Most galactic nuclei are now believed to harbour supermassive black holes 1 . Studies of stellar motions in the central few light-years of our Milky Way Galaxy indicate the presence of a dark object with a mass of ≈ 2.6 × 10 6 solar masses (refs 2, 3). This object is spatially coincident with Sagittarius A * (Sgr A * ), the unique compact radio source located at the dynamical centre of our Galaxy. By analogy with distant quasars and nearby active galactic nuclei (AGN), Sgr A * is thought to be powered by the gravitational potential energy released by matter as it accretes onto a supermassive black hole 4, 5 . However, Sgr A * is much fainter than expected in all wavebands, especially in X-rays, casting some doubt on this model. Recently, we reported the first strong evidence of X-ray emission from Sgr A * (ref. 6). Here we report the discovery of rapid X-ray flaring from the direction of Sgr A * . These data provide compelling evidence that the X-ray emission is coming from accretion onto a supermassive black hole at the Galactic Centre, and the nature of the variations provides strong constraints on the astrophysical processes near the event horizon of the black hole.Our view of Sgr A * in the optical and ultraviolet wavebands is blocked by the large visual extinction, AV ≈ 30 magnitudes 7 , caused by dust and gas along the line of sight. Sgr A * has not been detected in the infrared due to its faintness and to the bright infrared background from stars and clouds of dust 8 . Detection of X-rays from Sgr A * is therefore essential to constrain the spectrum at energies above the radio-tosubmillimetre band and to test the supermassive-black-hole accretion-flow paradigm 5 .We first observed the Galactic Centre on 21 September 1999 with the imaging array of the Advanced CCD Imaging Spectrometer (ACIS-I) aboard the Chandra X-ray Observatory 9 and discovered an X-ray source coincident within 0. 35 ± 0. 26 (1σ) of the radio source 6 . The luminosity in 1999 was very weak, LX ≈ 2 × 10 33 erg s −1 in the 2-10 keV band, after correction for the inferred neutral hydrogen absorption column NH ≈ 1 × 10 23 cm −2 . This is far fainter than previous X-ray observatories could detect 6 .We observed the Galactic Centre a second time with Chandra/ACIS-I from

Abstract. We present the results of deep, high-resolution, multi-band optical observations of the field of the young (∼5000 yr) 16 ms X-ray pulsar PSR J0537−6910 performed with the Advanced Camera for Surveys (ACS) aboard the Hubble Space Telescope (HST). Although a few new potential counterparts have been detected within or close to the revised Chandra X-ray error circle ( 1 ) of the pulsar, only two of them (with magnitudes m 814W ≈ 23.9 and m 814W ≈ 24.2) show indications of a peculiar spectrum which could be related to optical emission from the pulsar. This might be true also for a third, fainter, candidate detected only in one filter (with magnitude m 814W ≈ 26.7). If either of the two brighter candidates is indeed the actual counterpart, the optical output of PSR J0537−6910 would make it similar to young Crab-like pulsars. If not, it would mean that PSR J0537−6910 is significantly underluminous with respect to all pulsars detected in the optical.

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