Hi,
thank you for the quick replies. The exact name of what I bought (on ebay) is "15 Mhz HC49/4H crystal HC49S xtal Oscillator", here the link:
As per the name I was expecting an oscillator, but my googling this afternoon made me think I have just a crystal.
Would you have any info / hints on how can I achieve the goal of getting a signal in the Arduino whenever a pulse happens? I guess I will have a build a big circuit around it..
Nevertheless, if I am greatly mistaken by trying to use a crystal / oscilloscope to just signal on an Arduino pin periodically... just tell me 
Thank you!