NGC 6440 is a globular cluster near the center of the Galaxy, at a distance of 8.5 kpc and reddened by (Ortolani et al. 1994). A bright X-ray source was detected near this cluster with OSO-7 and with UHURU from 1971 December 17 to 1972 January 21 (Markert et al. 1975, Forman et al. 1976). UHURU observations obtained before 1971 Oct 23 and after 1972 Mar 1 did not detect the source (Forman et al. 1976). During the outburst the transient X-ray source had a virtually constant luminosity of about , in the 2-11 keV band. Before and after the outburst the flux was less than 5% of this. (We use the conversion of UHURU to flux given by Bradt & McClintock 1983; and the absorption column determined by in 't Zand et al. 1999.)
A dim source was detected in the core of NGC 6440 with the Einstein satellite, and again with ROSAT, at a luminosity of , in the 0.5-2.5 keV band (Hertz & Grindlay 1983; Johnston et al. 1995); after conversion to the 2-11 keV band, this corresponds to of the outburst flux.
On 1998 August 22 a bright transient source appeared again in NGC 6440, observed with BeppoSAX. The position coincides with the globular cluster within the accuracy of 1´. This time the outburst lasted rather shorter: the source had a luminosity of on Aug 22, on Aug 26, and on Sep 1, in the 2-10 keV band (in 't Zand et al. 1999, also see Fig. 3).
Like persistent bright X-ray sources, transients occur more often in globular clusters per unit of stellar mass than in the galactic disk. To understand this overabundance one would like to study these sources at optical and ultraviolet wavelengths. So far, no transient X-ray source in a globular cluster has been optically identified. Such identification is difficult because the relatively large error circle of the X-ray position contains a large number of stars. We therefore obtained a ROSAT HRI observation as soon as possible after the detection of the transient with BeppoSAX, in the hope of improving the X-ray position. The optical brightness of soft X-ray transients is known to vary in tandem with the X-ray luminosity (for a review, see e.g. Chen et al. 1997). We therefore obtained optical images of NGC 6440 to look for objects that vary in tandem with the X-ray flux, in the hope of identifying the optical counterpart of the transient.
In Sect. 2 we describe the results of the new ROSAT HRI observation, and also analyse archival ROSAT data of NGC 6440. In Sect. 3 we describe the optical observations and the search for an optical counterpart to the X-ray source. Our results and their implications are discussed in Sect. 4.
© European Southern Observatory (ESO) 2000
Online publication: July 13, 2000