Calvera: A low-mass strangeon star torqued by debris disk?

TL;DR

This paper proposes that Calvera is a low-mass strangeon star with an inactive magnetosphere, explaining its unique X-ray emission and spectral properties through a joint spectral fit with a strangeon star atmosphere model.

Contribution

It introduces a novel interpretation of Calvera as a low-mass strangeon star with a dead magnetosphere, supported by spectral fitting and comparison with similar neutron star observations.

Findings

Calvera may have a high temperature of 0.67 keV.

It could have a small radius less than 4 km.

It likely has a magnetic field below 10^11 G and is influenced by fallback disk accretion.

Abstract

Calvera is a $59\,\mathrm{ms}$ isolated pulsar, being unique due to its non-detection in radio, optical and gamma-rays but the purely thermal emission in soft X-rays. It is suggested that Calvera could be an ordinary middle-aged pulsar with significant magnetospheric activity at a large distance. Alternatively, it is proposed in this paper that Calvera is a low-mass strangeon star with inactive magnetosphere (dead). In this scenario, we jointly fit the spectra obtained by the {\it XMM-Newton} Observatory and the {\it Chandra} X-ray Observatory with the strangeon star atmosphere model. The spectral model is successful in explaining the radiation properties of Calvera and X-ray Dim Isolated Neutron Stars, both showing similar observation features. Within the dead pulsar picture, Calvera might be of high temperature at $0.67\,\mathrm{keV}$, possessing a small stellar radius $R\la4\,\mathrm{km}$ and a presumably small magnetic field $B\la10^{11}\,\mathrm{G}$ and is probably braked by the fall-back disk accretion. Future advanced facilities may provide unique opportunities to know the real nature of Calvera.