Search

An analysis of the energy spectra of cosmic rays and particularly the precise data from the AMS-02 experiment support the view about the important role of the Local Bubble in the nearby interstellar medium. It is suggested that the bulk of CR below about 200 GV of rigidity (momentum/charge ratio) comes from the modest number of supernova remnants in the Local Bubble which appear to have occurred some 10 6 years ago and contributed to its formation. At higher rigidities the contribution from a ‘Local Source’, a single supernova remnant generated some 10 5 years ago seems to dominate up to, at least 1000 GV. [ABSTRACT FROM AUTHOR]

Charge transfer, or charge exchange, describes a process in which an ion takes one or more electrons from another atom. Investigations of this fundamental process have accompanied atomic physics from its very beginning, and have been extended to astrophysical scenarios already many decades ago. Yet one important aspect of this process, i.e. its high efficiency in generating X-rays, was only revealed in 1996, when comets were discovered as a new class of X-ray sources. This finding has opened up an entirely new field of X-ray studies, with great impact due to the richness of the underlying atomic physics, as the X-rays are not generated by hot electrons, but by ions picking up electrons from cold gas. While comets still represent the best astrophysical laboratory for investigating the physics of charge transfer, various studies have already spotted a variety of other astrophysical locations, within and beyond our solar system, where X-rays may be generated by this process. They range from planetary atmospheres, the heliosphere, the interstellar medium and stars to galaxies and clusters of galaxies, where charge transfer may even be observationally linked to dark matter. This review attempts to put the various aspects of the study of charge transfer reactions into a broader historical context, with special emphasis on X-ray astrophysics, where the discovery of cometary X-ray emission may have stimulated a novel look at our universe. [ABSTRACT FROM AUTHOR]

Interstellar material (ISMa) is observed both inside and outside of the heliosphere. Relating these diverse sets of ISMa data provides a richer understanding of both the interstellar medium and the heliosphere. The galactic environment of the Sun is dominated by warm, low-density, partially ionized interstellar material consisting of atoms and dust grains. The properties of the heliosphere are dependent on the pressure, composition, radiation field, ionization, and magnetic field of ambient ISMa. The very low-density interior of the Local Bubble, combined with an expanding superbubble shell associated with star formation in the Scorpius-Centaurus Association, dominate the properties of the local interstellar medium (LISM). Once the heliosphere boundaries and interaction mechanisms are understood, interstellar gas, dust, pickup ions, and anomalous cosmic rays inside of the heliosphere can be directly compared to ISMa outside of the heliosphere. Our understanding of ISMa at the Sun is further enriched when the circumheliospheric interstellar material is compared to observations of other nearby ISMa and the overall context of our galactic environment. The IBEX mission will map the interaction region between the heliosphere and ISMa, and improve the accuracy of comparisons between ISMa inside and outside the heliosphere. [ABSTRACT FROM AUTHOR]

The hot Local Bubble surrounding the solar neighborhood has been primarily studied through observations of its soft X-ray emission. The measurements were obtained by attributing all of the observed local soft X-rays to the bubble. However, mounting evidence shows that the heliosphere also produces diffuse X-rays. The source is solar wind ions that have received an electron from another atom. The presence of this alternate explanation for locally produced diffuse X-rays calls into question the existence and character of the Local Bubble. This article addresses these questions. It reviews the literature on solar wind charge exchange (SWCX) X-ray production, finding that SWCX accounts for roughly half of the observed local 1/4 keV X-rays found at low latitudes. This article also makes predictions for the heliospheric O VI column density and intensity, finding them to be smaller than the observational error bars. Evidence for the continued belief that the Local Bubble contains hot gas includes the remaining local 1/4 keV intensity, the observed local O VI column density, and the need to fill the local region with some sort of plasma. If the true Local Bubble is half as bright as previously thought, then its electron density and thermal pressure are $1/\sqrt{2}$ as great as previously thought, and its energy requirements and emission measure are 1/2 as great as previously thought. These adjustments can be accommodated easily, and, in fact, bring the Local Bubble’s pressure more in line with that of the adjacent material. Suggestions for future work are made. [ABSTRACT FROM AUTHOR]

The major sources of the Soft X-ray Background (SXRB), besides distinct structures as supernovae and superbubbles (e.g. Loop I), are: (i) an absorbed extragalactic emission following a power law, (ii) an absorbed thermal component (∼2×106 K) from the galactic disk and halo, (iii) an unabsorbed thermal component, supposedly at 106 K, attributed to the Local Bubble and (iv) the very recently identified unabsorbed Solar Wind Charge-eXchange (SWCX) emission from the heliosphere and the geocorona. We study the SWCX heliospheric component and its contribution to observed data. In a first part, we apply a SWCX heliospheric simulation to model the oxygen lines (3/4 keV) local intensities during shadowing observations of the MBM 12 molecular cloud and a dense filament in the south galactic hemisphere with Chandra, XMM-Newton, and Suzaku telescopes. In a second part, we present a preliminary comparison of SWCX model results with ROSAT and Wisconsin surveys data in the 1/4 keV band. We conclude that, in the 3/4 keV band, the total local intensity is entirely heliospheric, while in the 1/4 keV band, the heliospheric component seems to contribute significantly to the local SXRB intensity and has potentially a strong influence on the interpretation of the ROSAT and Wisconsin surveys data in terms of Local Bubble hot gas temperature. [ABSTRACT FROM AUTHOR]

Recent 3D high-resolution simulations of the interstellar medium in a star forming galaxy like the Milky Way show that supernova explosions are the main driver of the structure and evolution of the gas. Its physical state is largely controlled by turbulence due to the high Reynolds numbers of the average flows. For a constant supernova rate a dynamical equilibrium is established within 200 Myr of simulation as a consequence of the setup of a galactic fountain. The resulting interstellar medium reveals a typical density/pressure pattern, i.e. distribution of so-called gas phases, on scales of 500–700 pc, with interstellar bubbles being a common phenomenon just like the Local Bubble and the Loop I superbubble, which are assumed to be interacting. However, modeling the Local Bubble is special, because it is driven by a moving group, passing through its volume, as it is inferred from the analysis of Hipparcos data. A detailed analysis reveals that between 14 and 19 supernovae have exploded during the last 15 Myr. The age of the Local Bubble is derived from comparison with H i and UV absorption line data to be 14.5± Myr. We further predict the merging of the two bubbles in about 3 Myr from now, when the interaction shell starts to fragment. The Local Cloud and its companion H i clouds are the consequence of a dynamical instability in the interaction shell between the Local and the Loop I bubble. [ABSTRACT FROM AUTHOR]

The ISSI Workshop “From the Heliosphere to the Local Bubble” has brought together space physicists who work on the in-situ study of the interaction of the heliosphere with the surrounding circum-heliospheric interstellar medium (CHISM) and astrophysicists interested in the interstellar medium itself and in the wider neighborhood known as the Local Bubble. This paper contains a summary of the workshop except for the big picture presentations given at the last day. It contains highlights as viewed by the author and is organized to lead the reader from the heliosphere, via the immediate solar neighborhood into the Local Bubble, with the attempt of a brief outlook into the future. [ABSTRACT FROM AUTHOR]

Thermal plasma emission in the soft X-ray band (0.1 - 2.0 keV) is believed to be responsible for the bulk of the X-ray intensity seen from the Local Bubble, a low-density cavity extending over ∼ 70 - 200 pc around the Sun. The state of the hot plasma is still a matter of discussion as previous instrumentation like aboard ROSAT was not able to unambiguously distinguish between equilibrium and non-equilibrium emission models and thus to pin-point the origin of the Local Bubble. Recent missions like DXS, XQC. and XMM-Newton have shed more light on this subject and observations indicate that collisional ionization equilibrium with solar abundances cannot explain the data: lines appear at positions and with intensities in contradiction to standard models. Analysis of EPIC-pn data of X-ray shadowing observations (MBM 12, Ophiuchus molecular cloud) suggest a component with higher temperature (kT ∼ 0.14 keV) besides the standard kT ∼ 0.09 keV plasma. [ABSTRACT FROM AUTHOR]