The Magnificent Seven X-ray Isolated Neutron Stars Revisited. I. Improved Timing Solutions and Pulse Profile Analysis

Abstract

We present the first systematic X-ray pulse timing analysis of the six members of the so-called "Magnificent Seven" nearby thermally-emitting isolated neutron stars (XINS) with detected pulsations. Using the extensive collection of archival XMM-Newton, Chandra, and NICER observations spanning over two decades, we obtain the first firm measurement of the spin-down rate for RX J2143.0+0654, while for the rest we improve upon previously published spin ephemerides and extend them by up to an additional decade. Five of the XINS follow steady spin-down with no indication of major anomalies in their long-term timing behavior; the notable exception is RX J0720.4-3125, for which, in addition to confirming the previously identified glitch, we detect a second spin derivative. The high quality folded X-ray pulse profiles produced with the updated timing solutions exhibit diverse and complex morphologies, as well as striking energy dependence. These peculiarities cannot be readily explained by blackbody-like isotropic emission and simple hot spot configurations, hinting at the presence of complex multi-temperature surface heat distributions and highly anisotropic radiation patterns, such as may arise from a strongly magnetized atmospheric layer.