THE DISTRIBUTION OF RADIOACTIVE ⁴⁴Ti IN CASSIOPEIA A

Abstract

The distribution of elements produced in the innermost layers of a supernova explosion is a key diagnostic for studying the collapse of massive stars. Here we present the results of a 2.4 Ms NuSTAR observing campaign aimed at studying the supernova remnant Cassiopeia A (Cas A). We perform spatially resolved spectroscopic analyses of the 44 Ti ejecta, which we use to determine the Doppler shift and thus the three-dimensional (3D) velocities of the 44 Ti ejecta. We find an initial 44 Ti mass of (1.54±0.21) ×10 −4 M e , which has a present-day average momentum direction of 340°±15°projected onto the plane of the sky (measured clockwise from celestial north) and is tilted by 58°±20°into the plane of the sky away from the observer, roughly opposite to the inferred direction of motion of the central compact object. We find some 44 Ti ejecta that are clearly interior to the reverse shock and some that are clearly exterior to it. Where we observe 44 Ti ejecta exterior to the reverse shock we also see shock-heated iron; however, there are regions where we see iron but do not observe 44 Ti. This suggests that the local conditions of the supernova shock during explosive nucleosynthesis varied enough to suppress the production of 44 Ti by at least a factor of two in some regions, even in regions that are assumed to be the result of processes like α-rich freezeout that should produce both iron and titanium.

The two promising scenarios to explode core collapse supernovae

“…In any case, I argue that strong pre-explosion outbursts require the presence of a binary companion. Grefenstette et al (2017) present the distribution of 44 Ti in the Cassiopeia A SNR (also Lee et al 2017). Wongwathanarat et al (2015) present numerical simulations based on neutrino-driven explosion, and argue that they reproduce the protrusions and the distribution of some metals in Cassiopeia A.…”

“…Indeed, Orlando et al (2016) had to introduce large-scale anisotropies (that I attribute to jets) to reproduce the structure of Cassiopeia A. Grefenstette et al 2017). Lower panel: Numerical simulations based on the neutrino-driven explosion mechanism (from Wongwathanarat et al 2015).…”

Comparing Neutron Star Kicks to Supernova Remnant Asymmetries

“…The origin of these asymmetries remains unclear: they may reflect the asymmetries inherent to the explosion mechanism, or they may arise from interactions with an inhomogeneous medium. For example, kinematic studies demonstrate large-scale asymmetries in SNR ejecta (e.g., SN 1987A: Boggs et al 2015; Cas A: Rest et al 2011;Grefenstette et al 2017), while observations and simulations show that interaction with a dense medium can alter the morphology and thermodynamic properties of SNRs (e.g., Tenorio-Tagle et al 1985;Lazendic & Slane 2006;Slane et al 2015).…”

“…The metals synthesized in the explosions are shock-heated to X-ray emitting temperatures, and X-ray observations of SNRs have revealed complex morphologies that may reflect asymmetries in SN explosion mechanisms (see Weisskopf & Hughes 2006 and Vink 2012 for a review). For example, the young SNR Cassiopeia A (Cas A) has asymmetric velocity gradients in its ejecta knots and jets that are attributed to its explosion (DeLaney et al 2010;Fesen et al 2006;Hwang & Laming 2012;Milisavljevic & Fesen 2015;Grefenstette et al 2017).…”

Gamma-ray line measurements from supernova explosions

“…Results were reported from OSSE on CGRO , RXTE (Rothschild et al 1999), Beppo-Sax (Vink et al 2001), and both INTE-GRAL instruments IBIS and SPI (Renaud et al 2006a;Martin et al 2009;Siegert et al 2015), culminating with NuSTAR's observations where NuSTAR's X-ray mirror allowed to spatially resolve this emission (Grefenstette et al 2014) . This first image of 44 Ti gamma-ray line emission (Grefenstette et al 2014) provides a key lesson on supernova Gamma-ray spectroscopy nucleosynthesis and morphology, with interesting deviations from a spherical explosion (Grefenstette et al 2017) (see also Grefenstette et al, Janka et al, Fryer et al, and Wongwhatanarath, these proceedings). INTEGRAL/SPI data are being accumulated to refine spectroscopic results, as only SPI is capable to measure both the hard X-ray and gamma ray lines from the 44 Ti decay chain .…”