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Simultaneous observations of PSR B0823+26 with ESA's XMM-Newton, the GiantMetrewave Radio Telescope and international stations of the Low Frequency Arrayrevealed synchronous X-ray/radio switching between a radio-bright (B) mode anda radio-quiet (Q) mode. During the B mode we detected PSR B0823+26 in 0.2$-$2keV X-rays and discovered pulsed emission with a broad sinusoidal pulse,lagging the radio main pulse by 0.208 $\pm$ 0.012 in phase, with high pulsedfraction of 70$-$80%. During the Q mode PSR B0823+26 was not detected in X-rays(2 $\sigma$ upper limit a factor ~9 below the B-mode flux). The total X-rayspectrum, pulse profile and pulsed fraction can globally be reproduced with amagnetized partially ionized hydrogen atmosphere model with three emissioncomponents: a primary small hot spot ($T$$\sim$3.6$\times10^6$ K, $R$$\sim$17m), a larger cooler concentric ring ($T$$\sim$1.1$\times10^6$ K, $R$$\sim$280m) and an antipodal hot spot ($T$$\sim$1.1$\times10^6 $ K, $R$$\sim$100 m), forthe angle between the rotation axis and line of sight direction $\sim66^\circ$.The latter is in conflict with the radio derived value of $(84\pm0.7)^\circ$.The average X-ray flux within hours-long B-mode intervals varied by a factor$\pm$20%, possibly correlated with variations in the frequency and lengths ofshort radio nulls or short durations of weak emission. The correlatedX-ray/radio moding of PSR B0823+26 is compared with the anti-correlated modingof PSR B0943+10, and the lack of X-ray moding of PSR B1822-09. We speculatethat the X-ray/radio switches of PSR B0823+26 are due to variations in the rateof accretion of material from the interstellar medium through which it ispassing.

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