One of the two breakout reactions from the hot CNO ( HCNO ) cycle is ^ { 15 } \textrm { O } ( \alpha, \gamma ) { } ^ { 19 } \textrm { Ne } , which at low temperatures depends strongly on the resonance strength of the 4.033 MeV state in ^ { 19 } \textrm { Ne } .
An experimental upper limit has been placed on its strength , but the lower limit on the resonance strength and thereby the astrophysical reaction rate is unconstrained experimentally .
However , this breakout reaction is crucial to the thermonuclear runaway which causes type I X-ray bursts on accreting neutron stars .
In this paper we exploit astronomical observations in an attempt to constrain the relevant nuclear physics and deduce a lower limit on the reaction rate .
Our sensitivity study implies that if the rate were sufficiently small , accreting material would burn stably without bursts .
The existence of type I X-ray bursts and superbursts consequently suggests a lower limit on the ^ { 15 } \textrm { O } ( \alpha, \gamma ) { } ^ { 19 } \textrm { Ne } reaction rate at low temperatures .