Search of nearby resolved neutron stars among optical sources

Abstract

Neutron stars are identified as pulsars, X-ray binary components, central objects of supernovae remnants, or isolated thermally emitting sources, and at distances beyond 120 pc. A population extrapolation suggests 10$^3$ objects within that boundary. Potentially, neutron stars could continuously emit gravitational waves at sensitivity reach of present instrumentation. As part of our Search for the Nearest Neutron Stars ``Five Seasons'' project, we search for nearby resolved neutron stars. Based on expected fluxes and magnitudes of thermally cooling neutron stars and pulsars, we selected sources in Gaia DR3. The sources have $G$-band absolute magnitudes $M_G>16$ mag, parallax signal-to-noise ratios greater than two, and colours $G_{BP}-G<0.78$ and $G-G_{RP}<0.91$ mag for power-law emitters of flux $F_{\nu} \propto \nu^{-\alpha_{\nu}}$ with spectral indices $\alpha_{\nu}<3$. The photometric region overlaps with that of white dwarfs, in confluence with most known pulsars in binaries having white dwarf companions. We looked for counterparts in gamma-ray, X-ray, ultraviolet, radio, optical, and infrared catalogues. We find about two X-ray-, 15 ultraviolet-, one radio probable counterparts, and at least four sources with power-law profiles at the ultraviolet-optical(-infrared). Because the sources have $G\gtrapprox20$ mag, we rely on Gaia DR3 single-source parameters. We identify possible binaries based on photoastrometric parameters, visual companions, and flux excesses. Some emission components suggest small thermal radii. Source types, neutron star content, and properties require further inquiry.