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Sgr A*, the compact radio source, believed to be the counterpart of themassive black hole at the galactic nucleus, was observed to undergo rapid andintense flaring activity in X-rays with Chandra in October 2000. We report herethe detection with XMM-Newton EPIC cameras of the early phase of a similarX-ray flare from this source, which occurred on September 4, 2001. The source2-10 keV luminosity increased by a factor about 20 to reach a level of 410^{34} erg s^{-1} in a time interval of about 900 s, just before the end ofthe observation. The data indicate that the source spectrum was hard during theflare. This XMM-Newton observation confirms the results obtained by Chandra andsuggests that, in Sgr A*, rapid and intense X-ray flaring is not a rare event.This can constrain the emission mechanism models proposed for this source, andalso implies that the crucial multiwavelength observation programs planned toexplore the behaviour of the radio/sub-mm and hard X-ray/gamma-ray emissionsduring the X-ray flares, have a good chance of success.Comment: 18 pages, 6 color figures, final version, accepted on October 24,  2002, to appear in ApJ, v584 n2 ApJ February 20, 2003 issu

A New X‐Ray Flare from the Galactic Nucleus Detected with theXMM‐NewtonPhoton Imaging Cameras