416 research outputs found
MERLIN imaging of the maser flare in Markarian 348
MERLIN images of Mrk 348 at 22 GHz show water maser emission at 0.02 - 0.11
Jy, within approximately 0.8 pc of the nucleus. This is the first direct
confirmation that molecular material exists close to the Seyfert 2 nucleus. Mrk
348 was observed in 2000 May one month after Falcke et al. (2000) first
identified the maser in single-dish spectra. The peak maser flux density has
increased about threefold. The masing region is < 0.6 pc in radius. The flux
density of radio continuum emission from the core has been rising for about 2
years. The maser-core separation is barely resolved but at the 3 sigma
significance level they are not coincident along the line of sight. The masers
lie in the direction of the northern radio lobes and probably emanate from
material shocked by a jet with velocity close to c. The correlation between the
radio continuum increase and maser flare is explained as arising from high
level nuclear activity through a common excitation mechanism although direct
maser amplification of the core by masers tracing a Keplerian disc is not
completely ruled out.Comment: Accepted by MNRAS Letters, 5 pages, 3 figures, corrections in text
and figur
Collisionless microinstabilities in stellarators II - numerical simulations
Microinstabilities exhibit a rich variety of behavior in stellarators due to
the many degrees of freedom in the magnetic geometry. It has recently been
found that certain stellarators (quasi-isodynamic ones with maximum-
geometry) are partly resilient to trapped-particle instabilities, because
fast-bouncing particles tend to extract energy from these modes near marginal
stability. In reality, stellarators are never perfectly quasi-isodynamic, and
the question thus arises whether they still benefit from enhanced stability.
Here the stability properties of Wendelstein 7-X and a more quasi-isodynamic
configuration, QIPC, are investigated numerically and compared with the
National Compact Stellarator Experiment (NCSX) and the DIII-D tokamak. In
gyrokinetic simulations, performed with the gyrokinetic code GENE in the
electrostatic and collisionless approximation, ion-temperature-gradient modes,
trapped-electron modes and mixed-type instabilities are studied. Wendelstein
7-X and QIPC exhibit significantly reduced growth rates for all simulations
that include kinetic electrons, and the latter are indeed found to be
stabilizing in the energy budget. These results suggest that imperfectly
optimized stellarators can retain most of the stabilizing properties predicted
for perfect maximum- configurations.Comment: 15 pages, 40 figure
A time-delay determination from VLA light curves of the CLASS gravitational lens B1600+434
We present Very Large Array (VLA) 8.5-GHz light curves of the two lens images
of the Cosmic Lens All Sky Survey (CLASS) gravitational lens B1600+434. We find
a nearly linear decrease of 18-19% in the flux densities of both lens images
over a period of eight months (February-October) in 1998. Additionally, the
brightest image A shows modulations up to 11% peak-to-peak on scales of days to
weeks over a large part of the observing period. Image B varies significantly
less on this time scale. We conclude that most of the short-term variability in
image A is not intrinsic source variability, but is most likely caused by
microlensing in the lens galaxy. The alternative, scintillation by the ionized
Galactic ISM, is shown to be implausible based on its strong opposite frequency
dependent behavior compared with results from multi-frequency WSRT monitoring
observations (Koopmans & de Bruyn 1999). From these VLA light curves we
determine a median time delay between the lens images of 47^{+5}_{-6} d (68%)
or 47^{+12}_{-9} d (95%). We use two different methods to derive the time
delay; both give the same result within the errors. We estimate an additional
systematic error between -8 and +7 d. If the mass distribution of lens galaxy
can be described by an isothermal model (Koopmans, de Bruyn & Jackson 1998),
this time delay would give a value for the Hubble parameter, H_0=57^{+14}_{-11}
(95% statistical) ^{+26}_{-15} (systematic) km/s/Mpc (Omega_m=1 and
Omega_Lambda=0). Similarly, the Modified-Hubble-Profile mass model would give
H_0=74^{+18}_{-15} (95% statistical) ^{+22}_{-22} (systematic) km/s/Mpc. For
Omega_m=0.3 and Omega_Lambda=0.7, these values increase by 5.4%. ... (ABRIDGED)Comment: 14 pages, 6 figures, accepted for publication in Astronomy &
Astrophysics (Figs 1 and 3 with degraded resolution
MERLIN observations of Stephan's Quintet
We present MERLIN L-band images of the compact galaxy group, Stephan's
Quintet. The Seyfert 2 galaxy, NGC 7319, the brightest member of the compact
group, is seen to have a triple radio structure typical of many extra-galactic
radio sources which have a flat spectrum core and two steep spectrum lobes with
hot spots. The two lobes are asymmetrically distributed on opposite sides of
the core along the minor axis of the galaxy. Ultraviolet emission revealed in a
high resolution HRC/ACS HST image is strongly aligned with the radio plasma and
we interpret the intense star formation in the core and north lobe as an event
induced by the collision of the north radio jet with over-dense ambient
material. In addition, a re-mapping of archive VLA L-band observations reveals
more extended emission along the major axis of the galaxy which is aligned with
the optical axis. Images formed from the combined MERLIN and archive VLA data
reveal more detailed structure of the two lobes and hot spots.Comment: Completely revised version with new HST data included, to appear in
MNRA
VLA 8.4-GHz monitoring observations of the CLASS gravitational lens B1933+503
The complex ten-component gravitational lens system B1933+503 has been
monitored with the VLA during the period February to June 1998 with a view to
measuring the time delay between the four compact components and hence to
determine the Hubble parameter. Here we present the results of an `A'
configuration 8.4-GHz monitoring campaign which consists of 37 epochs with an
average spacing of 2.8 days. The data have yielded light curves for the four
flat-spectrum radio components (components 1, 3, 4 and 6). We observe only
small flux density changes in the four flat-spectrum components which we do not
believe are predominantly intrinsic to the source. Therefore the variations do
not allow us to determine the independent time delays in this system. However,
the data do allow us to accurately determine the flux density ratios between
the four flat-spectrum components. These will prove important as modelling
constraints and could prove crucial in future monitoring observations should
these data show only a monotonic increase or decrease in the flux densities of
the flat-spectrum components.Comment: Accepted for publication in MNRAS. 5 pages, 2 included PostScript
figure
IRAM observations of JVAS/CLASS gravitational lenses
We have searched for molecular absorption lines at millimeter wavelengths in
eleven gravitational lens systems discovered in the JVAS/CLASS surveys of flat
spectrum radio sources. Spectra of only one source 1030+074 were obtained in
the 3-, 2- and 1.3-millimeter band at the frequencies corresponding to common
molecular transitions of CO and HCO+ as continuum emission was not found in any
of the other sources. We calculated upper limits to the column density in
molecular absorption for 1030+074, using an excitation temperature of 15 K, to
be N_{CO} < 6.3 x 10^{13} cm^{-2} and N_{HCO+} < 1.3 x 10^{11} cm^{-2},
equivalent to hydrogen column density of the order N_H < 10^{18} cm^{-2},
assuming standard molecular abundances. We also present the best upper limits
of the continuum at the lower frequency for the other 10 gravitational lenses.Comment: 6 pages, 1 figure, to appear in MNRA
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