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

Abstract

Calvera is a 59 ms isolated pulsar, being unique due to its non-detection in radio, optical and gammarays 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 (Shibanov et al. 2016). 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 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 showing similar observation features. Within the dead pulsar picture, Calvera might be of high temperature at 0.67 keV, possessing a small stellar radius R 4 km and a presumably small magnetic field B 10 11 G and and is probably braked by the fall-back disk accretion. Future advanced facilities may provide unique opportunities to know the real nature of Calvera.