3,512 research outputs found
B2 1144+35B, a giant low power radio galaxy with superluminal motion. Orientation and evidence for recurrent activity
The goal of this work is a detailed study of the nearby, low power radio
galaxy B2 1144+35B. For this purpose, we performed new Very Long Baseline Array
(VLBA) and Very Large Array (VLA) observations. This source has several
properties (bright, nearby, large range of spatial structures, visible
counterjet, etc.) that make it an excellent astrophysical laboratory for the
study of the evolution of radio jets. Here we report the detection of motion in
the counterjet at 0.23 0.07 c, which allows us to estimate the
orientation of the parsec-scale jet at 33 from the line of
sight, with an intrinsic velocity of (0.94)c. We also report
on a brightening of the core at high frequencies which we suggest could be the
result of a new component emerging from the core. High dynamic range VLBA
observations at 5 GHz reveal, for the first time, extended emission connecting
the core with the bright complex of emission that dominates the flux density of
the parsec scale structure at frequencies below 20 GHz. The evolution of this
bright complex is considered and its slow decline in flux density is
interpreted as the result of an interaction with the interstellar medium of the
host galaxy.Comment: 7 pages, 6 b&w figures. A&A in pres
A sample of low-redshift BL Lacertae objects. II. EVN and MERLIN data and multi-wavelength analysis
We present new radio observations of 9 members of a sample of 29 nearby (z <
0.2) BL Lac objects. The new data have been obtained with the European VLBI
Network and/or the MERLIN at 1.6 and 5 GHz and complement previous
observations. For one object, the TeV source Mrk 421, we also present deep
multi-epoch VLBA and Global VLBI data, which reveal a resolved diffuse jet,
with clear signatures of limb brightening. We use the new and old data to
estimate physical parameters of the jets of the sample from which the subset
with new radio data is drawn. We derive Doppler factors in the parsec scale
radio jet in the range ~2 < delta < ~9. Using HST data, we separate the
contribution of the host galaxy from that of the active core. From the measured
and de-beamed observables, we find a weak correlation between radio power and
black hole mass, and a tight correlation between radio and optical core
luminosities. We interpret this result in terms of a common synchrotron origin,
with little contribution from a radiatively efficient accretion disk. The BL
Lacs in our sample have de-beamed properties similar to low power radio
galaxies, including the fundamental plane of black hole activity.Comment: Accepted by ApJ. 17 page
Squeezed Thermal Vacuum and the Maximum Scale for Inflation
We consider the stimulated emission of gravitons from an initial state of
thermal equilibrium, under the action of the cosmic gravitational background
field. We find that the low-energy graviton spectrum is enhanced if compared
with spontaneous creation from the vacuum; as a consequence, the scale of
inflation must be lowered, in order not to exceed the observed CMB quadrupole
anisotropy. This effect is particularly important for models based on a
symmetry-breaking transition which require, as initial condition, a state of
thermal equilibrium at temperatures of the order of the inflation scale.Comment: 13 pages, plain tex, three figures available upon request, to appear
in Phys.Rev.D, CERN-TH.6836/9
Primordial Magnetic Fields From String Cosmology
Sufficiently large seeds for generating the observed (inter)galactic magnetic
fields emerge naturally in string cosmology from the amplification of
electromagnetic vacuum fluctuations due to a dynamical dilaton background. The
success of the mechanism depends crucially on two features of the so-called
pre-big-bang scenario, an early epoch of dilaton-driven inflation at very small
coupling, and a sufficiently long intermediate stringy era preceding the
standard radiation-dominated evolution.Comment: 12 pages, latex, two figures available by fax upon reques
Radio Halo and Relic Candidates from the NRAO VLA Sky Survey
We present the first results of the search of new halo and relic candidates
in the NRAO VLA Sky Survey. We have inspected a sample of 205 clusters from the
X-ray-brightest Abell-type clusters presented by Ebeling et al (1996), and
found 29 candidates. Out of them, 11 clusters are already known from the
literature to contain a diffuse cluster-wide source, while in 18 clusters this
is the first indication of the existence of this type of sources. We classify
these sources as halos or relics according to their location in the cluster
center or periphery, respectively. We find that the occurrence of cluster halos
and relics is higher in clusters with high X-ray luminosity and high
temperature. We also confirm the correlation between the absence of a cooling
flow and the presence of a radio halo at the cluster center.Comment: 25 Pages, 4 Figures included in the text, Figures 3a to 3g in
separated files. New Astronomy in pres
Gravitational Radiation from String Cosmology
A spectrum of relic stochastic gravitational radiation, strongly tilted
towards high frequencies, and characterized by two basic parameters is shown to
emerge in a class of string theory models. We estimate the required sensitivity
for detection of the predicted gravitational radiation and show that a region
of our parameter space is within reach for some of the
plannedgravitational-wave detectors.Comment: 4 pages, uuencoded p
- âŠ