Even though written keep session alive for all browsers but in IE8 session objects will be null after 20 minutes what is the reason of failing from below code?

    function KeepSessionAlive() {
            try {
                var xmlhttp;
                if (window.XMLHttpRequest) {
                    // code for IE7+, Firefox, Chrome, Opera, Safari
                    xmlhttp = new XMLHttpRequest();
                }
                else {
                    // code for IE6, IE5
                    xmlhttp = new ActiveXObject("Microsoft.XMLHTTP");
                }

                xmlhttp.open("GET", "KeepSession.aspx", false);
                xmlhttp.send();

       }
            catch (e) {
            }
        }

        function startPinging() {
            window.setInterval(KeepSessionAlive, 300000);
        }

When it calls KeepSession.aspx page load event in IE It will be null