X-ray studies of supernova remnants: A different SPECIAL FEATURE view of supernova explosions Carles Badenes1 Benoziyo Center for Astrophysics, Weizmann Institute of Science, Israel; and School of Physics and Astronomy, Tel-Aviv University, Israel Edited by Neta A. Bahcall, Princeton University, Princeton, NJ, and approved February 2, 2010 (received for review December 15, 2009) The unprecedented spatial and spectral resolutions of Chandra explosion and the events leading up to it. Understandably, the have revolutionized our view of the X-ray emission from super- Chandra observations of SNRs have drawn a great deal of atten- nova remnants. The excellent datasets accumulated on young, tion from the SN community, becoming a prime benchmark for ejecta-dominated objects like Cas A or Tycho present a unique different aspects of SN theory, in particular the state-of-the-art opportunity to study at the same time the chemical and physical three-dimensional models of core collapse (CC) and Type Ia structure of the explosion debris and the characteristics of the SN explosions. However, the interpretation of these exceptional circumstellar medium sculpted by the progenitor before the datasets is far from being a trivial task. explosion. Supernova remnants can thus put strong constraints In this review, I summarize the main results relevant to SN on fundamental aspects of both supernova explosion physics research obtained from SNR observations by Chandra and other and stellar evolution scenarios for supernova progenitors. This X-ray satellites, point out the current challenges in the field, and view of the supernova phenomenon is completely independent discuss future prospects to meet these challenges. The unique of, and complementary to, the study of distant extragalactic super- capabilities of Chandra have obviously led to substantial progress novae at optical wavelengths. The calibration of these two tech- in many other aspects of SNR research, most notably the physics niques has recently become possible thanks to the detection and of cosmic ray acceleration in SNR shocks (1–4). These issues are spectroscopic follow-up of supernova light echoes. In this paper, outside the scope of the present work, which focuses on SNRs as I review the most relevant results on supernova remnants obtained a tool to study SN explosions. For brevity, I will discuss almost during the first decade of Chandra and the impact that these exclusively the so-called “historical” SNRs, young objects whose ASTRONOMY results have had on open issues in supernova research. ages are known well enough to model their dynamics with a cer- tain degree of confidence. I will also concentrate on results from supernovae ∣ supernova remnants ∣ nucleosynthesis ∣ X-rays nondispersive X-ray spectroscopy, leaving out SNR observations conducted with the high resolution gratings onboard Chandra and he vast majority of the hundreds of SNe discovered each year XMM-Newton. Tat optical wavelengths explode very far away. High quality light curves and spectra of some of these supernovae (SNe) Chandra View of Young SNRs are obtained routinely from ground-based observatories, but Power of the Data. The excellent quality of the data generated by the large distances involved restrict the view of the supernova Chandra for bright SNRs in our Galaxy is showcased by the false (SN) ejecta to a single line of sight through the receding photo- color images that have become perhaps the mission’s most recog- sphere, and usually preclude a detailed study of the immediate nizable and widespread visual result (Fig. 1). For a SNR with an surroundings of the exploding star. Only the occasional nearby angular diameter of 60 like Cas A, Chandra can resolve more than objects like SN 1987A allow us to probe the explosion and its 105 individual regions. Deep exposures of bright objects usually surroundings with better resolution, advancing our knowledge provide enough photon statistics to obtain a good spectrum from more effectively than many dozens of distant SNe. most of these regions at the moderate spectral resolution of CCD The high quality X-ray observations of young ejecta- detectors (E∕ΔE ≈ 10–60), allowing the detection of K-shell dominated supernova remnant (SNRs) performed by Chandra emission from abundant elements like O, Si, S, Ar, Ca, and Fe. and XMM-Newton over the last decade have presented us with a different view of the SN phenomenon, a view that to a large Basic Concepts of NEI Plasmas. The density of the plasma inside extent complements the shortcomings of the optical studies of SNRs is low enough for the ages of young objects like Cas A SNe. After the light from the SN fades away, the ejecta expand or Tycho to be smaller than the ionization equilibrium timescale. and cool down until their density becomes comparable to that of The X-ray emitting plasma heated by the shocks is therefore in a the surrounding material, either the interstellar medium (ISM) or state of nonequilibrium ionization, or NEI (5). This means that a more or less extended circumstellar medium (CSM) modified the ionization state of any given fluid element is determined not by the SN progenitor. At this point, a double shock structure is only by its electron temperature Te, but also by its ionization formed and the SNR phase itself begins. The forward shock or timescale net, where ne is the electron number density and t is blast wave moves into the ambient medium, while the reverse the time since shock passage. The thermal X-ray spectrum from shock moves into the SN ejecta. Because the ambient densities a young SNR is thus intimately related to its dynamic evolution −3 −24 −3 are low (n ∼ 1 cm or ρ ∼ 10 gcm ), the shocks are fast en- through the individual densities and shock passage times of each −1 ough (v ≳ 1; 000 km s ) to heat the material to X-ray emitting fluid element. This has important implications for the ejecta temperatures. These thermal X-rays from the high-Z elements emission, because of the large differences in chemical composi- synthesized in the SN explosion are usually the dominant compo- tion across the SN debris, and the fact that the electron pool is nent in the X-ray spectra of young SNRs. completely dominated by the contributions from high-Z ele- Chandra The ability of to perform spatially resolved spectro- ments, making ne a strong function of the ionization state (6). scopy on arcsecond scales reveals the structure of this shock- heated SN ejecta in Galactic SNRs with an unprecedented level of detail. Because the morphology and spectral properties of the Author contributions: C.B. wrote the paper. SNRs also provide important clues about the structure of the ISM The authors declare no conflict of interest. or CSM that is interacting with the ejecta, young SNRs are This article is a PNAS Direct Submission. the ideal setting to study simultaneously the aftermath of a SN 1E-mail: [email protected]. www.pnas.org/cgi/doi/10.1073/pnas.0914189107 PNAS ∣ April 20, 2010 ∣ vol. 107 ∣ no. 16 ∣ 7141–7146 Downloaded by guest on October 2, 2021 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 ∼80 in diameter, Kepler is ∼40, and Cas A is ∼60. 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). Under these circumstances, the quantitative analysis of X-ray Type Ia SNe and Their SNRs spectra from young SNRs becomes a challenging endeavor. In Open Issues in Type Ia SNe. Type Ia SNe are believed to be the ther- order to derive magnitudes that are relevant to SN physics, like monuclear explosion of a C þ O white dwarf (WD) that is desta- the kinetic energy Ek or the ejected mass of each chemical ele- bilized when its mass becomes close to the Chandrasekhar limit ment, it is necessary to build a hydrodynamic model of the entire by accretion of material from a binary companion. After the cen- SNR. One must know when each fluid element was shocked, what tral regions ignite, the burning front propagates outwards, con- its chemical composition is, how much of the SN ejecta is still suming the entire star and leading to a characteristic ejecta 56 unshocked at the present time, etc. Individual fitting of each structure, with ∼0.7M⊙ of Fe-peak nuclei (mostly the Ni that magnitude becomes impractical, because they are all related to powers the light curve) in the inside and about an equal amount each other, and in order to understand the X-ray spectrum of of intermediate mass elements (mostly Si, S, Ar, and Ca) in the 56 a particular SNR, one has to understand the object as a whole. outside. The amount of Ni can be as high as ∼1M⊙ in the Further complications stem from two sources. One is a technical, brightest Type Ia SNe, and as low as ∼0.3M⊙ in the dimmest Type but important, problem: the uneven quality of atomic data in Ia SNe, but the large scale stratification seems to be retained in X-ray emission codes for NEI plasmas (7, 8).