Direct Confirmation of the Asymmetry of the Cas a Supernova With Light Echoes

Abstract

We report the first detection of asymmetry in a supernova (SN) photosphere based on SN light echo (LE) spectra of Cas A from the different perspectives of dust concentrations on its LE ellipsoid. New LEs are reported based on difference images, and optical spectra of these LEs are analyzed and compared. After properly accounting for the effects of finite dust-filament extent and inclination, we find one field where the He I λ5876 and Hα features are blueshifted by an additional ∼4000 km s −1 relative to other spectra and to the spectra of the Type IIb SN 1993J. That same direction does not show any shift relative to other Cas A LE spectra in the Ca II near-infrared triplet feature. We compare the perspectives of the Cas A LE dust concentrations with recent three-dimensional modeling of the SN remnant (SNR) and note that the location having the blueshifted He I and Hα features is roughly in the direction of an Fe-rich outflow and in the opposite direction of the motion of the compact object at the center of the SNR. We conclude that Cas A was an intrinsically asymmetric SN. Future LE spectroscopy of this object, and of other historical SNe, will provide additional insight into the connection of explosion mechanism to SN to SNR, as well as give crucial observational evidence regarding how stars explode.

Three-dimensional neutrino-driven supernovae: Neutron star kicks, spins, and asymmetric ejection of nucleosynthesis products

“…However, diagnosing momentum asymmetries of the ejecta may be difficult except in cases where the center of mass of the gaseous SN remnant is clearly displaced relative to the NS position or where the gas cloud exhibits extreme deformation without gradients of the environmental conditions being responsible for the asymmetric ejecta expansion. The SN remnants Cassiopeia A (Isensee et al 2010;DeLaney et al 2010;Rest et al 2011;Hwang & Laming 2012) and Puppis A (Petre et al 1996;Winkler & Petre 2007;Katsuda et al 2008Katsuda et al , 2010Becker et al 2012) may be such lucky cases, because in the former all the SN ejecta in the remnant, whose mass is dominated by oxygen, while in the latter fast-moving, oxygen-rich optical filaments and knots seem to have been recoiled opposite to the direction of the high-velocity compact stellar remnant. In general, however, a major problem for observing momenta asymmetries linked to an asymmetric beginning of the explosion may result from the fact that the outgoing SN shock transfers the bulk of the explosion energy and ejecta momentum to the He and H shells, which dominate the ejecta mass in events other than stripped SNe.…”

“…In particular, it might allow one to discriminate our gravitational tug-boat mechanism from scenarios like the neutrino-driven kicks discussed by Fryer & Kusenko (2006), who expect the stronger explosion (and thus enhanced explosive nucleosynthesis) in the direction of the NS motion. High-resolution spectral investigations of young SN remnants like Puppis A (Katsuda et al 2008(Katsuda et al , 2010, G11.2-0.3 (Moon et al 2009), and Cassiopeia A (Isensee et al 2010;DeLaney et al 2010;Rest et al 2011;Hwang & Laming 2012) in different wavebands may offer a promising perspective.…”

Asymmetry in the Outburst of Sn 1987a Detected Using Light Echo Spectroscopy

“…The spectral types of the Cas A and Tycho SNe have also been identified using this method of targeted LE spectroscopy (Rest et al 2008b;Krause et al 2008aKrause et al , 2008b. Rest et al (2011a) used LE spectroscopy to detect asymmetries in the outburst of the Cas A SN, while Rest et al (2012a) recently obtained LE spectroscopy from the "Great Eruption" of η Carinae. We refer the reader to Rest et al (2012b) for a review of LE spectroscopy emphasizing these more recent results.…”

“…In the case of historical LEs, one has to use the photometric and spectroscopic history of a different SN to model the LE spectrum (see, e.g., Rest et al 2011a where the Cas A outburst is modeled with the light curve and spectra of SN 1993J andSN 2003bg). For SN 1987A, however, the exact spectral and photometric history of the LE source is known with high precision, allowing observed LE spectra to be compared unambiguously to an isotropic scenario.…”

Light Echoes of Transients and Variables in the Local Universe

“…A more detailed discussion of the interplay between dust width, dust inclination, and seeing can be found in Rest et al (2011b). Such accurate modeling of the dust is essential when looking for small differences in spectral features, e.g., when comparing spectra of LEs from the same object as is done in Rest et al (2011a).…”