The Optical Counterpart of the NGC 6624 X-Ray Burster
Session 7 -- X-Ray Binaries and Gamma-Ray Binaries
Display presentation, Monday, 9:20-6:30, Heller Lounge Room

## [7.01] The Optical Counterpart of the NGC 6624 X-Ray Burster

Ivan R. King, S. Adam Stanford (U.C. Berkeley)

On a pair of 30-min HST FOC images taken at 1400 \AA\ (F140W), we have identified the optical counterpart of the X-ray burster in the globular cluster NGC 6624; this object completely dominates these UV images. Its flux agrees with the UV flux seen by Rich et al. \ (1993,ApJ,406,489) with the large aperture of IUE. In the blue (F430W) the object is at $B \simeq 18.6$, while in the $V$ band (F480LP) we can find no trace of it. The $1400-B$ color is consistent with a Rayleigh--Jeans spectrum. (For an interpretation of this radiation as X-ray energy reprocessed by the accretion disk around the LMXB and by the binary companion, see a separate paper by Arons and King at this meeting.) The X-ray source is now found to be only 0.3 arcsec from the cluster center, increasing the likelihood that the bizarre $\dot P$ of the binary is influenced by gravitational acceleration. The counterpart of the LMXB is surrounded by several brighter red giants, one only 80 mas away, so that it cannot be observed from the ground. Our new astrometry corrects the previously published positions of the cluster center and places the counterpart within 2 $\sigma$ of the X-ray position. The optical counterpart is very close to the radio position of Johnston and Kulkarni (1992,ApJL,393,L17), but that position is now recognized to refer to a coincidentally neighboring pulsar rather than to the LMXB. Further analysis of the UV light will be pursued with HST's High Speed Photometer.