RX J0420.0-5022: an isolated neutron star candidate with evidence for 22.7s X-ray pulsations

We report the discovery of a new isolated neutron star candidate, RX J0420. 0-5022, showing evidence (4 sigma) for 22.7 s X-ray pulsations in ROSAT dat a. NTT observations of the field around the soft X-ray source do not reveal any likely optical counterpart blighter than B = 25.25 implying an X-ray t o optical flux ratio of >10(3.3) and ruling out other possible kinds of X-r ay emitters. The X-ray spectrum of RX J0420.0-5022 can be described with bl ackbody emission with temperature kT of similar to 57 eV and four ROSAT det ections are consistent with no flux variations on time scales of years. The X-ray pulsations, if confirmed, make RX J0420.0-5022 the second long-perio d isolated neutron star candidate after RX J0720.3-3125. As for this latter source similar conclusions about the magnetic field strength of the neutro n star can be drawn depending on evolutionary scenarios.