The Optical Identification of the X-ray Burster X1746-370 in the Globular Cluster NGC6441

Abstract

We find convincing observational evidence to confirm the optical identification of the X-ray burster X1746-370 located in the globular cluster NGC6441. Chandra/HRC-I imaging yields a much improved X-ray position for the source, which we show to be fully consistent with our rederived position of a UV-excess star, U1, in the same astrometric reference frame. In addition, the smaller Chandra X-ray error circle excludes the only other blue stars previously identified in the old Einstein circle. We have also obtained Hubble Space Telescope/STIS time-resolved optical spectra of star U1. Although there are no strong line features, the flux distribution demonstrates U1 to be unusually bright in the blue and faint in the red, consistent with earlier WFPC2 photometry. More notably, the flux level of the continuum is seen to vary significantly compared to stars of similar brightness. Indeed, the lightcurve can plausibly be fit by a 5.73 hr period sinusoid, which is the period of the recurring X-ray dips seen in this source. The presence of modulations in both wavelengths strengthens the case for an orbital origin, and therefore deepens the puzzle of the unusual energy independent X-ray dips. Lastly, we note that X1746-370 remains the longest period confirmed X-ray burster in a globular cluster, and the only one with a period typical of the galactic population as a whole.