Pulsars are known to be rapidly spinning and strongly magnetized neutron stars so they can behave like a unipolar inductor. Without the charge screening young pulsars can easily develop huge potential drop along the open field lines (cf. Manchester & Taylor 1977, Lyne & Graham-Smith 1998 or a general review). Such large potential drop can accelerate charged particles to extremely relativistic speed. Since the magnetic field in the open field lines are still sufficiently strong so charged particles are confined to move along the field lines, hence high energy curvature photons are emitted. It has been shown that pulsar magnetosphere is filled with charge separated plasma, whose charge distribution is given by the Goldreich-Julian charge density
, where
is the angular velocity vector and
is the local magnetic field vector (Goldreich & Julian 1969).
When
is perperdicular to
, the charge density is zero and such region is called null charge surface and charged carriers on two sides of the null charge surface are opposite. With this charge distribution the electric field along the magnetic field is screened out. However Cheng et al. (1986a) argued that when the current pass through the null charge surface, opposite charged carriers are removed from the vicinity of null charge surface and hence vacuum region will be formed. Charged particles in this vacuum region will be accelerated to extremely relativistic speed so this region is also called “Outergap” accelerator. The question is if the current continue to pass through the null charge region will the vacuum region to grow unlimitedly. The answer is “no” because the energy of the curvature photons is proportional to the size of vacuum region. When the energy of curvature photons is energetically enough, curvature photons and soft photons from the stellar surface can be converted into electron/positron pairs, which can limit the size of the Outergap. Zhang & Cheng (1997) argued that the soft photons from the surface should be dominated by thermal photons emitted by the hot polar cap, which is heated by the return current from the Outergap. In this particular model, the fractional size of the Outergap (
where
where
It has been shown that many observed features from gamma-ray pulsars can be explained in terms of the Outergap models (e.g. Cheng et al. 1986b, 2000, Romani 1996, Zhang & Cheng 1997, Takata et al. 2004, Hirotani 2006, 2008, Wang et al. 2010)
2. OUTERGAP CLOSED BY MAGNETIC PAIR CREATION
Takata et al. (2010) suggest that if the surface multiple field is much stronger than the dipolar field near the stellar surface in particular they can bend those open field lines connecting to the Outergap sideward, then the curvature photons emitted by the return current from the Outergap can become pairs near the surface and their pitch angles can be larger than 90 degrees(cf. Fig. 2 of Takata et al. 2010). In this situation, the magnetic pairs can stream back to the outermagnetosphere and restrict the size of the Outergap. The Outergap size restricted by the magnetic pair creation process is given by
where
, where Ω is the angular velocity,
where
where
3. SOFT GAMMA-RAY COMPONENT IN MILLISECOND PULSARS
Cheng et al. (2000) have shown that most pairs should be created around the null charge surface. In this Outergap model, the current flow inside the Outergap is dominated by the outflow current from the null charge surface toward the light cylinder whereas the inflow current dominates from the null charge surface toward the star. It has been shown by Hirotani (2005) that the inner boundary of the Outergap is not located at the null charge surface when the outergap current is not zero. Roughly speaking the location of the inner boundary can be estimated as
where
where
, which gives the characteristic curvature photon energy of inflow as
where
where
, where α is the inclination angle. Since the caustic effect of inflow radiation is small, this soft gammaray component should occur very near the radio pulse emitted from the polar cap. Unlike in the case of millisecond pulsars this soft gamma-ray component is very difficult to be observed in canonical pulsars because most of inflow curvature photons will be converted into pairs by the strong magnetic field (cf. Cheng & Zhang 1999, Wang et al. 2013a). Unless the inclination angle and the viewing are both small most inflow curvature photons will be reprocessed into hard X-rays and PSR1509-58 is the representative example (Wang et al. 2013a). The detail fitting of energy dependent light curves of millisecond pulsars can be found in Wang et al. (2013b).
4. ORBITAL MODULATED GAMMA-RAY FROM BLACK WIDOW SYSTEMS
It is believed that most of pulsar spin-down power eventually wi l l be converted from low frequency electromagnetic dipole radiation into the particle kinetic energy of the pulsar wind. However the exact conversion process from EM wave energy into particle energy is still unclear. In the Crab nebula in a distance ~3 × 1017 from the pulsar almost all EM wave energy becomes particle kinetic energy (Kennel & Coroniti 1984ab). On the other hand, by fitting the pulsed TeV data of the Crab pulsar/nebula detected by MAGIC Aharonian et al. (2012) have argued that only a few times of the light cylinder radii from the star a good fraction of spin-down power has already been in the kinetic energy of particles. It is still controversial if the pulsed TeV gamma-rays from the direction of the Crab pulsar are emitted in the magnetosphere or outside the light cylinder. PSR B1259-63/LS 2883 is one of most studied gamma-ray binaries, which is a binary system in which a 48 ms pulsar orbits around a Be star in a high eccentric orbit with a long orbital period of about 3.4 yr. It is special for having asymmetric two-peak profiles in both the X-ray and TeV light curves (Johnston et al. 1994, 1996, 2005, Aharonian et al. 2005, 2009, Chernyakova et al. 2006, 2009, Uchiyama et al. 2009). Recently, an unexpected GeV flare has also been detected by the Fermi gamma-ray observatory several weeks after the last periastron passage (Abdo et al. 2011, Tam et al. 2011). Although X-rays and TeV gamma-rays are generally expected to be emitted from the shock region (Tavani & Arons 1997, Takata & Taam 2009), its aysmmetric light curves are very difficult to be explained. In order to explain its asymmetric two-peak multi-wavelength light curves, Kong et al. (2011, 2012) argue that one of important factors to cause such asymmetric light curve result from the fact that the particle kinetic energy of the pulsar wind is position dependent. They argue that the particle kinetic energy of the pulsar wind should gradually increase and can be approximately described by a power law as
where
where
is the
Cheng et al. (2010) have estimated bulk Lorentz factor of the pulsar wind from millisecond pulsars
where
is number ratio between
where
The inverse Compton scattering between the optical photons from the companion star and the pulsar wind can produce high energy photons with the characteristic energy given by
The luminosity of the inverse Compton scattering is given by
where
is the optical photon density at distance r from the companion star. If the line of sight is sufficiently closed to the companion star, i.e.
We argue that the reason why millisecond pulsars can maintain a surface quadruple field with strength ~1011 G results from interpinning of quantized flux tubes and quantized vortex lines. The magnetic flux tubes are dragged toward the spin-axis during the accretion spin-up phase. Consequently an extremely strong quadruple can be formed in the boundary between the inner crust and the core. This multiple field is very important to ensure the magnetic pair creation process can occur even in millisecond pulsars with dipolar field ~108 G. In the Outergap models we predict that a sub-GeV component should exist in the vicinity of radio pulse, which is emitted by the inflow current. But the characteristic energy and luminosity of this component are expected to be lower than the main outflow components. We also predict that an orbital modulated gamma-ray component could be produced by the inverse Compton scattering between the pulsar wind and the optical photons from the companion star.