Calvera: A Low-mass Strangeon Star Torqued by Debris Disk?

Calvera is a unique 59 {ms} isolated pulsar, because it cannot be detected by radio, optical, and gamma-rays; however, it is detectable through 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 an inactive magnetosphere (dead). In this scenario, we jointly fit the spectra obtained by the XMM-Newton Observatory and the 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 of which show similar observation features. Within the dead pulsar picture, Calvera might have a high temperature at 0.67 {keV}, possess a small stellar radius, R≲ 4 {km}, presumably have a small magnetic field, B≲ {10}¹¹ {{G}}, and is probably braked by the fallback disk accretion. Future advanced facilities may provide unique opportunities to understand the real nature of Calvera.