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

We report the discovery of a new isolated neutron star candidate, RX J0420.0–5022, showing evidence (4σ) for 22.7 s X-ray pulsations in ROSAT data. NTT observations of the field around the soft X-ray source do not reveal any likely optical counterpart brighter than B = 25.25 implying an X-ray to optical flux ratio of > 103.3 and ruling out other possible kinds of X-ray emitters. The X-ray spectrum of RX J0420.0–5022 can be described with blackbody emission with temperature kT of ∼57 eV and four ROSAT detections 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-period isolated neutron star candidate after RX J0720.4–3125. As for this latter source similar conclusions about the magnetic field strength of the neutron star can be drawn depending on evolutionary scenarios.