Hipparchus was Pisistratus' son and shared power in Athens with his brother Hippias, whose tyranny got significantly harsher following the murder of Hipparchus.
Bouchard, Elsa. "Murder of Hipparchus by Harmodius and Aristogiton". The Literary Encyclopedia. First published 01 March 2010
[http://www.litencyc.com/php/stopics.php?rec=true&UID=11173, accessed 18 May 2013.]