Son of Antiochus VII & Cleopatra Thea who challenged his half-brother, Antiochus VIII, after his mother's death (ca. 118 BCE) His persistence was rewarded by a treaty (116 BCE) that granted his right to rule half the Seleucid kingdom.  The partition did not end the rivalry between the half-brothers, however, & the lingering feud allowed Johanan Hyrcanus to declare Judean independence & conquer Samaria & Idumea.  Within a year of his brother's death, Antiochus was captured & executed by his nephew Seleucus VI, leading to open civil war among the children of the sibling rivals.

References: Josephus, Antiquities 13.270-282, 325, 366-369.
                   Justin, Epitome 39.2-4.
                  Appian, History of Rome: Syrian Wars 68.

Other online resources:

• Antiochus IX Cyzicenus - Jona Lendering chronicles events & lists primary resources [Livius: Articles on Ancient History].

• Antiochus IX Cyzicenus - article by Louis Ginzberg in the 1901 Jewish Encyclopedia.

• Antiochus IX Cyzicenus - entry in Wikipedia's web.

Silver tetradrachma with profile of a bearded Antiochus IX on the face & the standing figure of Athena holding a spear in one hand & winged Victory in the other.  The inscriptions reads: Basileos Antiochou Philopatoros ["of King Antiochus, who favors his father"].