XMM-Newton Observations of Radio Pulsars B0834+06 and B0826-34 and Implications for the Pulsar Inner Accelerator

We report the X-ray observations of two radio pulsars with drifting subpulses, B0834+06 and B0826-34, using XMM-Newton. PSR B0834+06 was detected with a total of 70 counts from the three EPIC instruments over 50 ks exposure time. Its spectrum is best described as that of a blackbody (BB), with temperature T_s = (2.0^{+ 2.0}_-0.9) × 10⁶ K and bolometric luminosity L_b = (8.6^{+ 14.2}_-4.4) × 10²⁸ erg s^-1. As is typical in pulsars with BB thermal components in their X-ray spectra, the hot-spot surface area is much smaller than that of the canonical polar cap, implying a non-dipolar surface magnetic field much stronger than the dipolar component derived from the pulsar spin-down (in this case about 50 times smaller and stronger, respectively). The second pulsar, PSR B0826-34, was not detected over the 50 ks exposure time, giving an upper limit for the bolometric luminosity L_b <= 1.4 × 10²⁹ erg s^-1. We use these data, as well as the radio emission data concerned with drifting subpulses, to test the partially screened gap (PSG) model of the inner accelerator in pulsars. This model predicts a simple and very intuitive relationship between the polar cap thermal X-ray luminosity (L_b) and the "carousel" period (P₄) for drifting subpulses detected in the radio band. The PSG model has been previously successfully tested with four radio pulsars whose L_b and P₄ were both measured: PSR B0943+10, PSR B1133+16, PSR B0656+14, and PSR B0628-28. The XMM-Newton X-ray data of PSR B0834+16 reported here are also in agreement with the model prediction, and the upper limit derived from the PSR B0826-34 observation does not contradict it. We also include two other pulsars, PSR B1929+10 and B1055-52, whose L_b and/or P₄ data became available just recently. These pulsars also follow the prediction of the PSG model. The clear prediction of the PSG model is now supported by all pulsars whose L_b and P₄ are measured and/or estimated.