A Physical Model of Pulsar X-ray Filaments

Abstract

We present a model for pulsar filaments - a class of narrow X-ray nebulae misaligned with the proper motion, powered by pulsar-generated $e^\pm$. We suggest that cosmic ray-enhanced turbulence drives pitch-angle scattering and dominates $e^\pm$ motion along the filament; highly amplified magnetic fields are not required. A simulation built on this picture, using analytic approximations for the turbulence growth and cosmic ray evolution, generates images and spectra matching observations of the three best-measured filaments. The model structure depends on interstellar medium properties, and fits to filament data require values similar to observed ISM values. In this model a substantial fraction of the filament $e^\pm$ escape, free-streaming for many pc, in contrast to the suppressed cosmic ray diffusion near pulsar TeV halos. Accordingly, nearby low-power filament-generating pulsars may make out-sized contributions to the local positron spectrum. Future X-ray observatories can make the sensitive spectral maps required to test this particle escape.

Figures (5)

• Figure 1: Comparisons between the model (bottom) and data (middle) for the three brightest X-ray filaments, assuming inclination $\iota=120^\circ$. Images and one-dimensional histograms (top) integrated over width are shown. The pulsar is moving up in all images.
• Figure 2: 68 and 95% posterior contours for our fit to observed images of Guitar, Lighthouse, and J2030. The band indicates typical ISM $\beta_A$. The Guitar may live in an especially low ionization zone (see text). All filaments appear to be well-described by similar $f_\perp$.
• Figure 3: Top: The Lighthouse model filament growth over time, with injection cutoff at $t=t_\mathrm{cool}$. Stars on the left edge indicate the pulsar's current position and position at cutoff. A reference line (15$^\circ$ from the pulsar position) shows how the peak of an old filament drifts outward with time. Bottom:Chandra data of the Lighthouse filament.
• Figure 4: Top: The histogram shows the steady state particle spectrum for the Guitar filament simulation. The dashed line indicates the injected spectrum, and solid colored lines are reference power laws described in the text. Bottom: The synchrotron photon spectrum, with reference power law components. The photon index $\Gamma$ for a power law fit to the segment of the spectrum in the Chandra band (shaded) is consistent with measurements (orange line).
• Figure 5: Photon spectral indices $\Gamma$ fitted to the Guitar filament model as a function of position.