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Asymmetric Type-Ia Supernova Origin of W49B As Revealed from Spatially Resolved X-Ray Spectroscopic Study Ping Zhou1,2 and Jacco Vink1,3

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Asymmetric Type-Ia Supernova Origin of W49B As Revealed from Spatially Resolved X-Ray Spectroscopic Study Ping Zhou1,2 and Jacco Vink1,3 A&A 615, A150 (2018) Astronomy https://doi.org/10.1051/0004-6361/201731583 & © ESO 2018 Astrophysics Asymmetric Type-Ia supernova origin of W49B as revealed from spatially resolved X-ray spectroscopic study Ping Zhou1,2 and Jacco Vink1,3 1 Anton Pannekoek Institute, University of Amsterdam, PO Box 94249, 1090 GE Amsterdam, The Netherlands e-mail: p.zhou,[email protected] 2 School of Astronomy and Space Science, Nanjing University, Nanjing 210023, PR China 3 GRAPPA, University of Amsterdam, PO Box 94249, 1090 GE Amsterdam, The Netherlands Received 17 July 2017 / Accepted 23 March 2018 ABSTRACT The origin of the asymmetric supernova remnant (SNR) W49B has been a matter of debate: is it produced by a rare jet-driven core- collapse (CC) supernova, or by a normal supernova that is strongly shaped by its dense environment? Aiming to uncover the explosion mechanism and origin of the asymmetric, centrally filled X-ray morphology of W49B, we have performed spatially resolved X-ray spectroscopy and a search for potential point sources. We report new candidate point sources inside W49B. The Chandra X-ray spectra from W49B are well-characterized by two-temperature gas components ( 0:27 keV + 0.6–2.2 keV). The hot component gas shows a large temperature gradient from the northeast to the southwest and is over-ionized∼ in most regions with recombination timescales 11 3 of 1–10 10 cm− s. The Fe element shows strong lateral distribution in the SNR east, while the distribution of Si, S, Ar, Ca is relatively× smooth and nearly axially symmetric. Asymmetric Type-Ia explosion of a Chandrasekhar-mass white dwarf (WD) well- explains the abundance ratios and metal distribution of W49B, whereas a jet-driven explosion and normal CC models fail to describe the abundance ratios and large masses of iron-group elements. A model based on a multi-spot ignition of the WD can explain the observed high MMn=MCr value (0.8–2.2). The bar-like morphology is mainly due to a density enhancement in the center, given the good spatial correlation between gas density and X-ray brightness. The recombination ages and the Sedov age consistently suggest a revised SNR age of 5–6 kyr. This study suggests that despite the presence of candidate point sources projected within the boundary of this SNR, W49B is likely a Type-Ia SNR, which suggests that Type-Ia supernovae can also result in mixed-morphology SNRs. Key words. ISM: individual objects: W49B – ISM: supernova remnants – nuclear reactions, nucleosynthesis, abundances – white dwarfs 1. Introduction and a shell-type morphology in the radio. Initially, it was noted that mixed-morphology SNRs show thermal emission in the The study of supernova remnants (SNRs) provides information interior originating from low-abundant hot gas (Rho & Petre about both the supernova explosions themselves, and about the 1998; Jones et al. 1998). W49B was included in the list, but environments in which the supernova explosions took place. appeared to have enhanced abundances. However, increasingly The environment often carries important information about the more of the originally mixed-morphology SNRs appeared to also supernova progenitor itself, such as whether it formed in a have enhanced abundances in their interiors (Lazendic & Slane star-forming region, and whether the progenitor shaped its own 2006). It is clear that W49B, together with some other metal- environment with a stellar wind. In particular, massive stars are rich cases such as Sgr A East (Sakano et al. 2004; Park et al. known to create large wind-blown bubbles of several tens of 2005) stand out. In most reviews of mixed-morphology SNRs, parsecs in size (Weaver et al. 1977; Chevalier 1999). W49B is listed as a mixed-morphology SNR (Lazendic & Slane The morphology and spectra of SNRs are determined by the 2006; Vink 2012; Zhang et al. 2015; Dubner & Giacani 2015), combined effects of both the intrinsic explosion properties and and the definition of mixed-morphology SNR is in those cases the ambient medium in which they involve. Unfortunately, how- based solely on different radio and X-ray morphology, and the ever, it is sometimes difficult to disentangle the effects of explo- fact that the X-ray emission is thermal in nature. It is thought that sion properties and the environment in which they occurred. mixed-morphology SNRs evolve in denser environments, and There are properties that can be firmly attributed to the explosion since massive stars are associated with molecular cloud environ- properties, but also properties that may be attributed to either the ments, it is usually assumed that these SNRs are remnants of CC explosion characteristics or to the environment. For example, for supernovae. Indeed, some of the mixed-morphology SNRs have young SNRs it is clear that the abundance pattern provides clear associated young pulsars, proving that these SNRs are indeed CC signatures of the type of explosion, with Type-Ia supernovae pro- SNRs (e.g., W44, IC 443, Wolszczan et al. 1991; Olbert et al. ducing more iron-group elements (IGEs), whereas core-collapse 2001). (CC) SNRs are more abundant in oxygen, neon, and magnesium Thermonuclear (or Type-Ia) supernova progenitors are (Hughes et al. 1995; Vink 2012). carbon–oxygen white dwarfs (WDs), which take a longer time Mixed-morphology SNRs are a special class of SNRs char- to evolve (>40 Myr), and, moreover, only explode if they accrete acterized by bright thermal X-ray emission from their center, sufficient matter from a companion star (the single-degenerate Article published by EDP Sciences A150, page 1 of 14 A&A 615, A150 (2018) scenario; Whelan & Iben 1973), or merge with a companion Although the X-ray spectrum of W49B shows the SNR to be carbon–oxygen WD (the double-degenerate scenario; Webbink very iron-rich, it is usually assumed that it is a CC SNR, like 1984). By the time they explode, their ambient medium does most mixed-morphology SNRs, albeit a peculiar one. Hwang not necessarily contain any information anymore about their et al.(2000) expressed some doubts, suggesting that neither a progenitors. The exact origin of Type-Ia supernovae is still a CC origin, nor a Type-Ia origin could explain the measured source of debate (see reviews Branch et al. 1995; Hillebrandt & abundances. The brightness of the Fe–K lines, but also the Niemeyer 2000; Livio 2000; Wang & Han 2012; Maoz et al. peculiar, jet-like morphology of the ejecta, has been interpreted 2014, and references therein), but also the manner in which the as evidence that W49B is the result of a hypernova explosion WDs explode is uncertain, with models involving deflagration (Keohane et al. 2007). On the other hand, Miceli et al.(2006) (Nomoto et al. 1984), competing with so-called delayed det- compared the observed abundances with yields for hypernova onation (DDT) models (Khokhlov 1991). In general, Type-Ia and supernova nucleosynthesis and found better agreement for SNRs are often to be found in less dense regions of the Galaxy. the abundances of W49B with models with a normal explosion For example, SN 1006 is found high above the Galactic plane energy (1051 erg). More recently, Lopez et al.(2013b), assuming (b = 14:6◦, corresponding to 560 pc at a distance of 2:18 W49B to be a CC SNR, presented evidence that the supernova 0:08 kpc, Winkler et al. 2003). The∼ less disturbed media in which± produced a black hole rather than a neutron star (NS). The rea- they are often found may account for the generally more symmet- son is that they did not find evidence for a cooling NS, similar to ric morphology, as compared to CC SNRs (Lopez et al. 2011). the X-ray point source in Cas A (Tananbaum 1999). On the other hand, the more symmetric morphologies of Type- The study presented here was prompted by the many pecu- Ia SNRs may also be caused by intrinsically more symmetric liarities of W49B. Most notably, we were puzzled by the fact that explosions. black holes are thought to be the end products of the most mas- The idea that Type-Ia progenitors do not shape the super- sive stars (>25 M , e.g., Heger et al. 2003), but W49B seems nova environments has recently been challenged. For example, to be evolving in a cavity of only 5 pc radius (Keohane et al. it is clear that Kepler’s SNR (Vink 2016, for a review), a Type- 2007). In contrast, a progenitor more∼ massive than 25 M is Ia SNR (Kinugasa & Tsunemi 1999; Reynolds et al. 1994), is expected to create a cavity with a radius of at least 20 pc (Chen evolving inside a bow-shock-shaped high-density region caused et al. 2013). by the wind from a progenitor system (Chiotellis et al. 2012). In With our study we therefore tried to investigate, a) whether contrast, the likely Type-Ia SNR RCW 86 (see Gvaramadze et al. a cooling NS may, after all, be present, given that W49B pro- 2017, for a recent paper suggesting a CC origin) seems to evolve vides a spatially non-uniform X-ray background that could hide inside the low-density environment created by a powerful low- a point source, and that the interstellar absorption is relatively 22 2 density wind (Williams et al. 2011; Broersen et al. 2014). Type-Ia high (NH > 10 cm− ; and b) whether W49B is indeed a CC SNR Tycho is suggested to be overrunning a slowly expand- SNR or even a jet-driven CC SNR, as often assumed. ing molecular bubble created by its progenitor’s outflow (Zhou To answer these questions we reanalyzed the archival et al.
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