[Paper Review] Do SGRs/AXPs and radio AXPs have the same nature?

Jaziel G. Coelho, M. Malheiro|arXiv (Cornell University)|Mar 4, 2013

Pulsars and Gravitational Waves Research1 references3 citations

TL;DR

This paper investigates whether soft gamma repeaters/anomalous X-ray pulsars (SGRs/AXPs) and radio-loud AXPs share the same physical origin. It finds that radio AXPs follow a linear log-log correlation between X-ray luminosity (L_X) and rotational energy loss rate (Ė_rot), similar to normal neutron star pulsars, while non-radio SGRs/AXPs do not, implying distinct physical mechanisms—radio AXPs are likely rotation-powered neutron stars, whereas others may be magnetized white dwarfs.

ABSTRACT

SGRs/AXPs are assumed to be a class of neutron stars (NS) powered by magnetic energy and not by rotation, as normal radio 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 AXPs in comparison to the others of this class. In this work, we show that the radio AXPs obey a linear log-log relation between $L_X$ and $\dot{E}_{ m rot}$, very similar to the one of normal X-ray pulsars, a correlation not seen for the others SGRs/AXPs. This result suggests a different nature between the radio AXPs comparing to the others SGRs/AXPs.

Motivation & Objective

• To determine whether radio-loud AXPs and non-radio SGRs/AXPs share the same physical nature.
• To investigate the origin of X-ray luminosity in SGRs/AXPs, particularly whether it arises from rotational energy loss or magnetic energy dissipation.
• To test whether the observed L_X–Ė_rot correlation supports a neutron star or white dwarf pulsar model for these sources.
• To assess whether the X-ray efficiency η_X = L_X / Ė_rot is consistent with known pulsar behavior or requires alternative models.
• To clarify the role of magnetic field strength and spin-down power in distinguishing between neutron star and white dwarf interpretations.

Proposed method

• Analysis of the log-log correlation between X-ray luminosity (L_X) and rotational energy loss rate (Ė_rot) for radio AXPs and non-radio SGRs/AXPs.
• Comparison of the observed L_X–Ė_rot relation in radio AXPs with that of known X-ray and gamma-ray neutron star pulsars using published data.
• Fitting a power-law relation: log L_X = log(5.9×10⁴) + 0.814 log Ė_rot to the radio AXP data to quantify the correlation.
• Evaluation of X-ray efficiency η_X = L_X / Ė_rot for both radio and non-radio SGRs/AXPs to assess energy source dominance.
• Use of magnetic dipole moment estimates (m ~ 10³² emu for radio AXPs, up to 10³⁶ emu for non-radio sources) to support neutron star vs. white dwarf models.
• Contrast of phenomenological behavior: steady high L_X with weak Ė_rot dependence in non-radio SGRs/AXPs vs. strong L_X–Ė_rot correlation in radio AXPs.

Experimental results

Research questions

• RQ1 Do radio AXPs follow the same L_X–Ė_rot correlation as normal neutron star pulsars?
• RQ2 Is the X-ray efficiency η_X = L_X / Ė_rot consistent with rotational energy powering in radio AXPs?
• RQ3 Why do non-radio SGRs/AXPs show a nearly constant L_X despite varying Ė_rot, unlike radio AXPs?
• RQ4 Can the high X-ray luminosity of non-radio SGRs/AXPs be explained by rotational energy loss, or does it require magnetic energy dissipation?
• RQ5 Do the observed properties of radio AXPs support a neutron star origin over a white dwarf pulsar model?

Key findings

• Radio AXPs exhibit a linear log-log correlation between X-ray luminosity (L_X) and rotational energy loss rate (Ė_rot), with the relation log L_X = log(5.9×10⁴) + 0.814 log Ė_rot.
• This correlation is very similar to that observed in normal X-ray and gamma-ray neutron star pulsars, supporting a rotation-powered neutron star origin for radio AXPs.
• Non-radio SGRs/AXPs do not show a significant correlation between L_X and Ė_rot, indicating a different energy source mechanism.
• The X-ray efficiency η_X ≈ 0.1–0.2 for radio AXPs is consistent with neutron star pulsars, not magnetars powered by magnetic energy.
• The high magnetic dipole moment (m ~ 10³² emu) in radio AXPs supports their identification as neutron stars, not white dwarfs.
• The steady, high X-ray luminosity in non-radio SGRs/AXPs, independent of Ė_rot, is better explained by magnetic energy dissipation in massive white dwarfs with m ~ 10³⁴–10³⁶ emu.