HaloSat Observations of Heliospheric Solar Wind Charge Exchange

Abstract

X-ray emission from solar wind charge exchange (SWCX) produced in interplanetary space contaminates every astrophysical observation, regardless of the line of sight. Unfortunately, the primary SWCX emission lines also happen to be important diagnostics of astrophysical plasmas. Models of SWCX emission are limited by two main uncertainties: the local solar wind fluxes along the line of sight and the charge exchange cross sections. The He cone, a localized density enhancement of helium neutrals, is the only heliospheric SWCX emission feature that is small enough and bright enough to be observationally isolated from the X-ray background and the broader SWCX emission. HaloSat, an X-ray CubeSat mission, has recently completed two series of specialized observations, near and far from the ecliptic plane, during two Earth transits of the He cone. These observations were used to test the predictions of an SWCX emission model against the emission observed at low ecliptic latitude, where the solar wind data are monitored, and at high ecliptic latitude, where the solar wind data are extrapolated. The measured SWCX emission for the set of observations near the ecliptic plane was consistent with the line intensities predicted by the model but underpredicted for the set of observations at high ecliptic latitude near the south ecliptic pole. Additionally, high-temperature Galactic halo emission components are reported for both spectral sets.

Widespread Detection of Two Components in the Hot Circumgalactic Medium of the Milky Way

“…This is in large part due to the splitting of the single CGM APEC into two, which consistently reduces the temperature of the warm component. The HaloSat based results from Ringuette et al (2021) are somewhat inconsistent, although the fields in that paper are closer to the Galactic plane than the fields studied in this paper (b = −16 • , −24 • ). They reported CGM temperatures of kT = 0.26 +0.03 −0.02 , 0.262 +0.016 −0.014 keV and kT = 1.01 +0.13 −0.11 , 1.03 +0.23 −0.19 keV.…”

“…However, that work notes that the M star contribution drops off steeply, diminishing exponentially over just a few degrees of latitude. Previ-ous investigation of stellar contributions to the HaloSat spectra (Ringuette et al 2021) has not supported a significant contribution from stars. The fields studied in this paper are even higher latitude than the fields from Ringuette et al (2021).…”

“…Previ-ous investigation of stellar contributions to the HaloSat spectra (Ringuette et al 2021) has not supported a significant contribution from stars. The fields studied in this paper are even higher latitude than the fields from Ringuette et al (2021). Due to the selection of only high latitude fields for our CGM analysis, it is unlikely that M stars are a significant contributor to our hot component.…”

“…The warm component is inconsistent with the range of temperatures seen for the warm component in this paper, while the hotter component is not consistent with the stacked spectra, but is within the range of temperatures seen in individual fields. It is important to note that Ringuette et al (2021) does not include the changes made to the instrument response outlined in this paper's appendices, and models some of the foreground and background components differently due to very different scientific objectives.…”

Widespread Detection of Two Components in the Hot Circumgalactic Medium of the Milky Way

“…For each HaloSat exposure, we estimated the relevant ACE data that apply to the local ion flux along the line of sight by calculating the propagation time from ACE's position using real-time solar wind speed data. Further detail on this process is described in Kaaret et al (2020) and Ringuette et al (2021). This was included in the model fit as estimates of O VII and O VIII emission, as a pair of Gaussians (XSPEC model Gauss), at 0.5634 keV and 0.6531 keV, respectively.…”