On the nature of the 25-min periodicity from 4U 0142+614 : a nearby, slowly spinning neutron star/Be system ?

Abstract

The unidentified X-ray source 4U 0142+614 has an unusually soft spectrum and has shown a periodic modulation at approximately 25 minutes, which could be interpreted as the orbital period of a Low Mass X-ray Binary. Recently a new X-ray source, probably a Be/neutron star system, has been discovered at an angular distance of only 24 arcmin from 4U 0142+614. We consider the possibility that the newly discovered source, RX J0146.9+6121, is responsible for the periodicity previously attributed to 4U 0142+614. If this interpretation is correct, RX J0146.9+6121 would be the most slowly rotating neutron star known. We present also new spectroscopic data on the optical counterpart of RX J0146.9+6121, that strengthen the similarity with other Be/neutron star binaries. The nature of both sources is discussed in the light of the new interpretation.