OGLE-2007-BLG-224/MOA-2007-BLG-163 is a remarkable microlensing event towards the Galactic bulge, which peaked on May 12, 2007. The light curve reached a peak magnification of ~3700, which is the highest magnification ever observed. The color and magnitude of the source indicate that it is a G-dwarf in the Galactic bulge, and source radius crossing time of t* = 8.2 minutes implies that the lens-source relative proper motion is 45 mas/yr. This indicates that the lens must either be a very nearby brown dwarf, or a halo star. The HST observations proposed here provide a very good chance to distinguish between these two possibilities and to determine the distance and mass of the lens. If it is a nearby brown dwarf, it is likely to be associated with a high-proper-motion star that is found close to the source. HST observations taken at two epochs will resolve out the geocentric and the heliocentric the proper-motions, thus providing unambiguous proof that it is a substellar lens. If the lens is a halo star, then it should be detectable as it separates from the source star over the next year. This would be the first definitive detection of a lens star in the Galatic halo and it would help to resolve the long running controversy over whether a significant fraction of the microlensing events seen towards the Magellanic Clouds are due to lens objects in the halo. Thus, it will either be the first clear proof of a substellar lens in the Galactic disk, or the first clear detection of a halo lens.

We will use WFPC2 to image the field in V and I filters, with the object placed in the PC chip. Given its magnitude of I~18.1, we will require about ~(5 +5) min exposures in V and I filters, which should allow us to see fainter objects in the field without saturating the source. We will also use NICMOS/NIC1 to image the field in J and H bands. The observations will be repeated about 8 months later. Note that the galactic bulge can be observed from about February to June, which allows the observations to be taken 8 months apart, in the 2-gyro mode. COOL STARS 0 false 0.0 0.0 false false false false false SUPPORTED EXT-STAR G V-IV J2000 18 05 41.0500 -28 45 36.20 J2000 WFPC2 Imaging No F555W WFPC2 Imaging No F814W NIC1 Imaging No No F110W NIC1 Imaging No No F160W POS-TARG DITHER 0.3535 LINE false false 2 45.0 WFPC2-LINE 1 false POS-TARG DITHER DITHER 0.334461 NIC-XSTRIP-DITH false false 2 45.0 POS-TARG NIC-SPIRAL-DITH 0.0 2 false (Tic-Visit :proposal-id 11311 :visit-id "01" :proposal-version -1 :trans-version "20070325" :date "25-May-2007 11:09:54.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 1 :saa-model "26" :si-bright-earth-limb-angle 20.0 :data-volume 179264 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 11311 :visit-id "01" :obset-id "01" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 11311 :visit-id "01" :orbit-number 1 :pcs-time 355 :si-vis-utilization 2460 :al-overhead-at-end 180 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3149 :pcs-gap 0 ) false 1 0 (Tic-Visit :proposal-id 11311 :visit-id "02" :proposal-version -1 :trans-version "20070325" :date "25-May-2007 11:09:47.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 1 :saa-model "23" :si-bright-earth-limb-angle 20.0 :data-volume 66256 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 11311 :visit-id "02" :obset-id "02" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 11311 :visit-id "02" :orbit-number 1 :pcs-time 355 :si-vis-utilization 2682 :al-overhead-at-end 0 :internal-dur-at-end 112 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3149 :pcs-gap 0 ) false 1 0 (Tic-Visit :proposal-id 11311 :visit-id "03" :proposal-version -1 :trans-version "20070709" :date "22-Jan-2008 16:54:24.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 1 :saa-model "26" :si-bright-earth-limb-angle 20.0 :data-volume 179264 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 11311 :visit-id "03" :obset-id "03" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 11311 :visit-id "03" :orbit-number 1 :pcs-time 355 :si-vis-utilization 2460 :al-overhead-at-end 180 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3149 :pcs-gap 0 ) false 1 0 (Tic-Visit :proposal-id 11311 :visit-id "04" :proposal-version -1 :trans-version "20070709" :date "23-Jan-2008 12:40:52.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 1 :saa-model "23" :si-bright-earth-limb-angle 20.0 :data-volume 64048 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 11311 :visit-id "04" :obset-id "04" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 11311 :visit-id "04" :orbit-number 1 :pcs-time 355 :si-vis-utilization 2426 :al-overhead-at-end 0 :internal-dur-at-end 110 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3149 :pcs-gap 0 ) false 1 0