In order to form ionic bonds (bonds of metals and non metals together), the metal atom must give up electrons to the non-metal and turns into a ion (cation specifically).
Since calcium is a metal, when it forms ionic bond with bromine, a non-metal, calcium atoms must give up electrons. As you may know, each electron carries a negative charge. Therefore, when a atom loses negative charged electrons, they become positive, due to the remaining protons are more than electrons (protons carry positive charge).
Therefore, the answer is a positive ion.