arXiv subjects

J. Martin Laming

Publications and source records attributed to J. Martin Laming.

At least 73 records · Page 4Linked to original sources

arXiv2009

Million Degree Plasmas in Extreme Ultraviolet (EUV) Astrophysics

Million degree plasmas are ubiquitous in the Universe, and examples include the atmospheres of white dwarfs; accretion phenomena in young stars, cataclysmic variables and active galactic nuclei; the coronae of stars; and the interstellar medium of our own galaxy and of others. The bulk of radiation from million degree plasmas is emitted at extreme ultraviolet (EUV) wavelengths, which includes critical spectral features containing diagnostic information often not available at other wavelengths. With underpinning by a mature instrument technology, there is great opportunity here for exciting discoveries.

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arXiv2009

Non-WKB Models of the FIP Effect: Implications for Solar Coronal Heating and the Coronal Helium and Neon Abundances

We revisit in more detail a model for element abundance fractionation in the solar chromosphere, that gives rise to the "FIP Effect" in the solar corona and wind. Elements with first ionization potential below about 10 eV, i.e. those that are predominantly ionized in the chromosphere, are enriched in the corona by a factor 3-4. We model the propagation of Alfven waves through the chromosphere using a non-WKB treatment, and evaluate the ponderomotive force associated with these waves. Under solar conditions, this is generally pointed upwards in the chromosphere, and enhances the abundance of chromospheric ions in the corona. Our new approach captures the essentials of the solar coronal abundance anomalies, including the depletion of He relative to H, and also the putative depletion of Ne, recently discussed in the literature. We also argue that the FIP effect provides the strongest evidence to date for energy fluxes of Alfven waves sufficient to heat the corona. However it appears that these waves must also be generated in the corona, in order to preserve the rather regular fractionation pattern without strong variations from loop to loop observed in the solar corona and slow speed solar wind.

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arXiv2008

The Heating of Thermal Electrons in Fast Collisionless Shocks: The Integral Role of Cosmic Rays

Understanding the heating of electrons to quasi-thermal energies at collisionless shocks has broad implications for plasma astrophysics. It directly impacts the interpretation of X-ray spectra from shocks, is important for understanding how energy is partitioned between the thermal and cosmic ray populations, and provides insight into the structure of the shock itself. In Ghavamian, Laming & Rakowski (2007) we presented observational evidence for an inverse square relation between the electron-to-proton temperature ratio and the shock speed at the outer blast waves of supernova remnants in partially neutral interstellar gas. There we outlined how lower hybrid waves generated in the cosmic ray precursor could produce such a relationship by heating the electrons to a common temperature independent of both shock speed and the strength of the ambient magnetic field. Here we explore the mechanism of lower hybrid wave heating of electrons in more detail. Specifically we examine the growth rate of the lower hybrid waves for both the kinetic (resonant) and reactive cases. We find that only the kinetic case exhibits a growing mode. At low Alfvén Mach numbers (~15) the growth of lower hybrid waves can be faster than the magnetic field amplification by modified Alfvén waves.

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arXiv2007

Ion Charge States in Halo CMEs: What can we Learn about the Explosion?

We describe a new modeling approach to develop a more quantitative understanding of the charge state distributions of the ions of various elements detected in situ during halo Coronal Mass Ejection (CME) events by the Advanced Composition Explorer (ACE) satellite. Using a model CME hydrodynamic evolution based on observations of CMEs propagating in the plane of the sky and on theoretical models, we integrate time dependent equations for the ionization balance of various elements to compare with ACE data. We find that plasma in the CME ``core'' typically requires further heating following filament eruption, with thermal energy input similar to the kinetic energy input. This extra heating is presumably the result of post eruptive reconnection. Plasma corresponding to the CME ``cavity'' is usually not further ionized, since whether heated or not, the low density gives freeze-in close the the Sun. The current analysis is limited by ambiguities in the underlying model CME evolution. Such methods are likely to reach their full potential when applied to data to be acquired by STEREO when at optimum separation. CME evolution observed with one spacecraft may be used to interpret CME charge states detected by the other.

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arXiv2007

Ion Charge States in the Fast Solar Wind: New Data Analysis and Theoretical Refinements

We present a further investigation into the increased ionization observed in element charge states in the fast solar wind compared to its coronal hole source regions. Once ions begin to be perpendicularly heated by ion cyclotron waves and execute large gyro-orbits, density gradients in the flow can excite lower hybrid waves that then damp by heating electrons in the parallel direction. We give further analysis of charge state data from polar coronal holes at solar minimum and maximum, and also from equatorial coronal holes. We also consider further the damping of lower hybrid waves by ions and the effect of non-Maxwellian electron distribution functions on the degree of increased ionization, both of which appear to be negligible for the solar wind case considered here. We also suggest that the density gradients required to heat electrons sufficiently to further ionize the solar wind can plausibly result from the turbulent cascade of MHD waves.

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arXiv2007

Analytic Approach to the Stability of Standing Accretion Shocks: Application to Core-Collapse Supernovae

We explore an analytic model of the accretion shock in the post bounce phase of a core-collapse supernova explosion. We find growing oscillations of the shock in the l=1 and l=2 modes, in agreement with a variety of existing numerical simulations. For modest values of the ratio of the outer accretion shock to that of the inner boundary to the shocked flow, the instability appears to derive from the growth of trapped sound waves, whereas at higher values, postshock advection clearly plays a role. Thus the model described here may relate to the different mechanisms of instability recently advocated by Blondin & Mezzacappa, and by Foglizzo and collaborators.

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arXiv2006

A Physical Relationship Between Electron-Proton Temperature Equilibration and Mach Number in Fast Collisionless Shocks

The analysis of Balmer-dominated optical spectra from non-radiative (adiabatic) SNRs has shown that the ratio of the electron to proton temperature temperature at the blast wave is close to unity at v_s <= 400 km/s, but declines sharply down to the minimum value of (m_e/m_p) dictated by the jump conditions at shock speeds exceeding 2000 km/s. We propose a physical model for the heating of electrons and ions in non-cosmic ray dominated, strong shocks (v_s > 400 km/s) wherein the electrons are heated by lower hybrid waves immediately ahead of the shock front. These waves arise naturally from the cosmic ray pressure gradient upstream from the shock. Our model predicts a nearly constant level of electron heating over a wide range of shock speeds, producing a relationship T_e/T_p ~ 1/v_s**2 (~ 1 /M**2) that is fully consistent with the observations.

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arXiv2006

The Polar Regions of Cassiopeia A: The Aftermath of a Gamma Ray Burst?

Probably not, but it is interesting nevertheless to investigate just how close Cas A might have come to generating such an event. Focusing on the northeast jet filaments, we analyze the polar regions of the recently acquired very deep 1 Ms Chandra X-ray observation. We infer that the so-called "jet" regions are indeed due to jets emanating from the explosion center, and not due to polar cavities in the circumstellar medium at the time of explosion. We place limits on the equivalent isotropic explosion energy in the polar regions (around 2.3 x 10^52 ergs), and the opening angle of the x-ray emitting ejecta (around 7 degrees), which give a total energy in the NE jet of order 10^50 ergs; an order of magnitude or more lower than inferred for "typical" GRBs. While the Cas A progenitor and explosion exhibit many of the features associated with GRB hosts, e.g. extensive presupernova mass loss and rotation, and jets associated with the explosion, we speculate that the recoil of the compact central object, with velocity 330 km/s, may have rendered the jet unstable. In such cases the jet rapidly becomes baryon loaded, if not truncated altogether. Although unlikely to have produced a gamma ray burst, the jets in Cas A suggest that such outflows may be common features of core-collapse SNe.

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arXiv2004

Outflows Near An Accreting Black Hole: Ionization and Temperature Structure

We calculate the temperature and ionization balance in an outflow from an accreting black hole under illumination by hard radiation from the central object. Electron scattering of the Fe K_alpha photons within the highly ionized expanding flow leads to a decrease of their energy (redshift) which is first order in v/c, when v is the outflow velocity and v is much less than the speed of light c. This photon redshift is an intrinsic property of any outflow for which divergence is positive. We also find that the equivalent widths of red-skewed K_alpha originated in the wind is of order of keV. We conclude that redshifted lines are intrinsic properties of the powerful outflows that are observed in many compact objects. Downscattering of the primary line photons generated in the outflow (a more natural and probable mechanism than the general relativistic effects in the innermost part of the accretion flow) leads to the formation of red-skewed lines.

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arXiv2004

A Million-Second Chandra View of Cassiopeia A

We introduce a million-second observation of the supernova remnant Cassiopeia A with the Chandra X-ray Observatory. The bipolar structure of the Si-rich ejecta (NE jet and SW counterpart) is clearly evident in the new images, and their chemical similarity is confirmed by their spectra. These are most likely due to jets of ejecta as opposed to cavities in the circumstellar medium, since we can reject simple models for the latter. The properties of these jets and the Fe-rich ejecta will provide clues to the explosion of Cas A.

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arXiv2004

A Unified Picture of the FIP and Inverse FIP Effects

We discuss models for coronal abundance anomalies observed in the coronae of the sun and other late-type stars following a scenario first introduced by Schwadron, Fisk & Zurbuchen of the interaction of waves at loop footpoints with the partially neutral gas. Instead of considering wave heating of ions in this location, we explore the effects on the upper chromospheric plasma of the wave ponderomotive forces. These can arise as upward propagating waves from the chromosphere transmit or reflect upon reaching the chromosphere-corona boundary, and are in large part determined by the properties of the coronal loop above. Our scenario has the advantage that for realistic wave energy densities, both positive and negative changes in the abundance of ionized species compared to neutrals can result, allowing both FIP and Inverse FIP effects to come out of the model. We discuss how variations in model parameters can account for essentially all of the abundance anomalies observed in solar spectra. Expected variations with stellar spectral type are also qualitatively consistent with observations of the FIP effect in stellar coronae.

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arXiv2003

On Collisionless Electron-Ion Temperature Equilibration in the Fast Solar Wind

We explore a mechanism, entirely new to the fast solar wind, of electron heating by lower hybrid waves to explain the shift to higher charge states observed in various elements in the fast wind at 1 A.U. relative to the original coronal hole plasma. This process is a variation on that previously discussed for two temperature accretion flows by Begelman & Chiueh. Lower hybrid waves are generated by gyrating minor ions (mainly alpha-particles) and become significant once strong ion cyclotron heating sets in beyond 1.5 R_sun. In this way the model avoids conflict with SUMER electron temperature diagnostic measurements between 1 and 1.5 R_sun. The principal requirement for such a process to work is the existence of density gradients in the fast solar wind, with scale length of similar order to the proton inertial length. Similar size structures have previously been inferred by other authors from radio scintillation observations and considerations of ion cyclotron wave generation by global resonant MHD waves.

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arXiv2003

On the Determination of Ejecta Structure and Explosion Asymmetry from the X-ray Knots of Cassiopeia A

We present a detailed analysis of Chandra X-ray spectra from individual ejecta knots in the supernova remnant Cassiopeia A. The spectra are fitted to give the electron temperature and (single) ionization age. These quantities are compared with the predictions of self similar hydrodynamic models incorporating time dependent ionization and radiation losses, and Coulomb electron-ion equilibration behind the reverse shock, for a variety of different ejecta density profiles described by a uniform density core and a power law envelope. We find that the ejecta close to the "jet" region in the NE, but not actually in the jet itself, have a systematically shallower outer envelope than ejecta elsewhere in the remnant, and we interpret this as being due to more energy of the initial explosion being directed in this polar direction as opposed to equatorially. The degree of asymmetry we infer is at the low end of that generally modelled in asymmetric core-collapse simulations, and may possibly be used to rule out highly asymmetric explosion models.

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arXiv2003

Where was the Iron Synthesized in Cassiopeia A?

We investigate the properties of Fe-rich knots on the east limb of the Cassiopeia A supernova remnant using observations with Chandra/ACIS and analysis methods developed in a companion paper. We use the fitted ionization age and electron temperature of the knots to constrain the ejecta density profile and the Lagrangian mass coordinates of the knots. Fe-rich knots which also have strong emission from Si, S, Ar, and Ca are clustered around mass coordinates q~0.35-0.4 in the shocked ejecta; for ejecta mass 2M_sun, this places the knots 0.7-0.8 M_sun out from the center (or 2-2.1 M_sun, allowing for a 1.3 M_sun compact object). We also find an Fe clump that is evidently devoid of line emission from lower mass elements, as would be expected if it were the product of alpha-rich freeze out; the mass coordinate of this clump is similar to those of the other Fe knots.

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arXiv2003

Slow Temperature Equilibration Behind the Shock Front of SN 1006

We report on the observation of OVII Doppler line broadening in a compact knot at the edge of SN 1006 detected with the Reflective Grating Spectrometer on board XMM-Newton. The observed line width of sigma = 3.4+/-0.5 eV at a line energy of 574 eV indicates an oxygen temperature of kT = 528+/-150 keV. Combined with the observed electron temperature of 1.5 keV the observed broadening is direct evidence for temperature non-equilibration in high Mach number shocks, and slow subsequent equilibration. The OVII line emission allows an accurate determination of the ionization state of the plasma, which is characterized by a relatively high forbidden line contribution, indicating log(n_e t) ~ 9.2.

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arXiv2002

On the magnetic fields and particle acceleration in Cas A

We investigate the non-thermal X-ray emission from Cas A. For the hard X-ray continuum we test the model proposed by Laming, which invokes 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, which suggest that most of the electron populations are too much affected by synchrotron losses to produce X-ray synchrotron emission. An exception is the X-ray emission from the rim surrounding Cas A, seen in Chandra continuum images. The width of this rim of 1.5" to 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 about 57 - 40 TeV. This is lower than the average magnetic field, but higher than what may be expected from shocked interstellar medium, suggesting a high pre-shock magnetic field, or rapid, post shock, field amplification. 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, whereas the inverse Compton emission comes 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 HEGRA with inverse Compton, 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 roughly 70 cm^{-3}. Pion decay is therefore likely to be the dominant emission from Cas A at TeV energies.

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arXiv2002

On the Dynamical Overstability of Radiative Blast Waves: The Atomic Physics of Shock Stability

Atomic physics calculations of radiative cooling are used to calculate criteria for the overstability of radiating shocks. Our calculations explain the measurement of shock overstability by Grun et al. and explain why the overstability was not observed in other experiments. The methodology described here can be especially useful in astrophysical situations where the relevant properties leading to an overstability can be measured spectroscopically, but the effective adiabatic index is harder to determine.

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arXiv2002

The detection of the Sc-44 nuclear de-excitation lines and hard X-ray emission from Cas A

We discuss the detection of Sc-44 (product of Ti-44) in the supernova remnant Cas A with the BeppoSAX-PDS instrument, and elaborate on the nature of the hard X-ray continuum, which is not only important for correctly estimating the Sc-44 flux, but is interesting in its own right. We apply the lower hybrid wave electron acceleration model to the hard X-ray data and use upper limits on the cosmic ray injection spectrum to infer that B > 2 mG. We conclude with the prospects of observing nuclear decay lines and electron-positron annihilation in Cas A with INTEGRAL.

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