Eli Michael

The X-Ray Remnant of SN 1987A

We present high-resolution Chandra observations of the remnant of SN 1987A in the Large Magellanic Cloud. The high angular resolution of the Chandra X-Ray Observatory permits us to resolve the X-ray remnant. We find that the remnant is shell-like in morphology, with X-ray peaks associated with some of the optical hot spots seen in Hubble Space Telescope images. The X-ray light curve has deviated from the linear flux increase observed by ROSAT, with a 0.5-2.0 keV luminosity of 1.5 × 1035 ergs s-1 in 2000 January. We set an upper limit of 2.3 × 1034 ergs s-1 on the observed luminosity of any embedded central source (0.5-2 keV). We also present a high-resolution spectrum and show that the X-ray emission is thermal in origin and is dominated by highly ionized species of O, Ne, Mg, and Si.

Modeling the Hubble Space Telescope ultraviolet and optical spectrum of spot 1 on the circumstellar ring of SN 1987A

We report and interpret Hubble Space Telescope (HST) Space Telescope Imaging Spectrograph (STIS) long-slit observations of the optical and ultraviolet (1150-10270 A) emission line spectra of the rapidly brightening spot 1 on the equatorial ring of SN 1987A between 1997 September and 1999 October (days 3869-4606 after outburst). The emission is caused by radiative shocks created where the supernova blast wave strikes dense gas protruding inward from the equatorial ring. We measure and tabulate line identifications, fluxes, and, in some cases, line widths and shifts. We compute flux correction factors to account for substantial interstellar line absorption of several emission lines. Nebular analysis shows that optical emission lines come from a region of cool (Te ≈ 104 K) and dense (ne ≈ 106 cm-3) gas in the compressed photoionized layer behind the radiative shock. The observed line widths indicate that only shocks with shock velocities Vs < 250 km s-1 have become radiative, while line ratios indicate that much of the emission must have come from yet slower (Vs 135 km s-1) shocks. Such slow shocks can be present only if the protrusion has atomic density n 3 × 104 cm-3, somewhat higher than that of the circumstellar ring. We are able to fit the UV fluxes with an idealized radiative shock model consisting of two shocks (Vs = 135 and 250 km s-1). The observed UV flux increase with time can be explained by the increase in shock surface areas as the blast wave overtakes more of the protrusion. The observed flux ratios of optical to highly ionized UV lines are greater by a factor of ~2-3 than predictions from the radiative shock models, and we discuss the possible causes. We also present models for the observed Hα line widths and profiles, which suggest that a chaotic flow exists in the photoionized regions of these shocks. We discuss what can be learned with future observations of all the spots present on the equatorial ring.

The X-Ray Spectrum of Supernova Remnant 1987A

We discuss the X-ray emission observed from supernova remnant 1987A with the Chandra X-Ray Observatory. We analyze a high-resolution spectrum obtained in 1999 October with the high-energy transmission grating (HETG). From this spectrum we measure the strengths and an average profile of the observed X-ray lines. We also analyze a high signal-to-noise ratio CCD spectrum obtained in 2000 December. The good statistics (?9250 counts) of this spectrum and the high spatial resolution provided by the telescope allow us to perform spectroscopic analyses of different regions of the remnant. We discuss the relevant shock physics that can explain the observed X-ray emission. The X-ray spectra are well fitted by plane-parallel shock models with postshock electron temperatures of ?2.6 keV and ionization ages of ?6 ? 1010 cm-3 s. The combined X-ray line profile has a FWHM of ?5000 km s-1, indicating a blast-wave speed of ?3500 km s-1. At this speed, plasma with a mean postshock temperature of ?17 keV is produced. This is direct evidence for incomplete electron-ion temperature equilibration behind the shock. Assuming this shock temperature, we constrain the amount of collisionless electron heating at the shock front at Te0/Ts = 0.11. We find that the plasma has low metallicity (abundances are ?0.1-0.4 solar) and is nitrogen enriched (N/O ? 0.8 by number), similar to abundances found for the equatorial ring. Analysis of the spectra from different regions of the remnant reveals slight differences in the parameters of the emitting plasma. The plasma is cooler near the optical spot 1 (at position angle ?30?) and in the eastern half of the remnant, where the bright optical spots are found, than in the western half, consistent with the presence of slower (?500 km s-1) shocks entering denser ring material. There is an overall flux asymmetry between the two halves, with the eastern half being 15%-50% brighter (depending on how the center of the remnant is defined). However, our spectroscopic analysis shows that less than 5% of the overall X-ray emission could come from a slow shock component. Therefore the flux asymmetry cannot fully be due to X-rays produced by the blast wave entering the ring, but rather indicates an asymmetry in the global interaction with the circumstellar material interior to the ring.

Hubble Space Telescope Observations of High‐Velocity Lyα and Hα Emission from Supernova Remnant 1987A: The Structure and Development of the Reverse Shock

We present two-dimensional line profiles of high-velocity (~+/-12,000 km s-1) Lyα and Hα emission from supernova remnant 1987A obtained with the Space Telescope Imaging Spectrograph between 1997 September and 2001 September (days 3869-5327 after the explosion). This emission comes from hydrogen in the debris that is excited and ionized as it passes through the remnants reverse shock. We use these profiles to measure the geometry and development of the reverse-shock surface. The observed emission is confined within ~+/-30° about the remnants equatorial plane. At the equator, the reverse shock has a radius of ~75% of the distance to the equatorial ring. We detect marginal differences (6%+/-3%) between the location of the reverse-shock front in the northeast and southwest parts of the remnant. The radius of the reverse shock surface increases for latitudes above the equator, a geometry consistent with a model in which the supernova debris expands into a bipolar nebula. Assuming that the outer supernova debris has a power-law density distribution, we can infer from the reverse-shock emission light curve an expansion rate (in the northeast part of the remnant) of 3700+/-900kms-1, consistent with the expansion velocities determined from observations in radio (Manchester et al.) and X-ray (Park et al.; Michael et al.) wavelengths. However, our most recent observation (at day 5327) suggests that the rate of increase of mass flux across the northeast sector of the reverse shock has accelerated, perhaps because of deceleration of the reverse shock caused by the arrival of a reflected shock created when the blast wave struck the inner ring. Resonant scattering within the supernova debris causes Lyα photons created at the reverse shock to be directed preferentially outward, resulting in a factor of ~5 difference in the observed brightness of the reverse shock in Lyα between the near and far sides of the remnant. Accounting for this effect, we compare the observed reverse-shock Lyα and Hα fluxes to infer the amount of interstellar extinction by dust as E(B-V)=0.17+/-0.01 mag. We also notice extinction by dust in the equatorial ring with E(B-V)~0.02-0.08 mag, which implies dust-to-gas ratios similar to that of the LMC. Since Hα photons are optically thin to scattering, the observed asymmetry in brightness of Hα from the near and far sides of the remnant represents a real asymmetry in the mass flux through the reverse shock of ~30%. We discuss future observational strategies that will permit us to further investigate the reverse-shock dynamics and resonant scattering of the Lyα line and to constrain better the extinction by dust within and in front of the remnant.

Analysis of Type IIn SN 1998S: Effects of Circumstellar Interaction on Observed Spectra

We present spectral analysis of early observations of the Type IIn supernova 1998S using the general non-local thermodynamic equilibrium atmosphere code PHOENIX. We model both the underlying supernova spectrum and the overlying circumstellar interaction region and produce spectra in good agreement with observations. The early spectra are well fitted by lines produced primarily in the circumstellar region itself, and later spectra are due primarily to the supernova ejecta. Intermediate spectra are affected by both regions. A mass-loss rate of order ~ 0.0001-0.001 M☉ yr-1 is inferred for a wind speed of 100-1000 km s-1. We discuss how future self-consistent models will better clarify the underlying progenitor structure.

High-Velocity Lyα Emission from SNR 1987A

The high-velocity Lya emission from SN 1987A observed with the Space Telescope Imaging Spectrograph (STIS) evidently comes from a reverse shock formed where the outer envelope of SN 1987A strikes ionized gas inside the inner circumstellar ring. The observations can be explained by a simple kinematic model, in which the Lya emission comes from hydrogen atoms with radial velocity »15,000 km s 21 crossing a reverse shock in the shape of a slightly prolate ellipsoid with equatorial radius cm or »80% of the distance to the 17 4.8 # 10 inner surface of the inner ring. N v ll1239, 1243 emission, if present, has a net luminosity &30% times that of the Lya emission. Future STIS observations should enable us to predict the time of impact with the inner ring and to determine unambiguously whether or not N v emission is present. These observations will offer a unique opportunity to probe the structure of SN 1987A’s circumstellar environment and the hydrodynamics and kinetics of very fast shocks. Subject heading: circumstellar matter — hydrodynamics — supernovae: individual (SN 1987A) — ultraviolet: ISM

Laser experiments to simulate supernova remnants

An experiment using a large laser facility to simulate young supernova remnants (SNRs) is discussed. By analogy to the SNR, the laboratory system includes dense matter that explodes, expansion and cooling to produce energetic, flowing plasma, and the production of shock waves in lower-density surrounding matter. The scaling to SNRs in general and to SN1987A in particular is reviewed. The methods and results of x-ray radiography, by which the system in diagnosed, are discussed. The data show that the hohlraum used to provide the energy for explosion does so in two ways—first, through its radiation pulse, and second, through an additional impulse that is attributed to stagnation pressure. Attempts to model these dynamics are discussed.

[ITAL]Hubble Space Telescope[/ITAL] Spectroscopy of Spot 1 on the Circumstellar Ring of SN 1987A

We present ultraviolet and optical spectra of the first bright spot (P.A. = 29°) on SN 1987As equatorial circumstellar ring taken with the Space Telescope Imaging Spectrograph. We interpret this spot as the emission produced by radiative shocks that occur where the supernova blast wave strikes an inward protrusion of the ring. The observed line widths and intensity ratios indicate the presence of radiative shocks with velocities ranging from 100 to 250 km s-1 entering dense (104 cm-3) gas. These observations, and future observations of the development of the spectra and line profiles, provide a unique opportunity to study the hydrodynamics of radiative shocks.

Hubble Space Telescope Observations of the Shocks in Supernova Remnant SN 1987A

We present observations of supernova remnant SN 1987A made with the Hubble Space Telescope. The bright spot previously observed on the inner circumstellar ring by the Wide Field/Planetary Camera at a position angle ? 31? has brightened by nearly a factor of 2 in seven months. Space Telescope Imaging Spectrograph (STIS) spectra confirm that this spot is the result of a ~ 300 km s-1 shock entering the inner circumstellar ring at the first point of contact by the supernova blast wave. High-velocity (? 15,000 km s-1) Ly? and H? emission are apparent in the STIS spectra as well. This emission comes in part from neutral hydrogen in the debris crossing a reverse shock located at ? 75% of the radius of the inner boundary of the inner circumsteller ring and confined within ?30? of the equatorial plane.

New [ITAL]Hubble Space Telescope[/ITAL] Observations of High-Velocity L[CLC]y[/CLC]α and Hα in SNR 1987A

We describe and model high-velocity (≈ 15,000 km s-1) Lyα and Hα emission from the supernova remnant SNR 1987A seen in 1997 September and October with the Space Telescope Imaging Spectrograph. Part of this emission comes from a reverse shock located at ≈ 75% of the radius of the inner boundary of the inner circumstellar ring and confined within ±30° of the equatorial plane. Departure from axisymmetry in the Lyα and Hα emission correlates with that seen in nonthermal radio emission and reveals an asymmetry in the circumstellar gas distribution. We also see diffuse high-velocity Lyα emission from supernova debris inside the reverse shock that may be due to excitation by nonthermal particles accelerated by the shock.We describe and model high velocity (~15,000 km/s) Lyman alpha and Balmer alpha emission from supernova remnant 1987A seen in September and October 1997 with the Space Telescope Imaging Spectrograph. Part of this emission comes from a reverse shock located at ~75% of the radius of the inner boundary of the inner circumstellar ring and confined within 30 degrees of the equatorial plane. Departure from axisymmetry in the Lyman alpha and Balmer alpha emission correlates with that seen in nonthermal radio emission and reveals an asymmetry in the circumstellar gas distribution. We also see diffuse high velocity Lyman alpha emission from supernova debris inside the reverse shock that may be due to excitation by nonthermal particles accelerated by the shock.