Historians consider this long Last journal to be a breakthrough in their understanding of the Byzantium empire based on this information the journal is a what source

The correct answer to this open question is the following.

It seems that there context missing in this question.

The question starts with "Historians have recently discovered a historical journal published in the early twentieth century. The journal contains facts based on original sources about the downfall of the early Byzantium empire."

Historians consider this long last journal to be a breakthrough in their understanding of the Byzantium empire.

Based on this information the journal is a secondary source because it is based on research that was published in the early 20th century. This means that the authors based their research on other original or primary sources. So when historians or researchers use primary resources to develop their own research, this new material is considered to be secondary sources.

Original or primary sources are all kinds of evidence written or recorded by people who lived in those historical times, participated in the events, or witnessed the events.

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