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10244275762
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-1. The optical axes of the HRI and WFC are co-aligned to within 10 to 15′, providing simultaneous observations in the EUV and x-ray. The backgrounds in both instruments are dominated by particles, and must be removed by modeling [S. L. Snowden, personal communication and R. G. West, Thesis, University of Leicester (1993)].
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Snowden, S.L.1
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Thesis, University of Leicester
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West, R.G.1
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2 [W. Zhang et al., in Proc. SPIE 2006, 324 (1993)]. The PCA detects x-rays in the 2-to 60-keV band with an FOV diameter of about 2°. Background confusion noise dominates the observations in the PCA.
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10244273625
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note
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Whatever mechanism is responsible for the emission of x-rays from comets, it must be highly variable among comets or over time, since the Einstein IPC 5-σ upper limit on the 0.2 to 4.0 keV emission from C/Bradfield (5) is a factor of 70 below our lowest measured flux for Hyakutake in March 1996. after correcting for distance effects. The non-detection of C/Bradfield cannot be explained by large secular changes in solar x-ray flux, since the sun was near the maximum of the solar cycle and its x-ray emission in 1980. and near the minimum in 1996.
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Corrections for variations in exposure over the comet images were made by modeling the comet's motion across the detector FOV. Background maps were created using the well-characterized spacecraft models of Snowden and West (3). Comparison of the model background to the average total background measured over many orbits gave good agreement. The uncertainties in the light curve are dominated by an ∼5% uncertainty in the absolute value of the instrumental backgrounds.
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The flux in the 2 to 10 keV band was estimated by averaging the count rate [16.75 counts per second (cps)] during the 16-s when the PCA optical axis was closest to the comet's position, subtracting the average rate during the rest of the maneuver (16.01 cps), and dividing by the mean collimator efficiency (0.663) during the observation. We have assumed that the emission from the comet had the same physical size as during the ROSAT observation.
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A 90% confidence upper limit of 4.5 cps was calculated by adding 1.645 times the standard deviation of the average count rate for other 16-s intervals (1.38 cps), and thus includes variations due to fluctuations of the x-ray sky. Similarly, the 90% confidence limit for the 10 to 15 keV band is 1.54 cps. 3-σ upper limits were then calculated from the 90% confidence limit values by assuming gaussian behavior and multiplying by a factor of (3.0/1.28).
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In this case the HRI/WFC count rate ratio would be >3000, while the observed ratio is <50.
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10244275258
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in preparation
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C. M. Lisse et al., in preparation.
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Lisse, C.M.1
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10244273465
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personal communication
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S. Ostro, personal communication.
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Ostro, S.1
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personal communication
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H. A. Weaver, personal communication.
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Weaver, H.A.1
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GOES-8 data obtained from the World Wide Web at http://web.ngdc.noaa.gov/stp/GOES/goes.html.
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personal communication
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H. U. Schmidt, personal communication.
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Schmidt, H.U.1
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0000236003
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H. Balsiger et al., Nature 321, 330 (1986).
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Balsiger, H.1
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The GOES-8 spacecraft is located in geosychronous orbit inside the Earth's magnetosheath, and monitors the solar x-ray flux, solar wind, and solar magnetic field. However, Hyakutake was outside the Earth's magnetosheath, where the solar wind and magnetic field are very different. Thus, GOES-8 data can only give us pertinent information about the solar x-ray flux at the comet. The WIND spacecraft, another monitor of the solar wind, was also inside the magnetosheath at the end of March 1996. The IMP-8 spacecraft farther away from the Earth and comet was outside the magnetosheath, and has shown that there were no large changes in the solar magnetic field during UT 26 to 28 March 1996.
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-4 gauss.
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in preparation
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Y. R. Fernandez et al., in preparation.
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Fernandez, Y.R.1
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K. Dennerl et al., IAU Circular 6404, 6413 (1996); K. Dennerl et al., in preparation.
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in preparation
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K. Dennerl et al., IAU Circular 6404, 6413 (1996); K. Dennerl et al., in preparation.
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Dennerl, K.1
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note
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The authors are indebted to M. D. Desch, Y. R. Fernandez, J. Harrington, T. A. Livengood, D. A. Mendis, T. Northrup, H. U. Schmidt, S. L. Snowden, and D. K. Yeomans for their help and many useful discussions. This work was supported in the U.S. by NASA's Planetary Astronomy Program Grant #NAGW188 and by a cooperative research grant between NASA Goddard Spaceflight Center and the University of Maryland. The ROSAT project is supported by the German Bundesministerium für Bildung, Wissenschaft, Forschung und Technologie (BMBF/DARA) and the Max-Planck-Gesellschaft. The UK contribution to the ROSAT Project is supported by the Particle Physics and Astronomy Research Council.
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