Spin-Down Measurement of PSR J1852+0040 in Kesteven 79: Central Compact Objects as Anti-Magnetars

Using XMM-Newton and Chandra, we achieved phase-connected timing of the 105 ms X-ray pulsar PSR J1852+0040 that provides the first measurement of the spin-down rate of a member of the class of central compact objects (CCOs) in supernova remnants. We measure \dot{P} = (8.68 ± 0.09) × 10^{-18}, and find no evidence for timing noise or variations in X-ray flux over 4.8 year. In the dipole spin-down formalism, this implies a surface magnetic field strength B_s = 3.1 × 10¹⁰ G, the smallest ever measured for a young neutron star, and consistent with being a fossil field. In combination with upper limits on B_s from other CCO pulsars, this is strong evidence in favor of the "anti-magnetar" explanation for their low luminosity and lack of magnetospheric activity or synchrotron nebulae. While this dipole field is small, it can prevent accretion of sufficient fall-back material so that the observed X-ray luminosity of L_x = 5.3 × 10³³(d/7.1 kpc)² erg s^-1 must instead be residual cooling. The spin-down luminosity of PSR J1852+0040, \dot{E} = 3.0 × 10^{32} erg s^-1, is an order of magnitude smaller than L_x . Fitting of the X-ray spectrum to two blackbodies finds small emitting radii, R ₁ = 1.9 km and R ₂ = 0.45 km, for components of kT ₁ = 0.30 keV and kT ₂ = 0.52 keV, respectively. Such small, hot regions are ubiquitous among CCOs, and are not yet understood in the context of the anti-magnetar picture because anisotropic surface temperature is usually attributed to the effects of strong magnetic fields.