@Ashwin N. First thing real quick, I wouldn't overly concern yourself with CAP rates on residential properties. I'm not sure why that seems to be such a big thing right now.
At the end of the day you have to figure out your risk tolerance. Fitchburg is rough..I grew up about 25 mins from there so I know it decently well. You will deal with low income tenants. The city does, however, at least have a college so maybe you look into targeting students. My uncle owns a few properties within close proximity to the college and he does okay.
If you are just looking for a passive turnkey investment, personally, I would look at Worcester. There seems to be some development planned and you will just overall deal with a better tenant class. There is also a few good colleges in the area.
Look for solid neighborhoods, with high renter demand and you won't have to worry all that much about vacancy, especially in our markets. And don't just settle for an average property manager if you go that route.
Hope this helps!