13,592 research outputs found
Identification and control of structures in space
The derivation of the equations of motion for the Spacecraft Control Laboratory Experiment (SCOLE) is reported and the equations of motion of a similar structure orbiting the earth are also derived. The structure is assumed to undergo large rigid-body maneuvers and small elastic deformations. A perturbation approach is proposed whereby the quantities defining the rigid-body maneuver are assumed to be relatively large, with the elastic deformations and deviations from the rigid-body maneuver being relatively small. The perturbation equations have the form of linear equations with time-dependent coefficients. An active control technique can then be formulated to permit maneuvering of the spacecraft and simultaneously suppressing the elastic vibration
Compact Radio Cores in Seyfert Galaxies
We have observed a sample of 157 Seyfert galaxies with a 275 km baseline
radio interferometer to search for compact, high brightness temperature radio
emission from the active nucleus. We obtain the surprising result that compact
radio cores are much more common in Seyfert 2 than in Seyfert 1 galaxies, which
at first seems to be inconsistent with orientation unification schemes. We
propose a model, involving optical depth effects in the narrow-line region,
which can reconcile our result with the standard unified scheme. (Accepted for
publication in ApJ 1994 Sep 10)Comment: 21 pages and 7 figures, uuencoded tar-compressed postscript files,
ATP18
The ATLAS-SPT Radio Survey of Cluster Galaxies
Using a high-performance computing cluster to mosaic 4,787 pointings, we have
imaged the 100 sq. deg. South Pole Telescope (SPT) deep-field at 2.1 GHz using
the Australian Telescope Compact Array to an rms of 80 Jy and a resolution
of 8". Our goal is to generate an independent sample of radio-selected galaxy
clusters to study how the radio properties compare with cluster properties at
other wavelengths, over a wide range of redshifts in order to construct a
timeline of their evolution out to . A preliminary analysis of the
source catalogue suggests there is no spatial correlation between the clusters
identified in the SPT-SZ catalogue and our wide-angle tail galaxies.Comment: 9 pages, 4 figures. Submitted to Proceedings of Science for "The many
facets of extragalactic radio surveys: towards new scientific challenges",
Bologna, Italy 20-23 October 2015 (EXTRA-RADSUR2015
The lag and duration-luminosity relations of gamma-ray burst pulses
Relations linking the temporal or/and spectral properties of the prompt
emission of gamma-ray bursts (hereafter GRBs) to the absolute luminosity are of
great importance as they both constrain the radiation mechanisms and represent
potential distance indicators. Here we discuss two such relations: the
lag-luminosity relation and the newly discovered duration-luminosity relation
of GRB pulses. We aim to extend our previous work on the origin of spectral
lags, using the duration-luminosity relation recently discovered by Hakkila et
al. to connect lags and luminosity. We also present a way to test this relation
which has originally been established with a limited sample of only 12 pulses.
We relate lags to the spectral evolution and shape of the pulses with a linear
expansion of the pulse properties around maximum. We then couple this first
result to the duration-luminosity relation to obtain the lag-luminosity and
lag-duration relations. We finally use a Monte-Carlo method to generate a
population of synthetic GRB pulses which is then used to check the validity of
the duration-luminosity relation. Our theoretical results for the lag and
duration-luminosity relations are in good agreement with the data. They are
rather insensitive to the assumptions regarding the burst spectral parameters.
Our Monte Carlo analysis of a population of synthetic pulses confirms that the
duration-luminosity relation must be satisfied to reproduce the observational
duration-peak flux diagram of BATSE GRB pulses. The newly discovered
duration-luminosity relation offers the possibility to link all three
quantities: lag, duration and luminosity of GRB pulses in a consistent way.
Some evidence for its validity have been presented but its origin is not easy
to explain in the context of the internal shock model.Comment: 8 pages, 5 figures, 1 tabl
The relation between star formation rate and accretion rate in LINERs
It is argued that there is a linear correlation between star formation rate
(SFR) and accretion rate for normal bright active galactic nuclei (AGNs).
However, it is still unclear whether this correlation holds for LINERs, of
which the accretion rates are relatively lower than those of normal bright
AGNs. The radiatively inefficient accretion flows (RIAFs) are believed to be
present in these LINERs. In this work, we derive accretion rates for a sample
of LINERs from their hard X-ray luminosities based on spectral calculations for
RIAFs. We find that LINERs follow the same correlation between star formation
rate and accretion rate defined by normal bright AGNs, when reasonable
parameters are adopted for RIAFs. It means that the gases feed the black hole
and star formation in these low-luminosity LINERs may follow the same way as
that in normal bright AGNs, which is roughly consistent with recent numerical
simulations on quasar evolution.Comment: 15 pages, 3 figures, accepted for publication in PASP, in pres
Techniques development for whale migration tracking
Effort leading to the completion of development and fabrication of expansible whale harnesses and whale-carried instrument pods is described, along with details of the gear. Early preparative effort for a January-February 1974 field expedition is reported
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