BeppoSAX monitoring of the ``anomalous'' X-ray pulsar 4U 0142+61

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The 8.7s X-ray pulsar 4U 0142+61 was monitored by BeppoSAX between January 1997 and February 1998. This source belongs to the rapidly growing class of "anomalous" X-ray pulsars (AXPs) which have pulse periods in the 6-12s range and no plausible optical, infrared or radio counterparts. The BeppoSAX periods measurements show that 4U 0142+61 continues its spin-down at a nearly constant rate of about 2 times 10⁻¹²s s⁻¹. The 0.5-10 keV pulse shape is double peaked. The phase-averaged spectrum can be described by a steep absorbed power-law (Gamma about 4) plus a blackbody with kT = 0.4 keV. 4U 0142+61 was also detected serendipitously in a March 1996 RossiXTE observation pointed towards the nearby 1455s X-ray pulsar RX J0146.9+6121. The timing analysis results are reported for this observation and the spin history of 4U 0142+61 since 1979 is discussed.