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 0.1 to 1 keV for a ∼1 sr region of the sky centered at l = 90• , b = +60• using a 36 pixel array of microcalorimeters flown on a sounding rocket. With an energy resolution of 11 eV FWHM below 1 keV, the spectrum's observed line ratios help separate charge exchange contributions originating within the heliosphere from thermal emission of hot gas in the interstellar medium. The X-ray sensitivity below 1 keV was reduced by about a factor of four from contamination that occurred early in the flight, limiting the significance of the results. The observed centroid of helium-like O vii is 568 +2 −3 eV at 90% confidence. Since the centroid expected for thermal emission is 568.4 eV and for charge exchange is 564.2 eV, thermal emission appears to dominate for this line complex. The dominance of thermal emission is consistent with much of the high-latitude O vii emission originating in 2-3 × 10 6 K gas in the Galactic halo. On the other hand, the observed ratio of C vi Lyγ to Lyα is 0.3 ± 0.2. The expected ratios are 0.04 for thermal emission and 0.24 for charge exchange, indicating that charge exchange must contribute strongly to this line and therefore potentially to the rest of the ROSAT R12 band usually associated with 10 6 K emission from the Local Hot Bubble. The limited statistics of this experiment and systematic uncertainties due to the contamination require only >32% thermal emission for O vii and >20% from charge exchange for C vi at the 90% confidence level. An experimental gold coating on the silicon substrate of the array greatly reduced extraneous signals induced on nearby pixels from cosmic rays passing through the substrate, reducing the triggered event rate by a factor of 15 from a previous flight of the instrument.

The Fractional Ionization of the Warm Neutral Interstellar Medium

“…interstellar atoms (Cravens 2000;Lallement 2004;Pepino et al 2004;Koutroumpa et al 2006Koutroumpa et al , 2007Koutroumpa et al , 2009Peek et al 2011;Crowder et al 2012). For this reason, we ignore the weakly absorbed, nearly isotropic portion of the X-ray background and focus our attention to the component that exhibits a pattern in the sky that clearly shows absorption by gas in the Galaxy.…”

SEARCHING FOR keV STERILE NEUTRINO DARK MATTER WITH X-RAY MICROCALORIMETER SOUNDING ROCKETS

“…The XQC payload is a mature flight system with six flights between 1995 and 2014 (Crowder et al 2012). The XQC spectrometer is an array of 36 microcalorimeters using ion-implanted semiconductor thermistors each coupled to a 2 mm × 2 mm × 0.96 μm HgTe absorber on a 14 μm thick bg.…”

The Structure of the Local Hot Bubble

“…In order to improve our understanding of the local diffuse X-ray emission and the structure of the LHB, it is essential to remove the contamination of the SWCX. However, despite many efforts, an accurate estimation of the SWCX is quite difficult, especially in the 1/4 keV band, due to the poorly known cross sections for producing the many X-ray lines from SWCX, limited data on heavy ion fluxes in the Solar Wind, and the general spectral similarity of SWCX and thermal emission (Cravens 2000;Lallement 2004;Koutroumpa et al 2006;Snowden et al 2009;Henley & Shelton 2008;Crowder et al 2012;Yoshino et al 2009). Efforts to estimate the SWCX contribution to historical measurements for the diffuse X-ray background, such as in the ROSAT All-Sky Survey (RASS) are even more problematic due to the limited solar wind data (see Kuntz et al 2015, for further discussion).…”