Atomic and Ionized Microstructures in the Diffuse Interstellar Medium

It has been known for half a century that the interstellar medium (ISM) of our Galaxy is structured on scales as small as a few hundred kilometers, more than 10 orders of magnitude smaller than typical ISM structures and energy input scales. In this review we focus on neutral and ionized structures on spatial scales of a few to ∼104AU, which appear to be highly overpressured, as these have the most important role in the dynamics and energy balance of interstellar gas: the tiny scale atomic structures (TSASs) and extreme scattering events (ESEs) as the most overpressured example of the tiny scale ionized structures (TSISs). We review observational results and highlight key physical processes at AU scales. We present evidence for and against microstructures as part of a universal turbulent cascade and as discrete structures, and we review their association with supernova remnants, the Local Bubble, and bright stars. We suggest a number of observational and theoretical programs that could clarify the nature of AU structures. TSAS and TSIS probe spatial scales in the range of what is expected for turbulent dissipation scales and are therefore of key importance for constraining exotic and not-well-understood physical processes that have implications for many areas of astrophysics. The emerging picture is one in which a magnetized, turbulent cascade, driven hard by a local energy source and acting jointly with phenomena such as thermal instability, is the source of these microstructures.

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The interstellar medium in young supernova remnants: key to the production of cosmic X-rays and $\gamma $-rays

Astrophysics and Space Science ◽

10.1007/s10509-021-03960-4 ◽

2021 ◽

Vol 366 (6) ◽

Author(s):

Hidetoshi Sano ◽

Yasuo Fukui

Keyword(s):

Interstellar Medium ◽

Supernova Remnants ◽

Cosmic Ray ◽

High Energy ◽

X Rays ◽

Interstellar Gas ◽

High Energy Radiation ◽

Dense Cores ◽

The Relationship ◽

Turbulent Velocity Field

AbstractWe review recent progress in elucidating the relationship between high-energy radiation and the interstellar medium (ISM) in young supernova remnants (SNRs) with ages of ∼2000 yr, focusing in particular on RX J1713.7−3946 and RCW 86. Both SNRs emit strong nonthermal X-rays and TeV $\gamma $ γ -rays, and they contain clumpy distributions of interstellar gas that includes both atomic and molecular hydrogen. We find that shock–cloud interactions provide a viable explanation for the spatial correlation between the X-rays and ISM. In these interactions, the supernova shocks hit the typically pc-scale dense cores, generating a highly turbulent velocity field that amplifies the magnetic field up to 0.1–1 mG. This amplification leads to enhanced nonthermal synchrotron emission around the clumps, whereas the cosmic-ray electrons do not penetrate the clumps. Accordingly, the nonthermal X-rays exhibit a spatial distribution similar to that of the ISM on the pc scale, while they are anticorrelated at sub-pc scales. These results predict that hadronic $\gamma $ γ -rays can be emitted from the dense cores, resulting in a spatial correspondence between the $\gamma $ γ -rays and the ISM. The current pc-scale resolution of $\gamma $ γ -ray observations is too low to resolve this correspondence. Future $\gamma $ γ -ray observations with the Cherenkov Telescope Array will be able to resolve the sub-pc-scale $\gamma $ γ -ray distribution and provide clues to the origin of these cosmic $\gamma $ γ -rays.

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Constraining the coherence scale of the interstellar magnetic field using TeV gamma-ray observations of supernova remnants

Monthly Notices of the Royal Astronomical Society ◽

10.1093/mnras/staa1678 ◽

2020 ◽

Vol 496 (2) ◽

pp. 2448-2461 ◽

Cited By ~ 2

Author(s):

Matteo Pais ◽

Christoph Pfrommer ◽

Kristian Ehlert ◽

Maria Werhahn ◽

Georg Winner

Keyword(s):

Magnetic Field ◽

Interstellar Medium ◽

Supernova Remnants ◽

Gamma Ray ◽

Galactic Cosmic Rays ◽

Shock Acceleration ◽

Interstellar Gas ◽

Narrow Angle ◽

Magnetohydrodynamic Simulations ◽

Tev Gamma Rays

ABSTRACT Galactic cosmic rays (CRs) are believed to be accelerated at supernova remnant (SNR) shocks. In the hadronic scenario, the TeV gamma-ray emission from SNRs originates from decaying pions that are produced in collisions of the interstellar gas and CRs. Using CR-magnetohydrodynamic simulations, we show that magnetic obliquity-dependent shock acceleration is able to reproduce the observed TeV gamma-ray morphology of SNRs such as Vela Jr and SN1006 solely by varying the magnetic morphology. This implies that gamma-ray bright regions result from quasi-parallel shocks (i.e. when the shock propagates at a narrow angle to the upstream magnetic field), which are known to efficiently accelerate CR protons, and that gamma-ray dark regions point to quasi-perpendicular shock configurations. Comparison of the simulated gamma-ray morphology to observations allows us to constrain the magnetic coherence scale λB around Vela Jr and SN1006 to $\lambda _B \simeq 13_{-4.3}^{+13}$ pc and $\lambda _B \gt 200_{-40}^{+50}$ pc, respectively, where the ambient magnetic field of SN1006 is consistent with being largely homogeneous. We find consistent pure hadronic and mixed hadronic-leptonic models that both reproduce the multifrequency spectra from the radio to TeV gamma-rays and match the observed gamma-ray morphology. Finally, to capture the propagation of an SNR shock in a clumpy interstellar medium, we study the interaction of a shock with a dense cloud with numerical simulations and analytics. We construct an analytical gamma-ray model for a core collapse SNR propagating through a structured interstellar medium, and show that the gamma-ray luminosity is only biased by 30 per cent for realistic parameters.

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What Physical Processes Drive the Interstellar Medium in the Local Bubble?

Space Science Reviews ◽

10.1007/s11214-008-9425-1 ◽

2008 ◽

Vol 143 (1-4) ◽

pp. 263-276 ◽

Cited By ~ 14

Author(s):

D. Breitschwerdt ◽

M. A. de Avillez ◽

B. Fuchs ◽

C. Dettbarn

Keyword(s):

Interstellar Medium ◽

Physical Processes ◽

Local Bubble

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Modelling the Local Interstellar Medium As A Supernova Remnant In A Multiphase Gas

International Astronomical Union Colloquium ◽

10.1017/s0252921100098407 ◽

1984 ◽

Vol 81 ◽

pp. 287-296

Author(s):

Lennox L. Cowie

Keyword(s):

Interstellar Medium ◽

Supernova Remnant ◽

Thermal Evaporation ◽

Local Interstellar Medium ◽

Physical Processes ◽

Interstellar Gas ◽

Reasonable Time ◽

Ergodic Hypothesis ◽

Global Properties ◽

The Galaxy

Trying to understand the local interstellar gas in detail may be a hopeless task for a theorist. In the interstellar medium as a whole, we can at least address global properties and perhaps come to some reasonable “time averaged” conclusions such as those of Cox and his collaborators (e.g. Cox and Smith 1974, Cox 1979) or McKee and Ostriker (1977). Even this is quite uncertain of course, both because the ISM gas has structure on scales from at least 1 Pc (and probably much smaller) all the way up to the size of the galaxy, and because none of us are quite sure which physical processes (such as thermal evaporation or heating of cooler gas by magnetohydrodynamic processes) are really important. However, in the local ISM things are significantly worse in that we no longer have even the ergodic hypothesis available to us – rather we have to try and deal with individual events and structures. On the other hand, we do have more detailed observations and hence a laboratory to try to decide on the importance of the various physical processes.

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Modeling the Local Bubble Using Multiple Supernova Remnants

International Astronomical Union Colloquium ◽

10.1017/s0252921100070858 ◽

1997 ◽

Vol 166 ◽

pp. 133-136

Author(s):

R.K. Smith ◽

D.P. Cox

Keyword(s):

Interstellar Medium ◽

Supernova Remnants ◽

Local Interstellar Medium ◽

Local Bubble ◽

One Dimensional ◽

X Ray ◽

Hot Gas ◽

Sky Survey ◽

Non Equilibrium

AbstractWe have modeled the Local Bubble (LB) using a one-dimensional hydrocode (ODIN) that can simulate multiple supernova remnants, with non-equilibrium ion evolution and dust. Our model assumes that the local interstellar medium was a cool (104 K) gas approximately 5-10 Myr ago; it was then disturbed by 2 or 3 supernovae exploding within 20-30 pc of each other over a period of 2-4 million years. The LB is the leftover hot gas from these explosions. The model predicts the x-ray emission from such a bubble, as well as ionic abundances for hot gas ions such as O VI. These are compared to the soft x-ray data from the Wisconsin all-sky survey and the ROSAT PSPC.

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The Interstellar Gas in the Line of Sight to ϵ Canis Majoris

International Astronomical Union Colloquium ◽

10.1017/s0252921100036137 ◽

1996 ◽

Vol 152 ◽

pp. 299-299

Author(s):

C. Gry ◽

L. Lemonon ◽

A. Vidal-Madjar ◽

M. Lemoine ◽

R. Ferlet

Keyword(s):

Interstellar Medium ◽

Hubble Space Telescope ◽

Line Of Sight ◽

Interstellar Gas ◽

Local Bubble ◽

Gas Density ◽

Neutral Gas ◽

Turbulence Velocity ◽

Main Components ◽

Local Cloud

We analyse Hubble Space Telescope GHRS observations of the interstellar medium in the direction to ϵ CMa, the strongest EUV source in the sky located 200 pc away in a region deficient in neutral gas. We show that the neutral gas density is the lowest yet measured in a galactic sight-line. The line of sight contains three main components among which the Local Cloud, and we derive their column densities, their velocity their temperature and their turbulence velocity. We discuss the ionization of the Local Cloud and we show that we detect the conductive interface between diffuse local cloud and the hot local bubble.

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