An XMM-Newton Search for X-Ray Emission from the Microlensing Event MACHO-96-BLG-5

MACHO-96-BLG-5 was a microlensing event observed toward the bulge of the Galaxy with an exceptionally long duration of ~970 days. The microlensing parallax fit parameters were used to estimate a lens mass M=6⁺¹⁰_-3 M_solar, corresponding to a distance d in the range 0.5-2 kpc. The upper limit on the absolute brightness for main-sequence stars of the same mass is less than 1 L_solar, so the lens is a good black hole candidate. Such a black hole would accrete from the interstellar medium, thereby emitting in the X-ray band. Here we report the analysis of a deep XMM-Newton observation toward the MACHO-96-BLG-5 lens position. Only an upper limit (99.8% confidence level) to the X-ray flux from the lens position, 9.10×10^-15 to 1.45×10^-14 ergs cm^-2 s^-1 in the 0.2-10 keV energy band, is obtained, allowing us to constrain the putative black hole's accretion parameters.