ASP.Net MVC Identity: Login after registration success

I managed to save google profile info into user db table but when redirecting to an authorize page it is asking for login username and password, why?

[Route("google-response")]
public async Task<ActionResult> GoogleResponse()
{
     var google_csrf_name = "g_csrf_token";
     var cookie = Request.Cookies[google_csrf_name];
     if (cookie == null)
         return StatusCode((int)HttpStatusCode.BadRequest);

     var requestbody = Request.Form[google_csrf_name];

     if (requestbody != cookie)
         return StatusCode((int)HttpStatusCode.BadRequest);

     var idtoken = Request.Form["credential"];
     GoogleJsonWebSignature.Payload payload = await GoogleJsonWebSignature.ValidateAsync(idtoken).ConfigureAwait(false);

     TempData["email"] = payload.Email;

     //return Json(payload);

     var user = new IdentityUser { UserName = payload.Email, Email = payload.Email };
     await _userManager.CreateAsync(user).ConfigureAwait(false);
     return RedirectToAction("Index","Category");

how to sign the user in before redirecting?