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

The unidentified X-ray source RX J0720.4-3125 is a candidate isolated neutronstar showing evidence for pulsed emission with an 8.39 s period and a spectrumconsistent with a blackbody at kT=80 eV (Haberl et al. 1996, 1997). We showthat this source is most likely an old isolated neutron star accreting fromsurrounding media. We then argue that unless it was born with a long spinperiod (P_i >~ 0.5 seconds) and weak field (B_i <~ 10^10 G), the magnetic fieldon this star must have decayed. With B_i ~ 10^12 G, we find decay timescales >~10^7 yrs for power law decay or >~ 10^8 yrs for exponential decay. A measuredperiod derivative Pdot <~ 10^{-16} s s^{-1} would be consistent with an oldaccreting isolated neutron star. Both power law and exponential decay modelscan give a Pdot ~ 10^{-16} s s^{-1}, though a Pdot substantially less than thiswould be indicative of exponential field decay.Comment: 19 pages, 1 figure, Latex; accepted for publication in The Astrophysical Journal Letter