THE DETECTION OF A GAMMA-RAY TRANSIENT ASSOCIATED WITH GW170817 HERALDED THE NEW ERA OF MULTI-MESSENGER ASTROPHYSICS. HOWEVER ITS ORIGIN HAS YET TO BE FULLY UNDERSTOOD. THE TIMING AND ENERGETICS OF THE GAMMA-RAY TRANSIENT WERE CONSISTENT WITH A SIMPLE COCOON MODEL BUT THE SPECTRUM SHOWED SIGNS OF DISSIPATION AND PAIR SATURATION. A SHOCK BREAKOUT CAN EXPLAIN THE SPECTRUM BUT REQUIRES THE PRESENCE OF AD-HOC EJECTA FROM THE MERGER AND AN UNCOMFORTABLE FINE-TUNING BETWEEN THE COLLAPSE-TO-JET DELAY AND THE JET TRANSIT TIME. WE ARGUE THAT AN UNSTEADY CENTRAL ENGINE DRIVES MULTIPLE COCOON SHELLS WHOSE COLLISIONS CREATE A NATURAL SHOCK BREAKOUT. WE PROPOSE TO DERIVE THE BREAKOUT PROPERTIES ANALYTICALLY AND PERFORM NUMERICAL SIMULATIONS TO CONFIRM THE VALIDITY OF THE MOST INTERESTING CASES.

Oregon State University, Corvallis OR

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