# NuSTAR Hard X-ray Observations of the Energetic Millisecond Pulsars PSR B1821-24, PSR B1937+21, and PSR J0218+4232

## TL;DR

This study uses NuSTAR to perform detailed timing and spectral analysis of three energetic millisecond pulsars, detecting pulsed X-ray emission up to ~50 keV and providing insights into their broadband spectral properties.

## Contribution

First-time detection of pulsed X-ray emission up to ~50 keV in these pulsars with improved background reduction and phase-resolved spectroscopy across 0.5-79 keV.

## Key findings

- Detected pulsed emission up to ~50 keV, ~20 keV, and ~25 keV for the three pulsars.
- Found no spectral turnover or break in the 0.5-79 keV range, extending the power-law continuum.
- Extrapolation suggests a spectral turnover between 100 keV and 100 MeV to match gamma-ray observations.

## Abstract

We present Nuclear Spectroscopic Telescope Array (NuSTAR) hard X-ray timing and spectroscopy of the three exceptionally energetic rotation-powered millisecond pulsars PSRs B1821-24, B1937+21, and J0218+4232. By correcting the NuSTAR onboard clock for phase and frequency drifts between observation intervals, we are able to recover the intrinsic hard X-ray pulse profiles of all three pulsars with a resolution down to < 15 microsec. The substantial reduction of background emission relative to previous broadband X-ray observations allows us to detect for the first time pulsed emission up to ~50 keV, ~20 keV, and ~25 keV, for the three pulsars, respectively. We conduct phase-resolved spectroscopy in the 0.5-79 keV range for all three objects, obtaining the best measurements yet of the broad-band spectral shape and high-energy pulsed emission to date. We find extensions of the same power-law continua seen at lower energies, with no evidence for a spectral turnover or break. Extrapolation of the X-ray power-law spectrum to higher energies reveals that a turnover in the 100 keV to 100 MeV range is required to accommodate the high-energy gamma-ray emission observed with Fermi-LAT, similar to the spectral energy distribution observed for the Crab pulsar.