0MQ / NetMQ - pub-sub - How can a subscriber know it the publisher is down?

Question

Using NetMQ (port from ZeroMQ) with C#.

The publisher and subscriber live on different web services.

At an underdetermined time, a publisher will broadcast a message to all subscribers.

If the web service which holds the publisher goes down, how can the subscriber know about that?

I am trying to avoid using a polling mechanism to solve this problem e.g. broadcast a message every x seconds, if subscriber doesn't get a message in x seconds, you may assume that server is down.

Example Publisher:

    private PublisherSocket _publisherSocket;

    public void StartPublisher()
    {
        _publisherSocket = new PublisherSocket();

        _publisherSocket.Bind("tcp://localhost:1234");

    }

    public void SendMessage(string message)
    {
        _publisherSocket
            .SendMoreFrame("superMessage")
            .SendFrame(message);
    }

Example Subscriber:

    public void StartSubscriber()
    {
        _subscriberSocket.Connect("tcp://localhost:1234");
        _subscriberSocket.Subscribe("superMessage");

        while (_subscriberIsActive)
        {
            // Blocks until a message is received
            var messages = _subscriberSocket.ReceiveMultipartStrings();

            Console.WriteLine(messages[1]);
        }
    }