189 research outputs found
Cosmological Constant, Conical Defect and Classical Tests of General Relativity
We investigate the perihelion shift of the planetary motion and the bending
of starlight in the Schwarzschild field modified by the presence of a
-term plus a conical defect. This analysis generalizes an earlier
result obtained by Islam (Phys. Lett. A 97, 239, 1983) to the case of a pure
cosmological constant. By using the experimental data we obtain that the
parameter characterizing the conical defect is less than
and , respectively, on the length scales associated with such
phenomena. In particular, if the defect is generated by a cosmic string, these
values correspond to limits on the linear mass densities of and
, respectively.Comment: 9 pages, no figures, revte
Discovery of Radio Outbursts in the Active Nucleus of M81
The low-luminosity active galactic nucleus of M81 has been monitored at
centimeter wavelengths since early 1993 as a by-product of radio programs to
study the radio emission from Supernova 1993J. The extensive data sets reveal
that the nucleus experienced several radio outbursts during the monitoring
period. At 2 and 3.6 cm, the main outburst occurred roughly in the beginning of
1993 September and lasted for approximately three months; at longer
wavelengths, the maximum flux density decreases, and the onset of the burst is
delayed. These characteristics qualitatively resemble the standard model for
adiabatically expanding radio sources, although certain discrepancies between
the observations and the theoretical predictions suggest that the model is too
simplistic. In addition to the large-amplitude, prolonged variations, we also
detected milder changes in the flux density at 3.6 cm and possibly at 6 cm on
short (less than 1 day) timescales. We discuss a possible association between
the radio activity and an optical flare observed during the period that the
nucleus was monitored at radio wavelengths.Comment: To appear in The Astronomical Journal. Latex, 18 pages including
embedded figures and table
23 GHz VLBI Observations of SN 2008ax
We report on phase-referenced 23 GHz Very-Long-Baseline-Interferometry (VLBI)
observations of the type IIb supernova SN 2008ax, made with the Very Long
Baseline Array (VLBA) on 2 April 2008 (33 days after explosion). These
observations resulted in a marginal detection of the supernova. The total flux
density recovered from our VLBI image is 0.80.3 mJy (one standard
deviation). As it appears, the structure may be interpreted as either a
core-jet or a double source. However, the supernova structure could be somewhat
confused with a possible close by noise peak. In such a case, the recovered
flux density would decrease to 0.480.12 mJy, compatible with the flux
densities measured with the VLA at epochs close in time to our VLBI
observations. The lowest average expansion velocities derived from our
observations are km s (case of a double
source) and km s (taking the weaker source
component as a spurious, close by, noise peak, which is the more likely
interpretation). These velocities are 7.3 and 2 times higher, respectively,
than the maximum ejecta velocity inferred from optical-line observations.Comment: 4 pages, 3 figures. Accepted in A&A on 24/03/200
A sample of radio-loud QSOs at redshift ~ 4
We obtained spectra of 60 red, starlike objects (E< 18.8) identified with
FIRST radio sources, S_{1.4GHz} > 1 mJy. Eight are QSOs with redshift z>
3.6.Combined with our pilot search (Benn et al 2002), our sample of 121
candidates yields a total of 18 z > 3.6 QSOs (10 of these with z > 4.0). 8% of
candidates with S_{1.4GHz} 10
mJy are QSOs with z > 3.6. The surface density of E 1mJy,
z> 4 QSOs is 0.003 deg^{-2}. This is currently the only well-defined sample of
radio-loud QSOs at z ~ 4 selected independently of radio spectral index. The
QSOs are highly luminous in the optical (8 have M_B < -28, q_0 = 0.5, H_0 = 50
kms^{-1}Mpc^{-1}). The SEDs are as varied as those seen in optical searches for
high-redshift QSOs, but the fraction of objects with weak (strongly
self-absorbed) Ly alpha emission is marginally higher (3 out of 18) than for
high-redshift QSOs from SDSS (5 out of 96).Comment: Accepted for publication in MNRAS, 9 pages, Latex, 5 postscript
figures, 1 landscape table (postscript
Radio sources at low Galactic latitudes
We present high-resolution radio observations of a sample of 65 radio sources
at low Galactic latitudes. The sources were all observed at 5 GHz with the Very
Large Array A-array. MERLIN observations at 5 GHz of the ultracompact HII
region G34.26+0.15 and one of the extragalactic sources, B1857-000, are also
presented, as are GMRT observations of HI in the direction of three sources,
B1801-203, B1802-196 and B1938+229. These observations were made with the
objectives of (i) finding compact components suitable for studying the effects
of interstellar scattering at lower frequencies, (ii) identifying high
surface-brightness lobes of background radio sources to probe the Galactic
magnetic field on different scales via polarization observations, and (iii)
searching for young supernova remnants. We discuss the nature of the sources
found to have shell or shell-like structure and exhibiting both thermal and
non-thermal spectra. Of the remaining sources, B1749-281 is coincident within
the positional errors of a known pulsar, not detected earlier at 5 GHz. The
rest are likely to be background extragalactic objects.Comment: 12 pages, 6 figures (most with multiple images), 1 table. Accepted
for publicaton in MNRA
The Nature of Composite LINER/HII Galaxies, As Revealed from High-Resolution VLA Observations
A sample of 37 nearby galaxies displaying composite LINER/HII and pure HII
spectra was observed with the VLA in an investigation of the nature of their
weak radio emission. The resulting radio contour maps overlaid on optical
galaxy images are presented here, together with an extensive literature list
and discussion of the individual galaxies. Radio morphological data permit
assessment of the ``classical AGN'' contribution to the global activity
observed in these ``transition'' LINER galaxies. One in five of the latter
objects display clear AGN characteristics: these occur exclusively in
bulge-dominated hosts.Comment: 31 pages, 27 figures, accepted by ApJ
8.4GHz VLBI observations of SN2004et in NGC6946
We report on 8.4GHz Very Long Baseline Interferometry (VLBI) observations of
the type II-P supernova SN2004et in the spiral galaxy NGC 6946, made on 20
February 2005 (151 days after explosion). The Very Large Array (VLA) flux
density was 1.230.07 mJy, corresponding to an isotropic luminosity at
8.4GHz of (4.450.3) erg s Hz and a brightness
temperature of (1.30.3) K. We also provide an improved
source position, accurate to about 0.5 mas in each coordinate. The VLBI image
shows a clear asymmetry. From model fitting of the size of the radio emission,
we estimate a minimum expansion velocity of 15,7002,000 km s. This
velocity is more than twice the expected mean expansion velocity estimated from
a synchrotron self-absorbed emission model, thus suggesting that synchrotron
self-absorption is not relevant for this supernova. With the benefit of an
optical spectrum obtained 12 days after explosion, we favor an emission model
which consists of two hot spots on an underlying expanding shell of width
comparable to that of SN 1993J.Comment: Accepted for publication in A&A (22/05/07
Selective serotonin reuptake inhibitors in the treatment of generalized anxiety disorder
Selective serotonin reuptake inhibitors have proven efficacy in the treatment of panic disorder, obsessive–compulsive disorder, post-traumatic stress disorder and social anxiety disorder. Accumulating data shows that selective serotonin reuptake inhibitor treatment can also be efficacious in patients with generalized anxiety disorder. This review summarizes the findings of randomized controlled trials of selective serotonin reuptake inhibitor treatment for generalized anxiety disorder, examines the strengths and weaknesses of other therapeutic approaches and considers potential new treatments for patients with this chronic and disabling anxiety disorder
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