The population of millisecond pulsars (MSPs) has been expanded considerably in the last decade. Not only is their number increasing, but also various classes of them have been revealed. Among different classes of MSPs, the behaviours of black widows and redbacks are particularly interesting. These systems consist of an MSP and a low-mass companion star in compact binaries with an orbital period of less than a day. In this article, we give an overview of the high energy nature of these two classes of MSPs. Updated catalogues of black widows and redbacks are presented and their X-ray/γ-ray properties are reviewed. Besides the overview, using the most updated eight-year Fermi Large Area Telescope point source catalog, we have compared the γ-ray properties of these two MSP classes. The results suggest that the X-rays and γ-rays observed from these MSPs originate from different mechanisms. Lastly, we will also mention the future prospects of studying these spider pulsars with the novel methodologies as well as upcoming observing facilities.
What Are Millisecond Pulsars?Rotation-powered pulsars, which act as unipolar inductors by coupling the strong magnetic field and fast rotation, radiate at the expense of their rotational energy. As a result, the rotation of a pulsar gradually slows down as it ages. When the rotation becomes too slow to sustain the particles' acceleration in their magnetospheres, the radiation beam shuts down. In such a case, we say a pulsar is "dead". While their magnetospheric particle accelerators have been turned off, other emission mechanisms (e.g., accretion) can still possibly work in these dead pulsars.By the time of writing, there are 2702 pulsars in total, including radio pulsars, radio-quiet γ-ray pulsars and magnetars [1]. In Figure 1, we show the distribution of their period P and period derivativė P. Applying a clustering analysis in this 2D parameter space with k-means partitioning, the whole population can be divided into two parts. The largest one is displayed as black dots in Figure 1. This partition spans a range of P ∼ 0.02 − 23.5 s andṖ ∼ 5 × 10 −18 − 5 × 10 −10 s/s. This group includes canonical pulsars as well as magnetars. Assuming the surface magnetic field is dipolar and the rotational energy of the pulsar goes entirely to the dipolar radiation, one is able to estimate their surface magnetic field strength B s and their characteristic age τ as: