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 greater than or similar to 0.5 s) and a weak field (B-i less than or similar to 10(10) G), the magnetic field on this star must have decayed. With B-i similar to 10(12) G, we find decay timescales of greater than or similar to 10(7) yr for power-law decay and greater than or similar to 10(8) yr for exponential decay. A measured period derivative (P) over dot less than or similar to 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 (P) over dot similar to 10(-16) s s(-1), though a (P) over dot substantially less than this would be indicative of exponential field decay.