44,121 research outputs found
Strong-field gravitational-wave emission in Schwarzschild and Kerr geometries: some general considerations
We show how the concurrent implementation of the exact solutions of the
Einstein equations, of the equations of motion of the test particles, and of
the relativistic estimate of the emission of gravitational waves from test
particles, can establish a priori constraints on the possible phenomena
occurring in Nature. Two examples of test particles starting at infinite
distance or from finite distance in a circular orbit around a Kerr black hole
are considered: the first leads to a well defined gravitational wave burst the
second to a smooth merging into the black hole. This analysis is necessary for
the study of the waveforms in merging binary systems.Comment: Resubmitted to PRD after Referee repor
High Angular Resolution Observations of the Collimated Jet Source Associated with a Massive Protostar in IRAS 16547-4247
A triple radio source recently detected in association with the luminous
infrared source IRAS 16547-4247 has been studied with high angular resolution
and high sensitivity with the Very Large Array at 3.6 and 2 cm. Our
observations confirm the interpretation that the central object is a thermal
radio jet, while the two outer lobes are most probably heavily obscured HH
objects. The thermal radio jet is resolved angularly for the first time and
found to align closely with the outer lobes. The opening angle of the thermal
jet is estimated to be , confirming that collimated outflows can
also be present in massive protostars. The proper motions of the outer lobes
should be measurable over timescales of a few years. Several fainter sources
detected in the region are most probably associated with other stars in a young
cluster.Comment: 9 pages, 2 figure
The Arecibo Galaxy Environment Survey VII : A Dense Filament With Extremely Long HI Streams
We present completed observations of the NGC 7448 galaxy group and background
volume as part of the blind neutral hydrogen Arecibo Galaxy Environment Survey
(AGES). Our observations cover a region spanning 5x4 degrees, over a redshift
range of approximately -2,000 < cz < 20,000 km/s. A total of 334 objects are
detected, mostly in three overdensities at cz 7,500, cz 9,600 and
cz 11,400 km/s. The galaxy density is extremely high (15 per square
degree) and many (24%) show signs of extended HI emission, including some
features as much as 800 kpc in projected length. We describe the overall
characteristics of this environment : kinematics, typical galaxy colours and
mass to light ratios, and substructure. To aid in the cataloguing of this data
set, we present a new FITS viewer (FRELLED : Fits Realtime Explorer of Low
Latency in Every Dimension). This incorporates interactive source cataloguing
tools which increase our source extraction speed by approximately a factor of
50.Comment: 15 pages, 13 figures, accepted for publication in MNRA
VLA Observations of H I in the Helix Nebula (NGC 7293)
We report the detection of 21-cm line emission from H I in the planetary
nebula NGC 7293 (the Helix). The observations, made with the Very Large Array,
show the presence of a ring of atomic hydrogen that is associated with the
outer portion of the ionized nebula. This ring is most probably gas ejected in
the AGB phase that has been subsequently photodissociated by radiation from the
central star. The H I emission spreads over about 50 km/s in radial velocity.
The mass in H I is approximately 0.07 solar masses, about three times larger
than the mass in molecular hydrogen and comparable with the mass in ionized
hydrogen.Comment: 19 pages, 9 figure
Discovery of a deep Seyfert-2 galaxy at z = 0.222 behind NGC 300
We report on the unveiling of the nature of the unidentified X-ray source
3XMM J005450.3-373849 as a Seyfert-2 galaxy located behind the spiral galaxy
NGC 300 using Hubble Space Telescope data, new spectroscopic Gemini
observations and available XMM-Newton and Chandra data. We show that the X-ray
source is positionally coincident with an extended optical source, composed by
a marginally resolved nucleus/bulge, surrounded by an elliptical disc-like
feature and two symmetrical outer rings. The optical spectrum is typical of a
Seyfert-2 galaxy redshifted to z=0.222 +/- 0.001, which confirms that the
source is not physically related to NGC 300. At this redshift the source would
be located at 909+/-4 Mpc (comoving distance in the standard model). The X-ray
spectra of the source are well-fitted by an absorbed power-law model. By tying
between the six available spectra, we found a variable index
running from ~2 in 2000-2001 years, to 1.4-1.6 in the 2005-2014
period. Alternatively, by tying , we found variable absorption columns
of N_H ~ 0.34 x cm in 2000-2001 years, and 0.54-0.75 x
cm in the 2005-2014 period. Although we cannot distinguish
between an spectral or absorption origin, from the derived unabsorbed X-ray
fluxes, we are able to assure the presence of long-term X-ray variability.
Furthermore, the unabsorbed X-ray luminosities of 0.8-2 x 10 erg
s derived in the X-ray band are in agreement with a weakly obscured
Seyfert-2 AGN at .Comment: MNRAS, accepte
The runaway black hole GRO J1655-40
We have used the Hubble Space Telescope to measure the motion in the sky and
compute the galactocentric orbit of the black hole X-ray binary GRO J1655-40.
The system moves with a runaway space velocity of km s in a
highly eccentric () orbit. The black hole was formed in the
disk at a distance greater than 3 kpc from the Galactic centre and must have
been shot to such an eccentric orbit by the explosion of the progenitor star.
The runaway linear momentum and kinetic energy of this black hole binary are
comparable to those of solitary neutron stars and millisecond pulsars. GRO
J1655-40 is the first black hole for which there is evidence for a runaway
motion imparted by a natal kick in a supernova explosion.Comment: Published in Astronomy and Astrophysics. 5 pages, 2 color figures.
Color figure and animation can be found at
http://www.iafe.uba.ar/astronomia/mirabel/mirabel.html or
ftp://ftp.cea.fr/incoming/y2k01/mirabe
Disorder-induced double resonant Raman process in graphene
An analytical study is presented of the double resonant Raman scattering
process in graphene, responsible for the D and D features in the
Raman spectra. This work yields analytical expressions for the D and
D integrated Raman intensities that explicitly show the dependencies
on laser energy, defect concentration, and electronic lifetime. Good agreement
is obtained between the analytical results and experimental measurements on
samples with increasing defect concentrations and at various laser excitation
energies. The use of Raman spectroscopy to identify the nature of defects is
discussed. Comparison between the models for the edge-induced and the
disorder-induced D band intensity suggests that edges or grain boundaries can
be distinguished from disorder by the different dependence of their Raman
intensity on laser excitation energy. Similarly, the type of disorder can
potentially be identified not only by the intensity ratio
, but also by its laser energy
dependence. Also discussed is a quantitative analysis of quantum interference
effects of the graphene wavefunctions, which determine the most important
phonon wavevectors and scattering processes responsible for the D and
D bands.Comment: 10 pages, 4 figure
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