The Galactic Golden Child Cassiopeia A Through the Lens of JWST

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Title: A JWST Survey of the Supernova Remnant Cassiopeia A
Authors: Dan Milisavljevic et al.
First Author’s Institution: Purdue University
Status: Published in ApJL

Cassiopeia A, or Cas A for short, is in many ways the golden child of supernova remnant astronomy, a field that studies the hot clouds of gas and dust produced by the explosive deaths of stars. Beautiful and with a wealth of insights on stellar death to be gleaned from its billowy filaments, Cas A has mesmerised astronomers since its nascent glow first lit up the night sky in the late 17th century.

The hype behind Cas A has been sustained by the launch of new facilities, which provide increasingly detailed views of the dusty, gaseous nebula that was left behind by the cataclysmic explosion of a star 15–20 times the mass of the Sun. Luckily for the die-hard Cas A fans, JWST has obtained incredibly detailed images of the remnant’s near- and mid-infrared emission in the ~1.5 to ~26.7 microns (1 micron = 10^-6 meter) range using a variety of filters referred to by their JWST filter name (e.g., F120W). These images are the focus of today’s article that studies Cas A’s structure and searches for evidence of its central neutron star.

Using both the Near-Infrared Camera (NIRCam) and the Mid-Infrared Instrument (MIRI), the authors of today’s article have uncovered several surprising details about Cas A’s structure. The images are shown in Figure 1, with the NIRCam image above and the MIRI image below. These images are more spatially resolved than those seen previously using other facilities such as the Hubble Space Telescope, allowing the authors to identify a number of important features. For example, supernovae that produce remnants like Cas A are known to produce a shock wave from the star’s ejected material interacting with the surrounding gas. However, the authors detected a web-like ejecta structure that has not yet interacted with the supernova shock wave, arguing this is indicative of turbulent mixing processes that occurred shortly after the death of the star that produced Cas A. In addition, they resolved for the first time a feature they dubbed the “Green Monster,” which can be seen as the pock-marked, central green structure in the MIRI image in the bottom panel of Figure 1. They attributed the “Green Monster” to a thick sheet or sheets of infrared-emitting dust in the interior of the supernova remnant.

Not only did the authors consider Cas A’s structure in their article, they also searched for evidence of a central compact object such as a neutron star within its nebula. When massive stars die, they are expected to produce a compact object, be it either a black hole or neutron star. Therefore, astronomers are often eager to find evidence of these exotic objects in supernova remnant images. Previously, X-ray images from Chandra had revealed a point source at the centre of Cas A that was presumed to be its neutron star. Unfortunately, more recent visible and near-infrared Hubble and Spitzer images of Cas A did not show evidence of the reported central compact object. However, this result was not entirely unexpected given the expected low magnetic field strength of the neutron star or the high extinction (or dimming) of visible light, disfavouring the detection of the central compact object at wavelengths other than X-rays.

Given the non-detection in Hubble images, the authors of today’s article conducted a search for Cas A’s neutron star in their deeper JWST images. The results of this search are presented in Figure 2, which gives the flux density at the reported location of the neutron star as a function of frequency for the JWST, Hubble, Spitzer, and Chandra images of Cas A. In the figure, the red down arrows represent the JWST upper limits, meaning that no emission from the neutron star was detected using the deep JWST observations, but that emission is still possible at fluxes below these values. Figure 2 also shows the upper limits established from previous Spitzer and Hubble observations in grey, as well as the flux of the neutron star detected in Chandra X-ray observations in black. Additionally, the authors compared their results to three different models that could describe the neutron star emission given these upper limits, including power laws with indices of −2 and −1 in yellow and blue, respectively, and a blackbody with a temperature of 900K in red. While no neutron star was apparent in the JWST observations, the authors calculated a flux density upper limit of 20 nanoJanskys at a wavelength of 3 microns.

Thanks to the new JWST observations, there’s plenty of content to keep Cas A enthusiasts interested. Not only did the detailed images reveal new (and ominous) structures such as the “Green Monster,” but they also established deep upper limits on the infrared emission from the neutron star seen in X-ray observations. With additional JWST observations of Cas A, our picture of the galactic golden child will become even more intricate, so stay tuned.

Original astrobite edited by Ivey Davis.

About the author, Sonja Panjkov:

I’m a second-year PhD student at the University of Melbourne. My research focuses on the high-energy emission from the supernova remnants in the Magellanic Clouds. In my spare time, I enjoy hanging out with my cats and going to see live music.