17,990 research outputs found
A survey of stellar X-ray flares from the XMM-Newton serendipitous source catalogue: Hipparcos-Tycho cool stars
The X-ray emission from flares on cool (i.e. spectral-type F-M) stars is
indicative of very energetic, transient phenomena, associated with energy
release via magnetic reconnection. We present a uniform, large-scale survey of
X-ray flare emission. The XMM-Newton Serendipitous Source Catalogue and its
associated data products provide an excellent basis for a comprehensive and
sensitive survey of stellar flares - both from targeted active stars and from
those observed serendipitously in the half-degree diameter field-of-view of
each observation. The 2XMM Catalogue and the associated time-series
(`light-curve') data products have been used as the basis for a survey of X-ray
flares from cool stars in the Hipparcos Tycho-2 catalogue. In addition, we have
generated and analysed spectrally-resolved (i.e. hardness-ratio), X-ray
light-curves. Where available, we have compared XMM OM UV/optical data with the
X-ray light-curves. Our sample contains ~130 flares with well-observed
profiles; they originate from ~70 stars. The flares range in duration from ~1e3
to ~1e4 s, have peak X-ray fluxes from ~1e-13 to ~1e-11 erg/cm2/s, peak X-ray
luminosities from ~1e29 to ~1e32 erg/s, and X-ray energy output from ~1e32 to
~1e35 erg. Most of the ~30 serendipitously-observed stars have little
previously reported information. The hardness-ratio plots clearly illustrate
the spectral (and hence inferred temperature) variations characteristic of many
flares, and provide an easily accessible overview of the data. We present flare
frequency distributions from both target and serendipitous observations. The
latter provide an unbiased (with respect to stellar activity) study of flare
energetics; in addition, they allow us to predict numbers of stellar flares
that may be detected in future X-ray wide-field surveys. The serendipitous
sample demonstrates the need for care when calculating flaring rates.Comment: 26 pages, 24 figures. Additional tables and figures available as 4
ancillary files. To be published in Astronomy and Astrophysic
Optimized pulse sequences for suppressing unwanted transitions in quantum systems
We investigate the nature of the pulse sequence so that unwanted transitions
in quantum systems can be inhibited optimally. For this purpose we show that
the sequence of pulses proposed by Uhrig [Phys. Rev. Lett. \textbf{98}, 100504
(2007)] in the context of inhibition of environmental dephasing effects is
optimal. We derive exact results for inhibiting the transitions and confirm the
results numerically. We posit a very significant improvement by usage of the
Uhrig sequence over an equidistant sequence in decoupling a quantum system from
unwanted transitions. The physics of inhibition is the destructive interference
between transition amplitudes before and after each pulse.Comment: 5 figure
Electromagnetic cascade in high energy electron, positron, and photon interactions with intense laser pulses
The interaction of high energy electrons, positrons, and photons with intense
laser pulses is studied in head-on collision geometry. It is shown that
electrons and/or positrons undergo a cascade-type process involving multiple
emissions of photons. These photons can consequently convert into
electron-positron pairs. As a result charged particles quickly lose their
energy developing an exponentially decaying energy distribution, which
suppresses the emission of high energy photons, thus reducing the number of
electron-positron pairs being generated. Therefore, this type of interaction
suppresses the development of the electromagnetic avalanche-type discharge,
i.e., the exponential growth of the number of electrons, positrons, and photons
does not occur in the course of interaction. The suppression will occur when 3D
effects can be neglected in the transverse particle orbits, i.e., for
sufficiently broad laser pulses with intensities that are not too extreme. The
final distributions of electrons, positrons, and photons are calculated for the
case of a high energy e-beam interacting with a counter-streaming, short
intense laser pulse. The energy loss of the e-beam, which requires a
self-consistent quantum description, plays an important role in this process,
as well as provides a clear experimental observable for the transition from the
classical to quantum regime of interaction.Comment: 13 pages, 7 figure
Small Displacement, Long Life On-Orbit Compressor Design and Fabrication
The focus is the generation of technology and fabrication of prototype hardware applicable to seven Space Station compressor system applications. The compressors are of the single acting reciprocating piston type and, in general, may be termed miniature in size compared with normal commercially available equipment. The initial technology development is focused on improved valve designs, and the control of pulsations and heating effects in order to increase compressor efficiency and reduce cycle temperatures, thus permitting significantly increased stage pressure ratios. The initial test compressor was successfully operated at pressure ratios of up to 50:1, and this significant extension of allowable pressure ratio will result in a reduction of the number of required stages and, hence, total hardware thereby reducing system weight and volume. These experiments have also identified the need to employ low shaft speeds, on the order of 250 to 500 rpm, to enhance heat transfer and increase life. The prototype compressor currently being designed, is to be driven by a low-speed brushless dc motor sealed in a case common to the compressor drive mechanism case. The compressor and motor case will communicate with stage suction pressure so that any minor gas leakage past the piston rings will be returned to the suction. Emphasis in this prototype design is being placed on simplicity, durability, commonality of components, and high efficiency
Optimized laser pulse profile for efficient radiation pressure acceleration of ions
The radiation pressure acceleration regime of laser ion acceleration requires
high intensity laser pulses to function efficiently. Moreover the foil should
be opaque for incident radiation during the interaction to ensure maximum
momentum transfer from the pulse to the foil, which requires proper matching of
the target to the laser pulse. However, in the ultrarelativistic regime, this
leads to large acceleration distances, over which the high laser intensity for
a Gaussian laser pulse must be maintained. It is shown that proper tailoring of
the laser pulse profile can significantly reduce the acceleration distance,
leading to a compact laser ion accelerator, requiring less energy to operate.Comment: 10 pages, 4 figure
Partially Unbiased Entangled Bases
In this contribution we group the operator basis for d^2 dimensional Hilbert
space in a way that enables us to relate bases of entangled states with single
particle mutually unbiased state bases (MUB), each in dimensionality d. We
utilize these sets of operators to show that an arbitrary density matrix for
this d^2 dimensional Hilbert space system is analyzed by via d^2+d+1
measurements, d^2-d of which involve those entangled states that we associate
with MUB of the d-dimensional single particle constituents. The number
lies in the middle of the number of measurements needed for bipartite
state reconstruction with two-particle MUB (d^2+1) and those needed by
single-particle MUB [(d^2+1)^2].Comment: 5 page
- …