4,557 research outputs found
Orbital and Spin Parameter Variations of Partial Eclipsing Low Mass X-ray Binary X 1822-371
We report our measurements for orbital and spin parameters of X 1822-371 using its X-ray partial eclipsing profile and pulsar timing from data collected by the Rossi X-ray Timing Explorer (RXTE). Four more X-ray eclipse times obtained by the RXTE 2011 observations were combined with historical records to trace evolution of orbital period. We found that a cubic ephemeris likely better describes evolution of the X-ray eclipse times during a time span of about 34 years with a marginal second order derivative of s. Using the pulse arrival time delay technique, the orbital and spin parameters were obtained from RXTE observations from 1998 to 2011. The detected pulse periods show that the neutron star in X 1822-371 is continuously spun-up with a rate of s s. Evolution of the epoch of the mean longitude (i.e. ) gives an orbital period derivative value consistent with that obtained from the quadratic ephemeris evaluated by the X-ray eclipse but the detected values are significantly and systematically earlier than the corresponding expected X-ray eclipse times by s. This deviation is probably caused by asymmetric X-ray emissions. We also attempted to constrain the mass and radius of the neutron star using the spin period change rate and concluded that the intrinsic luminosity of X 1822-371 is likely more than ergs s.postprin
Can Social Capital be Transferred Cross the Boundary of the Real and Virtual Worlds? An Empirical Investigation of Twitter
Micro-blogs like Twitter are playing increasingly important roles in social life. Some key users of Twitter have drawn huge attention from other people. Their opinions have had significant influence on the rest of Twitter users. In other words, these people are highly reputable and have more social capital in the Twitter world. But what factors contribute to the social capital in a part of the virtual world like Twitter is still largely unknown. This paper investigates the source of social capital in the Twitter world. We identify two types of sources that influence a user’s social capital in the Twitter world: (1) inherited capital from outside the Twitter world; and (2) social activities conducted within the Twitter world. The results show that both inherited capital from outside, and activities within, the Twitter world, have positive influence on a user’s social capital in the Twitter world. Our results suggest that social capital can be transferred from the real world to the virtual one. Meanwhile the inherited social capital of a user from outside the Twitter world significantly impacts the level of activities the user undertakes in the Twitter world. For ordinary people, inherited social capital positively associates with the level of their social activities in the Twitter world. But for the most well known Twitter users, who are usually celebrities, this relationship is negative. Implications for research and practice are further discussed
Circular dichroism in angle-resolved photoemission spectroscopy of topological insulators
Topological insulators are a new phase of matter that exhibits exotic surface
electronic properties. Determining the spin texture of this class of material
is of paramount importance for both fundamental understanding of its
topological order and future spin-based applications. In this article, we
review the recent experimental and theoretical studies on the differential
coupling of left- versus right-circularly polarized light to the topological
surface states in angle-resolved photoemission spectroscopy. These studies have
shown that the polarization of light and the experimental geometry plays a very
important role in both photocurrent intensity and spin polarization of
photoelectrons emitted from the topological surface states. A general
photoemission matrix element calculation with spin-orbit coupling can
quantitatively explain the observations and is also applicable to topologically
trivial systems. These experimental and theoretical investigations suggest that
optical excitation with circularly polarized light is a promising route towards
mapping the spin-orbit texture and manipulating the spin orientation in
topological and other spin-orbit coupled materials.Comment: submitted to Phys. Status Solidi RR
A New Kilohertz Gravitational-Wave Feature from Rapidly Rotating Core-Collapse Supernovae
We present self-consistent three-dimensional core-collapse supernova
simulations of a rotating progenitor model with various initial
angular velocities from to rad s using a smoothed particle
hydrodynamics code, SPHYNX, and a grid-based hydrodynamics code, FLASH. We
identify two strong gravitational-wave features, with peak frequencies of
Hz and kHz in the first ms postbounce. We demonstrate
that these two features are associated with the deformation from the
proto-neutron star (PNS) modulation induced by the low- instability,
regardless of the simulation code. The Hz feature is present in models
with an initial angular velocity between and rad s, while
the kHz feature is present only in a narrower range, from to
rad s. We show that the kHz signal originates from the
high-density inner core of the PNS, and the deformation triggers a strong
asymmetric distribution of electron anti-neutrinos. In addition to the Hz
and kHz features, we also observe one weaker but noticeable
gravitational-wave feature from higher-order modes in the range between
and rad s. Its peak frequency is around Hz initially and
gradually increases to Hz. Therefore, in addition to the
gravitational bounce signal, the detection of the Hz, kHz, the
higher-order mode, and even the related asymmetric emission of neutrinos, could
provide additional diagnostics to estimate the initial angular velocity of a
collapsing core.Comment: 20 pages, 14 figures,. Accepted for publication in the Astrophysical
Journa
Construction and Performance of Quantum Burst Error Correction Codes for Correlated Errors
© 2018 IEEE. In practical communication and computation systems, errors occur predominantly in adjacent positions rather than in a random manner. In this paper, we develop a stabilizer formalism for quantum burst error correction codes (QBECC) to combat such error patterns in the quantum regime. Our contributions are as follows. Firstly, we derive an upper bound for the correctable burst errors of QBECCs, the quantum Reiger bound (QRB). Secondly, we propose two constructions of QBECCs: one by heuristic computer search and the other by concatenating two quantum tensor product codes (QTPCs). We obtain several new QBECCs with better parameters than existing codes with the same coding length. Moreover, some of the constructed codes can saturate the quantum Reiger bounds. Finally, we perform numerical experiments for our constructed codes over Markovian correlated depolarizing quantum memory channels, and show that QBECCs indeed outperform standard QECCs in this scenario
Phosphorous Diffuser Diverged Blue Laser Diode for Indoor Lighting and Communication.
An advanced light-fidelity (Li-Fi) system based on the blue Gallium nitride (GaN) laser diode (LD) with a compact white-light phosphorous diffuser is demonstrated for fusing the indoor white-lighting and visible light communication (VLC). The phosphorous diffuser adhered blue GaN LD broadens luminescent spectrum and diverges beam spot to provide ample functionality including the completeness of Li-Fi feature and the quality of white-lighting. The phosphorous diffuser diverged white-light spot covers a radiant angle up to 120(o) with CIE coordinates of (0.34, 0.37). On the other hand, the degradation on throughput frequency response of the blue LD is mainly attributed to the self-feedback caused by the reflection from the phosphor-air interface. It represents the current state-of-the-art performance on carrying 5.2-Gbit/s orthogonal frequency-division multiplexed 16-quadrature-amplitude modulation (16-QAM OFDM) data with a bit error rate (BER) of 3.1 × 10(-3) over a 60-cm free-space link. This work aims to explore the plausibility of the phosphorous diffuser diverged blue GaN LD for future hybrid white-lighting and VLC systems
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