XMM-Newton observations of PSR J0726-2612, a radio-loud XDINS

Abstract

We present the results of an XMM-Newton observation of the slowly rotating (P = 3.4 s), highly magnetized (B ≈ 3 × 10¹³ G) radio pulsar PSR J0726-2612. A previous X-ray observation with the Chandra satellite showed that some of the properties of PSR J0726-2612 are similar to those of the X-ray-dim isolated neutron stars (XDINSs), a small class of nearby slow pulsars characterized by purely thermal X-ray spectra and undetected in the radio band. We confirm the thermal nature of the X-ray emission of PSR J0726-2612, which can be fitted by the sum of two blackbodies with temperatures kT₁=0.074^+0.006_-0.011 keV and kT₂=0.14^+0.004_-0.002 keV and emitting radii R₁=10.4^+10.8_-2.8 km and R₂=0.5^+0.9_-0.3 km, respectively (assuming a distance of 1 kpc). A broad absorption line modeled with a Gaussian profile centered at 0.39^+0.02_-0.03 keV is required in the fit. The pulse profile of PSR J0726-2612 is characterized by two peaks with similar intensity separated by two unequal minima, a shape and pulsed fraction that cannot be reproduced without invoking magnetic beaming of the X-ray emission. The presence of a single radio pulse suggests that in PSR J0726-2612 the angles that the dipole axis and the line of sight make with the rotation axis, ξ and χ, respectively, are similar. This geometry differs from that of the two radio-silent XDINSs with double-peaked pulse profiles similar to that of PSR J0726-2612, for which ξ ∼ 90° and χ ∼ 45° have recently been estimated. These results strengthen the similarity between PSR J0726-2612 and the XDINSs and support the possibility that the lack of radio emission from the latter might simply be due to an unfavorable viewing geometry.