Laravel 5.2 - Auth::login not preserve when using new \App\User

I've used these code to authenticate username from external source, but laravel 5.2 not save the authentication, and request the external source every time.

class Authenticate
{
    public function handle($request, Closure $next, $guard = null)
        {
            if (Auth::guard($guard)->guest()) {
                if ($request->ajax()) {
                    return response('Unauthorized.', 401);
                } else {
                    $username = getFromExternalSource();
                    if($username==null){ redirect()->guest('auth/login'); }
                    $user = new \App\User(['username'=>'admin']);
                    Auth::login($user);
                }
            }

            return $next($request);
        }
}

But when I change Auth::login using $user model get from database it work, I don't know why:

$userDB = \App\User::where('username','=','admin')->first();
Auth::login($userDB);

via Chebli Mohamed