How long does soul trap last on the bound sword? Because last night while playing I struck a bear with the sword, then casted thunderbolt on it a couple time and killed it, never touching it again while still picking up the soul. So does anyone have hard numbers on the bound sword's soul trap duration?

Don't believe there is an actual duration associated with the bound sword spell. You only capture the soul if you make the kill using the bound sword. At least that is my experience.

That's what I thought as well, but have found that when I hit an enemy they kinda turn a light shade of purple and I'm able to nab the soul even if my summon or a different spell kills the enemy.

Hmm, now that I think about it I have captured at least one soul when a guard killed a mob. I would wonder if the duration for a bound sword hit would be set to the same as the lowest enchanted weapon to have the soul capture enchantment which I believe is 3 seconds. Also I haven't played using followers or summons yet.

Edit: Guess we'll need to wait for the CK so we can check the values of the enchantments.

You're probably right with the 3 seconds which is nice because I always assumed it was "on contact" to capture the soul.