The origin of the local 1/4-keV X-ray flux in both charge exchange and a hot bubble

Abstract

The Solar neighborhood is the closest and most easily studied sample of the Galactic interstellar medium, an understanding of which is essential for models of star formation and galaxy evolution. Observations of an unexpectedly intense diffuse flux of easilyabsorbed ¼-keV X rays 1,2 , coupled with the discovery that interstellar space within ~100 pc of the Sun is almost completely devoid of cool absorbing gas 3 led to a picture of a "local cavity" filled with X-ray emitting hot gas dubbed the local hot bubble 4-6 . This model was recently upset by suggestions that the emission could instead be produced readily within the solar system by heavy solar wind ions charge exchanging with neutral H and He in interplanetary space 7-11 , potentially removing the major piece of evidence for the existence of million-degree gas within the Galactic disk 12-15 . Here we report results showing that the total solar wind charge exchange contribution is 40%±5% (stat) ±5% (sys) of the ¼ keV flux in the Galactic plane. The fact that the measured flux is not dominated by charge exchange supports the notion of a million-degree hot bubble of order 100 pc extent surrounding the Sun.When the highly ionized solar wind interacts with neutral gas, an electron may hop from a neutral to an outer orbital of an ion, in what is known as charge exchange. The electron then cascades to the ground state of the ion, often emitting soft X rays in the process 16 . The calculations of X-ray intensity from solar wind charge exchange depend on limited information on heavy ion fluxes and even more uncertain atomic cross sections. The DXL sounding rocket mission 17 was launched from White Sands Missile Range in New Mexico on December 12, 2012 to make an empirical measurement of the charge exchange flux by observing a region of higher interplanetary neutral density (with a correspondingly higher charge exchange rate) called the "helium focusing cone". Neutral interstellar gas flows at ~25 km s −1 through the solar system due to the motion of the Sun through a small interstellar cloud. This material, mostly hydrogen atoms with about 15% helium, flows from the Galactic direction (l,b)~(3 o ,16 o ), placing the Earth downstream of the Sun in early December 18 . The trajectories of the neutral interstellar helium atoms are governed primarily by gravity, executing hyperbolic Keplerian orbits and forming a relatively high-density focusing cone downstream of the Sun about 6 o below the ecliptic plane (Fig. 1) 19 . Interstellar hydrogen, on the other hand, also experiences significant impact from radiation pressure and ionization, creating a neutral hydrogen cavity around the Sun, with negligible focusing.

Wide field‐of‐view soft X‐ray imaging for solar wind‐magnetosphere interactions

“…Local galaxy (DXL) sounding rocket which launched from White Sands Missile Range in December 2012 [Galeazzi et al, 2014]. A schematic diagram of this instrument used for soft X-ray imaging is given in Figure 3.…”

“…To obtain values of the charge exchange target densities (variable Q from above), an established model of the exosphere [Hodges, 1994] is used with the global BATSRUS MHD model [Powell et al, 1999;Tóth et al, 2005Tóth et al, , 2012. In order to obtain observable counts from the model, one must specify an instrumental response.…”

Star formation in a diffuse high-altitude cloud?

“…Implying that HRK 81.4-77.8 is most likely located in between MBM 12 and both IVCs within a distance bracket of 300 pc to 500 pc. While the lower distance limit is affected by systematic uncertainties in evaluating I local (Galeazzi et al 2014), the depth of the strong soft X-ray shadow sets a robust upper distance limit, which is of greater importance here for our discussion. Adopting this distance bracket we have to consider HRK 81.4-77.8 as a low-extinction, diffuse interstellar cloud located within the Milky Way's disk-halo interface.…”

“…Because of systematic uncertainties in determining the absolute quantity of I local (Galeazzi et al 2014;Puspitarini et al 2014) we explore the error-range of this distance estimate. For this aim we inspect the local count rate towards -the Draco cloud (Snowden et al 2000 shadow #116, I local = (4.8±0.2)×10 −4 cts s −1 arcmin −2 at a distance between 463 pc and 618 pc Gladders et al 1998) and -IVC 135+54 (Snowden et al 2000 shadow #182, I local = (5.8 ± 0.2) × 10 −4 cts s −1 arcmin −2 at (395 ± 95) pc Benjamin et al 1996).…”

Probing the Local Bubble with Diffuse Interstellar Bands

“…While the putative million-degree gas was not seen in the expected extreme-UV emission by the CHIPS satellite (Hurwitz et al 2005) nor in O vi 1032 Å absorption (Cox 2005;Barstow et al 2010), recent works making use of soft X-ray shadows, Planck mm-wavelength data, and Local Bubble maps have now considerably reinforced the idea that there is such hot gas within the cavity (Snowden et al 2015). The picture is complicated by the interaction between the heliosphere and surrounding ISM, such as charge exchange between the solar wind ions and interstellar atoms (e.g., Lallement et al 2014b;Zirnstein et al 2015; but see Galeazzi et al 2014), indicating local "fluff" of a more neutral kind (Frisch 1998). Any novel way or tracer that can be used to map the Local Bubble is therefore highly desirable.…”