OBSERVED LIMITS ON CHARGE EXCHANGE CONTRIBUTIONS TO THE DIFFUSE X-RAY BACKGROUND

Abstract

We present a high resolution spectrum of the diffuse X-ray background from\n0.1 to 1 keV for a ~1 region of the sky centered at l=90, b=+60 using a\n36-pixel array of microcalorimeters flown on a sounding rocket. With an energy\nresolution of 11 eV FWHM below 1 keV, the spectrum's observed line ratios help\nseparate charge exchange contributions originating within the heliosphere from\nthermal emission of hot gas in the interstellar medium. The X-ray sensitivity\nbelow 1 keV was reduced by about a factor of four from contamination that\noccurred early in the flight, limiting the significance of the results. The\nobserved centroid of helium-like O VII is 568+2-3 eV at 90% confidence. Since\nthe centroid expected for thermal emission is 568.4 eV while for charge\nexchange is 564.2 eV, thermal emission appears to dominate for this line\ncomplex, consistent with much of the high-latitude O VII emission originating\nin 2-3 x 10^6 K gas in the Galactic halo. On the other hand, the observed ratio\nof C VI Ly gamma to Ly alpha is 0.3+-0.2. The expected ratios are 0.04 for\nthermal emission and 0.24 for charge exchange, indicating that charge exchange\nmust contribute strongly to this line and therefore potentially to the rest of\nthe ROSAT R12 band usually associated with 10^6 K emission from the Local Hot\nBubble. The limited statistics of this experiment and systematic uncertainties\ndue to the contamination require only >32% thermal emission for O VII and >20%\nfrom charge exchange for C VI at the 90% confidence level. An experimental gold\ncoating on the silicon substrate of the array greatly reduced extraneous\nsignals induced on nearby pixels from cosmic rays passing through the\nsubstrate, reducing the triggered event rate by a factor of 15 from a previous\nflight of the instrument.\n