an additional physical criterion to pinpoint the right object. It can
be radio spectral index, variability, polarization, or compactness, needing
high angular resolution in SKA1-MID; timing studies can also reveal pulsars,
which are often found from dedicated searches of unidentified gamma-ray
sources. SKA will be the ideal instrument for investigating these
characteristics in conjunction with CTA.
(abridged)Comment: 12 pages, to be published in the proceedings of "Advancing
Astrophysics with the Square Kilometre Array", PoS(AASKA14)15
HI emission and absorption in nearby, gas-rich galaxies II. -- sample completion and detection of intervening absorption in NGC 5156
We present the results of a survey for intervening 21cm HI absorption in a
sample of 10 nearby, gas-rich galaxies selected from the HI Parkes All-Sky
Survey (HIPASS). This follows the six HIPASS galaxies searched in previous work
and completes our full sample. In this paper we searched for absorption along
17 sightlines with impact parameters between 6 and 46 kpc, making one new
detection. We also obtained simultaneous HI emission-line data, allowing us to
directly relate the absorption-line detection rate to the HI distribution. From
this we find the majority of the non-detections in the current sample are
because sightline does not intersect the HI disc of the galaxy at sufficiently
high column density, but that source structure is also an important factor.
The detected absorption-line arises in the galaxy NGC 5156 () at an
impact parameter of 19 kpc. The line is deep and narrow with an integrated
optical depth of 0.82 km s. High resolution Australia Telescope Compact
Array (ATCA) images at 5 and 8 GHz reveal that the background source is
resolved into two components with a separation of 2.6 arcsec (500 pc at the
redshift of the galaxy), with the absorption likely occurring against a single
component. We estimate that the ratio of the spin temperature and covering
factor, , is approximately 950 K in the outer disc of NGC
5156, but further observations using VLBI would allow us to accurately measure
the covering factor and spin temperature of the gas.Comment: 31 pages, 11 figure
Milliarcsecond Structures of Variable Peaked-Spectrum Sources
Spectral variability offers a new technique to identify small scale
structures from scintillation, as well as determining the absorption mechanism
for peaked-spectrum (PS) radio sources. In this paper, we present very long
baseline interferometry (VLBI) imaging using the Long Baseline Array (LBA) of
two PS sources, MRC0225-065 and PMNJ0322-4820, identified as spectrally
variable from observations with the Murchison Widefield Array (MWA). We compare
expected milliarcsecond structures based on the detected spectral variability
with direct LBA imaging. We find MRC0225-065 is resolved into three components,
a bright core and two fainter lobes, roughly 430pc projected separation. A
comprehensive analysis of the magnetic field, host galaxy properties, and
spectral analysis implies that MRC0225-065 is a young radio source with recent
jet activity over the last 10^2-10^3years. We find PMNJ0322-4820 is unresolved
on milliarcsecond scales. We conclude PMNJ0322-4820 is a blazar with flaring
activity detected in 2014 with the MWA. We use spectral variability to predict
morphology and find these predictions consistent with the structures revealed
by our LBA images.Comment: Accepted for publication in PASA. 11 pages, 4 figure
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