RX J0720.4-3125 - a free precessing X-ray pulsarquestionMark

Within seven radioquiet, thermallyemitting isolated NSs, RXJ 0720 is unique inthat it shows longterm (virgulyrs) variations in its spectral parameters. Applying aphasecoherent timing solution with constant spin down leaves large residualswhich may follow a cyclic pattern. The reasons for this behaviour is a matter ofdebate. Both a glitch and precession have been invoked, but present data do notallow to reach a firm conclusion. Most recent XMM observations show that the residuals probably reached a minimum of the cycle, while the spectral parametersevolution slowed down. The proposed observations will prove if the timing residuals follow a cyclic pattern and weather the spectral and timing variationsare related. The outcome will rule out one of the two proposed scenarios truncated!, Please see actual data for full text [truncated!, Please see actual data for full text]