182 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
Hydrothermal quartz microtextures and depositional processes revealed by SEM-CL Imaging
SUMMARY -SEM-CL imaging of hydrothermal quartz from the Te Kopia geothermal field (New Zealand), complemented by fluid inclusion analysis, has revealed a complex history of crystal growth, dissolution and overprinting, unseen by other observational techniques. CL-dark quartz (characterised by euhedral growth zones) grew into fluid-filled space, at least m below the present ground surface, in a 195 reservoir. Movement on the Paeroa Fault provided pathways for fluids to move through the system, which resulted in further quartz precipitation, but SEM-CL evidence also shows that the quartz was partly dissolved, with later overprinting and void-filling by CL-bright quartz
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
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
Intraday variability of AGNs in the southern hemisphere
Understanding of the spectral and polarimetric characteristics of rapidly scintillating blazars is fundamental in order to describe both the innermost (sub-pc) regions of these compact objects and the interstellar medium responsible for the scintillation. A multi frequency analysis of the intraday variability in PMN J1326-5256, based on the combination of Australia Telescope Compact Array observations with the data from the monitoring projects at the University of Tasmania, will be described. Some implications concerning the structure of compact radio cores and the properties of the interstellar medium will be discussed
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
SHRIMP ion probe zircon geochronology and Sr and Nd isotope geochemistry for southern Longwood Range and Bluff Peninsula intrusive rocks of Southland, New Zealand
Permian–Jurassic ultramafic to felsic intrusive complexes at Bluff Peninsula and in the southern Longwood Range along the Southland coast represent a series of intraoceanic magmatic arcs with ages spanning a time interval of 110 m.y. New SHRIMP U-Pb zircon data for a quartz diorite from the Flat Hill complex, Bluff Peninsula, yield an age of 259 ± 4 Ma, consistent with other geochronological and paleontological evidence confirming a Late Permian age. The new data are consistent with an age of c. 260 Ma for the intrusive rocks of the Brook Street Terrane. SHRIMP U-Pb zircon ages for the southern Longwood Range confirm that intrusions become progressively younger from east to west across the complex. A gabbro at Oraka Point (eastern end of coastal section) has an age of 245 ± 4 Ma and shows virtually no evidence of zircon inheritance. The age is significantly different from that of the Brook Street Terrane intrusives. Zircon ages from the western parts of the section are younger and more varied (203–227 Ma), indicating more complex magmatic histories. A leucogabbro dike from Pahia Point gives the youngest emplacement age of 142 Ma, which is similar to published U-Pb zircon ages for the Anglem Complex and Paterson Group on Stewart Island
A seasonal cycle and an abrupt change in the variability characteristics of the intraday variable source S4 0954+65
The BLLac object S4 0954+65 is one of the main targets of the Urumqi
monitoring program targeting IntraDay Variable (IDV) sources. Between August
2005 and December 2009, the source was included in 41 observing sessions,
carried out at a frequency of 4.8 GHz. The time analysis of the collected light
curves, performed by applying both a structure function analysis and a
specifically developed wavelet-based algorithm, discovered an annual cycle in
the variability timescales, suggesting that there is a fundamental contribution
by interstellar scintillation to the IDV pattern of the source. The combined
use of the two analysis methods also revealed that there was a dramatic change
in the variability characteristics of the source between February and March
2008, at the starting time of a strong outburst phase. The analysis' results
suggest that the flaring state of the source coincides with the appearance of
multiple timescales in its light curves, indicating that changes in the
structure of the relativistically moving emitting region may strongly influence
the variability observed on IDV timescales.Comment: 9 pages, 8 figures and 3 tables. Accepted for publication in
Astronomy and Astrophysic
Milliarcsecond-Scale Structure in the Gamma-Ray Loud Quasar PKS 1622-297
We have made a high-resolution VLBI observation of the gamma-ray loud quasar
PKS 1622-297 with the HALCA spacecraft and ground radio telescopes at 5 GHz in
1998 February, almost three years after the source exhibited a spectacular GeV
gamma-ray flare. The source shows an elongated structure toward the west on the
parsec scale. The visibility data are well modeled by three distinct
components; a bright core and two weaker jet components. Comparison with
previous observations confirms that the jet components have an apparent
superluminal motion up to 12.1 h^{-1}c, with the inner jet components having
lower superluminal speeds. We apply the inverse Compton catastrophe model and
derive a Doppler factor, \delta, of 2.45, which is somewhat lower than that of
other gamma-ray loud active galactic nuclei (AGNs), suggesting the source was
in a more quiescent phase at the epoch of our observation. As an alternative
probe of the sub-parsec scale structure, we also present the results from
multi-epoch ATCA total flux monitoring, which indicate the presence of
persistent intraday variability consistent with refractive interstellar
scintillation. We examine the gamma-ray emission mechanism in the light of
these observations.Comment: 10 pages, 6 figures, 3 tables, to appear in PASJ, Vol.58, No.
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