Rachel Uchitel -- Ex-Husband Matt Hahn Arrested for Harassment ... 'I Hope You Die'

Rachel Uchitel's ex-husband was arrested for calling her multiple times, and at least in one case, wishing she dies during her looming brain surgery ... TMZ has learned.

We're told Rachel claimed Matt Hahn called her repeatedly on 9/11, allegedly saying among other things, "I hope you die." Rachel told cops he said this in front of their 3-year-old daughter, who was with him at the time.

Rachel will undergo serious brain surgery on Tuesday.

Hahn -- a former Penn State running back -- was arrested by NYPD for aggravated harassment. Rachel now has an order of protection.