A SPIN-DOWN VARIATION IN THE 6-SECOND X-RAY PULSAR 1E-1048.1-5937

The optically unidentified X-ray source 1E 1048.1-5937 belongs to a sm all group of pulsars characterized by spin periods in the small 5-9 s range and clearly distinct from the class of ''normal'' pulsars in hig h-mass X-ray binaries. Among the different explanations proposed for t hese anomolous pulsars, there are also models involving isolated compa ct objects. 1E 1048.1-5937 has been repeatedly observed with ROSAT in 1992-1993. While all the previous observations, spanning about 10 yr, were consistent with a constant spin-down of similar to 5 x 10(-4) s y r(-1), the new ROSAT data indicate that after 1988 the spin-down rate has almost doubled. This finding excludes some of the single-star mode ls and gives support to the scenarios involving accretion, most likely from a low-mass companion.