J. Takata, K. S. Cheng, R. E. Taam
The $Fermi$-LAT has revealed that rotation powered millisecond pulsars (MSPs)
are a major contributor to the Galactic $\gamma$-ray source population. Such
pulsars may also be important in modeling the quiescent state of several low
mass X-ray binaries (LMXBs), where optical observations of the companion star
suggest the possible existence of rotation powered MSPs. To understand the
observational properties of the different evolutionary stages of MSPs, the
X-ray and $\gamma$-ray emission associated with the outer gap model is
investigated. For rotation powered MSPs, the size of the outer gap and the
properties of the high-energy emission are controlled by either the
photon-photon pair-creation process or magnetic pair-creation process near the
surface. For these pulsars, we find that the outer gap model controlled by the
magnetic pair-creation process is preferable in explaining the possible
correlations between the $\gamma$-ray luminosity or non-thermal X-ray
luminosity versus the spin down power. For the accreting MSPs in quiescent
LMXBs, the thermal X-ray emission at the neutron star surface resulting from
deep crustal heating can control the conditions in the outer gap. We argue that
the optical modulation observed in the quiescent state of several LMXBs
originates from the irradiation of the donor star by $\gamma$-rays from the
outer gap. In these systems, the irradiation luminosity required for the
optical modulation of the source such as SAX J1808.4-3658 can be achieved for a
neutron star of high mass. Finally, we discuss the high-energy emission
associated with an intra-binary shock in black widow systems, e.g. PSR
B1957+20.
View original:
http://arxiv.org/abs/1111.3451
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