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 μJy 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 σ upper limit on the period derivative leads, in the dipole spin-down formalism, to an energy loss rate Ė < 7 × 10 33 ergs s -1 , surface magnetic field strength B p < 1.5 × 10 11 G, and characteristic age τ c ≡ P /2 > 8 Myr. This value of τ 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 ≈ 3 × 10 33 ( d /7.1 kpc) 2 ergs s -1 is a large fraction of Ė, which challenges the rotation-powered assumption. Instead, its high blackbody temperature kT BB = 0.46 ± 0.04 keV, small blackbody radius R BB ≈ 0.8 km, and large pulsed fraction f p ≈ 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 (τ c > 40 Myr) or recycled pulsars. Finally, such a CCO, if born in SN 1987A, could explain the nondetection of a pulsar there.