----- Attempting Submission 1 (Wed Jul 02 16:55:10 GMT 2008) -----
Phase II Proposal 11627 (v407vul_17_PhaseII_dith.apt) successfully submitted.
----- Attempting Submission 2 (Wed Oct 08 15:01:14 GMT 2008) -----
----- Attempting Submission 3 (Mon Jun 29 15:51:59 GMT 2009) -----Assigned ID: 404
----- Attempting Submission 1 (Fri Mar 07 17:21:32 GMT 2008) -----
Phase I Proposal 404 (v407vul_17.apt) successfully submitted.Resolving the puzzling nature of the ultra-compact binary V407 VulRecently, three variable stars have been identified as likely accreting binary systems with ultra-short orbital periods of 10 minutes or less. This makes them the closest binaries known as well as strong sources of gravitational wave emission. These objects have thus been identified as crucial calibration sources for the LISA mission. V407 Vul diplays a 9.5 minute period at optical and X-ray wavelengths. Several different binary geometries have been proposed to explain this variable star with all but one scenario identifying the 9.5 minute period with the binary orbital period. Our time-resolved spectroscopy and photometry from the ground revealed that the optical light is dominated by a stationary G9 star which would be inconsistent with the mass donor star of a compact binary. We propose ACS/HRC imaging in order to confirm the tentative 0.03" mis-alignment we have picked up between this G9 star and the underlying variable. Confirmation of a spatial separation between the variable and this field star would conclusively rule out that the G star is part of a low-inclination binary and thus verify it as a LISA source. In addition, we propose UV imaging that will allow us to determine the NUV luminosity of the variable star and thereby reliably infer the distance and the nature (and thus the gravitational wave signal) of the ultra-compact binary, as either mass transfer or magnetic interaction powers the X-ray source. Only HST imaging can provide rigorous proof for an ultra-compact binary interpretation for V407 Vul and thus validate it as LISA verification sourceWe plan to employ the high resolution channel of ACS and obtain high-resolution images of the
V407 Vul field in three filters. The plate-scale of 0.027”/pixel permits us to accurately measure
the positions of our target as well as several reference stars within the HRC field of view (Fig. 3).
Our goal is to compare the relative positions of the optical counterparts in the different filters with
accuracies well below the pixel scale.
Given the 53 minute visibility window and the 6.1 minute guide star acquisition overhead,
46.9 minutes are available for ACS exposures in the first orbit. With two planned exposures in
the first orbit, which each will be split into 3 to allow dithering, the ACS overheads amount to 9
minutes, leaving 37.9 minutes of effective exposure time. The second and third orbits will consist
of sets of three exposures each (42.8mins of net exposure time).
Our ’red’ image will employ the F775W (i) filter. Using the ACS imaging ETC and R=19.2
for V407 Vul, we obtain a S/N of ~160 in 180s. This exposure will be completely dominated by
light from the G9-star and will serve as the reference point for its position.
Our ’blue’ exposure will employ the F330W (U) filter. We chose the U filter to make sure
that the G9-star contribution is very small compared to the underlying blue source which based
on our ground-based observations is estimated to contribute more than 45% of the light in U. For
comparison, the variable only contributes 15% in the g-band, and thus we need to go bluer to
measure its position accurately. The U-band exposures will span the remaining time left in the
first orbit (38.9 min) as well as the second orbit of our visit, delivering a combined S/N of 60-100
for a 105K black-body that contributes 45% to the U flux. The range in S/N reflects the expected
reddening range as derived from our Gemini observations (0.6 < E(B−V ) < 1.4). It is important
to get a good S/N in this U-band exposure so as to be able to derive an accurate position for the
blue variable and confirm the suggested separation of 0.03” relative to the G9 star. With the S/N
of our exposures, we will be able to centroid our sources to better than 0.1 HRC pixels and can
measure this separation directly.
Finally, we will use the third orbit of our visit to obtain a near-UV exposure using COS.
In one orbit we will achieve a S/N of 30-80 which depends on the actual reddening towards the
variable and we have allowed for the full range of E(B-V). This third filter is mainly to constrain
the reddening and temperature of the underlying variable.
With this strategy, we can achieve our two key goals in three HST orbits. The three images
will allow us to measure the spatial separation between the blue variable and the G9 star as well as
determine the reddening towards the blue variable, which is important for the implied luminosity
of the system. These two measurements will finally settle the question of whether V407 Vul is a 9
minute ultra-compact binary accreting at high rates due to gravitational wave losses.HOT STARS12false0.00.0falsefalsefalsefalsefalseSUPPORTEDSTARINTERACTING BINARYu'=21.5 g'=20.3RXJ1914.3+2456J200019 14 26.1000+24 56 44.00J2000ACS/HRCImagingNoNoF775WACS/HRCImagingNoNoF330WCOS/NUVImagingNoNoMIRRORAPOS-TARGDITHER0.145LINEfalsefalse346.84WFC3-UVIS-DITHER-LINE2falseThe scheduling data was generated by CASM 17.2.CASM was run in Phase II mode./tmp/OPVisit19031.xml(Tic-Visit
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