15074( 2) - 12-Jul-2017 17:11:52 - [ 1] HUBBLE SPACE TELESCOPE OBSERVING PROGRAM 15074 Version: 2 Check-in Time: 12-Jul-2017 17:11:52 STScI Edit Number: 0 Title Identifying the donor star of the most extreme ULX pulsar ------------------------------------------------------------------------------------ Type Cycle GO 25 ------------------------------------------------------------------------------------ Investigators Contact? PI: Dr. Marianne Heida California Institute of Technology Y CoI: Dr. Fiona A. Harrison California Institute of Technology N CoI: Dr. Daniel K Stern Jet Propulsion Laboratory N CoI: Dr. Dominic Walton University of Cambridge N CoI: Dr. Felix Fuerst ESA-European Space Astronomy Centre N CoI: Dr. Murray Brightman California Institute of Technology N ------------------------------------------------------------------------------------ Abstract 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. ------------------------------------------------------------------------------------ Observations Description ------------------------ 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 15074( 2) - 12-Jul-2017 17:11:52 - [ 2] visibility window for NGC 5907 of 58 minutes, so the four exposures we need fit in two orbits. ------------------------------------------------------------------------------------ 15074( 2) - 12-Jul-2017 17:11:52 - [ 3] TARGET LIST Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 NGC5907-ULX EXT-STAR, ACCRETION RA=15H15M58.6000S, J2000 V = 30 DISK, NEUTRON STAR DEC=+56D18'10.00" Reference Frame: Other: SIMBAD Extended: NO Comments: This object was generated by the targetselector and retrieved from the SIMBAD database. ------------------------------------------------------------------------------------------------------------------------------------ 15074( 2) - 12-Jul-2017 17:11:52 - [ 4] Patterns ------------------------------------------------------------------------------------------------------------------------------------ Pattern_Number: 1 Primary_Pattern Secondary_Pattern Pattern_Type WFC3-IR-DITHER-BOX-MIN Pattern_Purpose DITHER Number_Of_Points 4 Point_Spacing 0.572 Line_Spacing 0.365 Coordinate_Frame POS-TARG Pattern_Orient 18.528 Angle_Between_Sides 74.653 Center_Pattern NO ------------------------------------------------------------------------------------------------------------------------------------ 15074( 2) - 12-Jul-2017 17:11:52 - [ 5] Visit: 01 Visit Requirements: On Hold Comments: Additional Comments: Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exp | Target | Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Num | Name | Config | Mode | or FOV |Element |Waveln.| Parameters |Exp| (Total) | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 NGC5907-ULX WFC3/IR MULTIAC IR F160W NSAMP=15, SAMP-SEQ=SPARS100 1 DEF PATTERN 1 1 CUM (5611.747 S) ------------------------------------------------------------------------------------------------------------------------------------ Sub Exposures ------------------------------------------------------------------------------------------------------------------------------------ Target | Exp |Instr | Oper. | Aper |Spectral|Cent.|Primary |Secondary |Iteration |CR-SPLIT |Orbit |Duration Name | Num |Config | Mode | or FOV |Element |Wave.|Pattern Pos |Pattern Pos |Num |Num |Number | ------------------------------------------------------------------------------------------------------------------------------------ NGC5907-ULX 1 WFC3/IR MULTIAC IR F160W 1 none none none 1 N/A CUM NGC5907-ULX 1 WFC3/IR MULTIAC IR F160W 2 none none none 1 N/A CUM NGC5907-ULX 1 WFC3/IR MULTIAC IR F160W 3 none none none 2 N/A CUM NGC5907-ULX 1 WFC3/IR MULTIAC IR F160W 4 none none none 2 N/A CUM 15074( 2) - 12-Jul-2017 17:11:52 - [ 6] Summary Form for Proposal 15074 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures IR ------------------------------------------------------------------------------------------------------------------------------------ Configurations WFC3/IR ------------------------------------------------------------------------------------------------------------------------------------ Opmodes MULTIACCUM ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters NSAMP=15, SAMP-SEQ=SPARS100 ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements PATTERN 1 1 ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements F160W ------------------------------------------------------------------------------------------------------------------------------------ Target Names NGC5907-ULX ------------------------------------------------------------------------------------------------------------------------------------