Can SGRs/AXPs originate from neutron star binaries?

TL;DR

This paper proposes that SGRs and AXPs originate from normal neutron stars in binary systems, with different accretion processes explaining their observed properties, challenging the magnetar model.

Contribution

It introduces a novel binary system origin model for SGRs/AXPs, emphasizing accretion onto dense crusts and thermal energy storage as key mechanisms.

Findings

SGRs may result from re-exploded neutron stars in binaries.

AXPs could be neutron stars with massive accretion disks.

The model explains spin-period clustering through binary interactions.

Abstract

Soft gamma repeaters (SGRs) and anomalous X-ray pulsars (AXPs) are two groups of enigmatic objects, which have been extensively investigated in past few decades. Based on the ample information about their timing behaviors, spectra, and variability properties, it was proposed that SGRs/AXPs are isolated neutron stars (NSs) with extremely strong magnetic fields, the so-called magnetars. Nonetheless, some alternative models are probably equally convincing such as those proposing that they are accreting NSs with a fall-back disk or rotation-powered magnetized and massive white dwarfs. The nature and nurture of SGRs/AXPs remain controversial. In this paper, we propose that SGRs/AXPs can originate from normal NSs in binary systems. SGRs are a class of objects containing a neutron-drip core and denser crust with a stiffer equation of state, which is formed from re-explosion of normal NSs in binary systems. It is the accretion onto the denser crust that contributes to the observed hard emissions. AXPs are a group of NSs which reserve huge thermal energy in an insulating layer above the surface. This structure accretes material from a massive disk, manifesting themselves via release of deposited energy. The spin-period clustering is due to either the brake of a slowly rotating envelope or the frictional drag during the common-envelope phase.