High-Speed Photometry of the UV Counterpart to 4U 1820-30

Details High-Speed Photometry of the UV Counterpart to 4U 1820-30 High-Speed Photometry of the UV Counterpart to 4U 1820-30

Jul 1996

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996hst..prop.6831f&link_type=abstract

HST Proposal ID #6831

Computer Science

Hst Proposal Id #6831 Binaries And Star Formation

Scientific paper

4U 1820-30, the 11 minute binary in the globular cluster NGC 6624, is one of the most extraordinary objects in the galaxy; it is also an exemplary low-mass x-ray binary, for it displays nearly all of the behaviors that have been observed in these objects. The x-ray flux of this binary is modulated at its 11- minute orbital period, and cycles between high and low luminosity states at an approximate period of 176 days. While in its high-state the x-ray flux shows high frequency variability similar to that seen in QPO's; in the low-state x- ray bursts recur on periods of hours and high-frequency noise is strong but flat. The binary is also a luminous UV source due to the reprocessing of the x-ray emission by the accretion disk. We propose to use the FOS in a novel mode to study the time variability of the UV radiation and through it, the geometry of the accretion disk. By replacing the FOS grating with the acquisition mirror and increasing the FOS sampling speed to 30 Hz we will gain both the quantum efficiency and rapid sampling necessary to measure the transmission function of x-ray to UV time variability. This function will in turn provide a direct estimate of the size and geometry of the disk. The reprocessing of x-rays on the face of the companion should also produce a detectable modulation of the UV light the measurement of which will determine the inclination of the orbit on the sky, the time of ascending node, and the orbital phase of the x-ray variations.

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