X-ray studies of supernova remnants: a different view of supernova explosions

Abstract

The unprecedented spatial and spectral resolutions of Chandra have revolutionized our view of the X-ray emission from supernova remnants. The excellent datasets accumulated on young, ejecta-dominated objects like Cas A or Tycho present a unique opportunity to study at the same time the chemical and physical structure of the explosion debris and the characteristics of the circumstellar medium sculpted by the progenitor before the explosion. Supernova remnants can thus put strong constraints on fundamental aspects of both supernova explosion physics and stellar evolution scenarios for supernova progenitors. This view of the supernova phenomenon is completely independent of, and complementary to, the study of distant extragalactic supernovae at optical wavelengths. The calibration of these two techniques has recently become possible thanks to the detection and spectroscopic follow-up of supernova light echoes. In this paper, I review the most relevant results on supernova remnants obtained during the first decade of Chandra and the impact that these results have had on open issues in supernova research.

Fig. 1.

Three-color images generated from deep Chandra exposures of the Tycho (Left; Credit: X-ray: NASA/CXC/SAO, Infrared: NASA/JPL-Caltech; Optical: MPIA, Calar Alto, O. Krause et al.), Kepler (Center; Credit: NASA/CXC/UCSC/L. Lopez et al.), and Cas A (Right; Credit: NASA/CXC/SAO/D. Patnaude et al.) SNRs. The details vary for each image, but red usually corresponds to low energy X-rays around the Fe-L complex (∼1 keV and below), green to midenergy X-rays around the Si K blend (∼2 keV), and blue to high energy X-rays in the 4–6 keV continuum bewteen the Ca K and Fe K blends. Images are not to scale: Tycho is ∼8^′ in diameter, Kepler is ∼4^′, and Cas A is ∼6^′. Total exposure times are 150, 750, and 1000 ks. Images courtesy of the Chandra X-ray Center; data originally published in (2), (11), and (12).

Fig. 2.

HD + NEI models for the Tycho SNR: Inferred distance as a function of the ambient medium density ρ_AM obtained by matching the angular sizes of the reverse shock (RS, dashed plots) and contact discontinuity (CD, solid plots) to a bright Ia SN model (DDTa, blue plots), and a dim Ia SN model (DDTg, red plots). The values of ρ_AM in the HD + NEI models of (8) that provide the best match to the X-ray emission from the SN ejecta are indicated by the striped vertical band. The value of ρ_AM required by the CR-modified shock models of (3) is indicated by a vertical dash-dotted line. The estimated value of the distance D from (34) is indicated by the horizontal solid line, with the dotted horizontal lines marking the upper and lower limits.

Fig. 3.

HD + NEI models for the Tycho SNR: Best-fit HD + NEI model to the total X-ray spectrum. The SN model is DDTc, a normal SN Ia model that synthesizes 0.74M_⊙ of ⁵⁶Ni, interacting with an AM of ρ_AM = 2 × 10^-24 g cm^-3. The contribution of the nonthermal emission from the blast wave is indicated by the dotted plot, the rest is thermal emission from the shocked ejecta (figure from ref. 8).

Fig. 4.

Direct comparison between SN and SNR spectroscopy for the LMC SNR 0509-67.5: The LE is well matched only by spectra of bright Type Ia SNe like SN 1998es, SN 1999aa, and SN 1999dq (figure from ref. 39).

Fig. 5.

Direct comparison between SN and SNR spectroscopy for the LMC SNR 0509-67.5: The X-ray spectrum of the SNR can only be reproduced with a bright Type Ia SN models that synthesizes 0.97M_⊙ of ⁵⁶Ni (figure adapted from ref. 30).

Fig. 6.

The Cas A SNR: Three-dimensional visualization of the distribution of ejecta, built by T. Delaney using Doppler shift mapping of multiwavelength data: green is X-ray emitting Fe; yellow is X-ray, optical and infrared emitting Ar and Si; red is infrared emitting unshocked ejecta; the pink dot represents the compact object. (Credit: NASA/CXC/MIT/T. Delaney et al.)

Fig. 7.

The Cas A SNR: Constraints on the power-law index of the ejecta structure obtained by comparing the fitted T_e and n_et values in small X-ray knots to HD + NEI models. The lines represent models appropriate for Cas A, with M_ej = 2M_⊙, E_k = 2 × 10⁵¹ erg and different power-law indexes (figure from ref. 61).