14724( 2) - 15-Jul-2016 18:07:29 - [ 1] HUBBLE SPACE TELESCOPE OBSERVING PROGRAM 14724 Version: 2 Check-in Time: 15-Jul-2016 18:07:29 STScI Edit Number: 0 Title Searching for a radio millisecond pulsar in a low-mass X-ray binary ------------------------------------------------------------------------------------ Type Cycle GO 24 ------------------------------------------------------------------------------------ Investigators Contact? PI: Dr. Nathalie Degenaar University of Cambridge Y CoI: Mr. Juan Venancio Hernandez University of Southampton N Santisteban CoI: Prof. Christian Knigge University of Southampton N CoI: Dr. Jon Matthew Miller University of Michigan N CoI: Dr. Rudy Wijnands Universiteit van Amsterdam N CoI: Dr. Craig Heinke University of Alberta N CoI: Dr. Diego Altamirano University of Southampton N CoI: Dr. Alessandro Patruno Universiteit Leiden N CoI: Dr. Jason Hessels Stichting Astronomisch Onderzoek in N Nederland (ASTRON) CoI: Dr. Adam Travis Deller Stichting Astronomisch Onderzoek in N Nederland (ASTRON) CoI: Ms. Amruta Jaodand Stichting Astronomisch Onderzoek in N Nederland (ASTRON) ------------------------------------------------------------------------------------ Abstract Low-mass X-ray binaries (LMXBs) and millisecond radio pulsars (MSRPs) are two different manifestations of neutron stars in binary systems. Their evolution paths are thought to be linked, but many questions about their connection remain. Recently, three neutron stars have been discovered to transition back and forth between LMXB and MSRP states, opening a new vista to investigate the link between these different manifestations. The neutron star LMXB SAX J1808.4-3658 is a strong candidate to belong to this new class of transitional objects. Here, we propose to exploit the unique UV capabilities of the HST to search for the presence of an accretion disk in the quiescent state of this system. This gives insight into whether it can turn on as a MSRP when not accreting and hence if it is indeed a transitional object. In addition, we will search for thermal emission from the surface of the unusually cold neutron star in SAX J1808.4-3658, which has important implications for the physics of ultra-dense matter. ------------------------------------------------------------------------------------ Observations Description ------------------------ Detecting a quiescent accretion disk or a hot neutron star in our target requires observations in the NUV/FUV bands. Observations with the Swift/UVOT in quiescence are not sensitive enough to detect the source in the NUV: u>20.5 mag (lambda_c~3465 A), and uvm2>21.5 mag (lambda_c~2246 A). This corresponds to NUV fluxes of <2E-17 erg/cm2/s/A (3465 A) and <1E-17 erg/cm2/s (2246 A), which is a factor of 5-20 above that expected for a quiescent accretion disk or a thermally emitting neutron star. Our science goals therefore require the unprecedented UV sensitivity of the HST. Given the unknown NUV/FUV magnitude of 14724( 2) - 15-Jul-2016 18:07:29 - [ 2] our target, we propose HST imaging observations. To be able to determine the shape of the UV spectral energy distribution and to measure UV colors, we request observations using different NUV/FUV filters. To achieve the best sensitivity (our target is not in a crowded field), we chose the WFC3/UVIS for the NUV, and the ACS/SBC for the FUV. For the WFC3/UVIS we will use 3 filters: F336W (lambda_c~3355 A), F275W (lambda_c~2704 A), and F225W (lambda_c~2359 A). For the ACS/SBC we use 1 filter: F150LP (lambda_c~1500 A). We used the online WFC3/UVIS and ACS/SBC exposure time calculators (v. 24.2). The distance to our target is 3.6 kpc, and the reddening is E(BV)= 0.23. Detecting a hot neutron star poses the strongest constraints on the science exposures. We assumed a black body of ~1E5 K (corresponding to the lowest neutron star temperatures in LMXBs), which yields a flux density of ~1E-18 erg/cm2/s/A. We aim for a SNR~10 at this flux density. For a remnant accretion disk we expect a higher UV flux and hence a higher SNR (a black body with a temperature of ~1E4 K, or a luminosity of ~8E30 erg/s, the coldest detected in quiescent LMXBs, would lead to FUV/NUV fluxes of ~5E-18 erg/cm2/s/A). We used dither patterns for all science observations and for the WFC3/UVIS exposure time calculation we took into account lamp flashes to ensure that the minimum background for proper CTE is reached. We require 2 orbits of ACS/SBC observations, and 2 orbits of WFC3/UVIS observations, so 4 orbits in total. Little UV variability is expected for our targets and therefore the observations are divided into two visits (one per instrument) to ease the constraints on scheduling. Our source is a transient X-ray source and can brighten to u~16.5 mag in outburst. The source is in outburst for ~1 month every ~2-5 years. The most recent outburst was in April 2015. Given the transient nature, we need to implement a safety procedure. We will obtain Swift observations (providing both X-ray and UV constraints) within 24 hr of the scheduled HST observations to ensure that our target is in quiescence. ------------------------------------------------------------------------------------ 14724( 2) - 15-Jul-2016 18:07:29 - [ 3] TARGET LIST Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 SAXJ1808.4-3 GALAXY, ACCRETION DISK RA=18H08M27.5400S +/- 0.02", J2000 V = 21 658 DEC=-36D58'44.30" +/- 0.2" Reference Frame: ICRS Extended: NO ------------------------------------------------------------------------------------------------------------------------------------ 14724( 2) - 15-Jul-2016 18:07:29 - [ 4] Patterns ------------------------------------------------------------------------------------------------------------------------------------ Pattern_Number: 1 Primary_Pattern Secondary_Pattern Pattern_Type ACS-SBC-DITHER-LINE Pattern_Purpose DITHER Number_Of_Points 3 Point_Spacing 0.472 Line_Spacing Coordinate_Frame POS-TARG Pattern_Orient 44.4 Angle_Between_Sides Center_Pattern NO ------------------------------------------------------------------------------------------------------------------------------------ Pattern_Number: 2 Primary_Pattern Secondary_Pattern Pattern_Type WFC3-UVIS-DITHER-BOX Pattern_Purpose DITHER Number_Of_Points 4 Point_Spacing 0.173 Line_Spacing 0.112 Coordinate_Frame POS-TARG Pattern_Orient 23.884 Angle_Between_Sides 81.785 Center_Pattern NO ------------------------------------------------------------------------------------------------------------------------------------ Pattern_Number: 3 Primary_Pattern Secondary_Pattern Pattern_Type WFC3-UVIS-DITHER-LINE Pattern_Purpose DITHER Number_Of_Points 2 Point_Spacing 0.145 Line_Spacing Coordinate_Frame POS-TARG Pattern_Orient 46.84 Angle_Between_Sides Center_Pattern NO ------------------------------------------------------------------------------------------------------------------------------------ Pattern_Number: 4 Primary_Pattern Secondary_Pattern Pattern_Type WFC3-UVIS-DITHER-LINE-3PT Pattern_Purpose DITHER Number_Of_Points 3 Point_Spacing 0.135 Line_Spacing Coordinate_Frame POS-TARG Pattern_Orient 46.84 Angle_Between_Sides Center_Pattern NO ------------------------------------------------------------------------------------------------------------------------------------ 14724( 2) - 15-Jul-2016 18:07:29 - [ 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 SAXJ1808.4- ACS/SBC ACCUM SBC F150LP 2 917 S PATTERN 1 1 3658 see below (5658 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 | ------------------------------------------------------------------------------------------------------------------------------------ SAXJ1808.4-3658 1 ACS/SBC ACCUM SBC F150LP 1 none 1 none 1 918.0 S SAXJ1808.4-3658 1 ACS/SBC ACCUM SBC F150LP 1 none 2 none 1 918.0 S SAXJ1808.4-3658 1 ACS/SBC ACCUM SBC F150LP 2 none 1 none 1 917.0 S SAXJ1808.4-3658 1 ACS/SBC ACCUM SBC F150LP 2 none 2 none 2 968.0 S SAXJ1808.4-3658 1 ACS/SBC ACCUM SBC F150LP 3 none 1 none 2 968.0 S SAXJ1808.4-3658 1 ACS/SBC ACCUM SBC F150LP 3 none 2 none 2 969.0 S 14724( 2) - 15-Jul-2016 18:07:29 - [ 6] Visit: 02 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 SAXJ1808.4- WFC3/UVI ACCUM UVIS F225W FLASH=10 1 631 S PATTERN 2 1 3658 S (2524 S) ------------------------------------------------------------------------------------------------------------------------------------ 2 SAXJ1808.4- WFC3/UVI ACCUM UVIS F275W FLASH=10 1 594 S PATTERN 4 2 3658 S (1782 S) ------------------------------------------------------------------------------------------------------------------------------------ 3 SAXJ1808.4- WFC3/UVI ACCUM UVIS F336W FLASH=10 1 348 S (696 PATTERN 3 3 3658 S 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 | ------------------------------------------------------------------------------------------------------------------------------------ SAXJ1808.4-3658 1 WFC3/UVI ACCUM UVIS F225W 1 none none none 1 N/A S SAXJ1808.4-3658 1 WFC3/UVI ACCUM UVIS F225W 2 none none none 1 N/A S SAXJ1808.4-3658 1 WFC3/UVI ACCUM UVIS F225W 3 none none none 1 N/A S SAXJ1808.4-3658 1 WFC3/UVI ACCUM UVIS F225W 4 none none none 1 N/A S SAXJ1808.4-3658 2 WFC3/UVI ACCUM UVIS F275W 1 none none none 2 N/A S SAXJ1808.4-3658 2 WFC3/UVI ACCUM UVIS F275W 2 none none none 2 N/A S SAXJ1808.4-3658 2 WFC3/UVI ACCUM UVIS F275W 3 none none none 2 N/A S SAXJ1808.4-3658 3 WFC3/UVI ACCUM UVIS F336W 1 none none none 2 N/A S SAXJ1808.4-3658 3 WFC3/UVI ACCUM UVIS F336W 2 none none none 2 N/A S 14724( 2) - 15-Jul-2016 18:07:29 - [ 7] Summary Form for Proposal 14724 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures SBC, UVIS ------------------------------------------------------------------------------------------------------------------------------------ Configurations ACS/SBC, WFC3/UVIS ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACCUM ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters FLASH=10 ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements PATTERN 1 1, PATTERN 2 1, PATTERN 3 3, PATTERN 4 2 ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements F150LP, F225W, F275W, F336W ------------------------------------------------------------------------------------------------------------------------------------ Target Names SAXJ1808.4-3658 ------------------------------------------------------------------------------------------------------------------------------------