930 research outputs found
Embedded expert system for space shuttle main engine maintenance
The SPARTA Embedded Expert System (SEES) is an intelligent health monitoring system that directs analysis by placing confidence factors on possible engine status and then recommends a course of action to an engineer or engine controller. The technique can prevent catastropic failures or costly rocket engine down time because of false alarms. Further, the SEES has potential as an on-board flight monitor for reusable rocket engine systems. The SEES methodology synergistically integrates vibration analysis, pattern recognition and communications theory techniques with an artificial intelligence technique - the Embedded Expert System (EES)
Correlated radio--X-ray variability of Galactic Black Holes: A radio--X-ray flare in Cygnus X-1
We report on the first detection of a quasi-simultaneous radio-X-ray flare of
Cygnus X-1. The detection was made on 2005 April 16 with pointed observations
by the Rossi X-ray Timing Explorer and the Ryle telescope, during a phase where
the black hole candidate was close to a transition from the its soft into its
hard state. The radio flare lagged the X-rays by approximately 7 minutes,
peaking at 3:20 hours barycentric time (TDB 2453476.63864). We discuss this lag
in the context of models explaining such flaring events as the ejection of
electron bubbles emitting synchrotron radiation.Comment: 4 pages, 4 figure
Limits on thermal variations in a dozen quiescent neutron stars over a decade
In quiescent low-mass X-ray binaries (qLMXBs) containing neutron stars, the
origin of the thermal X-ray component may be either release of heat from the
core of the neutron star, or continuing low-level accretion. In general, heat
from the core should be stable on timescales years, while continuing
accretion may produce variations on a range of timescales. While some quiescent
neutron stars (e.g. Cen X-4, Aql X-1) have shown variations in their thermal
components on a range of timescales, several others, particularly those in
globular clusters with no detectable nonthermal hard X-rays (fit with a
powerlaw), have shown no measurable variations. Here, we constrain the spectral
variations of 12 low mass X-ray binaries in 3 globular clusters over
years. We find no evidence of variations in 10 cases, with limits on
temperature variations below 11% for the 7 qLMXBs without powerlaw components,
and limits on variations below 20% for 3 other qLMXBs that do show non-thermal
emission. However, in 2 qLMXBs showing powerlaw components in their spectra
(NGC 6440 CX 1 & Terzan 5 CX 12) we find marginal evidence for a 10% decline in
temperature, suggesting the presence of continuing low-level accretion. This
work adds to the evidence that the thermal X-ray component in quiescent neutron
stars without powerlaw components can be explained by heat deposited in the
core during outbursts. Finally, we also investigate the correlation between
hydrogen column density (N) and optical extinction (A) using our sample
and current models of interstellar X-ray absorption, finding .Comment: 16 pages, 5 figures, MNRAS, in pres
Discovery of Radio Outbursts in the Active Nucleus of M81
The low-luminosity active galactic nucleus of M81 has been monitored at
centimeter wavelengths since early 1993 as a by-product of radio programs to
study the radio emission from Supernova 1993J. The extensive data sets reveal
that the nucleus experienced several radio outbursts during the monitoring
period. At 2 and 3.6 cm, the main outburst occurred roughly in the beginning of
1993 September and lasted for approximately three months; at longer
wavelengths, the maximum flux density decreases, and the onset of the burst is
delayed. These characteristics qualitatively resemble the standard model for
adiabatically expanding radio sources, although certain discrepancies between
the observations and the theoretical predictions suggest that the model is too
simplistic. In addition to the large-amplitude, prolonged variations, we also
detected milder changes in the flux density at 3.6 cm and possibly at 6 cm on
short (less than 1 day) timescales. We discuss a possible association between
the radio activity and an optical flare observed during the period that the
nucleus was monitored at radio wavelengths.Comment: To appear in The Astronomical Journal. Latex, 18 pages including
embedded figures and table
A Search for Diffuse X-ray Emission from GeV Detected Galactic Globular Clusters
Recently, diffuse and extended sources in TeV gamma-rays as well as in X-rays
have been detected in the direction of the Galactic globular cluster (GC)
Terzan 5. Remarkably, this is among the brightest GCs detected in the GeV
regime. The nature of both the TeV and the diffuse X-ray signal from Terzan 5
is not settled yet. These emissions most likely indicate the presence of
several non-thermal radiation processes in addition to these giving rise to the
GeV signal.
The aim of this work is to search for diffuse X-ray emission from the GeV
detected GCs M 62, NGC 6388, NGC 6541, M 28, M 80 and NGC 6139 to compare the
obtained results with the signal detected from Terzan 5. This study will help
to determine whether Terzan 5 stands out amongst other GC or whether a whole
population of globular clusters feature similar properties.
None of the six GCs show significant diffuse X-ray emission on similar scales
as observed from Terzan 5 above the particle and diffuse galactic X-ray
background components. The derived upper limits allow to assess the validity of
different models that were discussed in the interpretation of the
multi-wavelength data of Terzan 5. A scenario based on synchrotron emission
from relativistic leptons provided by the millisecond pulsar population can not
be securely rejected if a comparable magnetic field strength as in Terzan 5 is
assumed for every GC. However, such a scenario seems to be unlikely for NGC
6388 and M 62. An inverse-Compton scenario relying on the presence of a
putative GRB remnant with the same properties as the one proposed for Terzan 5
can be ruled out for all of the six GCs. Finally, the assumption that each GC
hosts a source with the same luminosity as in Terzan 5 is ruled out for all GCs
but NGC 6139. (abridged)Comment: 8 pages, 1 Figure, accepted for publication by Astronomy &
Astrophysics, final version after language editin
A voting scheme for estimating the synchrony of moving-camera videos
Copyright © 2003 IEEERecovery of dynamic scene properties from multiple videos usually requires the manipulation of synchronous (simultaneously captured) frames. This paper is concerned with the automated determination of this synchrony when the temporal alignment of sequences is unknown. A cost function characterising departure from synchrony is first evolved for the case in which two videos are generated by cameras that may be moving. A novel voting method is then presented for minimising the cost function in the case where the ratio of the cameras' frame rates is unknown. Experimental results indicate this relatively general approach holds promise.Pooley, D.W.; Brooks, M.J.; van den Hengel, A.J.; Chojnacki, W
Variability of the extreme z=4.72 blazar, GB 1428+4217
We report X-ray and radio variability of GB 1428+4217 which confirm its
blazar nature. IR observations reveal a powerful optical-UV component, not
obscured by dust, which is suggestive of the presence of a billion solar mass
black hole, already formed by z ~ 5. A detailed comparison of the broad band
spectral properties of GB 1428+4217 with those of nearby blazars shows it to be
extreme, but nevertheless consistent with the trend found for nearby sources.Comment: MNRAS, in press - 5 pages, 5 figure
Orbital, precessional and flaring variability of Cygnus X-1
We present the results of a 2.5-year multiwavelength monitoring programme of
Cygnus X-1, making use of hard and soft X-ray data, optical spectroscopy,
UBVJHK photometry and radio data. In particular we confirm that the 5.6-day
orbital period is apparent in all wavebands and note the existence of a
wavelength-dependence to the modulation, in the sense that higher energies
reach minimum first. We also find a strong modulation at a period of 142 +/- 7
days, which we suggest is due to precession and/or radiative warping of the
accretion disc. Strong modulation of the hard and soft X-ray flux at this long
period may not be compatible with simple models of an optically thin accretion
flow and corona in the low state. We present the basic components required for
more detailed future modelling of the system - including a partially optically
thick jet, quasi-continuous in the low state, the base of which acts as the
Comptonising corona. In addition, we find that there are a number of flares
which appear to be correlated in at least two wavebands and generally in more.
We choose two of these flares to study in further detail and find that the hard
and soft X-rays are well-correlated in the first and that the soft X-rays and
radio are correlated in the second. In general, the optical and infrared show
similar behaviour to each other but are not correlated with the X-rays or
radio.Comment: Accepted for publication in MNRAS, 2 figures in colou
Faint X-ray Sources in the Globular Cluster Terzan 5
We report our analysis of a Chandra X-ray observation of the rich globular
cluster Terzan 5, in which we detect 50 sources to a limiting 1.0-6 keV X-ray
luminosity of 3*10^{31} ergs/s within the half-mass radius of the cluster.
Thirty-three of these have L_X>10^{32} ergs/s, the largest number yet seen in
any globular cluster. In addition to the quiescent low-mass X-ray binary (LMXB,
identified by Wijnands et al.), another 12 relatively soft sources may be
quiescent LMXBs. We compare the X-ray colors of the harder sources in Terzan 5
to the Galactic Center sources studied by Muno and collaborators, and find the
Galactic Center sources to have harder X-ray colors, indicating a possible
difference in the populations. We cannot clearly identify a metallicity
dependence in the production of low-luminosity X-ray binaries in Galactic
globular clusters, but a metallicity dependence of the form suggested by Jordan
et al. for extragalactic LMXBs is consistent with our data.Comment: 15 pages, 10 figures (3 color). Resubmitted to ApJ after
incorporating referee comments. v2: Added references to introductio
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