[Paper Review] A different nature between the radio AXPs in comparison to the others SGRs/AXPs

Jaziel G. Coelho, M. Malheiro|arXiv (Cornell University)|Jul 30, 2013

Pulsars and Gravitational Waves Research4 references3 citations

TL;DR

This paper proposes that radio-loud AXPs (anomalous X-ray pulsars) are rotation-powered neutron stars with magnetic fields similar to high-B pulsars, differing fundamentally from other SGRs/AXPs that are likely magnetized white dwarfs. It shows a linear log-log relation between X-ray luminosity and rotational energy loss in radio AXPs, supporting their neutron star nature, while non-radio SGRs/AXPs show constant X-ray luminosity independent of spin-down power, indicating a different origin.

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 $\dot{P}\sim10^{-15}$ 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 ($L_X>\dot{E}_{ m rot}$), 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 $\dot{E}_{ m rot}$ 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 $\dot{E}_{ m rot}$, 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 $\log \dot{E}_{ m rot}$, a phenomenology not shared by X-ray neutron star pulsars, suggesting a different nature for these sources.

Motivation & Objective

• To investigate the fundamental difference in nature between radio-loud AXPs and other SGRs/AXPs.
• To determine whether the X-ray luminosity in radio AXPs is powered by rotational energy loss, as in normal pulsars.
• To test the hypothesis that non-radio SGRs/AXPs are not magnetars but magnetized white dwarfs with persistent X-ray emission independent of spin-down.
• To clarify the physical origin of X-ray luminosity in SGRs/AXPs based on their radio activity and spin-down properties.

Proposed method

• The authors analyze the X-ray luminosity ($L_X$) and rotational energy loss ($\dot{E}_{\rm rot}$) of SGRs/AXPs from observational data.
• They apply a log-log linear fit to $L_X$ vs. $\dot{E}_{\rm rot}$ for radio-loud AXPs, finding $\log L_X = \log(6.2265 \times 10^7) + 0.7239 \log \dot{E}_{\rm rot}$, consistent with neutron star pulsars.
• They compare this relation to the fundamental plane of magnetars and to known high-B pulsars, showing radio AXPs align with rotation-powered neutron stars.
• They contrast this with the behavior of non-radio SGRs/AXPs, which show nearly constant $L_X$ across varying $\dot{E}_{\rm rot}$, suggesting a different energy source.
• They evaluate the X-ray efficiency $\eta_X = L_X / \dot{E}_{\rm rot}$ for both groups, finding values of $10^{-2} - 10^{-1}$ for radio AXPs, consistent with neutron stars, but too low for white dwarfs.
• They use the magnetar fundamental plane and observational constraints to argue that non-radio SGRs/AXPs are better explained as magnetized white dwarfs with large magnetic dipole moments ($10^{34} - 10^{36}$ emu).

Experimental results

Research questions

• RQ1 Do radio-loud AXPs follow the same X-ray luminosity–spin-down power relation as normal neutron star pulsars?
• RQ2 Why do non-radio SGRs/AXPs exhibit nearly constant X-ray luminosity despite varying rotational energy loss?
• RQ3 Is the X-ray emission in radio AXPs powered by rotational energy, as in normal pulsars, or by magnetic energy decay as in magnetars?
• RQ4 Can the observed X-ray efficiency of radio AXPs be explained by a neutron star model, or does it require a different compact object?
• RQ5 Do non-radio SGRs/AXPs have a different physical origin, such as being magnetized white dwarfs, rather than neutron stars?

Key findings

• Radio-loud AXPs follow a linear log-log relation between X-ray luminosity and rotational energy loss: $\log L_X = \log(6.2265 \times 10^7) + 0.7239 \log \dot{E}_{\rm rot}$, consistent with rotation-powered neutron stars.
• The X-ray efficiency $\eta_X = L_X / \dot{E}_{\rm rot}$ for radio AXPs is $10^{-2} - 10^{-1}$, which is higher than normal pulsars but consistent with neutron star models.
• Non-radio SGRs/AXPs show nearly constant X-ray luminosity across a wide range of $\dot{E}_{\rm rot}$, indicating a different energy source than rotational power.
• The observed X-ray luminosity of non-radio SGRs/AXPs is better explained by a magnetized white dwarf model with magnetic dipole moments of $10^{34} - 10^{36}$ emu.
• Radio AXPs are not magnetars in the traditional sense, as their X-ray emission is powered by rotational energy, not magnetic energy decay.
• The alignment of radio AXPs and high-B pulsars on the $L_X$–$\dot{E}_{\rm rot}$ plane supports their classification as rotation-powered neutron stars.