▪ Abstract Turbulence affects the structure and motions of nearly all temperature and density regimes in the interstellar gas. This two-part review summarizes the observations, theory, and simulations of interstellar turbulence and their implications for many fields of astrophysics. The first part begins with diagnostics for turbulence that have been applied to the cool interstellar medium and highlights their main results. The energy sources for interstellar turbulence are then summarized along with numerical estimates for their power input. Supernovae and superbubbles dominate the total power, but many other sources spanning a large range of scales, from swing-amplified gravitational instabilities to cosmic ray streaming, all contribute in some way. Turbulence theory is considered in detail, including the basic fluid equations, solenoidal and compressible modes, global inviscid quadratic invariants, scaling arguments for the power spectrum, phenomenological models for the scaling of higher-order structu...

http://www.ifa.hawaii.edu/%7Ereipurth/reviews/elmegreen1_araa.pdf

Interstellar Turbulence I: Observations and Processes

This article reviews the current knowledge and understanding of the interstellar medium of our galaxy. The author first presents each of the three basic constituents---ordinary matter, cosmic rays, and magnetic fields---of the interstellar medium, with emphasis on their physical and chemical properties as inferred from a broad range of observations. The interaction of these interstellar constituents, both with each other and with stars, is then discussed in the framework of the general galactic ecosystem.

The interstellar environment of our galaxy

We discuss the methods used to compile a high signal-to-noise dataset representative of both the instrumental and cosmic background signal measured at high galactic latitude by the XMM-Newton EPIC cameras. The characteristics of the EPIC background are described and the potential applications of the de- rived dataset in general science analysis are outlined. In the case of the cosmic X-ray background, the tran- sition between a hard power-law spectrum (due to the integrated emission of unresolved, largely extragalac- tic, point sources) and a softer thermal spectrum (produced by hot plasma associated with the Galactic plane and halo) is unambiguously detected around 1 keV. We derive a value for the intensity of the power-law component of 2:15 0:26 10 11 erg cm 2 s 1 deg 2 in the 2{10 keV band (normalisation at 1 keV of 11.1 photons cm 2 s 1 sr 1 keV 1 ). The implication is that recent, very deep Chandra observations have re- solved70{90% of the 2{10 keV background into discrete sources. Our measurement is towards the higher end of the range of quoted background normalisations.

/pdf/x-ray-background-measurements-with-xmm-newton-epic-4rrv7uta0q.pdf

X-ray background measurements with XMM-Newton EPIC

In this paper we present a new look at the distribution of emission responsible for the soft X-ray diffuse background (SXRB) using ROSAT all-sky survey data. This is prompted by the demonstration of the existence of extensive 1/4 keV emission in the Galactic halo that was not considered in the most successful previous model, in which the bulk of the observed 1/4 keV X-rays originated in a Local Hot Bubble (LHB) that surrounds the Sun. The basic distribution can be represented by two angularly varying Galactic components (the LHB and an X-ray halo) and an isotropic extragalactic component. In addition, there are the distinct enhancements of supernova remnants, superbubbles, and clusters of galaxies. Using the negative correlation between the SXRB and DIRBE-corrected IRAS 100 μm intensity, we find a LHB that is similar to previous models, although it is possibly more limited in extent. The emission of the LHB varies by a factor of ~3.3 with direction and produces ROSAT 1/4 keV intensities between ~250 and ~820 × 10-6 counts s-1 arcmin-2 (typically brighter at higher Galactic latitudes), with inferred emission measures of ~0.0018 to 0.0058 cm-6 pc (assuming thermal ionization equilibrium). The distribution of the emission temperature of the LHB is peaked at 106.07 K with a range of ±12% at FWHM. While this variation is small and is perhaps due to systematic uncertainties in the analysis, there is a suggestion of a cooler region in the anticenter direction consistent with previous studies. We derive halo plus extragalactic intensities that vary from ~400 to 3000 × 10-6 counts s-1 arcmin-2 outside the absorbing gas of the Galactic disk. As the low end of the range is comparable to what is expected for the extragalactic background, this implies that the halo emission varies considerably over the sky, with inferred emission measures ranging from near zero to >0.02 cm-6 pc. The distribution of emission temperatures in the halo, again derived from thermal equilibrium emission models, peaks near 106.02 K and varies over ±20% at FWHM.

https://iopscience.iop.org/article/10.1086/305135/pdf

Progress on Establishing the Spatial Distribution of Material Responsible for the 1/4 keV Soft X-Ray Diffuse Background Local and Halo Components

The soft X-ray background in the 0.1-1.0 keV band is known to be produced by at least three sources; the Local Hot Bubble (LHB), the extragalactic power law (EPL), and a seemingly galactic component that lies outside the bulk of the absorption that is due to the ISM of the galactic disk. This last component, which we call the Trans-Absorption Emission (TAE), has been modeled by a number of groups who have derived disparate measures of its temperature. The differences have arisen from differing assumptions about the structure of the emitting gas and unrecognized methodological difficulties. In particular, spectral fitting methods do not uniquely separate the TAE from the foreground emission that is due the LHB. This "degeneracy" can be resolved using the angular variation of the absorption of the TAE. We show that the TAE cannot be characterized by a single thermal component; no single-component model can be consistent with both the spectral energy distribution of the TAE emission and the angular variation due to absorption by the galactic disk. We use the angular anticorrelation of the ROSAT All-Sky Survey with the galactic absorption to separate local from distant emission components, and to fit the spectral energy distribution of the resulting distant emission. We find that the emission is best described by a two-thermal-component model with logT(sub S) = 6.06(sup +0.14, sub -0.12) and log T(sub H) = 6.42(sup +0.14, sub -0.12). This two-thermal-component TAE fits the ROSAT spectral energy distribution significantly better than single-component models, and is consistent with both angular variation and spectral constraints.

/pdf/deconstructing-the-spectrum-of-the-soft-x-ray-background-218xurx8fp.pdf

Deconstructing the Spectrum of the Soft X-Ray Background

Two rocket observations of very soft X-ray diffuse intensity show three emission regions of large angular extent. A large portion of the sky as seen with wide angular resolution is mapped. These three regions coincide with broad regions of small column densities of neutral hydrogen. It is suggested that a component of the interstellar medium is very hot and is responsible for the soft X-ray emission and the O VI.

Observation of features in the soft X-ray background flux

Soft cosmic X ray background flux from rocket observations on 21 September 1968 from White Sands, discussing flux origins

Soft X-ray background flux.

Extragalactic background soft X ray diffuse flux consistent with absorption by Small Magellanic Cloud

A Search for Absorption of the Soft X-Ray Diffuse Flux by the Small Magellanic Cloud.

Diffuse low energy cosmic X rays rocket measurements, noting excess over extrapolated power law valid for higher energies

Low-energy diffuse X-rays

Soft X rays in the energy regions below 284 eV and between 500 and 1000 eV have been detected on the vicinity of the North Polar Spur. The region of enhanced X-ray emission reported was observed from an Aerobee 150 rocket launched on May 29, 1970, from Woomera, Australia.

A. N. Bunner

Papers

Observation of features in the soft X-ray background flux

Soft X-ray background flux.

A Search for Absorption of the Soft X-Ray Diffuse Flux by the Small Magellanic Cloud.

Low-energy diffuse X-rays

Soft X-rays from the vicinity of the North Polar Spur.