0 Assigned ID: 763 ----- Attempting Submission 1 (Fri Apr 08 20:06:41 GMT 2016) ----- 0 ----- Attempting Submission 1 (Fri Jul 08 16:29:03 GMT 2016) ----- The superluminous supernova (SLSN) SN 2015bn is one of the three nearest objects of its kind to date, and is now the best studied. However, despite a wealth of observations within ~1 year of peak light, the explosion mechanism remains elusive. Here we propose to distinguish between the popular scenarios of magnetar spin-down, circumstellar interaction and pair-instability explosion, by obtaining the latest and deepest ever images of a SLSN at around 2 years after explosion. At this phase, competing models predict robust colour differences, which we can detect in 3 orbits of HST+ACS observations (with 3 more orbits in the following cycle required for host galaxy template subtraction, also leading to a detailed host analysis). In the decade or so since the first SLSNe were discovered, no event has had the combination of late-time luminosity, proximity and a sufficiently faint host to be able to follow to such a late phase. We stress that these observations are new, timely, and only possible for ~1 SLSN per decade. Furthermore, we have a robust magnitude prediction based on extensive data, and the colour test we wish to perform is powerful yet relatively straightforward, and only possible using HST. We will use 3 orbits in Cycle 24 to observe SN 2015bn with the Advanced Camera for Surveys in the F475W, F625W and F775W filters, and 3 orbits in Cycle 25 to obtain images of the host galaxy in the same filters and with the same depth, after the SN has disappeared (during the same months in Cycle 25, the SN flux will be negligible, at r >30). These filters have been specifically chosen to cover the wavelength range where we expect most of the SN light to be concentrated. Taking into account the visibility window per orbit of ~54 min based on the target declination, and the time needed for guide star acquisition, dithering (in a standard 4 point pattern) and per-exposure overheads, the available time for integration on the target is ~2100 s in each orbit. Using this exposure time in each filter (i.e. 3 orbits total), the SN should be detectable with S/N >~ 5. If it has faded faster than expected, we will stack the exposures in a deep 'white-light' image to recover the SN. Stellar Physics Galaxies 6 false false false false false false SUPPORTED GALAXY DWARF COMPACT SDSSJ113341.53+004333.3 PS15AE J2000 J2000 supernova: r = 28 AB mag, host: r = 22 AB mag ACS/WFC Imaging No No F475W ACS/WFC Imaging No No F625W ACS/WFC Imaging No No F775W ACS/WFC Imaging No No F475W ACS/WFC Imaging No No F625W ACS/WFC Imaging No No F775W POS-TARG DITHER 0.262 LINE 0.192 false false 68.14 4 18.39 ACS-WFC-DITHER-BOX 1 false 1467980592102 The scheduling data was generated by CASM 24.2. CASM was run in Phase II mode. (Tic-Visit :proposal-id 14743 :visit-id "01" :proposal-version -1 :trans-version "20160601" :date "08-Jul-2016 13:23:11.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 3 :saa-model "27" :si-bright-earth-limb-angle 20.0 :data-volume 3410688 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 14743 :visit-id "01" :obset-id "01" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 14743 :visit-id "01" :orbit-number 1 :pcs-time 333 :si-vis-utilization 2874 :al-overhead-at-end 0 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3210 :pcs-gap 0 ) (Tic-Orbit :proposal-id 14743 :visit-id "01" :orbit-number 2 :pcs-time 222 :si-vis-utilization 2986 :al-overhead-at-end 0 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3210 :pcs-gap 0 ) (Tic-Orbit :proposal-id 14743 :visit-id "01" :orbit-number 3 :pcs-time 222 :si-vis-utilization 2985 :al-overhead-at-end 0 :internal-dur-at-end 347 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3210 :pcs-gap 0 ) false 3 0