X-ray observations of the isolated black hole candidate OGLE-2011-BLG-0462 , and other collapsed objects discovered through gravitational microlensing

TL;DR

This study searches for X-ray emissions from isolated black hole candidates identified via gravitational microlensing, providing upper limits that support their black hole nature and refine understanding of their properties.

Contribution

The paper presents the first X-ray flux upper limits for OGLE-2011-BLG-0462 and other candidates, aiding in distinguishing black holes from neutron stars.

Findings

Upper limits on X-ray flux for OGLE-2011-BLG-0462 support black hole interpretation.

Constraints on X-ray luminosity are consistent with low radiative efficiency expected for black holes.

Limits on other candidates are less restrictive due to larger uncertainties.

Abstract

Isolated black holes and neutron stars can be revealed through the observation of long duration gravitational microlensing events. A few candidates have been found in surveys of stars in the direction of the Galactic bulge. Recently, thanks to the addition of astrometric information at milliarcsecond level, it has been possible to reduce the uncertainties in the masses and distances for some of these "dark" gravitational lenses and select the most promising candidates. These isolated compact objects might emit X-rays powered by accretion from the interstellar medium. Using data of the Chandra, XMM-Newton, and INTEGRAL satellites, we searched for X-ray emission in the isolated black hole candidate OGLE-2011-BLG-0462, and in several other putative collapsed objects found with gravitational microlensing. OGLE-2011-BLG-0462 has been recently interpreted as a 7.1 M_sun black hole at a distance of 1.6 kpc, although a different group obtained a mass range (1.6-4.4 M_sun) that cannot exclude a massive neutron star. We have derived upper limits on the flux from OGLE-2011-BLG-0462 of 9$\times10^{-15}$ erg cm$^{-2}$ s$^{-1}$ in the 0.5-7 keV range and $\sim2\times10^{-12}$ erg cm$^{-2}$ s$^{-1}$ in the 17-60 keV range. The implied X-ray luminosity is consistent with the small radiative efficiency expected for a black hole and disfavours a neutron star interpretation. Limits down to a factor about five lower are obtained for the soft X-ray flux of other candidates, but their interpretation is affected by larger uncertainties in the masses, distances and spatial velocities.