“… those Viking saga kings, Ragnar Lodbrok and Björn Järnsida. I’d like to know if there exists any evidence at all that these persons ever existed?”
In the present, the categories “real person” and “fictional character” are pretty distinct. But when we look retrospectively at the first historically documented centuries in any given area, things get fuzzy. And it’s even worse if we look at people who are supposed to have lived before the introduction of writing to an area, and who are mentioned in early or foreign texts. These centuries to either side of the introduction of writing is known as protohistory, and protohistorical information is strictly speaking not factual knowledge. Not because we know that it’s wrong, but because it is impossible to corroborate. Protohistory is information of indeterminate value, which is extremely frustrating to many amateur historians who Want To Believe.
Beowulf and Arthur are protohistorical figures. And Arthur illustrates another tendency: even if there’s is reason to believe that he existed, almost none of his most famous characteristics are likely to be historical truth. He probably wasn’t named Arthur, he had no round table, his queen wasn’t named Guinevere, his stronghold wasn’t named Camelot, none of his “knights” wore plate armour, and so on.
Then on the other hand there are all the countless real, living individuals of the past of whom we know the name, the approximate date and the region they lived in – and nothing more. Because when people start writing, they usually start small, and one of the first things they write is names. We’ve seen it recently on the Hogganvik rune stone. Skelbathewar and Naudigastir were in all likelihood as real as anybody who reads this, but we hardly know anything about them. They are not protohistorical, just extremely poorly documented.
But perhaps Felicia’s question was specifically about Ragnar Lodbrok and Björn Järnsida, not so much about generalities. Both are protohistorical figures: Ragnar Shaggy-breeches and Björn Ironside.
In Ragnar’s case, we’re dealing with a documented 9th century Viking leader of that given name onto whom later story-tellers painted thick layers of fiction, including his second name. Or perhaps it is meaningless to make that identification since the historical guy and the Ragnar of later legend have so little in common.
Björn is said to have been Ragnar’s son and is an even worse candidate for factuality than his dad. There is contemporaneous written evidence for a Ragnar at the right time and in the right area. Not so with Björn. And this is why historians have long since scrapped the idea of a “Munsö royal lineage” springing from him.
Source-critical historians try to avoid believing in anything until good-quality sources force them to. Such a historian needn’t spend much time thinking about Ragnar or Björn. Alternatively, you can have the opposite attitude: that you want to avoid disbelieving any statement in early sources unless contradictions force you. Trouble is, it’s possible (and quite easy) to write internally consistent fiction. Thus, in my opinion, only the source-critical attitude deserves the moniker vetenskap or Wissenschaft, that the English language unfortunately lacks.
Dear Reader Peter Olausson points to a piece where historian Dick Harrison says pretty much what I said above.