Beppo SAX monitoring of the “ anomalous ” X – ray pulsar 4 U 0142 + 61

The 8.7 s 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 6–12 s 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 ˙ P ∼ 2×10 −12 s 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 (Γ ∼ 4) plus a blackbody with kT ∼ 0.4 keV. 4U 0142+61 was also detected serendipitously in a March 1996 RXTE observation pointed towards the nearby 1455 s 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.