“…It is possibly for this reason that several alternative ideas to the SNR paradigm, proposed years ago to explain the origin of CRs, have been recently revived. The list includes the acceleration of particles at the Galactic centre (see 12 for an early reference, 255 for a recent one), at stellar winds (see 280 for an early reference, 281 for a recent one), in superbubbles (see 282,283 for early references, [259][260][261] for recent ones), or the explanation of at least part of the CR spectrum with a single (or very few) sources (see 284 for an early reference, 285 for a recent one). Finally, other classes of sources (such as X-ray binaries 286,287 or pulsar wind nebulae 288 ) have been considered, and attempts to develop truly heterodox scenarios were published, as we briefly discussed in Section 3.1.5.…”