Austin Bede Tomaney

Expanding the realm of microlensing surveys with difference image photometry

We present a new technique for monitoring microlensing activity even in highly crowded fields, and use this technique to place limits on low-mass MACHOs in the haloes of M31 and the Galaxy. Unlike present Galactic microlensing surveys, we employ a technique in which a large fraction of the stellar sample is compressed into a single CCD field, rather than spread out in a way requiring many different telescope pointings. We implement the suggestion by Crotts (1992) that crowded fields can be monitored by searching for changes in flux of variable objects by subtracting images of the same field, taken in time sequence, positionally registered, photometrically normalized, then subtracted from one another (or a sequence average). The present work tackles the most difficult part of this task, the adjustment of the point spread function among images in the sequence so that seeing variations play an insignificant role in determining the residual after subtraction. The interesting signal following this process consists of positive and negative point sources due to variable sources. The measurement of changes in flux determined in this way we dub difference image photometry (also called pixel lensing [Gould 1996]). - The matching of the image point spread function (PSF) is accomplished by a division of PSFs in Fourier space to produce a convolution kernel, in a manner explored for other reasons by Phillips & Davis (1995). In practice, we find the application of this method is difficult in a typical telescope and wide field imaging camera due to a subtle interplay between the spatial variation of the PSF associated with the optical design and the inevitable time variability of the telescope focus. Such effects lead to complexities...(abstract continues)

Results from a Survey of Gravitational Microlensing toward M31

We describe the results of a search for microlensing events affecting stars in the outer bulge and inner disk of M31, due both to masses in M31 and the Galaxy. These observations, from 1994 and 1995 on the Vatican Advanced Technology Telescope and KPNO 4 m telescope, are sufficient to rule out masses in the range ~0.003-0.08 M☉ as the primary constituents of the mass of M31 toward this field. Furthermore, we find six candidate events consistent with microlensing as a result of masses of about 1 M☉, but we suspect that some of these may be cases in which long-period red supergiant variables may be mistaken for microlensing events. Coverage from anticipated data should be helpful in determining whether these sources maintain a constant baseline, and therefore they are best described by microlensing events.

The physical properties of a slow nova in the bulge of M31

Observations are presented of an extremely slow nova that was discovered in the bulge of M31 in 1986. The evolution of the object was monitored with subsequent yearly spectra until 1990, as part of a general spectroscopic survey of novae in M31 underway at McDonald Observatory. The spectra cover the novas evolution in the nebular phase, and these observations have made this nova the most extensively observed extragalactic nova to date. This has provided a unique opportunity to make the first detailed comparison of the evolution and properties of an extragalactic nova with those in our own Galaxy