A new X-ray outburst in the globular cluster NGC 6440: SAX J1748.9-2021

For the second time in 27 years a bright transient X-ray source has been de tected coincident with the globular cluster NGC 6440. It was found to be ac tive in August, 1998, with the Wide Field Camera and the narrow field instr uments on the BeppoSAX spacecraft, and with the All-Sky Monitor and the Pro portional Counter Array on the RossiXTE spacecraft. Four X-ray bursts were detected, at least one of which shows the characteristics of a thermonuclea r flash on a neutron star, in analogy with some similar to 20 optically ide ntified low-mass X-ray binaries. The broad-band spectrum is hard as is comm on among low-mass X-ray binaries of lower luminosity (less than or similar to 10(37) erg s(-1)) and can be explained by a Comptonized model. During th e burst the > 30 keV emission brightened, consistent with part of the burst emission being Compton up scattered within similar to 10(11) cm.