Is Calvera a Gamma-ray Pulsar?

TL;DR

This paper critically examines the gamma-ray pulsar candidate Calvera, demonstrating that previous Fermi detections are likely spurious, leaving its true nature and properties uncertain and open to further investigation.

Contribution

The study refutes the claimed gamma-ray detection of Calvera, clarifying that the Fermi data do not support its identification as a gamma-ray pulsar.

Findings

Fermi ephemeris does not align X-ray pulse phases

Additional Fermi data do not confirm gamma-ray detection

Calvera's spin-down rate remains poorly constrained

Abstract

Originally selected as a neutron star (NS) candidate in the ROSAT All-Sky Survey, 1RXS J141256.0+792204 ("Calvera") was discovered to be a 59 ms X-ray pulsar in a pair of XMM-Newton observations (Zane et al. 2011). Surprisingly, their claimed detection of this pulsar in Fermi gamma-ray data requires no period derivative, severely restricting its dipole magnetic field strength, spin-down luminosity, and distance to small values. This implies that the cooling age of Calvera is much younger than its characteristic spin-down age. If so, it could be a mildly recycled pulsar, or the first "orphaned" central compact object (CCO). Here we show that the published Fermi ephemeris fails to align the pulse phases of the two X-ray observations with each other, which indicates that the Fermi detection is almost certainly spurious. Analysis of additional Fermi data also does not confirm the gamma-ray detection. This leaves the spin-down rate of Calvera less constrained, and its place among the families of NSs uncertain. It could still be either a normal pulsar, a mildly recycled pulsar, or an orphaned CCO.