arXiv subjects

J. Martin Laming

Publications and source records attributed to J. Martin Laming.

77 records · Page 5Linked to original sources

arXiv2001

Detection of the 67.9 keV and 78.4 keV lines associated with the radioactive decay of Ti-44 in Cassiopeia A

We report the detection of the Sc-44 nuclear decay lines at 67.9 keV and 78.4 keV associated with the nuclear decay of Ti-44 in Cassiopeia A. The line emission was observed by the PDS instrument on board BeppoSAX, which recently observed the supernova remnant for over 500 ks. The detection of the line emission with a flux of (2.1 +/-0.7) 10^{-5} ph/cm^2/s in each line (90% confidence) is at the 5 sigma significance level, if we can assume that the 12-300 keV continuum is adequately represented by a single power law. However, as the nature of the continuum is not clear we investigate various other possibilities. A more conservative estimate of the line flux is made by assuming that a power law continuum is at least a good approximation to the continuum emission for a narrower 30-100 keV energy range. With this limitation the measured line flux is (1.9 +/- 0.9) 10^{-5} ph/cm^2/s, with the detection still at the 3.4 sigma significance level. We suggest that together with the CGRO-COMPTEL measurement of the Ca-44 line at 1157 keV of (3.5 +/- 0.9) 10^{-5} ph/cm^2/s a flux for all three lines of (2.5 +/- 1.0) 10^{-5} ph/cm^2/s for Cas A can be adopted. This implies an initial Ti-44 mass of (0.8 - 2.5) 10^{-4} Msun.

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arXiv2001

The Electron Temperature and 44Ti Decay Rate in Cassiopeia A

The effects of plasma elemental composition and ionization state on the effective decay rate of 44Ti are investigated. We essentially follow the methods of the first authors to treat this topic, Mochizuki et al., but use more realistic plasma models, including radiative cooling, to compute the evolution of the charge state distribution behind the reverse shock. For uniform density ejecta (i.e. no clumps or bubbles) we find a negligible change to the decay rate of 44Ti. We discuss the effects of non-uniform ejecta. We also briefly consider the effects on these calculations of collisionless electron heating associated with weak secondary shocks propagating throughout the Cas A shell as a result of foward or reverse shock encounters with density inhomogeneities, recently suggested as an explanation for the hard X-ray tail seen in BeppoSAX and RXTE/OSSE spectra.

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arXiv2001

Accelerated Electrons in Cassiopeia A: Thermal and Electromagnetic Effects

We consider in more detail a model previously proposed for the hard X-ray (>10 keV) emission observed from the supernova remnant Cas A, whereby electrons are accelerated by lower-hybrid waves and radiate bremsstrahlung. We consider both cold and thermal plasma limits of the modified two-stream instability that generates the lower-hybrid waves, and by studying time dependent ionization balance for various components of the Cas A ejecta and shocked circumstellar medium, find locations within the shell where one or other of the instabilities may occur. Either instability can be effective, with the cold plasma instability imposing fewer constraints on the shocked reflected ion population responsible for exciting the waves. The instability must be located in the ejecta shocked at the earliest times and therefore closest to the contact discontinuity where magnetic fields are expected to be the strongest. The energy deposited in this ejecta by collisions between accelerated and ambient electrons is broadly consistent with that required to reheat this ejecta to observed temperatures.

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arXiv2001

Uncertainties in Dielectronic Recombination Rate Coefficients: Effects on Solar and Stellar Upper Atmosphere Abundance Determinations

We have investigated how the relative elemental abundances inferred from the solar upper atmosphere are affected by uncertainties in the dielectronic recombination (DR) rate coefficients used to analyze the spectra. We find that the inferred relative abundances can be up to a factor of ~5 smaller or ~1.6 times larger than those inferred using the currently recommended DR rate coefficients. We have also found a plausible set of variations to the DR rate coefficients which improve the inferred (and expected) isothermal nature of solar coronal observations at heights of >~ 50 arcsec off the solar limb. Our results can be used to help prioritize the enormous amount of DR data needed for modeling solar and stellar upper atmospheres. Based on the work here, our list of needed rate coefficients for DR onto specific isoelectronic sequences reads, in decreasing order of importance, as follows: O-like, C-like, Be-like, N-like, B-like, F-like, Li-like, He-like, and Ne-like. It is our hope that this work will help to motivate and prioritize future experimental and theoretical studies of DR.

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arXiv2000

Accelerated Electrons in Cassiopeia A: An Explanation for the Hard X-ray Tail

We propose a model for the hard X-ray (> 10 keV) emission observed from the supernova remnant Cas A. Lower hybrid waves are generated in strong (mG) magnetic fields, generally believed to reside in this remnant, by shocks reflected from density inhomogeneities. These then accelerate electrons to energies of several tens of keV. Around 4% of the x-ray emitting plasma electrons need to be in this accelerated distribution, which extends up to electron velocities of order the electron Alfven speed, and is directled along magnetic field lines. Bremsstrahlung from these electrons produces the observed hard x-ray emission. Such waves and accelerated electrons have been observed in situ at Comet Halley, and we discuss the viability of the extrapolation from this case to the parameters relevant to Cas A.

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