Abstract
We present deep XMM-Newton European Photon Imaging Camera spectroscopic and timing X-ray observations of the nearby solitary radio millisecond pulsar, PSR J0030+0451. Its emission spectrum in the 0.1–10 keV range is found to be remarkably similar to that of the nearest and best-studied millisecond pulsar, PSR J0437–4715, being well described by a predominantly thermal two-temperature model plus a faint hard tail evident above ∼2 keV. The pulsed emission in the 0.3–2 keV band is characterized by two broad pulses with pulsed fraction ∼60%–70%, consistent with a mostly thermal origin of the X-rays only if the surface polar cap radiation is from a light-element atmosphere. Modeling of the thermal pulses permits us to place constraints on the neutron star radius of R > 10.7 (95% confidence) and R > 10.4 km (at 99.9% confidence) for M = 1.4 M☉.
Highlights
PSR J0030+0451 is one of the nearest known rotationpowered “recycled” millisecond pulsars (MSPs) in the field of the Galaxy (D = 300 ± 90 pc; Lommen et al 2006), with a spin period P = 4.87 ms and intrinsic spindown rate P ≡ dP/dt = 1.0 × 10−20 s s−1, implying a surface dipole magnetic field strength B ≈ 2.7 × 108 Gauss, a characteristic age τ ≈ 7.8 Gyr, and spin-down luminosity E ≈ 3 × 1033 ergs s−1
Our H atmosphere polar cap model is in fair agreement with the observed continuum and pulsed emission from PSR J0030+0451
As with PSR J0437–4715 (Bogdanov et al 2007), the relatively large pulsed fractions require the existence of a light-element atmosphere on the stellar surface and cannot be reproduced by a blackbody model for realistic NS radii
Summary
X-ray observations have detected a number of rotationpowered MSPs that exhibit soft, presumably thermal, emission (Zavlin 2006; Bogdanov et al 2006a; Zavlin 2007) This radiation likely originates from the pulsar magnetic polar caps that are heated by energetic particles from the pulsar magnetosphere (see, e.g., Harding & Muslimov 2002). As this heat is confined to a small portion of the NS, study of the X-ray properties of MSPs could offer insight into key NS properties that are inaccessible by other observational means (e.g., radio pulse timing) such as the radiative properties of the NS surface, magnetic field geometry, and NS compactness
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