0 Assigned ID: 967 ----- Attempting Submission 1 (Fri Apr 07 22:00:38 GMT 2017) ----- The V-band magnitude of the target is a best guess because it hasn't been detected (there are shorter HST observations in the archive and there is certainly no bright source at that position). -151087047 ----- Attempting Submission 1 (Thu Jul 06 20:21:51 GMT 2017) ----- Ultraluminous X-ray sources (ULXs) were once among the most promising candidates for long sought after intermediate-mass black holes, owing to their high X-ray luminosities (>10^39 erg/s) and off-nuclear positions. NGC 5907 ULX-1 was a prime example, and since it regularly reaches 10^41 erg/s it was thought to harbour a black hole with a mass of at least 500 solar masses. But in an astonishing discovery, the source was found to exhibit pulsations in the X-rays on second-timescales, revealing it to be a pulsar powered by accretion onto a neutron star with only 1.4 solar masses. This discovery challenges every known theory of accretion onto a compact object, which in this object exceeds the Eddington limit by a factor of 500. It requires us to imagine extreme departures from known accretion theory and/or binary evolution scenarios. The fuel source should be a massive companion star in order to sustain the required mass accretion rate, however X-ray timing favors a low-mass star. With the ability to detect a massive star, a short HST/WFC3 NIR observation would solve this mystery. A detection of a supergiant donor would open the path to future dynamical mass measurements with JWST, while a non-detection would prove that this extreme ULX pulsar contains a low-mass donor star, forcing us to consider new evolutionary formation channels. The goal of this project is to either detect the donor star of the ULX pulsar in NGC 5907 (if it is an O or B supergiant) or set a strong limit on its magnitude (if it is a low-mass star). To do this we will observe the ULX with WFC3/NIR in one filter, F160W. With four exposures of 1403 seconds (using the SPARS100 readout with Nsamp = 15), using the WFC3-IR-DITHER-BOX-MIN dither pattern to optimally sample the PSF, we obtain a total exposure time of 5612 seconds. According to the ETC, this will give us a S/N ratio of 7-8, depending on the subtype, for a B supergiant with H = 25.5; thus we will significantly detect a supergiant even if the extinction is at the upper edge of the suggested range. Taking into account the overheads for guide star acquisition, the instrument overhead per exposure and the time needed to offset the satellite, the total time needed for two exposures is 55 minutes. This fits easily in the orbital visibility window for NGC 5907 of 58 minutes, so the four exposures we need fit in two orbits. Stellar Physics 6 false false false false false false SUPPORTED EXT-STAR ACCRETION DISK NEUTRON STAR NO This object was generated by the targetselector and retrieved from the SIMBAD database. J2000 J2000 The scheduling data was generated by CASM 25.0. CASM was run in Phase I mode. WFC3/IR Imaging No No F160W POS-TARG DITHER 0.572 LINE 0.365 false false 74.653 4 18.528 WFC3-IR-DITHER-BOX-MIN 1 false 1499371955693 /private/var/folders/cm/81_3bbvx469g0vfslx1s3zkm0000gn/T/OPVisit8264777952222126491.xml The scheduling data was generated by CASM 25.2. CASM was run in Phase II mode. (Tic-Visit :proposal-id 15074 :visit-id "01" :proposal-version -1 :trans-version "20170613" :date "06-Jul-2017 13:12:35.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 2 :saa-model "30" :si-bright-earth-limb-angle 20.0 :data-volume 1114160 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 15074 :visit-id "01" :obset-id "01" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 15074 :visit-id "01" :orbit-number 1 :pcs-time 333 :si-vis-utilization 2910 :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 3449 :pcs-gap 0 ) (Tic-Orbit :proposal-id 15074 :visit-id "01" :orbit-number 2 :pcs-time 222 :si-vis-utilization 2910 :al-overhead-at-end 0 :internal-dur-at-end 272 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3449 :pcs-gap 0 ) true 2 0