2008ApJ...672.1137R

We report the discovery of a compact object at high Galactic latitude. The object was initially identified as a ROSAT All-Sky Survey Bright Source Catalog X-ray source, 1RXS J141256.0+792204, statistically likely to possess a high X-ray to optical flux ratio. Further observations using Swift, Gemini-North, and the Chandra X-Ray Observatory refined the source position and confirmed the absence of any optical counterpart to an X-ray to optical flux ratio of F_X(0.1-2.4 keV)/F_V>8700 (3 σ). Interpretation of 1RXS J141256.0+792204–which we have dubbed Calvera–as a typical X-ray-dim isolated neutron star would place it at z~5.1 kpc above the Galactic disk–in the Galactic halo–implying that it either has an extreme space velocity (v_z≳5100 km/s) or has failed to cool according to theoretical predictions. Interpretations as a persistent anomalous X-ray pulsar or a ``compact central object'' present conflicts with these classes' typical properties. We conclude that the properties of Calvera are most consistent with those of a nearby (80-260 pc) radio pulsar, similar to the radio millisecond pulsars of 47 Tucanae, with further observations required to confirm this classification. If it is a millisecond pulsar, it is has an X-ray flux equal to the X-ray brightest millisecond pulsar (and so is tied for highest flux); the closest northern hemisphere millisecond pulsar; and potentially the closest known millisecond pulsar in the sky, making it an interesting target for X-ray study, a radio pulsar timing array, and LIGO.