0 Assigned ID: 1633 ----- Attempting Submission 1 (Fri Mar 01 18:47:38 GMT 2013) ----- HST Phase I Proposal 1633 (n5408_faruv.apt) successfully submitted. 0 ----- Attempting Submission 1 (Fri Jun 21 15:12:04 GMT 2013) ----- Unveiling the nature of ultraluminous X-ray sources via UV spectroscopy Our recent HST/Chandra study of the ultraluminous X-ray source (ULX) in NGC 5408 showed that the X-ray/UV/optical/NIR spectral energy distribution (SED) is consistent with emission from an irradiated accretion disk, while the UV/optical/NIR SED alone can also be explained as a B0I supergiant star. If the accretion disk dominates in the far-UV, then we expect to see a wealth of spectral lines features in the far-UV spectrum. If, instead, the far-UV light arises from an early B supergiant, we expect to see UV lines with strong P-Cygni profiles. To determine the physical origin of the UV/optical/NIR emission, we propose to take a deep look at the far-UV spectrum of the ULX using the unique capabilities of the Cosmic Origins Spectrograph. The spectrum obtained will definitively distinguish between an irradiated accretion disk and a supergiant companion star. Detection of UV lines is likely one of the only direct ways to constrain the physical nature of ultraluminous X-ray sources. This program consists of two COS observations (4 orbits each), to be carried out with the G140L spectral element at the central wavelength 1105 A. As recommended, we will be using all four FP-POS offsets. Given the well-known position of the target, the acquisition will be done using the ACQ/IMAGE mode. To look for variability, the second observation should be carried out at least 5 days after the first observation has been executed. RESOLVED STELLAR POPULATIONS 12 false true 0.0 0.0 false false false SUPPORTED UNIDENTIFIED ACCRETION DISK OPTICAL EMITTER ULTRAVIOLET EMITTER X-RAY EMITTER J2000 J2000 m_F225W (VegaMag) = 20.05 +/- 0.1 ; F_F225W = 4.05 +/- 0.25 10^-17 erg/s/cm^2/A COS/FUV Spectroscopic No No G140L 1105 COS/FUV Spectroscopic No No G140L 1105 /home/grise/Work/Proposal/HST/Cycle21/N5408X1/OP-cache/n5408_faruv-v01.xml Timing link suitability includes constraints ([02 After 01 by 5.0 to 60.0 days]) The scheduling data was generated by CASM 21.2. CASM was run in Phase II mode. 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CASM was run in Phase II mode. 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