NuSTAR observations of the young, energetic radio pulsar PSR B1509-58

TL;DR

NuSTAR observations of PSR B1509-58 reveal a curved X-ray spectrum best modeled by a log parabola, constraining the high-energy peak at 2.6 MeV, and show phase-dependent spectral variations and pulsed fractions.

Contribution

This study provides the first measurement of the pulsed fraction in hard X-rays and refines the spectral peak position, supporting a magnetospheric synchrotron emission model.

Findings

Spectrum is best fit by a log parabola, confirming spectral curvature.

Peak of broadband high-energy spectrum is at 2.6 MeV with improved precision.

Pulsed fraction varies with phase and constrains off-pulse emission properties.

Abstract

We report on Nuclear Spectroscopic Telescope Array (NuSTAR) hard X-ray observations of the young rotation-powered radio pulsar PSR B1509$-$58 in the supernova remnant MSH 15$-$52. We confirm the previously reported curvature in the hard X-ray spectrum, showing that a log parabolic model provides a statistically superior fit to the spectrum compared with the standard power law. The log parabolic model describes the NuSTAR data, as well as previously published gamma-ray data obtained with COMPTEL and AGILE, all together spanning 3 keV through 500 MeV. Our spectral modelling allows us to constrain the peak of the broadband high energy spectrum to be at 2.6$\pm$0.8 MeV, an improvement of nearly an order of magnitude in precision over previous measurements. In addition, we calculate NuSTAR spectra in 26 pulse phase bins and confirm previously reported variations of photon indices with phase. Finally, we measure the pulsed fraction of PSR B1509$-$58 in the hard X-ray energy band for the first time. Using the energy resolved pulsed fraction results, we estimate that the pulsar's off-pulse emission has a photon index value between 1.26 and 1.96. Our results support a model in which the pulsar's lack of GeV emission is due to viewing geometry, with the X-rays originating from synchrotron emission from secondary pairs in the magnetosphere.