Novalike Object in Centaurus

L. Kaluzienski and S. Holt, Goddard Space Flight Center, report that continued monitoring of the x-ray nova in Centaurus (IAUC 3360) with the Ariel 5 all-sky monitor has yielded the following improved position for the source: R.A. = 14h53m, Decl. = -32o.8 (90-percent confidence error +/- 0o.8; equinox 1950.0); this is consistent with its initial identification with Cen X-4. The source brightened from a level of ~ 1.4 times the Crab Nebula (3-6 keV) on May 14 to a peak value of ~ 4 times the Crab on May 17 and has remained at roughly this level through May 20. C. Canizares and J. McClintock, Massachusetts Institute of Technology; and J. Grindlay, Center for Astrophysics, report the discovery of what is presumably the optical counterpart of the above. The observations were carried out at the Cerro Tololo Interamerican Observatory. Inspection of a plate of the region obtained by M. Liller with the 400-cm telescope on May 19 revealed a star at R.A. = 14h55m19s.5, Decl. = -31o28'07" (+/- 5"; equinox 1950.0) that had brightened to ~ magnitude 13 from ~ magnitude 19 on the Palomar Sky Survey. Photoelectric photometry with the 150-cm telescope on May 20.15 UT gave mean values of V = 12.9, B-V = 0.0 +/- 0.1, U-B = -0.8 +/- 0.2. The B magnitude showed intrinsic variations over ~ 5 percent over ~ 1 hr. By May 21.20 the star had faded by ~ 0.5 magnitude in all colors. Two spectrograms obtained by L. Stryker with the 100-cm telescope show a nearly featureless continuum, possibly with very faint emission at He II 4686 A and H-alpha. The star is 0o.24 from the revised Vela-satellite x-ray position of Cen X-4, which has a quoted uncertainty of +/- 0o.2. The remarkable similarity to the x-ray transient A0620-00 = V616 Mon leaves little doubt that the star is the optical counterpart of Cen X-4. For reference the star is flanked by two ~ magnitude 14 stars 25" northeast and 40" southwest, respectively.