615 research outputs found
Multifrequency Observations of Giant Radio Pulses from the Millisecond Pulsar B1937+21
Giant pulses are short, intense outbursts of radio emission with a power-law
intensity distribution that have been observed from the Crab Pulsar and PSR
B1937+21. We have undertaken a systematic study of giant pulses from PSR
B1937+21 using the Arecibo telescope at 430, 1420, and 2380 MHz. At 430 MHz,
interstellar scattering broadens giant pulses to durations of secs,
but at higher frequencies the pulses are very short, typically lasting only
-secs. At each frequency, giant pulses are emitted only in narrow
(\lsim10 \mus) windows of pulse phase located -sec after the
main and interpulse peaks. Although some pulse-to-pulse jitter in arrival times
is observed, the mean arrival phase appears stable; a timing analysis of the
giant pulses yields precision competitive with the best average profile timing
studies. We have measured the intensity distribution of the giant pulses,
confirming a roughly power-law distribution with approximate index of -1.8,
contributing \gsim0.1% to the total flux at each frequency. We also find that
the intensity of giant pulses falls off with a slightly steeper power of
frequency than the ordinary radio emission.Comment: 21 pages, 10 Postscript figures; LaTeX with aaspp4.sty and epsf.tex;
submitted to Ap
Simultaneous Dual Frequency Observations of Giant Pulses from the Crab Pulsar
Simultaneous measurements of giant pulses from the Crab pulsar were taken at
two widely spaced frequencies using the real-time detection of a giant pulse at
1.4 GHz at the Very Large Array to trigger the observation of that same pulse
at 0.6 GHz at a 25-m telescope in Green Bank, WV. Interstellar dispersion of
the signals provided the necessary time to communicate the trigger across the
country via the Internet. About 70% of the pulses are seen at both 1.4 GHz and
0.6 GHz, implying an emission mechanism bandwidth of at least 0.8 GHz at 1 GHz
for pulse structure on time scales of one to ten microseconds.
The arrival times at both frequencies display a jitter of 100 microseconds
within the window defined by the average main pulse profile and are tightly
correlated. This tight correlation places limits on both the emission mechanism
and on frequency dependent propagation within the magnetosphere.
At 1.4 GHz the giant pulses are resolved into several, closely spaced
components. Simultaneous observations at 1.4 GHz and 4.9 GHz show that the
component splitting is frequency independent. We conclude that the multiplicity
of components is intrinsic to the emission from the pulsar, and reject the
hypothesis that this is the result of multiple imaging as the signal propagates
through the perturbed thermal plasma in the surrounding nebula. At both 1.4 GHz
and 0.6 GHz the pulses are characterized by a fast rise time and an exponential
decay time which are correlated. The pulse broadening with its exponential
decay form is most likely the result of multipath propagation in intervening
ionized gas.Comment: LaTeX, 18 pages, 7 figures, accepted for publication in The
Astrophysical Journa
What makes the Crab pulsar shine?
Our high time resolution observations of individual pulses from the Crab
pulsar show that the main pulse and interpulse differ in temporal behavior,
spectral behavior, polarization and dispersion. The main pulse properties are
consistent with one current model of pulsar radio emission, namely, soliton
collapse in strong plasma turbulence. The high-frequency interpulse is quite
another story. Its dynamic spectrum cannot easily be explained by any current
emission model; its excess dispersion must come from propagation through the
star's magnetosphere. We suspect the high-frequency interpulse does not follow
the ``standard model'', but rather comes from some unexpected region within the
star's magnetosphere. Similar observations of other pulsars will reveal whether
the radio emission mechanisms operating in the Crab pulsar are unique to that
star, or can be identified in the general population.Comment: 5 pages, 2 figures, to appear in proceedings of meeting "Forty Years
of Pulsars: Millisecond Pulsars, Magnetars and More", Montreal, August 200
Giant Radio Pulses from the Crab Pulsar
Individual giant radio pulses (GRPs) from the Crab pulsar last only a few
microseconds. However, during that time they rank among the brightest objects
in the radio sky reaching peak flux densities of up to 1500 Jy even at high
radio frequencies. Our observations show that GRPs can be found in all phases
of ordinary radio emission including the two high frequency components (HFCs)
visible only between 5 and 9 GHz (Moffett & Hankins, 1996). This leads us to
believe that there is no difference in the emission mechanism of the main pulse
(MP), inter pulse (IP) and HFCs. High resolution dynamic spectra from our
recent observations of giant pulses with the Effelsberg telescope at a center
frequency of 8.35 GHz show distinct spectral maxima within our observational
bandwidth of 500 MHz for individual pulses. Their narrow band components appear
to be brighter at higher frequencies (8.6 GHz) than at lower ones (8.1 GHz).
Moreover, there is an evidence for spectral evolution within and between those
structures. High frequency features occur earlier than low frequency ones.
Strong plasma turbulence might be a feasible mechanism for the creation of the
high energy densities of ~6.7 x 10^4 erg cm^-3 and brightness temperatures of
10^31 K.Comment: accepted by Advances in Space Research, to appear in the 35th COSPAR
assembly proceeding
The role of betaines in alkaline extracts of Ascophyllum nodosum in the reduction of Meloidogyne javanica and M. incognita infestations of tomato plants
L'application sur les racines de plants de tomates d'un extrait alcalin de l'algue brune #Ascophyllum nodosum, disponible dans le commerce, produit une rĂ©duction significative du nombre de juvĂ©niles de deuxiĂšme stade de #Meloidogyne javanica et #M. incognita$ pĂ©nĂ©trant dans les racines, en comparaison avec des plants de tomates traitĂ©s uniquement avec de l'eau. Le nombre d'oeufs rĂ©cupĂ©rĂ©s sur les plants traitĂ©s par l'extrait d'algue est Ă©galement significativement plus faible. Lorsque les trois bĂ©taĂŻnes essentielles (bĂ©taĂŻne d'acide-gamma-aminobutyrique, bĂ©taĂŻne d'acide delta-aminovalĂ©rique, glycine bĂ©taĂŻne) prĂ©sentes dans l'extrait d'algue sont appliquĂ©es - Ă des concentrations Ă©quivalent Ă celles de l'extrait - on observe Ă©galement une rĂ©duction significative de l'infestation par les nĂ©matodes et du nombre d'oeufs rĂ©cupĂ©rĂ©s. Il peut donc ĂȘtre conclu que les bĂ©taĂŻnes prĂ©sentes dans l'extrait d'algue jouent un rĂŽle important dans le processus occasionnant les effets observĂ©s. Lorsque le sol est humidifĂ©e par une application des constituants inorganiques de l'extrait, on observe Ă©galement une rĂ©duction significative du nombre d'oeufs rĂ©cupĂ©rĂ©s, cette rĂ©duction Ă©tant plus faible que celle produite par application de bĂ©taĂŻnes. (RĂ©sumĂ© d'auteur
High time-resolution observations of the Vela pulsar
We present high time resolution observations of single pulses from the Vela
pulsar (PSR B0833-45) made with a baseband recording system at observing
frequencies of 660 and 1413 MHz. We have discovered two startling features in
the 1413 MHz single pulse data. The first is the presence of giant micro-pulses
which are confined to the leading edge of the pulse profile. One of these
pulses has a peak flux density in excess of 2500 Jy, more than 40 times the
integrated pulse peak. The second new result is the presence of a large
amplitude gaussian component on the trailing edge of the pulse profile. This
component can exceed the main pulse in intensity but is switched on only
relatively rarely. Fluctutation spectra reveal a possible periodicity in this
feature of 140 pulse periods. Unlike the rest of the profile, this component
has low net polarization and emits predominantly in the orthogonal mode. This
feature appears to be unique to the Vela pulsar. We have also detected
microstructure in the Vela pulsar for the first time. These same features are
present in the 660 MHz data. We suggest that the full width of the Vela pulse
profile might be as large as 10 ms but that the conal edges emit only rarely.Comment: 6 pages, 5 figures, In Press with ApJ Letter
A Survey for Transient Astronomical Radio Emission at 611 MHz
We have constructed and operated the Survey for Transient Astronomical Radio
Emission (STARE) to detect transient astronomical radio emission at 611 MHz
originating from the sky over the northeastern United States. The system is
sensitive to transient events on timescales of 0.125 s to a few minutes, with a
typical zenith flux density detection threshold of approximately 27 kJy. During
18 months of around-the-clock observing with three geographically separated
instruments, we detected a total of 4,318,486 radio bursts. 99.9% of these
events were rejected as locally generated interference, determined by requiring
the simultaneous observation of an event at all three sites for it to be
identified as having an astronomical origin. The remaining 3,898 events have
been found to be associated with 99 solar radio bursts. These results
demonstrate the remarkably effective RFI rejection achieved by a coincidence
technique using precision timing (such as GPS clocks) at geographically
separated sites. The non-detection of extra-solar bursting or flaring radio
sources has improved the flux density sensitivity and timescale sensitivity
limits set by several similar experiments in the 1970s. We discuss the
consequences of these limits for the immediate solar neighborhood and the
discovery of previously unknown classes of sources. We also discuss other
possible uses for the large collection of 611 MHz monitoring data assembled by
STARE.Comment: 24 pages, 6 figures; to appear in PAS
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