17,492 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
Frequency dependence of pulsar radiation patterns
We report on new results from simultaneous, dual frequency, single pulse
observation of PSR B0329+54 using the Giant Metrewave Radio Telescope. We find
that the longitude separation of subpulses at two different frequencies (238
and 612 MHz) is less than that for the corresponding components in the average
profile. A similar behaviour has been noticed before in a number of pulsars. We
argue that subpulses are emitted within narrow flux tubes of the dipolar field
lines and that the mean pulsar beam has a conal structure. In such a model the
longitudes of profile components are determined by the intersection of the line
of sight trajectory with subpulse-associated emission beams. Thus, we show that
the difference in the frequency dependence of subpulse and profile component
longitudes is a natural property of the conal model of pulsar emission beam. We
support our conclusions by numerical modelling of pulsar emission, using the
known parameters for this pulsar, which produce results that agree very well
with our dual frequency observations.Comment: 24 pages, 8 figures. Accepted for publication in Ap
Interrelation between radio and X-ray signatures of drifting subpulses in pulsars
We examined a model of partially screened gap region above the polar cap, in
which the electron-positron plasma generated by sparking discharges coexists
with thermionic flow ejected by the bombardment of the surface beneath these
sparks. Our special interest was the polar cap heating rate and the subpulse
drifting rate, both phenomena presumably associated with sparks operating at
the polar cap. We investigated correlation between the intrinsic drift rate and
polar cap heating rate and found that they are coupled to each other in such a
way that the thermal X-ray luminosity from heated polar cap depends only
on the observational tertiary subpulse drift periodicity (polar cap
carousel time). Within our model of partially screened gap we derived the
simple formula relating and , and showed that it holds for
PSRs B094310 and B1133+16, which are the only two pulsars in which both
and are presently known.Comment: 4 page
Discovery of a remarkable subpulse drift pattern in PSR B0818-41
We report the discovery of a remarkable subpulse drift pattern in the
relatively less studied wide profile pulsar, B0818-41, using high sensitivity
GMRT observations. We find simultaneous occurrence of three drift regions with
two different drift rates: an inner region with steeper apparent drift rate
flanked on each side by a region of slower apparent drift rate. Furthermore,
these closely spaced drift bands always maintain a constant phase relationship.
Though these drift regions have significantly different values for the measured
P2, the measured P3 value is the same and equal to 18.3 P1. We interpret the
unique drift pattern of this pulsar as being created by the intersection of our
line of sight (LOS) with two conal rings on the polar cap of a fairly aligned
rotator (inclination angle alpha ~ 11 deg), with an ``inner'' LOS geometry
(impact angle beta ~ -5.4 deg). We argue that both the rings have the same
values for the carousel rotation periodicity P4 and the number of sparks Nsp.
We find that Nsp is 19-21 and show that it is very likely that, P4 is the same
as the measured P3, making it a truly unique pulsar. We present results from
simulations of the radiation pattern using the inferred parameters, that
support our interpretations and reproduce the average profile as well as the
observed features in the drift pattern quite well.Comment: 5 pages and 7 figures, Accepted for publication in MNRAS Letter
Mapping the star formation history of Mrk86: I. Data and models
We have obtained optical (BVR, [OIII]5007 and Halpha), near infrared (JHK)
imaging and long-slit optical spectroscopy for the Blue Compact Dwarf galaxy
Mrk86 (NGC2537). In this paper, the first of two, we present optical-near-
infrared colors and emission-line fluxes for the currently star-forming
regions, intemediate aged starburst and underlying stellar population. We also
describe the evolutionary synthesis models used in Paper II. The R and Halpha
luminosity distributions of the galaxy star-forming regions show maxima at
M_R=-9.5 and L_Halpha=10^37.3 erg s^-1. The underlying stellar population shows
an exponential surface brigthness profile with central value, mu_E,0=21.5 mag
arcsec^-2, and scale, alpha=0.88 kpc, both measured in the R-band image. In the
galaxy outer regions, dominated by this component, no significant color
gradients are observed. Finally, a set of evolutionary synthesis models have
been developed, covering a wide range in metallicity and burst strength.Comment: 21 pages, 14 figures, 2 landscape tables, accepted for publication in
Astronomy & Astrophysics Supplement Series, for higher resolution images see
ftp://cutrex.fis.ucm.es/pub/OUT/gil/PAPERS/aa00_I.ps.g
XMM-Newton Observations of Radio Pulsars B0834+06 and B0826-34 and Implications for Pulsar Inner Accelerator
We report the X-ray observations of two radio pulsars with drifting
subpulses: B0834 + 06 and B0826 - 34 using \xmm\. PSR B0834 + 06 was detected
with a total of 70 counts from the three EPIC instruments over 50 ks exposure
time. Its spectrum was best described as that of a blackbody (BB) with
temperature K and bolometric luminosity
of erg s. As it is typical in
pulsars with BB thermal components in their X-ray spectra, the hot spot surface
area is much smaller than that of the canonical polar cap, implying a
non-dipolar surface magnetic field much stronger than the dipolar component
derived from the pulsar spin-down (in this case about 50 times smaller and
stronger, respectively). The second pulsar PSR B0826 - 34 was not detected over
50 ks exposure time, giving an upper limit for the bolometric luminosity erg s. We use these data as well as the radio
emission data concerned with drifting subpulses to test the Partially Screened
Gap (PSG) model of the inner accelerator in pulsars.Comment: Accepted for publication by The Astrophysical Journa
Efficient single photon emission from a high-purity hexagonal boron nitride crystal
Among a variety of layered materials used as building blocks in van der Waals
heterostructures, hexagonal boron nitride (hBN) appears as an ideal platform
for hosting optically-active defects owing to its large bandgap ( eV).
Here we study the optical response of a high-purity hBN crystal under green
laser illumination. By means of photon correlation measurements, we identify
individual defects emitting a highly photostable fluorescence under ambient
conditions. A detailed analysis of the photophysical properties reveals a high
quantum efficiency of the radiative transition, leading to a single photon
source with very high brightness. These results illustrate how the wide range
of applications offered by hBN could be further extended to photonic-based
quantum information science and metrology.Comment: 5 pages, 4 figure
Spin-Exchange Interaction in ZnO-based Quantum Wells
Wurtzitic ZnO/(Zn,Mg)O quantum wells grown along the (0001) direction permit
unprecedented tunability of the short-range spin exchange interaction. In the
context of large exciton binding energies and electron-hole exchange
interaction in ZnO, this tunability results from the competition between
quantum confinement and giant quantum confined Stark effect. By using
time-resolved photoluminescence we identify, for well widths under 3 nm, the
redistribution of oscillator strengths between the A and B excitonic
transitions, due to the enhancement of the exchange interaction. Conversely,
for wider wells, the redistribution is cancelled by the dominant effect of
internal electric fields, which dramatically reduce the exchange energy.Comment: 14 pages, 3 figure
- âŠ