A High-Energy Post-Mortem of the Galactic Supernova Remnant G309.8+00.0

Title: A look at the high energy aspects of the supernova remnant G309.8+00.0 with eROSITA and Fermi-LAT

Authors: Miltiadis Michailidis, Gerd Pühlhofer, Andrea Santangelo, Manami Sasaki, Werner Becker

First Author’s Institution: Institut für Astronomie und Astrophysik Tübingen (IAAT), Germany

Status: Accepted for publication in Astronomy & Astrophysics [open access]

In today’s bite, we’re getting up close and personal with the cadaver of a dying star, known as a supernova remnant (SNR). These swirling nebulae of dust and gas emit brightly across the electromagnetic spectrum, from low-energy radio waves to high-energy gamma-rays. However, the identification of emission at different wavelengths is not always straightforward, particularly when an entire galaxy is blocking our view (here’s looking at you, Milky Way), as is the case for the Galactic SNR, G309.8+00.0, shown in Figure 1. 

Figure 1. The X-ray image of G309.8+00.0 obtained from eROSITA in the 1 – 2 keV band (blue), overlaid with the radio image at 843 MHz from previous studies. The X-ray emission is brighter in the lower half of the SNR. Figure 1 in the paper.

G309.8+00.0 is located in the Galactic plane, meaning there’s a whole lot of dust and dense material shrouding our view, in addition to countless other confounding sources that astronomers need to disentangle from the SNR signal. As such, G309.8+00.0 had only been observed at radio wavelengths, while its higher energy emission had remained a mystery. That is, until the authors of today’s paper conducted an in-depth study of its X-ray and gamma-ray emission, hoping to reveal the inner workings of this stellar corpse. 

The Nitty-Gritty of the X-ray Study

To get the best signal-to-noise possible, the authors combined data from the first four eROSITA all-sky surveys. eROSITA is an X-ray space telescope aboard the Russian-German Spektrum Roentgen Gamma (SRG) observatory which observes in the 0.2 – 10 keV energy range. From these observations, the authors were able to map for the first time the spatial distribution of the X-ray emission from G309.8+00.0, as shown in Figure 1. In the figure, red is the 843 MHz radio emission identified in previous studies, while blue is the emission detected by eROSITA in the 1 – 2 keV band. Clearly, the X-ray and radio emission are both associated with the SNR, but the X-ray emission is brighter in the lower half of the shell.

Next, the authors wanted to understand the nature of X-ray emission from the SNR. A great way of doing this is to model the X-ray spectrum, which tells you how much light is emitted as a function of wavelength, or equivalently, energy. The X-ray spectrum of G309.8+00.0 is given in Figure 2, which shows the actual data as the black markers and the best-fit model to the data in red. The other coloured lines are the individual components that together make up the best-fit model. Their results indicated that the X-ray spectrum is thermal in nature, meaning that it arises from hot gas rather than the acceleration of particles by magnetic fields. In addition, their best-fit model yielded a high absorption column density of ~ 3.1 x 1022 cm-2, reflective of the large quantity of material between us and the SNR due to its home in the Galactic plane. Finally, they estimated an age of (1 – 3.5) x 105 yr, making G309.8+00.0 a fairly old SNR. 

Figure 2. The X-ray spectrum of G309.8+00.0. The data from eROSITA are shown as the black markers, while the best-fit model is given by the red curve. The individual components that together make up the best fit spectrum are shown as the other coloured curves. Adapted from Figure 2 in the paper.

The Nuts and Bolts of the Gamma-ray Analysis

Figure 3. The gamma-ray emission from the source 4FGL J1349.5-6206c at 10 GeV. The more yellow the emission, the brighter it is. The yellow circle gives the point spread function (PSF), which indicates how a point source would appear blurred in the image due to the imaging capabilities of the Fermi-LAT. The blue contour indicates the radio-bright region of the SNR, as shown in Figure 1. Adapted from Figure 5 in the paper.

Next, the authors turned their attention to an even more energetic part of the electromagnetic spectrum, that of gamma-rays. To understand the emission from G309.8+00.0 in the 100 MeV – 100 GeV band, the authors used ~15.5 years of data from the Fermi Large Area Telescope (Fermi-LAT). Previously, an unidentified gamma-ray source 4FGL J1349.5-6206c was detected at the location of the SNR, which was confirmed by the authors of today’s paper using their deep observation shown at 10 GeV in Figure 3. From their analysis of the source, they suggested two possible explanations. Namely, that the emission arises from one large, single object, or that it is in fact due to four smaller sources. If the latter is true, one of these four point-like sources is spatially coincident with the same part of the SNR shell that is brightest in X-rays. This led the authors to suggest that a significant fraction (or even all) of the emission from 4FGL J1349.5-6206c is likely associated with the SNR G309.8+00.0. Given the old estimated age from their X-ray analysis ((1 – 3.5) x 105 yr), this would make G309.8+00.0 among the oldest gamma-ray bright SNRs ever detected.

Unfortunately, the authors were unable to constrain the mechanism responsible for the observed gamma-ray emission by modelling the spectrum. This was due to the complex environment and the narrow energy range in which the emission was observed. Despite this, the detection of high-energy X-rays and gamma-rays from G309.8+00.0 for the first time is an exciting result given its location in the bright and jam-packed galactic plane. However, with future, longer observations, more gruesome details regarding the stellar cadaver G309.8+00.0 might become available, providing the closure astronomers have been hoping for.

Astrobite edited by Janette Suherli

Featured image credit: Figure 1 in the paper (Michailidis et al. (2024))

About 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.

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