Shocked Interstellar Medium Research Articles

Probing the extended emission-line region in 3C 171 with high-frequency radio polarimetry

I present high-frequency polarimetric radio observations of the radio galaxy 3C 171, in which depolarization is associated with an extended emission-line region. The radio hotspots, known to be depolarized at low radio frequencies, become significantly polarized above (observer's frame) frequencies of 15 GHz. There is some evidence that some of the Faraday rotation structure associated with the emission-line regions is being resolved at the highest resolutions (0.1 arcsec, or 400 pc); however, the majority remains unresolved. Using the new radio data and a simple model for the nature of the depolarizing screen, it is possible to place some constraints on the nature of the medium responsible for the depolarization. I argue that it is most likely that the depolarization is due not to the emission-line material itself but to a second, less dense, hot phase of the shocked interstellar medium, and derive some limits on its density and temperature.

ITAL]Chandra[/ITAL] Observation of Supernova Remnant G54.1+0.3: A Close Cousin of the Crab Nebula

We present a Chandra Advanced CCD Imaging Spectrometer observation of supernova remnant G54.1+0.3. This supernova remnant is resolved into several distinct X-ray-emitting components: a central bright pointlike source (CXOU J193030.13+185214.1), a surrounding ring, bipolar elongations, plus low surface brightness diffuse emission. The spectra of these components are all well described by power-law models; the spectral index steepens with increasing distance from the pointlike source. We find no evidence for any thermal plasma emission that would correspond to shocked interstellar medium or ejecta. The observed morphological and spectral characteristics suggest that G54.1+0.3 is the closest cousin of the Crab Nebula-a pulsar wind nebula driven by a combination of equatorial and polar outflows from the putative pulsar represented by the pointlike X-ray source.

Afterglow Emission from Naked Gamma-Ray Bursts

We calculate the afterglow emission for gamma-ray bursts (GRBs) going off in an extremely low density medium, referred to as naked bursts. Our results also apply to the case where the external medium density falls off sharply at some distance from the burst. The observed afterglow flux in this case originates at high latitudes, i.e., where the angle between the fluid velocity and the observer line of sight is greater than Γ-1. The observed peak frequency of the spectrum for naked bursts decreases with observer time as t-1, and the flux at the peak of the spectrum falls off as t-2. The 2-10 keV X-ray flux from a naked burst of average fluence should be observable by the Swift satellite for time duration of about 103 longer than the burst variability timescale. The high-latitude emission contributes to the early X-ray afterglow flux for any GRB, not just naked bursts, and can be separated from the shocked interstellar medium emission by their different spectral and temporal properties. Measurements of the high-latitude emission could be used to map the angular structure of GRB-producing shells.

Gamma-ray bursts: post-burst evolution of fireballs

The post-burst evolution of fireballs that produce gamma-ray bursts (GRBs) is studied, assuming the expansion of fireballs to be adiabatic and relativistic. Numerical results as well as an approximate analytic solution for the evolution are presented. Owing to the adoption of a new relation between t, R and gamma, our results differ markedly from previous studies. Synchrotron radiation from the shocked interstellar medium is carefully calculated, using a conventional set of equations, The observed X-ray flux of GRB afterglows can be reproduced easily. Although the optical afterglows seem much more complicated, our results can still present a rather satisfactory agreement with observations. We also find that the expansion will no longer be highly relativistic about 4 d after the main GRB. We thus suggest that the marginally relativistic phase of the expansion should be investigated so as to check the afterglows observed a week or more later.

Approximate Analytical Expressions of the Wind-driven Bow Shock

view Abstract Citations (15) References (17) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Approximate Analytical Expressions of the Wind-driven Bow Shock Chen, Yang ; Bandiera, Rino ; Wang, Zhen-Ru Abstract Previous analytical works on the wind-driven thin-shell bow shock neglected the centrifugal force exerted on the shell material. We reanalyze the dynamic equations including the centrifugal force, and we present approximate formulae in a Cartesian coordinate system for the general quantities describing the thin-shell bow shock. The shocked interstellar medium and the shocked stellar wind are treated as mixed and separated, respectively. These formulae are more accurate than previous analytical results. Publication: The Astrophysical Journal Pub Date: October 1996 DOI: 10.1086/177818 Bibcode: 1996ApJ...469..715C Keywords: ISM: KINEMATICS AND DYNAMICS; METHODS: ANALYTICAL; SHOCK WAVES; STARS: EARLY-TYPE; STARS: MASS LOSS full text sources ADS |

GS64-01-97: Expanding Supershell in the Milky Way

The expansion of shocked interstellar medium around an OB association is an example of the astrophysical blastwave (Ostriker and McKee, 1988). The thin, cold, shell-like structure expanding supersonically accumulates the ambient medium. We assume that the shell has the zero thickness at radius of the shockwave rs and that it expands at velocity vs. The shell is divided into elements described with equations of motion: d/dt(Mvs) = dS[(P–Po) + novo(vs–vo)]–Mg(R,z), where M and dS are the mass and the surface of it, P, Po are the inside and outside pressures, no,vo are density and velocity of the ambient medium and g(R,z) is the gravitational acceleration in the Galaxy. R, z are the galactocentric cylindrical coordinates. The mass of an element increases as long as the expansion velocity component normal to the shell, v⊥ exceeds the velocity of sound in the ambient medium M = v⊥nodS. The 3D model using the above infinitesimally thin shell approximation was developed by Palouš (1990, 1992) and Silich (1992).

A Far-Ultraviolet Spectrum of the Puppis A Supernova Remnant Using the Hopkins Ultraviolet Telescope

We report the first successful far-ultraviolet spectroscopic observation of a filament in the Puppis A supernova remnant. We observed a position on the eastern side of the remnant near the brightest X-ray region using the Hopkins Ultraviolet Telescope during the Astro-2 space shuttle mission in 1995 March. The spectrum covers the 820-1840 A spectral range with ~3 A resolution and shows numerous lines of C, N, O, Ne, He, and possibly Si. Comparison of this spectrum to solar abundance shock models indicates a shock velocity of approximately 160-180 km s-1. A slight overabundance of N is possible for this position, consistent with a shocked interstellar medium picture for the observed filament. This is at odds with the much larger overabundance of N indicated by optical data from other regions of the outer shell in Puppis A. Comparison with optical CCD images obtained at Las Campanas Observatory and with Einstein HRI data indicates that the region observed corresponds to a very recent encounter between the blast wave and an interstellar cloud. The interaction of the shock wave with this cloud is responsible for the bright X-ray emission in this region.

Interaction of planetary nebulae with the interstellar medium - Theory

view Abstract Citations (64) References (37) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Interaction of planetary nebulae with the interstellar medium: theory. Soker, Noam ; Borkowski, Kazimierz J. ; Sarazin, Craig L. Abstract We performed hydrodynamical simulations of the interaction between planetary nebulae (PNs) and the interstellar medium (ISM), in order to provide a theoretical framework for observations of interacting PNs. Our results validate a thin-shell approximation originally developed by Smith [MNRAS, 175, 419 (1976)] to calculate the expected displacement of central stars from PN centers, for a typical nebula in which the shocked ISM gas cools down to a characteristic nebular temperature of ∼104 K. This simple picture breaks down for high-velocity PNs in a low density environment, where the shocked ISM cools slowly. In this case, a Rayleigh-Taylor instability develops, leading to shell fragmentation on roughly the same timescale on which the shell is decelerated. We use our results to interpret observations of two interacting nebulae of particular interest, NGC 246 and the PN in the globular cluster M22. Publication: The Astronomical Journal Pub Date: October 1991 DOI: 10.1086/115963 Bibcode: 1991AJ....102.1381S Keywords: Globular Clusters; Interstellar Matter; Planetary Nebulae; Stellar Physics; Astronomical Models; Hydrodynamics; Astrophysics full text sources ADS | data products SIMBAD (5)

X-ray emission from supernova remnants MSH 14 - 63 and RCW 103

Electron-ion collisions, heavy element enrichment of emissions from reverse-shocked ejecta, abundances in the interstellar medium (ISM), the correspondence between the projected and measured age, ISM particle density, and distance, and classification factors for Type I or II supernovae are examined using a nonequilibrium ionization (NEI) model. The NEI model accounts for the absence of collisional ionization equilibrium and possibly thermal equilibrium between electrons and ions behind the shock fronts, and describes the emission spectra from ejecta and the shocked ISM. When applied to data from the SNR MSH 14 - 63 a non-Coulomb, ion-electron interaction is obtained that is consistent with a shocked ISM. For the SNR RCW 103, however, the processes behind the shock front are not discernable, although the elemental abundances will have solar values if the interactions are non-Coulomb. The age of RCW 103 must be approximated from physical data, as Chinese astronomers would not have been able to see the event from their position on earth two millenia ago.

A high-resolution X-ray image of Puppis A - Inhomogeneities in the interstellar medium

Eleven HRI exposures from the Einstein Observatory are assembled into an 0.1-4 keV image of the Puppis A supernova remnant which displays a complex morphology that may reflect the structure of the shocked interstellar medium. In addition to showing a density gradient of a factor greater than four across the approximately 30 pc diameter of the remnant perpendicular to the galactic plane, a shell of X-ray emission is seen surrounding the northern half of Puppis A, coincident with the radio shell, whose edge brightness profile indicates direct hot plasma heating by the blast wave rather than evaporation from clouds. The interior structure of the supernova remnant suggests inhomogeneities whose sizes range over 0.1-5 pc, but with moderate density contrast. Although isolated clouds of 10-30/cu cm density are responsible for the two brightest X-ray features, they represent only a small fraction of the Puppis A mass.