Discovery of a 105 ms X-Ray Pulsar in Kesteven 79: On the Nature of Compact Central Objects in Supernova Remnants

Abstract

We report the discovery of 105 ms X-ray pulsations from the compact central object (CCO) in the supernova remnant Kes 79 using data acquired with the Newton X-Ray Multi-Mirror Mission (XMM-Newton). Two observations of the pulsar taken 6 days apart yield an upper limit on its spin-down rate of P˙<7×10^-14 s s^-1 and no evidence for binary orbital motion. The implied energy loss rate is E˙<2×10³⁶ ergs s^-1, the surface magnetic field strength is B_p<3×10¹² G, and the spin-down age is τ>24 kyr. The latter exceeds the remnant's estimated age, suggesting that the pulsar was born spinning near its current period. The X-ray spectrum of PSR J1852+0040 is best characterized by a blackbody model of temperature kT_BB=0.44+/-0.03 keV, radius R_BB~0.9 km, and L_bol=3.7×10³³ ergs s^-1 at d=7.1 kpc. The sinusoidal light curve is modulated with a pulsed fraction of >45%, suggestive of a small hot spot on the surface of the rotating neutron star. The lack of a discernible pulsar wind nebula is consistent with an interpretation of PSR J1852+0040 as a rotation-powered pulsar whose spin-down luminosity falls below the empirical threshold for generating bright wind nebulae, E˙_c~4×10³⁶ ergs s^-1. The age discrepancy implies that its E˙ has always been below E˙_c, perhaps a distinguishing property of the CCOs. Alternatively, the X-ray spectrum of PSR J1852+0040 suggests a low-luminosity anomalous X-ray pulsar (AXP), but the weak inferred B_p field is incompatible with a magnetar theory of its X-ray luminosity. We cannot exclude accretion from a fallback disk. The ordinary spin parameters discovered from PSR J1852+0040 highlight the difficulty that existing theories of isolated neutron stars have in explaining the high luminosities and temperatures of CCO thermal X-ray spectra.