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Astron. Astrophys. 345, 100-108 (1999)

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

J.J.M. in 't Zand 1, F. Verbunt 2, T.E. Strohmayer 3, A. Bazzano 4, M. Cocchi 4, J. Heise 1, M.H. van Kerkwijk 2, J.M. Muller 1,5, L. Natalucci 4, M.J.S. Smith 1,6 and P. Ubertini 4

1 Space Research Organization Netherlands, Sorbonnelaan 2, 3584 TA Utrecht, The Netherlands
2 Astronomical Institute, P.O. Box 80000, 3508 TA Utrecht, The Netherlands
3 NASA Goddard Space Flight Center, Code 666, Greenbelt, MD 20771, USA
4 Istituto di Astrofisica Spaziale (CNR), Area Ricerca Roma Tor Vergata, Via del Fosso del Cavaliere, I-00133 Roma, Italy
5 BeppoSAX Science Data Center, Nuova Telespazio, Via Corcolle 19, I-00131 Roma, Italy
6 BeppoSAX Science Operation Center, Nuova Telespazio, Via Corcolle 19, I-00131 Roma, Italy

Received 11 December 1998 / Accepted 10 February 1999


For the second time in 27 years a bright transient X-ray source has been detected coincident with the globular cluster NGC 6440. It was found to be active in August, 1998, with the Wide Field Camera and the narrow field instruments on the BeppoSAX spacecraft, and with the All-Sky Monitor and the Proportional Counter Array on the RossiXTE spacecraft. Four X-ray bursts were detected, at least one of which shows the characteristics of a thermonuclear flash on a neutron star, in analogy with some [FORMULA] optically identified low-mass X-ray binaries. The broad-band spectrum is hard as is common among low-mass X-ray binaries of lower luminosity ([FORMULA] erg s-1) and can be explained by a Comptonized model. During the burst the [FORMULA] keV emission brightened, consistent with part of the burst emission being Compton up scattered within [FORMULA] cm.

Key words: X-rays: stars – X-rays: bursts – Galaxy: globular clusters: individual: NGC 6440 – stars: neutron – stars: individual: SAX J1748.9-2021 – stars: individual: MX 1746-20

Send offprint requests to: J.J.M. in 't Zand (jeanz@sron.nl)

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© European Southern Observatory (ESO) 1999

Online publication: April 12, 1999