A supernova explosion ejects dense energetic plasma and this sweeps up the nearby interstel- lar medium. A shock wave is generated in the place where supernova ejecta and the interstel- lar medium collide. There, the first order Fermi acceleration (see Sect. 1.2.2) works and thus charged particles can be accelerated. Considering the shock speed, scale of the system, magnetic field and explosion rate, SNR can explain basically all the features of CRs at least below the knee energy. Fig. 1.1 shows the emission areas of VHE gamma-rays of the supernova remnant RX J1713-3946.
Pulsars, Pulsar Winds and Pulsar Wind Nebulae
Inside a pulsar magnetosphere, a persistent and strong electric field can exist. Also a high density of electrons and positrons exist there and these can be accelerated by the strong electric field. The details will be explained in the next chapter.
As described in Sect. 1.2.4, pulsars can accelerate particles by low frequency magnetic dipole radiation. This relativistic plasma wind is called a pulsar wind and the energy of electrons there can be as high as10
14
eV (see [10]).
At a certain distance from the pulsar, the pulsar wind will be terminated due to the interaction with interstellar plasma. At the termination point, a standing reverse shock is created. The shock accelerates electrons up to10
15
eV and randomizes their pitch angles. This results in the formation of an extended synchrotron source (see [10]) which is called a pulsar wind nebula (PWN).
In summary, one expects in a pulsar system both pulsed gamma-ray emission within the light cylinder and steady gamma-ray emission from the unshocked region and the termination shock region (PWN). Many of PWN and one pulsar have been detected in VHE gamma-rays.
1.4 Possible Acceleration Sites and Known VHE Gamma-ray Sources 21
Figure 1.10: Acceleration region around the pulsar. Inside the light cylinder (see Sect. 2.5.2) , the electric field accelerates particles. Outside the light cylinder, magnetic dipole radiation accelerates particles and produces strong pulsar winds. In addition, shock acceleration happens in the place where the pulsar winds produce termination shock, resulting in pulsar wind nebula. Figure adopted from [10].
Binary Systems
Binary systems may accelerate particles either in the stellar wind shock or in the microquasar jets (see the right panel of Fig. 1.11). Three VHE gamma-ray binaries have been discovered so far.
If both stars have strong plasma wind as do pulsars, Wolf-Rayet stars or OB stars, their winds collide and produce a strong shock (see the right panel of Fig. 1.11). Then particles can be accelerated by shock acceleration. In this case, since the binary orbit is usually eccentric, the modulations of gamma-ray flux is expected according to the orbital phase (see [12] and [16]).
If one star is a compact object like a neutron star or a black hole and the other star has a huge mass loss, accretion of the matter to the strong gravitational star may create an accretion disc and relativistic plasma jets. Such a system is called a microquasar (see the left panel of Fig. 1.11). Inside the microquasar jets, shock waves may exist and particles can be accelerated. Magnetic reconnection which accelerates the particles can also happen (see [64]).
22 1. Very High Energy Gamma Ray Astronomy
Figure 1.11: Schematic view of the binary system. Binary systems can accelerate particles either in the microquasar jet or in the wind collision shock. If one star is a compact object like a pulsar or a black hole and the other star has a large mass loss rate, the accretion of matter may generate relativistic jets where particles are accelerated (left, microquasar scenario). If both stars have strong stellar winds and they collide with each other, shocks will occur and particles will be accelerated there (right, wind collision scenario). Figure adopted from [133].
.
Open Clusters and Globular Clusters
In a young open cluster, there are numerous young massive stars which have a strong plasma wind. Because of the smaller distance between stars, the winds collide with each other or collec- tive winds collide with surrounding matters. Then, shock will be created which leads to particle acceleration (see e.g. [38]). Similarly, in a globular cluster, plasma winds from many pulsars can accelerate particles (see e.g. [39]). Currently no open/globular clusters have been detected in VHE gamma-rays.
Other Sources
Wolf-Rayet stars and OB stars have strong plasma winds and it is possible that termination shocks or turbulence can accelerate particles to high energy. Up till now only one VHE source has been found by HESS which may be associated with a Wolf-Rayet star (see [51]).
Our galaxy has in its center a supermassive black hole with10 6
solar masses. The compact radio source Sgr A* is associated with it. A VHE gamma-ray source is also found in there (see e.g. [17]) and hence, it is possible that particle acceleration takes place there. However, it should be noted that there are also quite a few different objects near the galactic center such as supernova remnant Sgr A East and pulsar wind nebula G359.95, which are within the error circle
1.4 Possible Acceleration Sites and Known VHE Gamma-ray Sources 23
of the VHE gamma-ray source position.