Whose bow did King Janaka make each of Sita's suitors try to string?

In the Ramayana, King Janaka possessed an extremely powerful bow belonging to Lord Shiva, called the Pinaka. He set it as the challenge for Sita's swayamvara (ceremony to choose a husband), stating that only the suitor who could string this divine bow would marry Sita. Rama successfully lifted and strung Shiva's bow, winning Sita's hand in marriage. Neither Vishnu, Parshuram, nor Janaka owned the bow that was used for this test.