Efficiency of Nonthermal Pulsed Emission from Eight MeV Pulsars

Abstract

We report on the properties of pulsed X-ray emission from eight MeV pulsars using XMM-Newton, NICER, NuSTAR, and HXMT data. For five of the eight MeV pulsars, the X-ray spectra can be fit by a broken power-law model with a break energy of ∼5–10 keV. The photon indices below and above the break energy are ∼1 and ∼1.5, respectively. In comparison with the X-ray emission of the Fermi-LAT pulsars, the MeV pulsars have a harder spectrum and a higher radiation efficiency in the 0.3–10 keV energy bands. When isotropic emission is assumed, the emission efficiency in the keV–MeV bands is estimated to be η _MeV ∼ 0.01–0.1, and this is similar to the efficiency of the GeV emission of the Fermi-LAT pulsars with a similar spin-down power. To explain the observed efficiency of the MeV pulsars, we estimate the required pair multiplicity as 10 ^4–7 , which depends on the emission process (curvature radiation or synchrotron radiation) and on the location in the magnetosphere. The high multiplicity indicates that the secondary pairs that are created by a pair-creation process of the GeV photons produce the X-ray/soft gamma-ray emission of the MeV pulsars. We speculate that the difference between MeV pulsars and Fermi-LAT pulsars can be attributed to the difference in viewing angle measured from the spin axis if the emission originates from a region inside the light cylinder (canonical gap model) or to the difference in the inclination angle of the magnetic axis if the emission is produced in the equatorial current sheet outside the light cylinder.

Keywords

• Neutron stars