DEEP OPTICAL OBSERVATIONS OF UNUSUAL NEUTRON STAR CALVERA WITH THE GTC ∗

Abstract

Calvera is an unusual, isolated neutron star with a pure thermal X-ray spectrum typical of central compact objects in supernova remnants. On the other hand, its rotation period and spin-down rate are typical of ordinary rotation-powered pulsars. It was discovered and studied through X-rays, and has not yet been detected in other spectral domains. We present deep optical imaging of the Calvera field, obtained with the Gran Telescopio Canarias, in the and bands. Within the vicinity of of Calvera, we detected two point-like objects that were invisible at previous shallow observations. However, accurate astrometry showed that neither of them can be identified with the pulsar. We put new upper limits of and on its optical brightness. We also reanalyzed all available archival X-ray data on Calvera. Comparison of the Calvera thermal emission parameters and upper limits on optical and non-thermal X-ray emission with respective data on rotation-powered pulsars shows that Calvera might belong to the class of ordinary middle-aged pulsars, if we assume that its distance is in the range of 1.5–5 kpc.