25,061 research outputs found
Evidence for Centrifugal Barrier in X-ray Pulsar GRO J1744-28
We present further observational evidence of the effects of a centrifugal
barrier in GRO J1744-28, based on continued monitoring of the source with RXTE.
For X-ray pulsars, the centrifugal barrier manifests itself in the cessation of
pulsed emission when the source becomes faint. We show that such phenomenon
occurred repeatedly for GRO J1744-28, following the decay of two X-ray
outbursts. This has allowed a direct measurement of the dipole field strength
for this pulsar. Here we argue that some of the other peculiar properties
observed of this source may also be related to an active centrifugal barrier.Comment: 4 pages. To appear in "Accretion Processes in Astrophysical Systems",
Proc. of the 8th Annual Astrophysics Conference in Maryland, S. S. Holt & T.
Kallman (eds.
News from a Multi-Wavelength Monitoring Campaign on Mrk 421
We conducted a daily monitoring campaign on Mrk 421 in 2003 and 2004 with the
Whipple 10 m telescope and the large-area instruments aboard RXTE,
simultaneously covering TeV and X-ray energies. Supporting observations at
optical and radio wavelengths were also frequently carried out. Mrk 421 was
observed over a wide range of fluxes (with a dynamic range of ~30 both at TeV
and X-ray energies). The source was relatively quiet in 2003 but became
unusually active in 2004, with flares reaching peak fluxes of ~80 mCrab in
X-rays and >3 Crab at TeV energies! We will describe the multiwavelength
campaign and present some preliminary results. We will also discuss the
implications of the results on the proposed emission models for TeV blazars.Comment: Proc. "International Symposium on High Energy Gamma-Ray Astronomy"
(Gamma-2004, Heidelberg, Germany), eds. F.A. Aharonian and H. Voelk, AIP
Conf. Ser. This preprint version contains 7 pages and 8 figure
X-ray Flaring Activity of Mrk 421
We report results from a systematic search for X-ray flares from Mrk 421,
using archival data from the Rossi X-ray Timing Explorer (RXTE) satellite. The
flares are clearly seen over a wide range of timescales. The quasi-continuous
coverage of the source with the All-Sky Monitor (ASM) reveals frequent
occurrence of major flares that last for months. On a few occasions, the source
was intensively monitored with the more sensitive pointing instruments aboard
RXTE. The data from these observations shows the presence of X-ray flares of
much shorter durations, ranging from weeks down to less than an hour. For the
first time, we clearly resolved the sub-hour flares from Mrk 421. Moreover,
Fourier analyses reveal variability on even shorter timescales, up to about
0.01 Hz. The source appears to behave differently in its spectral properties
during different flares, large or small, which is intriguing. While significant
hysteresis is observed to be associated with spectral evolution in some cases,
little is seen in other cases. Sometimes, the shape of the X-ray spectrum
hardly varies across a flare. Therefore, the phenomenology is complex. The
observed hierarchical structure of the X-ray flares seems to imply the
scale-invariant nature of the phenomenon, perhaps similar to solar flares or
rapid X-ray flares observed of stellar-mass black holes in this regard.
Combined with other results, the observed flaring timescales seriously
constrain the physical properties of X-ray emitting regions in the jets of Mrk
421.Comment: 24 pages, 10 figures, to appear in Ap
Energy-Efficient Multi-View Video Transmission with View Synthesis-Enabled Multicast
Multi-view videos (MVVs) provide immersive viewing experience, at the cost of
heavy load to wireless networks. Except for further improving viewing
experience, view synthesis can create multicast opportunities for efficient
transmission of MVVs in multiuser wireless networks, which has not been
recognized in existing literature. In this paper, we would like to exploit view
synthesis-enabled multicast opportunities for energy-efficient MVV transmission
in a multiuser wireless network. Specifically, we first establish a
mathematical model to characterize the impact of view synthesis on multicast
opportunities and energy consumption. Then, we consider the optimization of
view selection, transmission time and power allocation to minimize the weighted
sum energy consumption for view transmission and synthesis, which is a
challenging mixed discrete-continuous optimization problem. We propose an
algorithm to obtain an optimal solution with reduced computational complexity
by exploiting optimality properties. To further reduce computational
complexity, we also propose two low-complexity algorithms to obtain two
suboptimal solutions, based on continuous relaxation and Difference of Convex
(DC) programming, respectively. Finally, numerical results demonstrate the
advantage of the proposed solutions.Comment: 22 pages, 6 figures, to be published in GLOBECOM 201
On the Disappearance of Kilohertz Quasi-Periodic Oscillations at a High Mass Accretion Rate in Low-Mass X-ray Binaries
For all sources in which the phenomenon of kilo-Hertz quasi-periodic
oscillation (kHz QPO) is observed, the QPOs disappear abruptly when the
inferred mass accretion rate exceeds a certain threshold. Although the
threshold cannot at present be accurately determined (or even quantified)
observationally, it is clearly higher for bright Z sources than for faint atoll
sources. Here we propose that the observational manifestation of kHz QPOs {\em
requires} direct interaction between the neutron star magnetosphere and the
Keplerian accretion disk and that the cessation of kHz QPOs at high accretion
rate is due to the lack of such an interact when the Keplerian disk terminates
at the last stable orbit and yet the magnetosphere is pushed farther inward.
The threshold is therefore dependent of the magnetic field strength -- the
stronger the magnetic field the higher the threshold. This is certainly in
agreement with the atoll/Z paradigm, but we argue that it is also generally
true, even for individual sources within each (atoll or Z) category. For atoll
sources, the kHz QPOs also seem to vanish at low accretion rate. Perhaps the
``disengagement'' between the magnetosphere and the Keplerian disk also takes
place under such circumstances, because of, for instance, the presence of
quasi-spherical advection-dominated accretion flow (ADAF) close to the neutron
star. Unfortunately, in this case, the estimation of the accretion rate
threshold would require a knowledge of the physical mechanisms that cause the
disengagement. If the ADAF is responsible, the threshold is likely dependent of
the magnetic field of the neutron star.Comment: Minor revisions to match the published versio
Absolutely Maximally Entangled States: Existence and Applications
We investigate absolutely maximally entangled (AME) states, which are
multipartite quantum states that are maximally entangled with respect to any
possible bipartition. These strong entanglement properties make them a powerful
resource for a variety of quantum information protocols. In this paper, we show
the existence of AME states for any number of parties, given that the dimension
of the involved systems is chosen appropriately. We prove the equivalence of
AME states shared between an even number of parties and pure state threshold
quantum secret sharing (QSS) schemes, and prove necessary and sufficient
entanglement properties for a wider class of ramp QSS schemes. We further show
how AME states can be used as a valuable resource for open-destination
teleportation protocols and to what extend entanglement swapping generalizes to
AME states
- …