Phoenixes are not physical beings and when people say 'it exists on the astral' it means they exist spiritually. If you want to research phoenixes, look to the lore they come from.
Here's a starting point for phoenixes. Keep in mind that Wikipedia can be altered, but also look to the sources they cite and cross reference them with validated sources.
A little bit about the states of the brain.
A little bit on identification of birds.