With the advent of high-energy-density (HED) experimental facilities, such as high-energy lasers and fast Z-pinch, pulsed-power facilities, millimeter-scale quantities of matter can be placed in extreme states of density, temperature, and/or velocity. This has enabled the emergence of a new class of experimental science, HED laboratory astrophysics, wherein the properties of matter and the processes that occur under extreme astrophysical conditions can be examined in the laboratory. Areas particularly suitable to this class of experimental astrophysics include the study of opacities relevant to stellar interiors, equations of state relevant to planetary interiors, strong shock-driven nonlinear hydrodynamics and radiative dynamics relevant to supernova explosions and subsequent evolution, protostellar jets and high Mach number flows, radiatively driven molecular clouds and nonlinear photoevaporation front dynamics, and photoionized plasmas relevant to accretion disks around compact objects such as black holes and neutron stars.

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Experimental astrophysics with high power lasers and Z pinches

Technological advances in radio telescopes and X-ray instruments over the last 20 years have greatly increased the number of known supernova remnants (SNRs) and have led to a better determination of their properties. In particular, more SNRs now have reasonably determined distances. However, many of these distances were determined kinematically using old rotation curves (based on R☉ = 10 kpc and V☉ = 250 km s-1). A more modern rotation curve (based on R☉ = 8.5 kpc and V☉ = 220 km s-1) is used to verify or recalculate the distances to these remnants. We use a sample of 36 shell SNRs (37 including Cassiopeia A) with known distances to derive a new radio surface brightness-to-diameter (Σ-D) relation. The slopes derived here (β = -2.64 including Cas A, β = -2.38 without Cas A) are significantly flatter than those derived in previous studies. An independent test of the accuracy of the Σ-D relation was performed by using the extragalactic SNRs in the Large and Small Magellanic Clouds. The limitations of the Σ-D relation and the assumptions necessary for its use are discussed. A revised Galactic distribution of SNRs is presented based on the revised distances as well as those calculated from this Σ-D relation. A scaling method is employed to compensate for observational selection effects by computing scale factors based on individual telescope survey sensitivities, angular resolutions, and sky coverage. The radial distribution of the surface density of shell SNRs, corrected for selection effects, is presented and compared with previous works.

https://iopscience.iop.org/article/10.1086/306089/pdf

A New Σ-D Relation and Its Application to the Galactic Supernova Remnant Distribution

We investigate the nonthermal X-ray emission from Cas A, using BeppoSAX, Compton Gamma Ray Observatory OSSE, and Chandra data. For the hard X-ray continuum we test the model proposed by Laming, which invokes nonthermal bremsstrahlung from electrons accelerated by lower hybrid plasma waves. The justification for this model comes from our determination of a lower limit to the average magnetic field of B > 0.5 mG. For such high magnetic fields, the synchrotron losses are severe enough that most of the electron populations responsible for the radio emission have maximum electron energies well below the limit for which X-ray synchrotron emission is important. However, we do suggest that the rim surrounding Cas A, seen in Chandra continuum images, is X-ray synchrotron emission. The width of this rim of 15-4'' can be used to infer the magnetic field near the shock front, for which we estimate B = 0.08-0.16 mG and electron energies of ~57-40 TeV. This magnetic field strength is lower than the average magnetic field but higher than what may be expected from shocked interstellar medium, suggesting either a high magnetic field in the wind of the progenitor or rapid postshock field amplification by nonlinear growth of plasma waves. Combining the two magnetic field measurements, we have constructed a simple two-zone model. Most of the radio emission comes from inside Cas A, where the magnetic field is strong. In contrast, the inverse Compton emission is dominated by emission from near the shock front. Only for our lower limit on the magnetic field strength near the shock front is it possible to explain the recent detection of TeV emission by the High Energy Gamma Ray Astronomy experiment with inverse Compton emission, for which, in addition, we have to assume a rather high far-infrared photon density that should be twice as high as our best estimate of ~70 cm-3. Pion decay is therefore likely to be the dominant emission from Cas A at TeV energies.

https://iopscience.iop.org/article/10.1086/345832/pdf

On the Magnetic Fields and Particle Acceleration in Cassiopeia A

The millisecond-duration radio flashes known as fast radio bursts (FRBs) represent an enigmatic astrophysical phenomenon. Recently, the sub-arcsecond localization (∼100 mas precision) of FRB 121102 using the Very Large Array has led to its unambiguous association with persistent radio and optical counterparts, and to the identification of its host galaxy. However, an even more precise localization is needed in order to probe the direct physical relationship between the millisecond bursts themselves and the associated persistent emission. Here, wereport very-long-baseline radio interferometric observations using the European VLBI Network and the 305 m Arecibo telescope, which simultaneously detect both the bursts and the persistent radio emission at milliarcsecond angular scales and show that they are co-located to within a projected linear separation of ≲40 pc (≲12 mas angular separation, at 95% confidence). We detect consistent angular broadening of the bursts and persistent radio source (∼2–4 mas at 1.7 GHz), which are both similar to the expected Milky Way scattering contribution. The persistent radio source has a projected size constrained to be ≲ 0.7 pc (≲0.2 mas angular extent at 5.0 GHz) and a lower limit for the brightness temperature of Tb ≳ 5× 107 K. Together, these observations provide strong evidence for a direct physical link between FRB 121102 and the compact persistent radio source. We argue that a burst source associated with a low-luminosity active galactic nucleus or a young neutron star energizing a supernova remnant are the two scenarios for FRB 121102 that best match the observed data.

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The Repeating Fast Radio Burst FRB 121102 as Seen on Milliarcsecond Angular Scales

Supernova remnants are beautiful astronomical objects that are also of high scientific interest, because they provide insights into supernova explosion mechanisms, and because they are the likely sources of Galactic cosmic rays. X-ray observations are an important means to study these objects. And in particular the advances made in X-ray imaging spectroscopy over the last two decades has greatly increased our knowledge about supernova remnants. It has made it possible to map the products of fresh nucleosynthesis, and resulted in the identification of regions near shock fronts that emit X-ray synchrotron radiation. Since X-ray synchrotron radiation requires 10–100 TeV electrons, which lose their energies rapidly, the study of X-ray synchrotron radiation has revealed those regions where active and rapid particle acceleration is taking place.

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Supernova remnants: the X-ray perspective

The Three-dimensional Structure of the Cassiopeia A Supernova Remnant. I. The Spherical Shell

The Three Dimensional Structure Of Casseopeia A

At a distance of 3 kpc and age of about 330 years, Cas A is one of the premier laboratories of the kinematic and chemical structure of supernova remnants. The material in its fast-moving ejecta is unusually rich in oxygen and the silicon-group elements and contains little or no hydrogen (PEIMBERT and VAN DEN BERGH [1], CHEVALIER and KIRSHNER [2]). Although photographic studies of Cas A have been continuing for over 30 years, in this work we present the first complete spectroscopic map of its optical structure.

A. C. Fabian

Papers

The Three-dimensional Structure of the Cassiopeia A Supernova Remnant. I. The Spherical Shell

The Three Dimensional Structure Of Casseopeia A

The Optical Structure of Cassiopeia A