N132D: a cosmic-ray marathoner

September 2021

Previous | Index | Next

The atmosphere on Earth is continuously bombarded by energetic particles originating from outside the Solar System. Somewhat confusingly, we call these particles 'cosmic rays'. These particles are the reason why astronauts and pilots receive a higher radiation dose than do people on the Earth's surface. Cosmic rays are charged particles and do not travel through space in straight lines, as they are deflected by magnetic fields. However, cosmic rays colliding with gas in the Milky Way produce highly energetic radiation, called gamma rays, and by studying the gamma rays we learn about the original cosmic rays which were produced in sources at astronomical distances. Observing gamma-ray radiation is therefore important to understand the century-old problem of cosmic rays: What type(s) of sources are responsible for cosmic rays? By studying the cosmic rays in nearby and distant types of sources we may learn more about how cosmic rays in the Milky Way, including those observed on Earth, are generated.

Supernova remnants — the hot shells created by expanding supernova ejecta — are promising candidates for producing most of the cosmic rays in the Milky Way and similar galaxies. We know that the shocks at the boundaries of supernova shells accelerate cosmic rays, but we do not know when the cosmic-ray production in supernova remnants peaks and when the highest energies are reached: In the first year after the explosion when the shock speed is highest? Or in a few years when most of the explosion energy has been transferred to the gas surrounding the exploding star? It is generally thought that cosmic-ray acceleration is over its peak after a few hundred years. To answer these questions we need to study many supernova remnants, in different environments and with different ages.

The H.E.S.S. collaboration recently announced a study of the supernova remnant N132D [1] (see Figure 1), based on 252 hours of observations, that sheds new light on this question. Here we highlight this supernova remnant, which defies theoretical expectations. N132D is detected unambiguously in very-high-energy gamma rays which allows us to reliably compute a luminosity (i.e., radiated power) of this remnant at TeV energies.

Fig. 1: LMC N132D in X-rays, as observed with the Chandra X-ray telescope. Different colors show light emitted by different atoms: oxygen (red), iron (green), and neon (blue). Credits: NASA (HEASARC)

N132D is not located in our cosmic backyard, the Milky Way galaxy, but rather in the Large Magellanic Cloud galaxy (Figure 2). The Large Magellanic Cloud can be seen by naked eye from the Southern hemisphere. It is named after the explorer Ferdinand Magellan who sighted this galaxy — and the neighboring Small Magellanic Cloud — during his journey around the world in 1519. The Large Magellanic Cloud (LMC) is a small galaxy at a distance of 163,000 light-years and is captured by the gravity of the Milky Way. There are many supernova remnants in the LMC, including the famous SN1987A, which was the first naked-eye supernova since Kepler’s supernova in 1604 AD. But as far as gamma-ray emission from supernova remnants is concerned, N132D is the only supernova remnant in the LMC so far that has been detected in very-high-energy gamma rays. N132D is an important source, being bright in radio and X-rays, and belongs to the class of oxygen-rich supernova remnants. These are all remnants of stars that were born as rather massive stars, more than 18 times the mass of the Sun. Other oxygen-rich supernova remnants are Cas A and Puppis A, both of which are very bright radio sources. However, when it comes to the energy radiated in gamma-rays. N132D seems to beat all other known gamma-ray emitting supernova remnants. We only know of one other supernova remnant as luminous as N132D in gamma rays, J1640-465 (see the H.E.S.S. SOM 9/2013), and even the nearby, young and bright Galactic shell, J1713.7–3946 (see the H.E.S.S. SOM 9/2016), is less luminous (see Figure 3).

Fig. 2: Left: An image of the Large Magellanic Cloud (Credit: ESO). The zoom-in shows the gamma-ray map of the region containing N132D in false colors (taken from [1]).

On the one hand, the high gamma-ray luminosity may not be surprising, as N132D is also a bright supernova remnant in radio emission and X-rays. Moreover, N132D is a shell evolving in gas that is more than ten times denser than that surrounding other remnants. This helps the production of gamma-ray emission, as it arises when cosmic-ray particles collide with gas particles producing unstable particles called pions. These pions can only exist for a very short time until they fall apart and produce gamma rays. So, dense gas helps to make cosmic rays visible in gamma-ray light.

We observe N132D at an age of around 2500 yr after the explosion, and as mentioned above we expect supernova remnants at this age to be past their prime as cosmic-ray factories. But not N132D. In fact, H.E.S.S. detected gamma rays with energies as high as 10 TeV, indicating cosmic rays with energies approximately ten times higher, i.e. 100 TeV. For comparison, the highest proton energy achieved by a human made accelerator, the Large Hadron Collider at CERN, is 6.5 TeV. The 10 TeV gamma-ray energy detected from N132D is not as high as was observed from HESS J1702-420—see also our recent report on HESS J1702-420 [3] — but this may partially also be caused by the fact that N132D is much further away, and H.E.S.S. was not able to detect the fainter gamma-ray emission at higher energies. Moreover, the gamma-ray spectrum did not show any evidence for a steep decline of gamma rays at higher energies, suggesting that the spectra could continue above these energies. This is in contrast to some older supernova remnants seen in the Milky Way, such as the oxygen-rich Puppis A, which has an age not that different from N132D. Even more surprising, the young oxygen-rich remnant Cas A (340 yr) does show a decline in gamma-ray emission above 3 TeV. So as far as cosmic-ray acceleration goes, N132D seems to have more stamina than its younger cousin Cas A.

N132D suggests that not all supernova remnants sprint towards producing cosmic rays when they are young but that some are instead capable of sustaining cosmic-ray acceleration even when they are mature. So with N132D H.E.S.S. may have identified the first cosmic-ray marathon runner among supernova remnants.

Fig. 3: A comparison of the gamma-ray differential luminosity (EdL/dE) of five bright supernova remnants, all corrected for distance (adapted from [1]). The luminosity is compared to the luminosity of the Sun, but note that the Sun shines mostly in optical light and here the luminosity is in gamma rays. So in gamma rays, N132D is about 30 times more powerful than the Sun is in optical light! Of particular interest is that the gamma-ray luminosity of N132D is much higher than those of Cas A and Puppis A, despite all three being oxygen-rich supernova remnants. In addition, not only is N132D a marathon runner when it comes to cosmic-ray acceleration: the gamma rays it produces have also traveled the largest distance to reach us, having been underway for more than 163,000 years!

[1] H.E.S.S. Collaboration, Abdalla et al, 2021, "LMC N132D: A mature supernova remnant with a power-law gamma-ray spectrum extending beyond 8 TeV", A&A in press, arXiv:2108.02015

[2] H.E.S.S. Collaboration,Abramowski et al, 2015, "The exceptionally powerful TeV γ-ray emitters in the Large Magellanic Cloud", Science 347, Issue 6220

[3] H.E.S.S. Collaboration, Abdalla et al, 2021, "Evidence of 100 TeV gamma-ray emission from HESS J1702-420: a new PeVatron candidate", A&A in press, arXiv:2106.06405