Rates for Color-shifted Microlensing Events

Abstract

If the objects responsible for gravitational microlensing of Galactic-bulge stars are faint dwarfs, then blended light from the lens will distort the shape of the microlensing light curve and shift the color of the observed star during the microlensing event. In most cases, the resolution in current microlensing surveys is not accurate enough to observe this color-shift effect. However, such signatures could conceivably be detected with frequent followup observations of microlensing events in progress, providing the photometric errors are small enough. We calculate the expected rates for microlensing events where the shape distortions will be observable by such followup observations, assuming that the lenses are ordinary low-mass main-sequence stars in a rapidly rotating bar and in the disk. We adopt Galactic models consistent with observed microlensing timescale distributions, and consider separately the cases of self-lensing of the bulge, lensing of the bulge by the disk, and self-lensing of the disk, further differentiating between events where the source is a giant or a main-sequence star. We study the dependence of the rates for color-shifted microlensing events on the frequency of followup observations and on the precision of the photometry for a variety of waveband pairings. We find that for hourly observations in B and K with typical photometric errors of 0.01 mag, 28% of the events where a main-sequence bulge star is lensed, and 7% of the events where the source is a bulge giant, will give rise to a measurable color shift at the 95% confidence level. For observations in V and I, the fractions become 18% and 5%, respectively,

Light-curve classification in massive variability surveys - II. Transients towards the Large Magellanic Cloud

“…The amplified star will contribute most of the colour around the peak. The colour of a microlensing event can become redder or bluer, depending on the population of the blend, but it usually changes symmetrically about the peak with substantial correlation between passbands (see, for example, Di Stefano & Esin 1995;Buchalter, Kamionkowski & Rich 1996). Again, we build a training set with patterns of features extracted from simulated microlensing and supernova light curves.…”

Difference Image Analysis of Galactic Microlensing. I. Data Analysis

“…In this case the Ñux of the lens is not ampliÐed, and such blending is generally negligible for stars with masses consistent with microlensing results. The color shifts associated with this type of blending usually cannot be detected with the present level of photometry (Buchalter et al 1996). This case is also generally inseparable from blending with objects that are not the sources.…”

Color‐Shift Measurement in Microlensing‐induced Stellar Variation from Future Space‐based Surveys

“…Then, a color shift occurs as the relative brightnesses of the source and blended stars change during lensing magnification. Kamionkowski (1995) and Buchalter et al (1996) pointed out that if the object responsible for a lensing event is a star, it is possible to obtain information about the lens from the measured color shift. However, this idea could not be implemented for the actual analysis of lens populations.…”

“…We also investigate the properties of the color-shifted events by conducting a detailed simulation of Galactic lensing events under realistic observational conditions. The fraction of color-shifted events was previously estimated by Buchalter et al (1996). However, their work was intended as a template for general comparison with various observational assumptions and thus is not focused on a specific survey.…”