Binary Microlens Fits for MACHO-98-SMC-1

Last updated at June 15.97 UT; Caustic crossing expected June 19.2±1.5 UT.
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Figure 1 The current binary lens fit of MACHO/GMAN data for event MACHO-98-SMC-1.
The analysis presented here of MACHO/GMAN data has been performed by S.Rhie
and members of the MACHO team as a part of a "MACHO Affiliate" collaboration.
This is the latest binary lens fit for event MACHO 98-SMC-1 which was discovered on June 9 to be a caustic crossing binary event by the MACHO/GMAN group on the basis of follow-up observations on the CTIO 0.9m telescope. (See also IAUC 6935.) This latest fit indicates that a caustic crossing will occur at June 19.2±1.5 UT. (This error estimate is based upon our limited experience with caustic crossing predictions.) The two top panels contain data from the Mt. Stromlo 1.3m telescope in the MACHO-Red and MACHO-Blue bands while the bottom two lightcurves have data from the CTIO 0.9m in the R and B bands. Each lightcurve has a different magnification because the blending with unlensed stars in the different passbands is different. The CTIO data has less blending because more of the faint nearby stars are resolved. Note that the last three nights of CTIO-R data each contain 6 consecutive measurements. The times indicated on the x axis are days in June (UT) + 2341.

Photometric measurements during the caustic crossing are urgently requested because the timescale of the caustic crossing will indicate whether this binary lens resides in the SMC itself or in the Milky Way's dark halo. A finding chart is available here. Potential observers are urged to contact macho@astro.washington.edu to coordinate observations.

The magnification values in Figure 1 are normalized to the unlensed flux detected in each pass band. For the MACHO data the unlensed fluxes correspond to R = 21.7 and V = 21.8, so a magnification of A = 10 in the MACHO bands corresponds to R = 19.2 and V = 19.3.

Caustic Structure

Figure 2

Figure 2 is a plot of the caustic curves according to the latest for the SMC binary microlensing event. The straight line indicates the path of the source star, and the red dots indicate the location of source star on dates in June (0.0, 10.0, 20.0, and 30.0 UT). When the source passes inside one of the caustic curves, two new images are created making 5 images in all. The new images have formally infinite magnification when they are created (and when they disappear) which gives rise to the sharp caustic crossing features seen in the lightcurve.

Figure 3

Figure 3 is a close up of Figure 2 showing the central caustic in more detail. Note that the path of the source is predicted to cross the caustic at an angle of about 25 degrees. This tends to increase the uncertainty in the predicted time of the caustic crossing.

SMC vs. Halo Lens Lightcurves

Figure 4

Figure 4 shows the expected lightcurve differences for a lens in the SMC which takes 14 hours to cross a stellar diameter and a lens in the Galactic halo which can cross a stellar diameter in 30 minutes.

Fit Parameters

parameter      value      
t0 June  14.5 UT
t^hat 149 days
umin 0.043
epsilon1 0.276
sep 0.678
theta -0.205
Table 1 

Table 1 shows the parameters of the most recent fit. t0 is the time of closest approach between the source and the center of mass of the lens system. t^hat is the Einstein ring diameter crossing time, and umin is the distance of closest approach between the source and the center of mass. epsilon1 is the fraction of the total lens mass for mass 1 while sep and theta represent the lens separation in units of the Einstein ring radius and the angle between the lens axis and the path of the source.

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Sun Hong Rhie (srhie@nd.edu)