530 research outputs found
Preliminary observations on the use of a frame trawl in hydroacoustic surveys
Thirteen hauls were made during the hydroacoustic survey of the Ugandan waters of Lake Victoria from 7-19 February 1999. Ten of the hauls were made above the oxycline which was clearly visible as a strong echo on the echogram at between 25 and 35 m depth in most of the sampled areas. The remaining three hauls targeted the oxycline. Approximate equal weights of Rastrineobola argentia and Haplocromine cichlids were caught in total, but with marked differences between hauls. Near the surface R. argentia dominated the catches. In midwater Haplochromines were dominant. At the oxycline Caridina niloticus was abundan
The present status of the fish stocks in the experimental and commercial/artisanal gillnet fisheries in selected waters of Lake Victoria, Uganda
Lake Victoria had a multi-species fishery dominated until the 1970s by the tilapiine and the haplochromine cichlids, but with important subsidiary fisheries of more than 20 genera of non-cichlid fishes such as Bagrus, Barbus, C/arias, Mormyrus, Protopterus, Synodontis, etc (Kudhongania & Cordone 1974). From about 1930 to 1960, the fisheries of Lake Victoria were managed by controlling the mesh size of gill nets (Graham 1929). Gill nets of mesh sizes less than 127 mm (5 inches) stretched mesh had been prohibited on Lake Victoria because they cropped immature Oreochromis esculentus (Ngege) which were at that time the most important commercial species on the lake (Graham 1929). The catch in the legal 127 mm mesh nets per night was over 30 fish of Oreochromis esculentus prior to 1921; this dropped to 6 and 1.6 fish in 1928 and 1954, respectively,(Beauchamp1955),indicating overfishing ofthe stocks
The quasi-persistent neutron star soft X-ray transient 1M 1716-315 in quiescence
We report on our analysis of a 20 ksec Chandra X-ray observation of the
quasi-persistent neutron star soft X-ray transient (SXT) 1M1716-315 in
quiescence. Only one source was detected in the HEAO-I error region. Its
luminosity is 1.6E32-1.3E33 erg s-1. In this the range is dominated by the
uncertainty in the source distance. The source spectrum is well described by an
absorbed soft spectrum, e.g. a neutron star atmosphere or black body model. No
optical or near-infrared counterpart is present at the location of the X-ray
source, down to a magnitude limit of I> 23.5 and K_s> 19.5. The positional
evidence, the soft X-ray spectrum together with the optical and near-infrared
non-detections provide strong evidence that this source is the quiescent
neutron star SXT. The source is 10-100 times too bright in X-rays in order to
be explained by stellar coronal X-ray emission. Together with the interstellar
extinction measured in outburst and estimates for the source distance, the
reported optical and near-infrared limit give an upper limit on the absolute
magnitude of the counterpart of I>8.6 and K_s>5.1. This implies that the system
is either an ultra-compact X-ray binary having P_orb<1 hr or the companion star
is an M-dwarf. We reconstructed the long term X-ray lightcurve of the source.
1M1716-315 has been active for more than 12 years before returning to
quiescence, the reported Chandra observation started 16.9+-4.1 years after the
outburst ended.Comment: 7 pages, 4 figures, accepted to MNRA
X-ray Observations of XSS J12270-4859 in a New Low State: A Transformation to a Disk-Free Rotation-Powered Pulsar Binary
We present XMM-Newton and Chandra observations of the low-mass X-ray binary
XSS J12270--4859, which experienced a dramatic decline in optical/X-ray
brightness at the end of 2012, indicative of the disappearance of its accretion
disk. In this new state, the system exhibits previously absent
orbital-phase-dependent, large-amplitude X-ray modulations with a decline in
flux at superior conjunction. The X-ray emission remains predominantly
non-thermal but with an order of magnitude lower mean luminosity and
significantly harder spectrum relative to the previous high flux state. This
phenomenology is identical to the behavior of the radio millisecond pulsar
binary PSR J1023+0038 in the absence of an accretion disk, where the X-ray
emission is produced in an intra-binary shock driven by the pulsar wind. This
further demonstrates that XSS J12270-4859 no longer has an accretion disk and
has transformed to a full-fledged eclipsing "redback" system that hosts an
active rotation-powered millisecond pulsar. There is no evidence for diffuse
X-ray emission associated with the binary that may arise due to outflows or a
wind nebula. An extended source situated 1.5' from XSS J12270--4859 is unlikely
to be associated, and is probably a previously uncatalogued galaxy cluster.Comment: 8 pages, 6 figures; accepted for publication in the Astrophysical
Journa
The first Suzaku observation of SGR 1806-20
The soft gamma-ray repeater SGR 1806-20 has been attracting a lot of
attention owing to the fact that in December 2004 it emitted the most powerful
giant flare ever observed. Here we present the results of the first Suzaku
observation of SGR 1806-20, that seems to have reached a state characterized by
a flux close to the pre-flare level and by a relatively soft spectrum. Despite
this, the source remained quite active, as testified by several short bursts
observed by Suzaku. We discuss the broadband spectral properties of SGR 1806-20
in the context of the magnetar model, considering its recent theoretical
developments.Comment: 3 pages, 1 figure. Proceedings of the conference "40 Years of
Pulsars, Millisecond Pulsars, Magnetars and More", Montreal, August 12-17
2007. AIP, in pres
Space cowboys odyssey: beyond the Gould Belt
We present our new advanced model for population synthesis of close-by
cooling NSs. Detailed treatment of the initial spatial distribution of NS
progenitors and a detailed ISM structure up to 3 kpc give us an opportunity to
discuss the strategy to look for new isolated cooling NSs. Our main results in
this respect are the following: new candidates are expected to be identified
behind the Gould Belt, in directions to rich OB associations, in particular in
the Cygnus-Cepheus region; new candidates, on average, are expected to be
hotter than the known population of cooling NS. Besides the usual approach
(looking for soft X-ray sources), the search in 'empty' -ray error
boxes or among run-away OB stars may yield new X-ray thermally emitting NS
candidates.Comment: 3 pages, 2 figures, proceedings of the conference "40 Years of
Pulsars ", 12-17 August 2007, Montreal, Canad
A Search for Pulsed and Bursty Radio Emission from X-ray Dim Isolated Neutron Stars
We have carried out a search for radio emission from six X-ray dim isolated
neutron stars (XDINSs) observed with the Robert C. Byrd Green Bank Radio
Telescope (GBT) at 820 MHz. No bursty or pulsed radio emission was found down
to a 4sigma significance level. The corresponding flux limit is 0.01-0.04 mJy
depending on the integration time for the particular source and pulse duty
cycle of 2%. These are the most sensitive limits yet on radio emission from
these objects.Comment: 3 pages, 3 figures, to be appeared in the Proceedings of the
conference "40 Years of Pulsars: Millisecond Pulsars, Magnetars, and More"
held on August 12-17, 2007, McGill University, Montreal, Canad
The GBT350 Survey of the Northern Galactic Plane for Radio Pulsars and Transients
Using the Green Bank Telescope (GBT) and Pulsar Spigot at 350MHz, we have
surveyed the Northern Galactic Plane for pulsars and radio transients. This
survey covers roughly 1000 square degrees of sky within 75 deg < l < 165 deg
and |b| < 5.5 deg, a region of the Galactic Plane inaccessible to both the
Parkes and Arecibo multibeam surveys. The large gain of the GBT along with the
high time and frequency resolution provided by the Spigot make this survey more
sensitive by factors of about 4 to slow pulsars and more than 10 to millisecond
pulsars (MSPs), compared with previous surveys of this area. In a preliminary,
reduced-resolution search of all the survey data, we have discovered 33 new
pulsars, almost doubling the number of known pulsars in this part of the
Galaxy. While most of these sources were discovered by normal periodicity
searches, 5 of these sources were first identified through single, dispersed
bursts. We discuss the interesting properties of some of these new sources.
Data processing using the data's full-resolution is ongoing, with the goal of
uncovering MSPs missed by our first, coarse round of processing.Comment: To appear in the proceedings of "Forty Years of Pulsars: Millisecond
Pulsars, Magnetars and More" held in Montreal, Canada, August 12-17, 2007. 3
pages, 2 figure
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