The Kepler supernova remnant: a cosmic particle accelerator
The role of supernova remnants (expanding clouds produced by supernovae) in the acceleration of cosmic rays (high-energy particles present in various astrophysical environments) has been known since 1995. The discovery, made by astronomers from Kyoto University, was made possible by identifying the presence of non-thermal X-ray emission in the supernova remnant SN 1006. X-rays are a type of high-energy radiation that can be emitted by plasma at millions of degrees (thermal emission) or by high-energy particles in various processes (non-thermal emission). In particular, the non-thermal emission observed in SN 1006 was synchrotron radiation, produced when high-energy particles move along magnetic field lines.
An object of great interest for the study of these processes is the Kepler’s supernova remnant, produced by the type Ia supernova (i.e., one whose progenitor was a white dwarf that accreted matter from a companion star in a binary system) SN 1604. This supernova remnant is interacting with a dense cloud of circumstellar material in its northern region. This causes the properties of the supernova remnant and the expansion speed of the shock wave in that direction to differ from those in other regions of the supernova remnant. In fact, a 2022 study conducted by researchers from the University of Palermo and INAF – Osservatorio Astronomico di Palermo demonstrated that in Kepler’s supernova remnant, cosmic ray acceleration is more efficient in the northern region than in the southern regions.
With the aim of continuing the analyses begun in 2022, the research team led by V. Sapienza (University of Palermo and INAF – Osservatorio Astronomico di Palermo) analyzed an extensive set of X-ray observations obtained with NASA’s Chandra satellite of Kepler’s supernova remnant. Specifically, the authors analyzed certain filaments of the supernova remnant to measure their motion in the sky (proper motion) and determine the parameters of the synchrotron emission coming from these regions. The observations confirm the greater efficiency of the cosmic ray acceleration process in the northern region of the supernova remnant. This result was not obvious, as cosmic ray acceleration depends on the speed at which the shock wave of the supernova remnant propagates, which in this case is slower in the north due to the interaction with the circumstellar medium cloud. However, this interaction triggers turbulence that influences the local magnetic field, increasing the efficiency of the cosmic ray acceleration process. Consequently, the higher efficiency in the particle acceleration process observed in the northern regions of Kepler’s supernova remnant in 2022 arises from the balance between the effects of shock wave deceleration and the turbulent topology of the magnetic field. The study is described in the article “Time evolution of the synchrotron X-ray emission in Kepler’s SNR: the effects of turbulence and shock velocity“, recently published in “The Astrophysical Journal.” Co-authors include O. Petruk, S. Orlando, and F. Bocchino from INAF – Osservatorio Astronomico di Palermo, and M. Miceli from the University of Palermo.
The cover image (click here to view it in full) shows an X-ray image of Kepler’s supernova remnant obtained from the Chandra satellite.
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