X-ray timing of PSR J1852+0040 in Kesteven 79: Evidence of neutron stars weakly magnetized at birth

The 105 ms X-ray pulsar J1852+0040 is the central compact object (CCO) in supernova remnant Kes 79. We report a sensitive upper limit on its radio flux density of 12 mu Jy at 2 GHz using the NRAO Green Bank Telescope. Timing using the Newton X-Ray Multi-Mirror Mission (XMM-Newton) and the Chandra X-Ray Observatory over a 2.4 yr span reveals no significant change in its spin period. The 2 sigma upper limit on the period derivative leads, in the dipole spin-down formalism, to an energy loss rate. E < 7 x 10(33) ergs s(-1), surface magnetic field strength B-p < 1.5 x 10(11) G, and characteristic age tau(c) P/2P > 8 Myr. This value of tau(c) exceeds the age of the SNR by 3 orders of magnitude, implying that the pulsar was born spinning at its current period. However, the X-ray luminosity of L-bol approximate to 3 x 10(33)( d/7.1 kpc)(2) ergs s(-1) is a large fraction of E, which challenges the rotation-powered assumption. Instead, its high blackbody temperature kT(BB) 0.46 +/- 0.04 keV, small blackbody radius R-BB approximate to 0.8 km, and large pulsed fraction fp approximate to 80% may be evidence of accretion onto a polar cap, possibly from a fallback disk made of supernova debris. If B-p < 10(10) G, an accretion disk can penetrate the light cylinder and interact with the magnetosphere, while resulting torques on the neutron star remain within the observed limits. A weak B field is also inferred in another CCO, the 424 ms pulsar, from its steady spin and soft X-ray absorption lines. We propose this origin of radio-quiet CCOs: the magnetic field, derived from a turbulent dynamo, is weaker if the neutron star is formed spinning slowly, which enables it to accrete supernova debris. Accretion excludes neutron stars born with both B-p < 10(11) G and P > 0.1 s from radio pulsar surveys, where B-p < 10(11) G is not encountered except among very old (tau(c) > 40 Myr) or recycled pulsars. Finally, such a CCO, if born in SN 1987A, could explain the nondetection of a pulsar there.