10,198 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
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
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
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
What if pulsars are born as strange stars?
The possibility and the implications of the idea, that pulsars are born as
strange stars, are explored. Strange stars are very likely to have atmospheres
with typical mass of but bare polar caps almost
throughout their lifetimes, if they are produced during supernova explosions. A
direct consequence of the bare polar cap is that the binding energies of both
positively and negatively charged particles at the bare quark surface are
nearly infinity, so that the vacuum polar gap sparking scenario as proposed by
Ruderman & Sutherland should operate above the cap, regardless of the sense of
the magnetic pole with respect to the rotational pole. Heat can not accumulate
on the polar cap region due to the large thermal conductivity on the bare quark
surface. We test this ``bare polar cap strange star'' (BPCSS) idea with the
present broad band emission data of pulsars, and propose several possible
criteria to distinguish BPCSSs from neutron stars.Comment: 31 pages in Latex. Accepted by AstroParticle Physic
- …