This is version 2 of the Phase II proposal. The basic change from version 1 is that the 2 visits of 5 orbits
in version 1 have been broken down into 10 individual visits, grouped into two unbroken sequences of 5 visitis. ----- Submission 1 (Fri May 06 21:20:49 GMT 2005) -----
Phase II Proposal 10535 (10535) successfully submitted.
----- Submission 2 (Wed May 11 19:15:18 GMT 2005) -----
Assigned ID: 600
----- Submission 1 (Sat Jan 22 00:24:38 GMT 2005) -----
High Temperature Accretion Flows and Reprocessing in X-ray Binaries: The ADC Source 4U 1822-371The eclipsing low-mass X-ray binary 4U 1822-371 is the ideal system in which to observe hot accretion and the effects of strong irradiation. Its orbital period is 5.57~hr and its orbital inclination is high enough that flux from the neutron star is blocked by the edge-on accretion disk in the system. Because the neutron star is hidden its Accretion Disk Corona is visible. These ADCs are present in most if not all low-mass X-ray binaries but are generally not visible. 4U 1822-371 provides a unique opportunity to study an ADC and disk irradiation because the eclipse by the secondary star allows their geometry and flux to be mapped.
We will perform a multiwavelength light curve synthesis and eclipse analysis of 4U 1822-371. The scientific goal is to map the geometry and flux from its ADC and from its irradiated accretion disk. The crucial and irreplaceable role of HST is to provide the UV data needed to disentangle the ADC from the irradiated accretion disk. We will obtain time-resolved, objective-prism UV spectroscopy of 4U 1822-371 with the ACS/SBC with the PR130L prism, covering most of its orbit and all of the eclipse. This combination will yield excellent orbital light curves in the UV continuum and in the integrated line fluxes.We will use the ACS/SBC with the PR130L prism to obtain time-resolved spectroscopy of 4U 1822-371. The observations will consist of contiguous exposures, each with a 45-second integration, separated by a 40-second dead time. Every 16th exposure is 350 seconds long to allow the ACS buffer to dump. The UV eclipse should last about 1 hour, so this combination yields ~35 independent measurements through the eclipse. The UV spectrum of 4U 1822-371 has previously been measured with the FOS on HST (see Figure 4 in the Phase I proposal) and shows two strong emission lines, C IV at 1550 A and He II at 1640 A, superimposed on a blue continuum. Using fluxes measured from the FOS spectrum, the ACS Exposure Time Calculator yields 30 < S/N < 45 in the continuum at 1500 A in 45 seconds. This time resolution and S/N ratio are adequate to achieve our science goals.
The observations will take advantage of a coincidence: The orbital period of 4U 1822-371 is almost exactly 3.5 orbits of HST and, therefore, the fifth of 5 HST orbits precisely fills one of the gaps in the coverage of the star's light curve left by the first 4 HST orbits. The net effect is that one achieves the equivalent of ~146 minutes of continuous coverage of the 4U 1822-371 light curve plus ~50 minute samples of the rest of the orbit. Taking advantage of this coincidence imposes two scheduling constraint. First, the 60-minute eclipse of 4U 1822-371 must fall within the 146 minutes of continuous coverage. Second, each visit must consisting of 5 consecutive orbits, the maximum allowed between SAA passages. We also strongly prefer that the visibility period be greater than 56 minutes to avoid gaps in the overlapped coverage of the 4U 1822-371 light curve.
We must visit 4U 1822-371 twice because the light curve is known to show rapid random variability (``flickering''). Two measurements of the light curve are necessary to insure that models are not responding to non-repeating fluctuations in the light curves. There are no constraints on the relative times of the two visits.
Finally, roll angle is irrelevant; there are no bright stars near enough to contaminate the spectrum. Observing techniques often used for other kinds of ACS observations (eg, drizzling) are irrelevant or inappropriate for our observations.We have assumed that 4U 1822-371 is visible for 56 minutes/orbit when using the orbit planner. If the visibility is greater than 56 minutes for any of the actual orbits, we prefer to fill the orbit by adding more short (45-second) exposures, not by lengthening the existing exposures.
4U 1822-371 is bright. We have performed extensive modeling to test whether the observations exceed the bright object limit. The worst-case model corresponds to the brightest flux ever measured in the UV and to the 3-gyro mode. According to the ACS ETC, the brightest pixel for the PR130L prism will occur at the peak of the C IV emission line. The flux at the peak is 3.5 X 10^{-14} ergs/sec/cm^2/A, which yields a count rate of 9.5 cps/pixel. 4U 1822-371 shows some rapid variablity but its amplitude is always much less than 50% and would not raise the maximum count rate even to 14 cps/pixel. There is no evidence whatsoever for flares, outbursts, or sudden brightenings in either the continuum or the emission lines of 4U 1822-371. The are no nearby bright objects, so there is no danger of exceeding the global limit. Bottom line: The proposed observation is safely within the 50 cps/pixel bright object limit.HOT STARS12false0.0falsefalsefalsefalsefalseSUPPORTEDSTARINTERACTING BINARYACCRETION DISKLMXBContinuum flux at 1500 A is 3 X 10 ^{-14} ergs/cm/sec/AV691CRAJ200018 25 46.8000-37 06 19.00J2000ACS/SBCSpectroscopicNoNoPR130L
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