Parviz Ghavamian

Balmer-dominated Spectra of Nonradiative Shocks in the Cygnus Loop, RCW 86, and Tycho Supernova Remnants

We present an observational and theoretical study of the optical emission from nonradiative shocks in three supernova remnants: the Cygnus Loop, RCW 86, and Tycho. The spectra of these shocks are dominated by collisionally excited hydrogen Balmer lines, which have both a broad component caused by proton-neutral charge exchange and a narrow component caused by excitation of cold neutrals entering the shock. In each remnant, we have obtained the broad-to-narrow flux ratios of the H? and H? lines and measured the H? broad component width. A new numerical shock code computes the broad and narrow Balmer line emission from nonradiative shocks in partially neutral gas. The Balmer line fluxes are sensitive to Lyman line trapping and the degree of electron-proton temperature equilibration. The code calculates the density, temperature, and size of the postshock ionization layer and uses a Monte Carlo simulation to compute narrow Balmer line enhancement from Lyman line trapping. The initial fraction of the shock energy allocated to the electrons and protons (the equilibration) is a free parameter. Our models show that variations in electron-proton temperature equilibration and Lyman line trapping can reproduce the observed range of broad-to-narrow ratios. The results give 80%-100% equilibration in nonradiative portions of the northeast Cygnus Loop (vS ~ 300 km s-1), 40%-50% equilibration in nonradiative portions of RCW 86 (vS ~ 600 km s-1), and 20% equilibration in Tycho (vS ~ 2000 km s-1). Our results suggest an inverse correlation between magnetosonic Mach number and equilibration in the observed remnants.

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 (northeast jet and southwest 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.

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

The analysis of Balmer-dominated optical spectra from nonradiative (adiabatic) SNRs has shown that the ratio of the electron to proton temperature at the blast wave is close to unity at vS 400 km s-1 but declines sharply down to the minimum value of me/mp dictated by the jump conditions at shock speeds exceeding 2000 km s-1. We propose a physical model for the heating of electrons and ions in non-cosmic-ray-dominated, strong shocks (vS > 400 km s-1) 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 (Te/Tp)0 v (M-2) that is fully consistent with the observations.

Measuring the Cosmic-Ray Acceleration Efficiency of a Supernova Remnant

Cosmic Shock Waves Cosmic rays are high-energy charged particles that bombard Earth from all directions in the sky; those originating from within our Galaxy are thought to be accelerated in the shockwaves produced by supernova explosions. Helder et al. (p. 719, published online 25 June; see the Perspective by Raymond) measured the velocity of a section of the blast wave created by supernova RCW 86, an exploding star believed to have been witnessed by Chinese astronomers in 185 A.D., and the post-shock proton temperature. The post-shock proton temperature was much lower than would be expected without any cosmic ray acceleration, which implies that the pressure induced by cosmic ray exceeds the thermal pressure behind the shock. The pressure induced by cosmic rays produced by the explosion of a star exceeds the thermal pressure behind the shock wave. Cosmic rays are the most energetic particles arriving at Earth. Although most of them are thought to be accelerated by supernova remnants, the details of the acceleration process and its efficiency are not well determined. Here we show that the pressure induced by cosmic rays exceeds the thermal pressure behind the northeast shock of the supernova remnant RCW 86, where the x-ray emission is dominated by synchrotron radiation from ultrarelativistic electrons. We determined the cosmic-ray content from the thermal Doppler broadening measured with optical spectroscopy, combined with a proper-motion study in x-rays. The measured postshock proton temperature, in combination with the shock velocity, does not agree with standard shock heating, implying that >50% of the postshock pressure is produced by cosmic rays.

The optical spectrum of the sn 1006 supernova remnant revisited

We present the deepest optical spectrum acquired to date of Balmer-dominated shocks in the northwest rim of SN 1006. We detect the broad and narrow components of Hα, Hβ, and Hγ and report the first detection of the He I λ6678 emission line in this supernova remnant. We may have detected, at the 1.5 σ level, faint He II λ4686 emission. We measure a full width at half-maximum of 2290 ± 80 km s-1 in the broad-component Hα line, with broad-to-narrow flux ratios of 0.84 and 0.93 in Hα and Hβ, respectively. To match these observations, our nonradiative shock models require a low degree of electron-proton equilibration at the shock front, Te/Tp ≤ 0.07, and a shock speed of 2890 ± 100 km s-1 . These results agree well with an earlier analysis of ultraviolet lines from SN 1006. The He I/Hα and He I/He II flux ratios also indicate low equilibration. Furthermore, our models match the observations for mostly ionized (~90%) preshock H and mostly neutral (70%) preshock He, respectively. We conclude that the high H ionization fraction cannot be explained by either photoionization from the reverse shock or relic ionization from EUV photons released in the A.D. 1006 supernova. The most plausible explanation appears to be photoionization from the Galactic Lyman continuum.

EVIDENCE FOR PARTICLE ACCELERATION TO THE KNEE OF THE COSMIC RAY SPECTRUM IN TYCHO'S SUPERNOVA REMNANT

Supernova remnants (SNRs) have long been assumed to be the source of cosmic rays (CRs) up to the knee of the CR spectrum at 1015 eV, accelerating particles to relativistic energies in their blast waves by the process of diffusive shock acceleration (DSA). Since CR nuclei do not radiate efficiently, their presence must be inferred indirectly. Previous theoretical calculations and X-ray observations show that CR acceleration significantly modifies the structure of the SNR and greatly amplifies the interstellar magnetic field. We present new, deep X-ray observations of the remnant of Tychos supernova (SN 1572, henceforth Tycho), which reveal a previously unknown, strikingly ordered pattern of non-thermal high-emissivity stripes in the projected interior of the remnant, with spacing that corresponds to the gyroradii of 1014-1015 eV protons. Spectroscopy of the stripes shows the plasma to be highly turbulent on the (smaller) scale of the Larmor radii of TeV energy electrons. Models of the shock amplification of magnetic fields produce structure on the scale of the gyroradius of the highest energy CRs present, but they do not predict the highly ordered pattern we observe. We interpret the stripes as evidence for acceleration of particles to near the knee of the CR spectrum in regions of enhanced magnetic turbulence, while the observed highly ordered pattern of these features provides a new challenge to models of DSA.

RCW 86: A Type Ia Supernova in a Wind-Blown Bubble

We report results from a multi-wavelength analysis of the Galactic supernova remnant RCW?86, the proposed remnant of the supernova of 185 A.D. We show new infrared observations from the Spitzer Space Telescope and the Wide-Field Infrared Survey Explorer, where the entire shell is detected at 24 and 22 ?m. We fit the infrared flux ratios with models of collisionally heated ambient dust, finding post-shock gas densities in the non-radiative shocks of 2.4 and 2.0?cm?3 in the southwest (SW) and northwest (NW) portions of the remnant, respectively. The Balmer-dominated shocks around the periphery of the shell, large amount of iron in the X-ray-emitting ejecta, and lack of a compact remnant support a Type Ia origin for this remnant. From hydrodynamic simulations, the observed characteristics of RCW?86 are successfully reproduced by an off-center explosion in a low-density cavity carved by the progenitor system. This would make RCW?86 the first known case of a Type Ia supernova in a wind-blown bubble. The fast shocks (>3000?km?s?1) observed in the northeast are propagating in the low-density bubble, where the shock is just beginning to encounter the shell, while the slower shocks elsewhere have already encountered the bubble wall. The diffuse nature of the synchrotron emission in the SW and NW is due to electrons that were accelerated early in the lifetime of the remnant, when the shock was still in the bubble. Electrons in a bubble could produce gamma rays by inverse-Compton scattering. The wind-blown bubble scenario requires a single-degenerate progenitor, which should leave behind a companion star.

High Resolution Spectroscopy of Balmer-Dominated Shocks in the RCW 86, Kepler and SN 1006 Supernova Remnants ⋆

We report results from high resolution optical spectroscopy of three non-radiative galactic supernova remnants, RCW 86, Keplers supernova remnant and SN 1006. We have measured the narrow component Hα line widths in Balmer- dominated filaments in RCW 86 and SN 1006, as well as the narrow component width in a Balmer-dominated knot in Keplers SNR. The narrow component line widths measured in RCW 86 and Keplers SNR show FWHM of 30−40 km s −1 , similar to what has been seen in other Balmer-dominated remnants. Of the remnants in our sample, SN 1006 is the fastest shock (∼3000 km s −1 ). The narrow component Hα and Hβ lines in this remnant have a FWHM of merely 21 km s −1 . Comparing the narrow component widths measured in our sample with those measured in other remnants shows that the width of the narrow component does not correlate in a simple way with the shock velocity. The implications for the pre-heating mechanism responsible for the observed line widths are discussed.

Spitzer Space Telescope observations of Kepler's supernova remnant : A detailed look at the circumstellar dust component

We present 3.6-160 ?m infrared images of Keplers supernova remnant (SN 1604) obtained with the IRAC and MIPS instruments on the Spitzer Space Telescope. We also present MIPS SED low-resolution spectra in the 55-95 ?m region. The observed emission in the MIPS 24 ?m band shows the entire shell. Emission in the MIPS 70 ?m and IRAC 8 ?m bands is seen only from the brightest regions of 24 ?m emission, which also correspond to the regions seen in optical H? images. Shorter wavelength IRAC images are increasingly dominated by stars, although faint filaments are discernible. The SED spectrum of shows a faint continuum dropping off to longer wavelengths and confirms that strong line emission does not dominate the mid-IR spectral region. The emission we see is due primarily to warm dust emission from dust heated by the primary blast wave; no excess infrared emission is observed in regions where supernova ejecta are seen in X-rays. We use models of the dust to interpret the observed 70/24 ?m ratio and constrain the allowed range of temperatures and densities. We estimate the current mass of dust in the warm dust component to be 5.4 ? 10-4 M?, and infer an original mass of about 3 ? 10-3 M? before grain sputtering. The MIPS 160 ?m band shows no emission belonging to the remnant. We place a conservative but temperature-dependent upper limit on any cold dust component roughly a factor of 10 below the cold dust mass inferred from SCUBA observations. Finally, we comment on issues relevant to the possible precursor star and the supernova type.

Dust destruction in type Ia supernova remnants in the large magellanic cloud

We present first results from an extensive survey of Magellanic Cloud supernova remnants (SNRs) with the Spitzer Space Telescope. We describe IRAC and MIPS imaging observations at 3.6, 4.5, 5.8, 8, 24, and 70 μm of four Balmer-dominated Type Ia SNRs in the Large Magellanic Cloud (LMC): DEM L71 (0505-67.9), 0509-67.5, 0519-69.0, and 0548-70.4. None was detected in the four short-wavelength IRAC bands, but all four were clearly imaged at 24 μm, and two at 70 μm. A comparison of these images with Chandra broadband X-ray images shows a clear association with the blast wave, and not with internal X-ray emission associated with ejecta. Our observations are well described by one-dimensional shock models of collisionally heated dust emission, including grain size distributions appropriate for the LMC, grain heating by collisions with both ions and electrons, and sputtering of small grains. Model parameters are constrained by X-ray, optical, and far-ultraviolet observations. Our models can reproduce observed 70/24 μm flux ratios only by including sputtering, destroying most grains smaller than 0.03-0.04 μm in radius. We infer total dust masses swept up by the SNR blast waves, before sputtering, on the order of 10-2 M☉, several times less than those implied by a dust-to-gas mass ratio of 0.3% as often assumed for the LMC. Substantial dust destruction has implications for gas-phase abundances.