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

Abstract

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