7,063 research outputs found
Thermal Radiation from a Fluctuating Event Horizon
We consider a pointlike two-level system undergoing uniformly accelerated
motion. We evaluate the transition probability for a finite time interval of
this system coupled to a massless scalar field near a fluctuating event
horizon. Horizon fluctuations are modeled using a random noise which generates
light-cone fluctuations. We study the case of centered, stationary and Gaussian
random processes. The transition probability of the system is obtained from the
positive-frequency Wightman function calculated to one loop order in the noise
averaging process. Our results show that the fluctuating horizon modifies the
thermal radiation but leaves unchanged the temperature associated with the
acceleration.Comment: 8 pages, 3 figure
Boundary effects on radiative processes of two entangled atoms
We analyze radiative processes of a quantum system composed by two identical
two-level atoms interacting with a massless scalar field prepared in the vacuum
state in the presence of perfect reflecting flat mirrors. We consider that the
atoms are prepared in a stationary maximally entangled state. We investigate
the spontaneous transitions rates from the entangled states to the collective
ground state induced by vacuum fluctuations. In the empty-space case, the
spontaneous decay rates can be enhanced or inhibited depending on the specific
entangled state and changes with the distance between the atoms. Next, we
consider the presence of perfect mirrors and impose Dirichlet boundary
conditions on such surfaces. In the presence of a single mirror the transition
rate for the symmetric state undergoes a slight reduction, whereas for the
antisymmetric state our results indicate a slightly enhancement. Finally, we
investigate the effect of multiple reflections by two perfect mirrors on the
transition rates.Comment: submitted version to the journa
Investigating Galactic supernova remnant candidates with LOFAR
We investigate six supernova remnant (SNR) candidates --- G51.21+0.11,
G52.37-0.70, G53.07+0.49, G53.41+0.03, G53.84-0.75, and the possible shell
around G54.1-0.3 --- in the Galactic Plane using newly acquired LOw-Frequency
ARray (LOFAR) High-Band Antenna (HBA) observations, as well as archival
Westerbork Synthesis Radio Telescope (WSRT) and Very Large Array Galactic Plane
Survey (VGPS) mosaics. We find that G52.37-0.70, G53.84-0.75, and the possible
shell around pulsar wind nebula G54.1+0.3 are unlikely to be SNRs, while
G53.07+0.49 remains a candidate SNR. G51.21+0.11 has a spectral index of
, but lacks X-ray observations and as such requires further
investigation to confirm its nature. We confirm one candidate, G53.41+0.03, as
a new SNR because it has a shell-like morphology, a radio spectral index of
and it has the X-ray spectral characteristics of a
1000-8000 year old SNR. The X-ray analysis was performed using archival
XMM-Newton observations, which show that G53.41+0.03 has strong emission lines
and is best characterized by a non-equilibrium ionization model, consistent
with an SNR interpretation. Deep Arecibo radio telescope searches for a pulsar
associated with G53.41+0.03 resulted in no detection, but place stringent upper
limits on the flux density of such a source if it is beamed towards Earth.Comment: 9 pages, 4 figures, 1 tabl
The eccentric short-period orbit of the supergiant fast X-ray transient HD 74194 (=LM Vel)
Aims. We present the first orbital solution for the O-type supergiant star HD
74194, which is the optical counterpart of the supergiant fast X-ray transient
IGR J08408-4503. Methods. We measured the radial velocities in the optical
spectrum of HD 74194, and we determined the orbital solution for the first
time. We also analysed the complex H{\alpha} profile. Results. HD 74194 is a
binary system composed of an O-type supergiant and a compact object in a
short-period ( d) and high-eccentricity ()
orbit. The equivalent width of the H{\alpha} line is not modulated entirely
with the orbital period, but seems to vary in a superorbital period
( d) nearly 30 times longer than the orbital one.Comment: 4 pages, 6 figures, accepted for publication in A&
D'atri spaces of type k and related classes of geometries concerning jacobi operators
In this article we continue the study of the geometry of -D'Atri spaces,
( denotes the dimension of the manifold) began by
the second author. It is known that -D'Atri spaces, are related
to properties of Jacobi operators along geodesics, since she has shown
that , are invariant
under the geodesic flow for any unit tangent vector . Here, assuming that
the Riemannian manifold is a D'Atri space, we prove in our main result that
is also invariant under the geodesic flow if . In addition, other properties of Jacobi operators related to the
Ledger conditions are obtained and they are used to give applications to
Iwasawa type spaces. In the class of D'Atri spaces of Iwasawa type, we show two
different characterizations of the symmetric spaces of noncompact type: they
are exactly the -spaces and on the other hand they are -D'Atri
spaces for some In the last case, they are -D'Atri for all
as well. In particular, Damek-Ricci spaces that are -D'Atri
for some are symmetric.
Finally, we characterize -D'Atri spaces for all as the -spaces (geodesic symmetries preserve the principal curvatures of
small geodesic spheres). Moreover, applying this result in the case of 4%
-dimensional homogeneous spaces we prove that the properties of being a D'Atri
(1-D'Atri) space, or a 3-D'Atri space, are equivalent to the property of being
a -D'Atri space for all .Comment: 19 pages. This paper substitute the previous one where one Theorem
has been deleted and one section has been adde
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