How to measure impulse responses with the Sine Sweep, guide for correct I.R. measurements

That's called "time of flight", is the gap of silence before the direct sound.
The definition of most parameters given in the ISO 3382 standard does not take into any account the time of flight, as the "zero of time" is taken at the rising time of the peak of the direct sound.
Aurora locates this point by means of a trigger (Direct Sound Trigger), which by default is set to detect the first sample where the signal becomes larger than a value which is 20 dB smaller than the maximum. Of course you can change the trigger sensitivity by changing this value, in the initial screen of the acoustics parameters plugin.
Once the trigger point has been found, everything happening before it is just ignored in the computation of Clarity, Center Time, Definition, IACC, LF, etc...