12,060 research outputs found
Two-photon annihilation in the pair formation cascades in pulsar polar caps
The importance of the photon-photon pair production process () to form pair production cascades in pulsar
polar caps is investigated within the framework of the Ruderman-Sutherland
vacuum gap model. It is found that this process is unimportant if the polar
caps are not hot enough, but will play a non-negligible role in the pair
formation cascades when the polar cap temperatures are in excess of the
critical temperatures, , which are around when
s and will slowly increase with increasing periods. Compared with the
process, it is found that the two-photon annihilation process may
ignite a central spark near the magnetic pole, where sparks can not
be formed due to the local weak curvatures. This central spark is large if the
gap is dominated by the ``resonant ICS mode''. The possible connection of these
central sparks with the observed pulsar ``core'' emission components is
discussed.Comment: 7 pages, 3 Postscript figures, LaTex, accepted for publication in
Astronomy and Astrophysic
Electrical properties of breast cancer cells from impedance measurement of cell suspensions
Impedance spectroscopy of biological cells has been used to monitor cell status, e.g. cell proliferation, viability, etc. It is also a fundamental method for the study of the electrical properties of cells which has been utilised for cell identification in investigations of cell behaviour in the presence of an applied electric field, e.g. electroporation. There are two standard methods for impedance measurement on cells. The use of microelectrodes for single cell impedance measurement is one method to realise the measurement, but the variations between individual cells introduce significant measurement errors. Another method to measure electrical properties is by the measurement of cell suspensions, i.e. a group of cells within a culture medium or buffer. This paper presents an investigation of the impedance of normal and cancerous breast cells in suspension using the Maxwell-Wagner mixture theory to analyse the results and extract the electrical parameters of a single cell. The results show that normal and different stages of cancer breast cells can be distinguished by the conductivity presented by each cell. © 2010 IOP Publishing Ltd
PSR 0943+10: a bare strange star?
Recent work by Rankin & Deshpande strongly suggests that there exist strong
``micro-storms'' rotating around the magnetic axis of the 1.1s pulsar PSR
0943+10. Such a feature hints that most probably the large-voltage vacuum gap
proposed by Ruderman & Sutherland (RS) does exist in the pulsar polar cap.
However, there are severe arguments against the formation of the RS-type gap in
pulsars, since the binding energies of both the Fe ions and the electrons in a
neutron star's surface layer is too small to prevent thermionic ejection of the
particles from the surface. Here we propose that PSR 0943+10 (probably also
most of the other ``drifting'' pulsars) might be bare strange stars rather than
normal neutron stars, in which the ``binding energy'' at the surface is merely
infinity either for the case of ``pulsar'' or ``anti-pulsar''. It is further
proposed that identifying a drifting pulsar as an anti-pulsar is the key
criterion to distinguish strange stars from neutron stars.Comment: 4 pages, no figures, LaTeX, accepted 1999 July 9 by ApJ Letter
Is Gamma-ray Absorption by Induced Electric Fields Important in the Pulsar Magnetospheres?
Although the unified formula for gamma-ray absorption process involving both
the magnetic field and a perpendicular electric field derived by Daugherty &
Lerche (1975) is correct, we argued in this paper that their conclusion that
the induced electric fields are important in the pair formation process in the
pulsar magnetospheres is wrong and misleading. The key point is that usually
the direction of a gamma photon at the emission point observed in the
laboratory frame should be (v/c, 0, [1-(v/c)^2]^{1/2}) rather than (0, 0, 1),
where v is the co-rotating velocity. This emission direction is just the one
which results in zero attenuation coefficient of the gamma photon. Calculation
shows that after the photon has moved a distance, its direction lead to the
result that the induced electric field is also of minor importance. Thus only
gamma-B process is the important mechanism for the pair production in the
pulsar magnetospheres. The implications of the modification by ejecting the
induced electric field are also discussed.Comment: 4 pages, 2 Postscript figures, LaTeX, some miscomments on the
references of Harding et al are modified, Accepted for publication in
Astronomy and Astrophysics Letter
Searching for sub-millisecond pulsars from highly polarized radio sources
Pulsars are among the most highly polarized sources in the universe. The NVSS
has catalogued 2 million radio sources with linear polarization measurements,
from which we have selected 253 sources, with polarization percentage greater
than 25%, as targets for pulsar searches. We believe that such a sample is not
biased by selection effects against ultra-short spin or orbit periods. Using
the Parkes 64m telescope, we conducted searches with sample intervals of 0.05
ms and 0.08 ms, sensitive to submillisecond pulsars. Unfortunately we did not
find any new pulsars.Comment: 2 pages 1 figure. To appear in "Young Neutron Stars and Their
Environments" (IAU Symposium 218, ASP Conference Proceedings), eds F. Camilo
and B. M. Gaensle
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