Colour change measurements of gravitational microlensing events by using the difference image analysis method

Abstract

Detecting colour changes of a gravitational microlensing event induced by the limb‐darkened extended source effect is important for obtaining useful information about both the lens and the source star. However, precise measurements of the colour changes are hampered by blending, which also causes colour changes of the event. In this paper, we show that although the colour change measured from the subtracted image by using the recently developed photometric method of the ‘difference image analysis’ (DIA) differs from the colour change measured by using the conventional method based on the extraction of the individual source stars’ point spread functions, the curve of the colour changes (colour curve) constructed by using the DIA method enables one to obtain the same information about the lens and source star, but with significantly reduced uncertainties due to the absence of blending. We investigate the patterns of the DIA colour curves for both single lens and binary lens events by constructing colour change maps.

WeCAPP -Wendelstein Calar Alto pixellensing project I

“…Under certain circumstances chromatic light curves permit to constrain the physical properties of the source-lens system (e.g. Gould & Welch, 1996;Han & Park, 2001). Variable stars will generally change colour in a different way.…”

On the Possibility of an Astronomical Detection of Chromaticity Effects in Microlensing by Wormhole-Like Objects

“…To ease the comparison with the standard (i.e. possitive mass) case, we shall adopt the blending coefficients as f U = 1.39% and f I = 1.04%, and a source star with radius R * = 0.1 [7]. Our new results, including the effect of blending, and for different impact parameters k = b 0 /r * = B 0 /R * , are shown in Figures 1 and 2.…”

“…We have then adapted the numerical code used in Refs. [7] to the case of negative mass lenses. Figure 5 shows two particular examples obtained with this code.…”

“…where t exp is the exposure time. The noise comes from the lensed source as well as from the the blended background stars [7],…”

“…Detection of the extended source effects from colour measurements, instead of single band photometry, is interesting because of two facts (see Ref. [7] for further discussion). Firstly, by detecting the colour curves the extended nature of the source is revealed: if the source approaches very close to the lens caustics but do not cross them, the induced amplification can always be mimicked by changes in the lensing parameters of a point-like object.…”