As was pointed out the March 5th, 1979 GRB event seems to be the implied change.
This article links it to a possible magnetar in the Greater Magellanic Cloud.
This paper links soft gamma ray repeaters (SGR) with gamma ray bursters, stating its just a matter of direction; if you get hit full on it looks like a GRB, otherwise you see only the spillover, seen as a SGR.
I think the improbably event is that the burst hit us head on, at an order of magnitude or three greater than the real life burst. Apparently March 5 lasted only 2/10ths of a second - if it lasted 10 seconds it could have done more than swamp some astronomy instruments.
Anyway, the original post was for what the reaction would be. Panic (probable) or peaceful acceptance and a quiet passing into that good night (yeah, right).
If there are pocket of survivors I'd also go with an extreme paranoia and mystical mindset. Plenty of studies show that when people have to deal with events totally beyond their control they usually attribute events to non-logical sources; like evenly match sports teams with the members wearing lucky hats or socks or using lucky balls. The unfortunate Syrian situation was even more illustrative. Interviews with the anti-Assad forces were blaming Isreal and the West; they acknowledged that Assad was an enemy of Isreal, but through convoluted "logic" were able to convince themselves somehow Isreal and the US was behind their suffering. Politics aside that is the classic mark of people with nothing left and bewildered by events.
I'm sure survivor populations would be just as paranoid. Americans would blame Soviets, and visa versa. Traditional enemies would blame each other. If their scavenging teams met they would likely attack, and probably try to destroy enemy bases.
Homeline might think that Lucifer3 is empty but in fact have several pockets of very paranoid survivors, armed to the teeth and probably possessing nuclear weapons. Infinity agents blundering into THAT would have a heck of a time explaining themselves.