I think it is Linda Evans's The Far Edge of Darkness
ALL ROADS LEAD TO ROME
Sibyl Johnson—a graduate student In classical archeology, just this
side of her Ph.D., when an anachronism at her dig in Italy causes her
career to take a sudden lurch—and suddenly she's a slave in the very
society she was studying!
Charlie Flynn—a Miami cop, deep undercover on a Mafia sting operation
stumbled onto something he shouldn't have seen, end now he's scheduled
for a hit—in the Roman forum. Lagan McKee—once a commando, now a bum,
he got caught in a Florida thunderstorm and suddenly he was in
Alaska—five years after the storm struck. Now he's slated for a fatal
visit to the dark side of Classical Rome.
Three people, all castaways in time, all victims of the same evil
hand—all out for vengeance, on the Far Edge of Darkness.
This book was to have a sequel, but according to Ms Evans she ran into serious medical issues and the sequel remains unwritten. I do remember it as a good book, myself.
I initially thought this might be the book "Time Scout" by Robert Asprin and Linda Evans. Another in the series deals with Jack the Ripper, so it might be dark enough. It does involve the woman time traveller to Rome being raped.