Whether ultraluminous X-ray sources (ULXs) contain stellar-mass or intermediate-mass black holes (IMBHs) is an important, but as yet unresolved, astrophysical question. We have discovered variable optical emission from the ULX NGC 5408 X-1 that we interpret as reprocessed emission in an irradiated disk. We propose simultaneous observations with Chandra and HST to test this interpretation and place constraints on the geometry of the accretion disk. The observations should provide a means to discriminate between stellar-mass versus intermediate-mass black holes.

The observations consist of three simultaneous Chandra/HST observations to test the irradiated disk model and one HST-only observation to image the surrounding nebula in the [O III] emission line. For the observations simultaneous with Chandra, we will obtain images in the F225W, F336W, F547M, and F845M filters with the WFC3/UVIS and in the F105W and F160W filters with the WFC3/IR. For the HST-only observations, we will use the WFC3 with the F502N filter. Continuum subtraction will be done using the F547M images obtained in the other observations. The first three visits must be simultaneous with a corresponding Chandra observation in order to determine whether the optical continuum flux is correlated with the X-ray emission, as predicted in the irradiated disk model, and to better constrain the parameters of the irradiated disk model. For each of these visits, the one HST orbit should be scheduled to occur during the 12,000 second duration of the corresponding Chandra observations. UNRESOLVED STELLAR POPULATIONS AND GALAXY STRUCTURE 12 false 0.0 0.0 false false false false false false SUPPORTED UNIDENTIFIED ACCRETION DISK EMISSION LINE NEBULA X-RAY EMITTER J2000 14 03 19.6200 -41 22 58.54 J2000 WFPC2 Imaging No F336W WFPC2 Imaging No F255W WFPC2 Imaging No F547M ACS/SBC Imaging No No F140LP ACS/SBC Imaging No No F165LP WFPC2 Imaging No F502N POS-TARG DITHER 0.173 LINE 0.112 false false 81.785 4 23.884 WFC3-UVIS-DITHER-BOX 1 false POS-TARG DITHER 0.145 LINE false false 2 46.84 WFC3-UVIS-DITHER-LINE 2 false POS-TARG DITHER 0.636 LINE false false 2 41.788 WFC3-IR-DITHER-LINE 3 false The scheduling data was generated by CASM 17.2. CASM was run in Phase II mode. This visit should be scheduled to occur during the 12,000 second duration of the corresponding Chandra observation (Chandra Seq: 401067 ObsId: 11032). 1272776874000 17.8078015160161 (Tic-Visit :proposal-id 12021 :visit-id "01" :proposal-version -1 :trans-version "20090702" :date "29-Sep-2009 09:14:02.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 1 :saa-model "29" :si-bright-earth-limb-angle 20.0 :data-volume 731296 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 12021 :visit-id "01" :obset-id "01" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 12021 :visit-id "01" :orbit-number 1 :pcs-time 333 :si-vis-utilization 2889 :al-overhead-at-end 0 :internal-dur-at-end 725 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3351 :pcs-gap 0 ) true 1 0 The scheduling data was generated by CASM 17.2. CASM was run in Phase II mode. This visit should be scheduled to occur during the 12,000 second duration of the corresponding Chandra observation (Chandra Seq: 401068 ObsId: 11033). 1273933759000 45.354382090156605 (Tic-Visit :proposal-id 12021 :visit-id "02" :proposal-version -1 :trans-version "20090702" :date "29-Sep-2009 09:14:04.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 1 :saa-model "29" :si-bright-earth-limb-angle 20.0 :data-volume 731296 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 12021 :visit-id "02" :obset-id "02" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 12021 :visit-id "02" :orbit-number 1 :pcs-time 333 :si-vis-utilization 2889 :al-overhead-at-end 0 :internal-dur-at-end 725 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3351 :pcs-gap 0 ) true 1 0 The scheduling data was generated by CASM 17.2. CASM was run in Phase II mode. This visit should be scheduled to occur during the 12,000 second duration of the corresponding Chandra observation (Chandra Seq: 401069 ObsId: 11034). 1275061177000 66.1250328221189 (Tic-Visit :proposal-id 12021 :visit-id "03" :proposal-version -1 :trans-version "20090702" :date "29-Sep-2009 09:14:05.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 1 :saa-model "29" :si-bright-earth-limb-angle 20.0 :data-volume 731296 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 12021 :visit-id "03" :obset-id "03" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 12021 :visit-id "03" :orbit-number 1 :pcs-time 333 :si-vis-utilization 2889 :al-overhead-at-end 0 :internal-dur-at-end 725 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3351 :pcs-gap 0 ) true 1 0 The scheduling data was generated by CASM 18.3.1. CASM was run in Phase II mode. This visit should be scheduled to occur during the 12,000 second duration of the Chandra re-observation that was approved due to the HST problems. (Tic-Visit :proposal-id 12021 :visit-id "53" :proposal-version -1 :trans-version "20100505" :date "13-Jul-2010 13:22:13.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 1 :saa-model "29" :si-bright-earth-limb-angle 20.0 :data-volume 732976 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 12021 :visit-id "53" :obset-id "53" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 12021 :visit-id "53" :orbit-number 1 :pcs-time 333 :si-vis-utilization 2856 :al-overhead-at-end 0 :internal-dur-at-end 540 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3341 :pcs-gap 0 ) true 1 0 The scheduling data was generated by CASM 18.3.1. CASM was run in Phase II mode. (Tic-Visit :proposal-id 12021 :visit-id "04" :proposal-version -1 :trans-version "20100505" :date "13-Jul-2010 13:19:50.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 1 :saa-model "29" :si-bright-earth-limb-angle 20.0 :data-volume 18320 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 12021 :visit-id "04" :obset-id "04" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 12021 :visit-id "04" :orbit-number 1 :pcs-time 333 :si-vis-utilization 2758 :al-overhead-at-end 38 :internal-dur-at-end 59 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3341 :pcs-gap 0 ) true 1 0