High-energy radiation from millisecond pulsars

Abstract

As the successor of the Energetic Gamma Ray Experiment Telescope, the Large Area Telescope on board the Fermi Gamma-ray Space Telescope has measured the pulsations from over a hundred pulsars, a rapidly growing population in the gamma-ray sky. Pulsars spinning as fast as a few to ten milliseconds, the millisecond pulsars (MSPs), constitute over 30% of the population, with most of which found in binary systems. Many MSPs are found to have low-mass companions with masses less than ~ 0:1 of that of the Sun. Observations reveal that in these systems, the companion is irradiated by the radiation and/or the wind from the pulsar. It is believed that isolated MSPs in the Galactic field were once a member of such kind of systems, and have ablated their companion away. The gamma-ray emission from the original Black Widow pulsar was studied using data from the Fermi-LAT. The emission was found to depend on the orbital phase and an extra higher-energy component was observed near the inferior conjunction. The results can be explained by an inverse Compton (IC) process associated with the ultra-relativistic pulsar wind. PSR B1957+20 is the first black widow system from which evidence on interaction of unshocked pulsar wind is observed. In addition, diffuse X-rays were found from the globular cluster (GC) 47 Tucanae. It is believed that the observed gamma-rays and the diffuse X-rays can be produced via inverse Compton process between the background soft photons and the pulsar wind from the MSPs residing in the GC. The observation of diffuse X-rays from 47 Tucanae provides constraints on the energetics and the emission region for the IC scenario. They provide good examples and guides for search strategies of similar emissions in other black widow systems and globular clusters.