How do I pass an object into a timer event?

You can save string in some object and read it in event handler:

static string _value;

static void MyElapsedMethod(object sender, ElapsedEventArgs e)
{
    Console.WriteLine(_value);
}

UPDATE: same code via different syntax: timer.Elapsed += (s,e) => Console.WriteLine(_value);

UPDATE: Consider also using System.Threading.Timer instead

State state = new State();
Timer timer = new Timer(OnTimer, state, 0, 123);
state.Value = "FooBar"; // change state object

You can retrieve state in timer callback:

static void OnTimer(object obj)
{
    State state = obj as State;
    if (state == null)
        return;        

    Console.WriteLine(state.Value);
}

If you want to be able to unregister your "Elapsed" event handler again, you shouldn't use a delegate without remembering it in a variable.

So another solution could be to create a custom class based on Timer. Just add whatever members you like and get your custom Timer object back from the "sender" argument of the "Elapsed" event handler:

class CustomTimer : System.Timers.Timer
{
    public string Data;
}

private void StartTimer()
{
    var timer = new CustomTimer
    {
        Interval = 3000,
        Data = "Foo Bar"
    };

    timer.Elapsed += timer_Elapsed;
    timer.Start();
}

void timer_Elapsed(object sender, ElapsedEventArgs e)
{
    string data = ((CustomTimer)sender).Data;
}

This strategy of course works for other events and classes too, as long as the base class is not sealed.

The easiest way to do this is to change the event handler into an anonymous function. It allows you to pass the string at the point of declaration.

string theString = ...;
timer.Elapsed += (sender, e) => MyElapsedMethod(sender, e, theString);

static void MyElapsedMethod(object sender, ElapsedEventArgs e, string theString) {
  ...
}

 Timer aTimer = new Timer(300);
                aTimer.Elapsed += delegate { PublishGPSData(channel, locationViewModel); };
                // Hook up the Elapsed event for the timer. 
                aTimer.AutoReset = true;
                aTimer.Enabled = true;
private void PublishGPSData(IModel channel, LocationViewModel locationViewModel)
{
};