The heliospheric soft X-ray emission pattern during the ROSAT survey: Inferences on Local Bubble hot gas

Abstract

Solar wind interaction with neutral gas has been shown to generate soft X-rays, through the charge-exchange of highly ionized ions with neutral atoms and molecules in the interplanetary space and in the geocorona. The resulting diffuse emission can explain the non-cosmic long term enhancements (LTE) of ROSAT soft X-ray observations, which are produced by strong solar wind flux increases. This paper focuses on the emission pattern resulting from the impact on the interstellar gas of the average, “quiet” solar wind, i.e., the non variable signal “below” the LTEs. A sky map of the heliospheric emission is calculated, based on realistic hydrogen and helium atom distributions under the influence of solar conditions such as those prevailing during the ROSAT survey. It is shown that parallax effects linked to the ROSAT observing strategy have a strong influence on the resultant emission pattern. For a stationary solar wind the modeled emission has an anisotropy factor of $\simeq$1.8, and the emission maxima do not coincide with the interstellar wind axis direction. This emission is compared with the fraction of the ROSAT 0.25 keV emission assigned to hot ($T=10^{6}$ K) gas in the Local Interstellar Bubble and with the dense gas contour maps of the Bubble drawn from NaI absorption. Assuming constancy of the global pattern, but allowing for a variable scaling factor, the maximum heliospheric contribution is derived and this allows the placement of lower limits on the truly interstellar emission in many directions. After subtraction of the heliospheric emission, there is significant interstellar 0.25 keV emission from the Local Bubble “openings” to the lower halo, as expected, but also to neighbouring interstellar “bubbles”, as well as towards a number of regions along the galactic plane. There is evidence that there is hot gas emission attributable to our local cavity and generated throughout its volume, including along the galactic plane, but significantly weaker than initially thought. This would attenuate the discrepancy by a factor of about 4–5 between the hot gas pressure initially derived without heliospheric decontamination and the pressure of the embedded local diffuse clouds.