Scientists explore gamma-rays from “superbubble”

Study reveals production mechanism of very-high-energy radiation

The 'superbubble' 30 Doradus C, captured by the satellite Chandra. Credit: Nasa

An international scientific team, including Stefan Ohm from DESY, used NASA's Chandra X-ray Observatory to reveal how very-high-energy (VHE) gamma rays are produced by the 30 Doradus C “superbubble”, an astronomical phenomenon on the Southern sky and the only one of its kind visible in VHE gamma-rays. The energetic radiation is the product of the interaction of fast electrons with ambient light, as the team working with lead author Patrick Kavanagh from the Dublin Institute for Advanced Studies reports in the journal Astronomy & Astrophysics.

30 Doradus C is located in the Large Magellanic Cloud (LMC), a dwarf satellite galaxy of the Milky Way at a distance of about 170,000 light years. Even at this great distance it is visible to the naked eye in the southern hemisphere. Star formation is proceeding at a high rate in the LMC and very large stars (many tens the mass of the sun) are being born into new, massive stellar clusters. Collectively, through their powerful stellar winds and later their supernova remnants, these massive stellar populations blow huge superbubbles into the surrounding interstellar medium. It is at the blast wave of the interior supernova remnants that cosmic rays are thought to be accelerated.

VHE gamma rays are excellent tracers of cosmic-ray accelerators such as supernova remnants. Charged particles are accelerated to incredibly high velocities and VHE gamma rays can be produced either by accelerated electrons interacting with light, or accelerated protons interacting with gas. “While VHE gamma rays have been detected from 30 Doradus C before, it is not clear which mechanism dominates the gamma-ray production,” explains Ohm.

A key piece of evidence to address this question is the strength of the magnetic field near the acceleration site. If the magnetic field is high, then the interaction of accelerated protons with gas will dominate the gamma-ray emission. On the other hand, if the magnetic field is low, the interaction of accelerated electrons with light is more efficient. Crucially, as the electrons move away from the acceleration site, they will lose energy by emitting X-rays at a rate that depends on the strength of the magnetic field. The observed width of the X-ray emitting regions can therefore be used as a probe of the magnetic field strength.

Resolving these emission regions at the distance of the LMC in sufficient detail requires a very powerful telescope. The combination of Chandra's exquisite mirror and the Advanced CCD Imaging Spectrometer revealed the structure of the 30 Doradus C shell in unprecedented detail. This allowed the team to measure the widths of the emission region around the shell. “We could determine a generally low magnetic field strength, suggesting that the VHE gamma-rays produced in 30 Doradus C predominantly arise from the interaction of accelerated electrons with ambient light,” reports Ohm.



Magnetic field estimates from the X-ray synchrotron emitting rims of the 30 Dor C superbubble and the implications for the nature of 30 Dor C’s TeV emission; Patrick J. Kavanagh, Jacco Vink, Manami Sasaki, You-Hua Chu, Miroslav D. Filipovic ́, Stefan Ohm, Frank Haberl, Perica Manojlovic, Pierre Maggi; Astronomy & Astrophysics, 2018; DOI: 10.1051/0004-6361/201833659