0 Assigned ID: 429 ----- Attempting Submission 1 (Fri Apr 08 09:36:28 GMT 2016) ----- 0 ----- Attempting Submission 1 (Fri Jul 15 09:11:42 GMT 2016) ----- Searching for a radio millisecond pulsar in a low-mass X-ray binary 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. 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 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. Stellar Physics 12 false true false false false false SUPPORTED GALAXY ACCRETION DISK NO J2000 J2000 ACS/SBC Imaging No No F150LP WFC3/UVIS Imaging No No F225W WFC3/UVIS Imaging No No F275W WFC3/UVIS Imaging No No F336W POS-TARG DITHER 0.472 LINE false false 3 44.4 ACS-SBC-DITHER-LINE 1 false POS-TARG DITHER 0.173 LINE 0.112 false false 81.785 4 23.884 WFC3-UVIS-DITHER-BOX 2 false POS-TARG DITHER 0.145 LINE false false 2 46.84 WFC3-UVIS-DITHER-LINE 3 false POS-TARG DITHER 0.135 LINE false false 3 46.84 WFC3-UVIS-DITHER-LINE-3PT 4 false /Users/degenaar/Work/Proposals/HST/Cycle24/SAXJ1808/PHASE_II/OP-cache/saxj1808_imaging_phaseII-v01.xml The scheduling data was generated by CASM 24.2. CASM was run in Phase II mode. 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