As most readers know, the CTIO 0.9-m has been working in tandem with the MACHO microlensing survey telescope since 1994. The MACHO project officially terminates in January 2000, as do the microlensing follow up efforts of the Global Microlensing Alert Network (GMAN), for which the 0.9m has served as the flagship telescope. The purpose of this network has been to obtain high-accuracy photometry of microlensing-like variability without sacrificing the overall detection rate of the MACHO survey system. In this, the project has been extremely successful. All variants of exotic microlensing have been detected by the CTIO 0.9-m, and in most cases real-time recognition of these effects has been largely due to the CTIO data.
Specific examples include prediction of the behavior of event MACHO 95-BLG-30, where the lens actually transited the face of the lensed source. Our prediction was based largely on CTIO data, and allowed us to resolve subtle center-to-limb spectral changes as the lens preferentially magnified certain regions of the stellar atmosphere. The 0.9-m was also responsible for the real-time recognition of lens binarity in MACHO 98-SMC-1, the single most important lensing event seen to date. In this event, the source underwent a caustic crossing, where strong lensing features pass over the face of the source -- in fact, at its peak the source brightened by approximately 5 magnitudes. Resolution of the temporal width of this crossing by multiple telescopes allows us to say with strong confidence that this lens resides in the SMC. This has exciting implications for the nature of the lensing population, and the velocity structure of the SMC. We were also able to measure limb-darkening profiles of the lensed source, a metal-poor A star.
In all cases, the staff and visiting observers at CTIO have allowed enormous flexibility in the night-to-night scheduling of the telescope. And in many cases, this provided access to lightcurve fine structure which would otherwise pass unobserved. The MACHO Collaboration, in particular Chris Stubbs and Andy Becker, would like to thank all involved with the CTIO 0.9-m over the past five years for their contribution to dark matter science, and to the realization of gravitational microlensing as a viable tool to study Galactic and Magellanic populations, both dark and luminous.
With the end of the MACHO project, changes are in store for the CTIO 0.9-m telescope. In addition to implementing a synoptic/service/target-of-opportunity observing program (see related article in this newsletter), we are investigating options for a nightly monitoring project similar to MACHO.
Andy Becker (Macho Project, Washington),
Bob Schommer, CTIO (bschommer@noao.edu)