INVESTIGATING THE COSMIC-RAY IONIZATION RATE IN THE GALACTIC ISM WITH H$_3^+$ OBSERVATIONS

Abstract

Observations of H$_3^+$ in the Galactic diffuse interstellar medium (ISM) have led to various surprising results, including the conclusion that the cosmic-ray ionization rate ($\zeta_2$) is variable by over 1 order of magnitude between different diffuse cloud sight lines, with values as high as $8\times10^{-16}$~s$^{-1}$, and $3\sigma$ upper limits as low as $0.7\times10^{-16}$~s$^{-1}$. This variation is interesting, as it contradicts the typical assumption that the cosmic-ray spectrum is relatively uniform throughout the Galaxy. Instead, the flux of low-energy cosmic rays responsible for ionizing H$_2$ must be decreased in some regions due to particle propagations effects, and increased in other regions by local acceleration sites. Whether or not acceleration in and propagation from supernova remnants (thought to be the primary accelerators of Galactic cosmic rays) alone can account for such variability remains unknown. \hspace{0.25in} At present, the survey of H$_3^+$ in diffuse clouds consists of observations toward 52 sight lines, with detections in 20 of those. In an attempt to understand variations in the inferred ionization rates, I have studied the environments through which all of these sight lines pass. I have also observed H$_3^+$ in 6 sight lines that probe gas in close proximity to the supernova remnant IC 443. Ionization rates inferred in 2 of these sight lines are about $20\times10^{-16}$~s$^{-1}$, indicating a high flux of low-energy particles, but the other 4 sight lines do not show absorption from H$_3^+$, so the high ionization rate and particle flux seems very localized. Combining these results near a supernova remnant to those in the general ISM further enhances the variability seen in the cosmic-ray ionization rate, and requires that the concept of a uniform cosmic-ray spectrum be reviewed. %\hspace{0.25in} %The energy spectrum of cosmic-rays --- a product of particle acceleration and subsequent diffusion --- is generally assumed to be uniform throughout the Galaxy. As a result, the cosmic-ray ionization rate inferred in similar environments (e.g. in several diffuse clouds) should also be relatively constant. However, current estimates of the ionization rate in diffuse molecular clouds vary over the range $(1-8)\times10^{-16}$~s$^{-1}$. In addition, there are a few sight lines with $3\sigma$ upper limits of $\zeta_2<1\times10^{-16}$~s$^{-1}$, suggesting even lower ionization rates in some clouds. This roughly order of magnitude difference in the cosmic-ray ionization rate between sight lines contradicts the concept of a spatially uniform cosmic-ray flux. %\hspace{0.25in} %We present cosmic-ray ionization rates derived from several published and unpublished spectroscopic observations of H$_3^+$ in diffuse cloud sight lines. These ionization rates are then compared with various other parameters (Galactic latitude, Galactic longitude, hydrogen column density) in a search for correlations. Also, sight lines in close proximity are compared to each other to determine the variability of the ionization rate on small spatial scales.