Discovery of synchronous X-ray and radio moding of PSR B0823+26

Simultaneous observations of PSR B0823+26 with ESA's XMM-Newton, the Giant Metrewave Radio Telescope and international stations of the Low Frequency Array revealed synchronous X-ray/radio switching between a radio-bright (B) mode and a radio-quiet (Q) mode. During the B mode we detected PSR B0823+26 in 0.2 - 2 keV X-rays and discovered pulsed emission with a broad sinusoidal pulse, lagging the radio main pulse by 0.208 +/- 0.012 in phase, with high pulsed fraction of 70 - 80 per cent. During the Q mode PSR B0823+26 was not detected in X-rays (2 sigma upper limit a factor similar to 9 below the B-mode flux). The total X-ray spectrum, pulse profile and pulsed fraction can globally be reproduced with a magnetized partially ionized hydrogen atmosphere model with three emission components: a primary small hotspot (T similar to 3.6 x 10(6) K, R(-1)7 m), a larger cooler concentric ring (T similar to 1.1 x 10(6) K, R similar to 280 m) and an antipodal hotspot (T similar to 1.1 x 10(6) K, R similar to 100 m), for the angle between the rotation axis and line of sight direction similar to 66 degrees. The latter is in conflict with the radio derived value of (84 +/- 0.7)degrees. The average X-ray flux within hours-long B-mode intervals varied by a factor +/- 20 per cent, possibly correlated with variations in the frequency and lengths of short radio nulls or short durations of weak emission. The correlated X-ray/radio moding of PSR B0823+26 is compared with the anti-correlated moding of PSR B0943+ 10, and the lack of X-ray moding of PSR B1822-09. We speculate that the X-ray/radio switches of PSR B0823+26 are due to variations in the rate of accretion of material from the interstellar medium through which it is passing.