A different nature between the radio AXPs in comparison to the others SGRs/AXPs

Abstract

SGRs/AXPs are considered a subclass of pulsars powered by magnetic energy and not by rotation, as normal radio pulsars. They are understood as strongly magnetized neutron star, with large periods of rotation $P\sim (2-12)$ s, and large spin-down, with typical $\dot{P}\sim (10^{-13}-10^{-10})$ s/s in contrast to $\dot{P}\sim 10^{-15}$ s for ordinary pulsars. Their persistent X-ray luminosity, as well as the bursts and flares typical of these sources, are instead believed to be powered by the decay of their ultrastrong magnetic field. SGRs/AXPs typically have a larger X-ray luminosity that can not be explained by their spin-down luminosity ($L_X>{\dot{E}}_{\mathrm{rot}}$), unlike rotation-powered pulsars. However, the recent discovery of radio-pulsed emission in four of this class of sources, where the spin-down rotational energy lost ${\dot{E}}_{\mathrm{rot}}$ is larger than the X-ray luminosity $L_X$ during the quiescent state - as in normal pulsars - opens the question of the nature of these radio sources in comparison to the others of this class. In this contribution, we show that the radio SGRs/AXPs obey a linear log-log relation between $L_X$ and ${\dot{E}}_{\mathrm{rot}}$, very similar to the one satisfied by X-ray and gamma-ray neutron star pulsars, suggesting their neutron star nature. Furthermore, we show that almost all the high-B pulsars are also near the line found for the radio AXPs. In contrast, for almost all the others SGRs/AXPs, $\log L_X$ does not vary too much as function of $\log {\dot{E}}_{\mathrm{rot}}$, a phenomenology not shared by X-ray neutron star pulsars, suggesting a different nature for these sources.

Comments: Contribution for the Conference Proceedings of the Compact Stars in the QCD Phase Diagram III (CSQCD III), December 12-15, 2012, Guarujá, Brazil. http://www.astro.iag.usp.br/~foton/CSQCD3