In late December 2010 we discovered a transient ultraluminous X-ray source (ULX) in an inter-arm region of the nearby spiral galaxy M83, during the first in a series of Chandra observations. Most ULXs are associated with very young stars, and so finding one in an inter-arm region was unusual. In April 2010 we found from Gemini-South a faint blue object within the X-ray error circle that was conspicuously absent in earlier images---from Magellan (April 2009) and from WFC3 (this region of M83 had fortuitously been observed as part of the WFC3 ERS program in August 2009). Here we request 3 orbits of DD time for new broad-band images of this region with WFC3, to positively identify the optical counterpart of the ULX and measure its characteristics. Given a precise position, we can determine which, if any, of a few very faint red stars within the error circle, all barely detected in the ERS images, is the progenitor---presumably the donor star in an accreting binary system containing a black hole. From new post-outburst WFC3 photometry, we can infer the present temperature and characteristic size of the optical secondary, which is now glowing primarily due to X-ray heating of its surface. This unique opportunity for "before and after" pictures will enable us to set stringent limits on the mass of the black hole that forms the central engine for the ULX, and help to resolve the controversy that exists concerning the nature of ULXs. Since we don’t know how long the X-ray outburst will last, we request that the observations take place as soon as reasonably possible, and no later than the end of August 2011, when our final Chandra observation is scheduled.

We plan to observe a single field with WFC3 in 4 broadband filters, equivalent to U, B, V, and I. We use the same standard 3-point line dither pattern for each filter. We center the ULX at the optimum position in the UVIS2-M1K1C-SUB aperture (which reads out only a 1K x 1K section near the center of chip 2). This saves us on overhead, since a single readout is sufficient for all the frames. This then allows us to use shorter (than 350 sec) exposures for V, and longer ones for U and I. We request that the observation be scheduled as soon as possible--at latest before the end of August 2011, when our final Chandra observaton of M83 is scheduled. We also request to be notified as soon as the observation is scheduled, since we hope to arrange a near-simultaneous X-ray observation from Swift. RESOLVED STELLAR POPULATIONS 12 false 0.0 0.0 false false false false SUPPORTED GALAXY STARBURST This is the same position as observed with WFC3 in August 2009 as part of the Early Release Science program. J2000 WFC3/UVIS Imaging No No F336W WFC3/UVIS Imaging No No F438W WFC3/UVIS Imaging No No F555W WFC3/UVIS Imaging No No F814W POS-TARG DITHER 0.135 LINE false false 3 46.84 WFC3-UVIS-DITHER-LINE-3PT 3 false The scheduling data was generated by CASM 19.3. CASM was run in Phase II mode. (Tic-Visit :proposal-id 12683 :visit-id "01" :proposal-version -1 :trans-version "20110509" :date "14-Jul-2011 11:51:55.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 2 :saa-model "29" :si-bright-earth-limb-angle 20.0 :data-volume 208912 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 12683 :visit-id "01" :obset-id "01" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 12683 :visit-id "01" :orbit-number 1 :pcs-time 333 :si-vis-utilization 2924 :al-overhead-at-end 77 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3257 :pcs-gap 0 ) (Tic-Orbit :proposal-id 12683 :visit-id "01" :orbit-number 2 :pcs-time 222 :si-vis-utilization 3035 :al-overhead-at-end 77 :internal-dur-at-end 267 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3257 :pcs-gap 0 ) true 2 0