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S. Wachter, D.W. Hoard (CTIO), C. Bailyn, R. Jain (Yale), P. Kaaret, S. Corbel (CfA), R. Wijnands (MIT)
We present highlights from an optical monitoring program of X-ray transients designed to assemble long-term optical outburst and decay lightcurves that can be compared to X-ray lightcurves obtained from the RXTE All Sky Monitor or from monitoring campaigns with the RXTE pointed instruments. Since the optical radiation is thought to be largely due to reprocessing of X-rays in the disk, contemporaneous optical and X-ray observations offer insight into the outburst mechanism and accretion dynamics of these objects. Our optical data were obtained during 2000 Feb - Jul under the Synoptic, Service, and Target-of-Opportunity program on the Cerro Tololo Inter-American Observatory (CTIO) 0.9-m telescope and on the YALO 1-m telescope, also located at CTIO.
We will present lightcurves for SAX J1808.4-3658 and X1608-522. SAX J1808.4-3658, the transient 2.5 ms X-ray pulsar, displayed renewed activity in 2000 Jan - Feb. Our optical data show the same erratic variability as seen in X-rays. We will also present some of the first data of the quiescent optical counterpart. X1608-522 is a neutron star X-ray transient that undergoes large outbursts on a timescale of ~700 days. A new outburst of X1608-522 expected for early 2000 did not materialize; however, the optical counterpart was visible and we obtained I-band photometry for 68 days before it dropped below the detection limit of our telescopes. The X-ray data exhibit a series of mini-outbursts. Corresponding brightening of the counterpart is also seen in the optical. In addition, the optical lightcurve shows plateaus of emission with a mean level which drops after each mini-outburst, possibly indicating discrete ``emptying'' events in the accretion disk.
Cerro Tololo Inter-American Observatory is operated by AURA, Inc. under cooperative agreement with the NSF.