Deep optical observations of unusual neutron star Calvera with the GTC

TL;DR

This study presents deep optical imaging and reanalysis of X-ray data for neutron star Calvera, suggesting it may be an ordinary middle-aged pulsar if located within 1.5-5 kpc, despite its unusual properties.

Contribution

It provides the first deep optical limits for Calvera and compares its emission properties with typical pulsars, proposing a possible classification as a standard middle-aged pulsar.

Findings

No optical counterpart detected within the limits.

Calvera's emission parameters are consistent with middle-aged pulsars.

Optical brightness upper limits constrain its distance and nature.

Abstract

Calvera is an unusual isolated neutron star with pure thermal X-ray spectrum typical for central compact objects in supernova remnants. On the other hand, its rotation period and spin-down rate are typical for ordinary rotation-powered pulsars. It was discovered and studied in X-rays and not yet detected in other spectral domains. We present deep optical imaging of the Calvera field obtained with the Gran Telescopio Canarias in $g'$ and $i'$ bands. Within $\approx 1^{\prime\prime}$ vicinity of Calvera, we detected two point-like objects invisible at previous shallow observations. However, accurate astrometry showed that none of them can be identified with the pulsar. We put new upper limits on its optical brightness of $g' > 27.87$ and $i' > 26.84$. 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.