I work for a very large physician practice in Georgia that has privileges to treat/admit patients at two major area hospitals. When we initiate an admission, or plan a surgery for a patient, whether that be inpatient or outpatient, it is always our responsibility to obtain authorization (since we planned the course of treatment).
If a patient presents to the Emergency Department and is admitted by our physician through the ER, it is still the responsibility of that hospital to obtain authorization since they presented (unplanned) to the ER.
If another physician has admitted a patient and we happen to see that patient during the admission (perhaps we consult, perhaps we do a surgery, and continue seeing them until they are discharged), it is not our responsibility to obtain authorization, since our services should be covered under the "umbrella" of the authorization that is on file for that entire admission.
All of our insurance payors recognize these guidelines. Best of luck to you!