Pulsar wind nebulae in the MeV domain

In the Crab Nebula, a dramatic variability is observed near the cutoff of the synchrotron spectrum (GeV flares), the cutoff energy being determined by a balance between acceleration and the cooling losses. The synchrotron spectrum in X-rays and higher energies is consistent with Fermi acceleration at a relativistic shock, although questions remain about the magnetization regime. In most other young pulsar wind nebulae (PWNe), with lower magnetic fields, the maximum acceleration energy is set by the particle confinement criterion, which implies a synchrotron cutoff in the MeV domain for typical parameters. However, there is currently little or no data above about 100 keV. Measurement of the spectral cutoff in these objects would yield constraints on particle transport in relativistic shocks, and magnetic field in the inner region of PWNe (shedding light on the so-called ''σ problem''). Possible MeV variability around the synchrotron cutoff energy could illuminate the physics of Crab GeV flares. In addition, measurement of the synchrotron polarization near cutoff can give a unique view of the magnetic geometry of the acceleration region.

Image of the Crab Nebula taken with the NASA/ESA Hubble Space Telescope. The bluish glow inside the nebula comes from the synchrotron radiation of relativistic electrons. Credit: NASA, ESA and Allison Loll/Jeff Hester.

Required instrument performances:
Detection of bright PWNe other than the Crab Nebula and measurement of their spectral cutoffs would ideally require reaching a sensitivity of about 3x10-12 erg cm-2 s-1 throughout the MeV domain (given a typical spectral index of ~ 2.1). Resolving the emission region would require an angular resolution of ~10 arcsec or better; angular resolution < 1 degree (to be quantified further) will be required to overcome source confusion in the Galactic plane. A modest spectral resolution of ~10% will be adequate to characterize the energy cutoffs. Separating the PWN emission from the pulsed emission requires a timing resolution better than a few ms (for young pulsars) throughout their energy range (as well as contemporaneous ephemerides from other frequencies). High polarization fractions (~50%) are expected. But after the Crab Nebula, the next most intense PWN is MSH 15-52 with a flux of about 5 milliCrab; the corresponding minimum detectable polarization fraction is thus about 0.2%.

Performance parameter   Goal value Remarks and notes
(FWHM, deg)
  Sources known from other energies
Angular resolution
(FWHM, deg)
< 0.003
< 1
To resolve MeV emission region
To overcome confusion with nearby sources
Spectral resolution
(ΔE/E @ Energy)
Line sensitivity (@ Energy)
(cm-2.s-1, 3σ, 1 Ms)
Continuum sensitivity (in which energy band?)
(cm-2 s-1 keV-1, ΔE=E, 3σ, 1 Ms)
~ 3x10-9  @ 1 MeV Ideally going as E-2 throughout range
Timing performances < few ms To distinguish PWN from pulsar
Polarimetric capability
(Minimum Polarization Fraction for a Crab source in 1 Ms)
50% polarisation for a 5 mCrab source
Real-time data?