2002A&A...381..151S

The analysis of the recurrence time T_C of outbursts in Aql X-1, using the method of O-C residuals, revealed that the character of the O-C curves bears a striking similarity to that of dwarf novae. It means that variations of T_C are large, but generally not chaotic and long-term trends in the O-C curves can be resolved. The prevailing value of T_C is 211 days but the evolution of T_C shows several large jumps. The mean T_C varies by more than 1:2 and the switches occur within just a single epoch. A comprehensive study of the morphology of the light curves from ASM/RXTE revealed that although the respective outbursts in Aql X-1 differ in both their duration and the maximum intensity, their rising and decay branches display remarkable similarities. The rising branches are exponential. The slope of the decay is broken in some events, with the final phase having a steeper slope and being roughly linear. We argue that just the inner disk is irradiated during outburst and that viscous heating plays a big role. The evolution of the hardness ratios HR1 and HR2 displays common characteristics for the respective outbursts observed by ASM/RXTE. HR1 at the peak intensity of the more intense outbursts tends to be harder. Similar behaviour is also apparent for HR2. An asymmetric course (hysteresis) of HR2 with respect to the moment of maximum intensity is apparent for some outbursts, the largest value of HR2 occurring on the rising branch. The observed behaviour is discussed in terms of the thermal instability model and variations of the thermal and power-law spectral components.