32,923 research outputs found
Spark Model for Pulsar Radiation Modulation Patterns
A non-stationary polar gap model first proposed by Ruderman & Sutherland
(1975) is modified and applied to spark-associated pulsar emission at radio
wave-lengths. It is argued that under physical and geometrical conditions
prevailing above pulsar polar cap, highly non-stationary spark discharges do
not occur at random positions. Instead, sparks should tend to operate in well
determined preferred regions. At any instant the polar cap is populated as
densely as possible with a number of two-dimensional sparks with a
characteristic dimension as well as a typical distance between adjacent sparks
being about the polar gap height. Our model differs, however, markedly from its
original 'hollow cone' version. The key feature is the quasi-central spark
driven by pair production process and anchored to the local pole of a
sunspot-like surface magnetic field. This fixed spark prevents the motion of
other sparks towards the pole, restricting it to slow circumferential drift
across the planes of field lines converging at the local pole. We argue that
the polar spark constitutes the core pulsar emission, and that the annular
rings of drifting sparks contribute to conal components of the pulsar beam. We
found that the number of nested cones in the beam of typical pulsar should not
excced three; a number also found by Mitra & Deshpande (1999) using a
completely different analysis.Comment: 31 pages, 8 figures, accepted by Ap
Computing solutions of the modified Bessel differential equation for imaginary orders and positive arguments
We describe a variety of methods to compute the functions ,
and their derivatives for real and positive . These
functions are numerically satisfactory independent solutions of the
differential equation . In an accompanying paper
(Algorithm xxx: Modified Bessel functions of imaginary order and positive
argument) we describe the implementation of these methods in Fortran 77 codes.Comment: 14 pages, 1 figure. To appear in ACM T. Math. Sof
Unraveling the drift behaviour of the remarkable pulsar PSR B0826-34
We present new results from high sensitivity GMRT observations of PSR
B0826-34. We provide a model to explain the observed subpulse drift properties
of this pulsar, including the apparent reversals of the drift direction. In
this model, PSR B0826-34 is close to being an aligned rotator. We solve for the
emission geometry of this pulsar and show that the angle between the rotation
and the magnetic axes is less than 5 deg. We see evidence for as many as 6 to 7
drifting bands in the main pulse at 318 MHz, which are part of a circulating
system of about 15 spark-associated subpulse emission beams. We provide
quantitative treatments of the aliasing problem and various effects of
geometry. The observed drift rate is an aliased version of the true drift rate,
such that a subpulse drifts to the location of the adjacent subpulse (or a
multiple thereof) in about one pulsar period. We show that small variations, of
the order of 3-8%, in the mean drift rate are then enough to explain the
apparent reversals of drift direction. We find the mean circulation time of the
drift pattern to be significantly longer than the predictions of the original
RS75 model and propose an explanation for this, based on modified models with
temperature regulated partial ion flow in the polar vacuum gap. From the
variation of the mean subpulse separation across the main pulse window, we show
that the spark pattern is not centred around the dipole axis, but around a
point much closer (within a degree or so) to the rotation axis -- we discuss
the implication of this.Comment: 23 pages (including 9 figure). Submitted to Astronomy and
Astrophysics on November 11, 200
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