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 12μJy 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 2σ upper limit on the period derivative leads, in the dipole spin-down formalism, to an energy loss rate Ė < 7× 10 ergs s, surface magnetic field strength Bp < 1.5× 10 11 G, and characteristic age τc ≡ P/2Ṗ > 8 Myr. This value of τc 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 PSR J1852+0040, Lbol ≈ 3× 10 33 (d/7.1 kpc) ergs s, is a large fraction of Ė, which challenges the rotation-powered assumption. Instead, its high blackbody temperature kTBB = 0.46±0.04 keV, small blackbody radius RBB ≈ 0.8 km, and large pulsed fraction fp ≈ 80%, may be evidence of accretion onto a polar cap, possibly from a fallback disk made of supernova debris. If Bp < 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 1E 1207.4−5209, 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 NS is formed spinning slowly, which enables it to accrete SN debris. Accretion excludes neutron stars born with both Bp < 10 11 G and P > 0.1 s from radio pulsar surveys, where Bp < 10 11 G is not encountered except among very old (τc > 40 Myr) or recycled pulsars. Finally, such a CCO, if born in SN 1987A, could explain the non-detection of a pulsar there. Subject headings: ISM: individual (Kesteven 79, SN 1987A) — pulsars: individual (1E 1207.4−5209, CXOU J185238.6+004020, PSR J1852+0040) — stars: neutron