Hard X-ray emission from pulsar-wind nebulae

Abstract

Pulsar-wind nebulae emit an extremely broad spectrum of continuum radiation, from low radio frequencies to TeV gamma rays. The part of the spectral energy distribution (SED) from radio through MeV gamma rays is due to synchrotron emission from a distribution of relativistic electrons (or pairs) which can be described by one or more power laws. This spectrum exhibits that particle energy distribution, responsible also for the higher-energy (GeV–TeV) part of the SED, due to inverse-Compton upscattering of one of three photon fields: the synchrotron spectrum, the cosmic microwave background, or ambient optical/infrared photons. However, in a few sources, primary hadrons may produce GeV–TeV gamma rays through the decay of neutral pions produced in inelastic cosmic-ray collisions with thermal gas. The higher-energy end of the particle spectrum, producing synchrotron photons above approximately 10 keV, holds clues to the particle acceleration process. However, its detailed study requires imaging spectroscopy in this energy range, not available until the NuSTAR mission beginning in 2012, which performs true imaging between 3 and 78 keV with ${\sim}1^{\prime }$ angular resolution. I review NuSTAR observations of the first three pulsar-wind nebulae (PWNe) to be examined in this way: the Crab Nebula, G21.5–0.9 and MSH 15–52. All three show spectral structure not previously known: spectral steepening in certain locations and overall source shrinkage with increasing photon energy. The Crab Nebula has different shrinkage rates along the torus and along the northwest counter-jet. The latter rate is similar to that for both the other sources (FWHM $\propto E^{m}$ with $m\sim -0.2$). I discuss implications of these results for models of particle transport in PWNe.

Relativistic turbulence with strong synchrotron and synchrotron self-Compton cooling

“…Based on broadband observations (see, e.g., de Jager et al 1996;Aharonian et al 2006;Hester 2008;Abdo et al 2010), the Crab Nebula is believed to be filled with mildly magnetized (with estimates for the magnetization σ parameter ranging from 10 −3 to 1) ultra-relativistic electron-positron pair plasma (Rees & Gunn 1974;Kennel & Coroniti 1984a,b), see Arons (2012); Kargaltsev et al (2015); Reynolds (2016) for recent reviews. It shines to us brightly across the entire observationally accessible electromagnetic spectrum, with most of the power attributed to synchrotron emission, spanning from radio to highenergy (∼ 100 MeV) gamma-rays, with a second, somewhat weaker, distinct component at even higher (TeV) photon energies, attributed to inverse-Compton emission.…”

A Broadband X-Ray Study of the Rabbit Pulsar Wind Nebula Powered by PSR J1418-6058

“…In general, young rotation-powered PWNe (10 3 yr) such as the Crab Nebula and G21.5−0.9, compared to older nebulae, are bright in X-rays and relatively faint in the VHE band, implying that magnetic fields in young PWNe are likely strong (B ∼ 100 μG; Meyer et al 2010;Guest et al 2019). Particles in these PWNe lose energy efficiently via synchrotron radiation, which is observed as a spectral break and/or a PWN size decrease with increasing photon energy (e.g., Reynolds 2016). The X-ray-to-VHE flux ratios of numerous PWNe in different evolutionary stages (e.g., young, middle-aged, and relic PWNe) have been observed to decrease with their ages (Kargaltsev et al 2013).…”

X-Ray Characterization of the Pulsar PSR J1849−0001 and Its Wind Nebula G32.64+0.53 Associated with TeV Sources Detected by H.E.S.S., HAWC, Tibet ASγ, and LHAASO

“…However, the injection rates and energetics of electrons remain unclear. These quantities depend on the flow properties and energy-loss mechanisms within PWNe (e.g., Reynolds 2016), which can be investigated by modeling spatially varying properties and broadband SEDs of PWNe (e.g., Park et al 2023a) across different evolutionary stages. Of particular significance is investigating the most energetic electrons (>100 TeV), which is most effectively accomplished by analyzing their synchrotron X-ray emission, while the IC emission in TeV energies is suppressed by the Klein-Nishina effect.…”