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 massM = 6þ10 3 M, corresponding to a distance d in the range 0.5Y2 kpc. The upper limit on the absolute brightness formain-sequence stars of the same mass is less than 1 L, 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 fromthe lens position, 9.10 ; 1015 to 1.45 ; 1014 ergs cm2 s1 in the 0.2Y10 keVenergy band, is obtained, allowing us to constrain the putative black hole’s accretion parameters.
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