With the release of Windows Phone 8.1 SDK comes a new StatusBar. The StatusBar replaces the SystemTray from Windows Phone Silverlight Apps. Unlike the SystemTray, the StausBar can only be accessed via code and some functionality has changed.
StatusBar statusBar = Windows.UI.ViewManagement.StatusBar.GetForCurrentView();
// Hide the status bar
await statusBar.HideAsync();
//Show the status bar
await statusBar.ShowAsync();
Reference: Differences between the new StatusBar in Windows Phone XAML Apps and the SystemTray
Msdn Reference: StatusBar class