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21.
Bryan M. Gaensler Maura McLaughlin Stephen Reynolds Kazik Borkowski Nanda Rea Andrea Possenti Gianluca Israel Marta Burgay Fernando Camilo Shami Chatterjee Michael Kramer Andrew Lyne Ingrid Stairs 《Astrophysics and Space Science》2007,308(1-4):95-99
“Rotating RAdio Transients” (RRATs) are a newly discovered astronomical phenomenon, characterised by occasional brief radio
bursts, with average intervals between bursts ranging from minutes to hours. The burst spacings allow identification of periodicities,
which fall in the range 0.4 to 7 seconds. The RRATs thus seem to be rotating neutron stars, albeit with properties very different
from the rest of the population. We here present the serendipitous detection with the Chandra X-ray Observatory of a bright point-like X-ray source coincident with one of the RRATs. We discuss the temporal and spectral properties of
this X-ray emission, consider counterparts in other wavebands, and interpret these results in the context of possible explanations
for the RRAT population.
B.M.G. acknowledges the support of NASA through LTSA grant NAG5-13023 and of an Alfred P. Sloan Fellowship. 相似文献
22.
Orimoloye Israel R. Olusola Adeyemi O. Belle Johanes A. Pande Chaitanya B. Ololade Olusola O. 《Natural Hazards》2022,112(2):1085-1106
Natural Hazards - Droughts are particularly disastrous in South Africa and other arid regions that are water-scarce by nature due to low rainfall and water sources. According to some studies,... 相似文献
23.
Hydrogeology Journal - The water-level drawdown in pumping wells is the sum of two components: aquifer loss and well loss. The latter results from mostly turbulent and nonlaminar flow in and around... 相似文献
24.
R. Kaur Rudy Wijnands Alessandro Patruno Vincenzo Testa GianLuca Israel Nathalie Degenaar Biswajit Paul Brijesh Kumar 《Monthly notices of the Royal Astronomical Society》2009,394(3):1597-1604
We present results from our Chandra and XMM–Newton observations of two low-luminosity X-ray pulsators SAX J1324.4−6200 and SAX J1452.8−5949 which have spin periods of 172 and 437 s, respectively. The XMM–Newton spectra for both sources can be fitted well with a simple power-law model of photon index, Γ∼ 1.0 . A blackbody model can equally well fit the spectra with a temperature, kT ∼ 2 keV, for both sources. During our XMM–Newton observations, SAX J1324.4−6200 is detected with coherent X-ray pulsations at a period of 172.86 ± 0.02 s while no pulsations with a pulse fraction greater than 18 per cent (at 95 per cent confidence level) in 0.2–12 keV energy band are detected in SAX J1452.8−5949 . The spin period of SAX J1324.4−6200 is found to be increasing on a time-scale of which would suggest that the accretor is a neutron star and not a white dwarf. Using subarcsec spatial resolution of the Chandra telescope, possible counterparts are seen for both sources in the near-infrared images obtained with the son of infrared spectrometer and array camera (SOFI) instrument on the New Technology Telescope. The X-ray and near-infrared properties of SAX J1324.4−6200 suggest it to be a persistent high-mass accreting X-ray pulsar at a distance ≤8 kpc . We identify the near-infrared counterpart of SAX J1452.8−5949 to be a late-type main-sequence star at a distance ≤10 kpc, thus ruling out SAX J1452.8−5949 to be a high-mass X-ray binary. However, with the present X-ray and near-infrared observations, we cannot make any further conclusive conclusion about the nature of SAX J1452.8−5949 . 相似文献
25.
26.
Tame Gonzalez Rolando Cardenas Israel Quiros Yoelsy Leyva 《Astrophysics and Space Science》2007,310(1-2):13-18
In this work we investigate the evolution of matter (linear) density perturbations for quintessence models with a self-interaction
potential that is a combination of exponentials. One of the models is based on the Einstein theory of gravity, while the other
is based on the Brans-Dicke scalar tensor theory. We constrained the parameter space of the models by using the determinations
of the growth rate of perturbations derived from data of the 2-degree Field Galaxy Redshift Survey.
相似文献
27.
Ari Ben-Menahem Ezra Aboodi Moshe Vered Robert L. Kovach 《Physics of the Earth and Planetary Interiors》1977,14(4):P17-P27
The results of two millennia of earthquake documentation, a few decades of macroseismic and instrumental routine seismological observations and five months of microearthquake monitoring, are used to estimate the rate of seismic activity of the Dead Sea fault. It is found that these vastly diverse data which combine long- and short-term tectonic processes, are in good accord with the formula: where N is the annual number of events of local magnitude ML or greater. If this equation is extrapolated to ca. 2000 B.C., it yields a Richter magnitude Ms = 7 for the event of Sodom and Gomorrah which is believed to be associated with the strongest earthquake in the region during historical times.Comparing our findings with the results of other investigators in Turkey, Greece, Aegean Sea and Iran, we note that the b values along the Syrian-African rift zone (0.78–0.86) are smaller than those in Greece and its surrounding seas (0.94–1.16). 相似文献
28.
In this study, a set of 20 2D seismic lines, acquired over the Golan Heights basaltic plateau, was processed and analysed. Although the data were acquired and processed by standard techniques, in some cases good-quality seismic images were obtained under several hundred metres of basalts. We describe how the seismic characteristics of the top basalt layer were defined and show the effect of the numerous widespread volcanic sources on the quality of the final images. The new data reveal the first images of the sedimentary sequence under the basalt flows, and indicate that strands of the Dead Sea Transform extend into this area. The entire region was found to be very deformed. Several attractive traps for hydrocarbon exploration were also identified on the output sections. 相似文献
29.
The network-based approach to kinematic GPS positioning significantly increases the distance, over which carrier-phase ambiguity
resolution can be performed. This can be achieved either by introducing geometric conditions based on the fixed reference
locations, and/or through the use of reference network data to estimate the corrections to GPS observations that can be broadcast
to the users. The Multi Purpose GPS Processing Software (MPGPS) developed at The Ohio State University uses the multiple reference
station approach for wide area and regional differential kinematic GPS positioning. The primary processing algorithm uses
the weighted free-net (WFN) approach with the distance-dependent weighting scheme to derive optimal estimates of the user
coordinates and realistic accuracy measures. The WFN approach, combined with the single epoch (instantaneous) ambiguity resolution
algorithm is presented here as one approach to real-time kinematic (RTK) GPS. Since for baselines exceeding ~100 km, the instantaneous
ambiguity resolution may not always be possible due to the increasing observation noise and insufficient number of observations
to verify the integer selection, an alternative approach, based on a single- (or multiple-) baseline solution, supported by
a double-difference (DD) ionospheric delay propagated from the previous epoch is also presented. In this approach, some data
accumulation, supported by the network-derived atmospheric corrections, is required at the beginning of the rover data processing
to obtain the integer ambiguities; after this initialization period, the processing switches to the instantaneous RTK positioning
mode. This paper presents a discussion on the effects of the network geometry, station separation and the data reduction technique
on the final quality and reliability of the rover positioning solution. A 24-h data set of August 31, 2003, collected by the
Ohio Continuously Operating Reference Station (CORS) network was processed by both techniques under different network geometry
and reference station separation. Various solutions, such as (1) single-baseline solution for varying base-rover separation,
(2) multi-baseline solution with medium-range base separation (over 100 km), and (3) multi-baseline solution with long-range
base separation (up to 377 km), were obtained and compared for accuracy and consistency. The horizontal positioning accuracy
achieved in these tests, expressed as the difference between the estimated coordinates and the known rover coordinates, is
at the sub-decimeter level for the first approach, and at the centimeter-level for the second method, for baselines over 100 km.
In the vertical coordinate, decimeter- and sub-decimeter levels were achieved for the two approaches, respectively. Even though
all the results presented here were obtained in post-processing, both algorithms are suitable for real-time applications. 相似文献
30.
F.P. Israel 《Astronomy and Astrophysics Review》1998,8(4):237-278
Summary. At a distance of 3.4 Mpc, NGC 5128 (Centaurus A) is by far the nearest active radio galaxy. It is often considered to be
the prototype Fanaroff-Riley Class I ‘low-luminosity’ radio galaxy, and as such it plays an important role in our understanding
of a major class of active galaxies. Its proximity has spawned numerous detailed investigations of its properties, yielding
unrivalled but still incomplete knowledge of its structure and dynamics.
The massive elliptical host galaxy is moderately triaxial and contains a thin, strongly warped disk rich in dust, atomic and
molecular gas and luminous young stars. Its globular cluster ensemble has a bimodal distribution of metallicities. Deep optical
images reveal faint major axis extensions as well as a system of filaments and shells. These and other characteristics are
generally regarded as strong evidence that NGC 5128 has experienced a major merging events at least once in its past.
The galaxy has a very compact, subparsec nucleus exhibiting noticeable intensity variations at radio and X-ray wavelengths,
probably powered by accretion events. The central object may be a black hole of moderate mass. Towards the nucleus, rich absorption
spectra of atomic hydrogen and various molecular species suggest the presence of significant amounts of material falling into
the nucleus, presumably ‘feeding the monster’. Emanating from the nucleus are linear radio/X-ray jets, becoming subrelativistic
at a few parsec from the nucleus. At about 5 kpc from the nucleus, the jets expand into plumes. Huge radio lobes extend beyond
the plumes out to to 250 kpc. A compact circumnuclear disk with a central cavity surrounds the nucleus. Its plane, although
at an angle to the minor axis of the galaxy, is perpendicular to the inner jets. The jet-collimating mechanism, probably connected
to the circumnuclear disk, appears to precess on timescales of order a few times 10 years.
This review summarizes the present state of knowledge of NGC 5128 and its associated radio source Centaurus A. Underlying
physical processes are outside its scope: they are briefly referred to, but not discussed.
Received 30 December 1997 相似文献