X-ray and gamma-ray emission from millisecond pulsars

Abstract

Abstract. We present a self-consistent model to describe X-ray and γ-ray emission from millisecond pulsars (MSPs). The X-rays of MSPs are produced by the backflow of primary charged particles from the outer gap and most likely consist of three components, two thermal components and one power law component if there is a strong multipole magnetic field on the stellar surface. The backflow of ultra-relativistic particles emits photons with energies about several tens of GeV via curvature radiation. These photons cause an electromagnetic cascade about 2-3 stellar radii above the polar cap. The synchrotron radiation of these cascade e ± pairs produces hard X-rays with a power law index ∼1.5. Near 10 5 cm above the stellar surface, the primary charged particles encounter the strong surface magnetic field, which alters the local radius of curvature greatly, and they quickly loose more than half of their remaining energies to curvature radiation. These curvature photons heat up the polar cap area with a radius ∼10 5 cm, which produce the softer thermal X-ray component. Finally, the primary charged particles deposit their remaining energies in a much smaller polar cap area, which corresponds to the footprints of outer gap and produce the medium hard X-ray component. γ-rays are produced in the outer gap through synchro-curvature radiation. We have applied this model to the MSPs which emit pulsed X-rays and likely γ-rays such as PSR J0437-4715, PSR J2124-3358, PSR J0218+4232 and PSR B1821-24. Our results give an agreement between predicted spectrum and the observed spectrum of MSP emission.

Radio and Gamma-Ray Pulsed Emission From Millisecond Pulsars

“…In this model, the accelerating field is not screened and the entire open volume is available for particle acceleration and emission of gamma rays. Zhang & Cheng (2003) used the outer gap model with a multi-pole magnetic field to model the X-ray and γ -ray spectra for four MSPs, and the predicted results basically agree with the observations (Harding et al 2005). …”

Gamma-ray pulsars: A gold mine

“…10 4.1. The unexpected γ-ray millisecond pulsar revolution Even though the magnetic fields of MSPs are relatively low (see Figure 1, their short periods produce high enough electric fields to accelerate particles to γ-ray energies [71,72]. However, it was thought that MSPs cannot produce rich pair cascades unless there are distortions of the surface magnetic fields [73] (see section 5), so many were expected to be pair-starved [21].…”

An X-ray View of Radio Millisecond Pulsars

“…The large majority of the 47 Tuc MSPs (16 out of 19) appear to be soft, thermal X-ray sources with temperatures T eff ∼ (1 − 3) × 10 6 K, emission radii R eff ∼ 0.1 − 3 km, and luminosities L X ∼ 10 30−31 ergs s −1 [7]. The inferred effective emission areas imply that this radiation originates from the heated magnetic polar caps of the pulsar, as suggested by theoretical pulsar models [8,9]. The thermal nature of the observed X-ray emission was subsequently confirmed with Chandra HRC-S timing observations, which revealed relatively broad pulsations with relatively low (≤50%) pulsed fractions [10].…”