THE ENIGMATIC SOURCE RX J1856.5-3754: IS IT A MILLISECOND PULSARquestionMark

The nature of RX J1856.53754, the famous nearby radiosilent compact object,remains unknown because its rotational period has not been detected. However,the numerous Xray observations of this puzzling source have been sensitive toperiods only above 20 ms. We argue that the observed Xray and optical radiationof this source can be interpreted as thermal emission from peripheries of hotpolar caps of a millisecond pulsar, whose nonthermal emission in radio andXrays is not observable because of an unfavorable orientation of the radiationbeams. To verify this hypothesis, we propose to observe this target with theEPICpn instrument operating in timing mode. The proposed observation will besensitive to millisecond pulsations with a pulsed fraction as low as 3%. truncated!, Please see actual data for full text [truncated!, Please see actual data for full text]