Evidence for Magnetic Field Decay in RX J0720.4−3125

The unidentified X-ray source RX J0720.4-3125 is a candidate isolated neutron star showing evidence for pulsed emission with an 8.39 s period and a spectrum consistent with a blackbody at kT = 80 eV. We show that this source is most likely an old isolated neutron star accreting from surrounding media. We then argue that unless it was born with a long spin period ( P i ≳ 0.5 s) and a weak field ( B i ≲ 10 10 G), the magnetic field on this star must have decayed. With B i ~ 10 12 G, we find decay timescales of ≳10 7 yr for power-law decay and ≳10 8 yr for exponential decay. A measured period derivative ≲ 10 -16 s s -1 would be consistent with an old accreting isolated neutron star. Both power-law and exponential decay models can give a ~ 10 -16 s s -1 , though a substantially less than this would be indicative of exponential field decay.