Spin-down of the X-ray pulsar GX1 +4 during an extended low state

X-ray pulsars^1,2 are magnetized, spinning neutron stars accreting matter from their binary companions. Their pulse periods P, ranging over four orders of magnitude, increase and decrease in complex ways^1,3,4. The more luminous ones tend to show faster spin-up^1,5. A puzzle is that the spin-up timescales of many X-ray pulsars are much shorter than their binary-evolution timescales, thus apparently violating the steady-state condition. It has there-fore been suspected⁶ that there exist many 'turned-off X-ray pulsars currently spinning down undetected. An excellent test for this hypothesis became available using the X-ray pulsar GX1 +4, which used to show the fastest spin-up over a decade^1,7-10 and then faded away¹¹. Using the X-ray satellite Ginga¹², we detected GX1+4 at ~1/40 the previous intensity, and found that it now has an average spin-down trend. This discovery apparently supports the above hypothesis.