Event handling don't work with this

Why I can't get data from this in ivent handler and how cant it be fixed?

twittyApp.factory('Unshown', function () {
function Unshown() {
    this.allIsLoaded = false;
    this.loading = false;
    this.firstTime = true;
    this.scrollMarker = 100;
    this.loadedUnshownPages = 0;
    this.timeLineHiader = $cookies.get("lastReadedTweetId");
}
window.onscroll = function () {
     //why here this.scrollMarker is undefined?
};
return Unshown;
});