A millisecond pulsar's optical pulse suddenly changed shape—then switched back

PSR J1023+0038, a transitional millisecond pulsar, normally shows a stable double-peaked optical pulse with 0.3% brightness variation. Using the 6-m BTA telescope, observers caught the system flip to a single sinusoidal peak with 5% pulsed fraction for 220 seconds before returning to baseline. The shift suggests a sudden change in how the pulsar's wind interacts with surrounding material, but the mechanism remains unclear.