094348

We discovered in archival XMM-Newton data a puzzling phenomenon in PSRJ1311-3430 a black widow millisecond pulsar in a tight binary (P_B=93.8 min)with a very low-mass (M~0.01 Msun) He companion star. A series of six pulseswith a regular recurrence time of ~124 min is apparent in the 0.2-10 keV lightcurve. The X-ray spectrum of the pulses is consistent with a power law withphoton index ~1.5 also describing the quiescent emission. Simultaneousobservations in the U and g band only display the well-known orbital modulationof the system. The phenomenon defies any interpretation. We ask for 4 XMMobservations (30 ks each) to constrain the recurrence time duty cycle andmultiwavelength properties of the pulses with important implications for our overall understanding of millisecond pulsars. [truncated! Please see actual data for full text]