We need sticky topics.
Go to forums.
Search for abs show.
Go to google. Type "ab definition"
At bodyfat percentages that are low, abs will show, even in folks who are starving, and even in folks who weigh lots.
To answer your question, based on the extremely low amount of information presented, you are not lean enough. If you are leaner, your abs will show. You will get leaner by increasing your calories to allow you to work harder (you're very thin now), and allowing for a faster metabolism, gaining muscle weight to raise your base metabolism, to burn more fat at rest, and by consuming slightly fewer calories than you burn, and by metabolic activation via HIIT.
Review the other postings on this topic here on realjock.com, or bodybuilding.com.
If you are lean, and muscular, you will have abs, regardless of your weight. Right now, you are neither lean, nor muscular, but, you're underweight, and underfed, and your body is a fat storing machine because it's likely in famine mode from not eating enough. Eat.
In this picture, I'm 190#, at 5"5' and not without "abs."
You MUST eat to get lean and stay lean and support an active metabolism.