Xperia J ST26i email notification sound without SIM card in case of incoming Gmail on my Xperia X

[SonyUser2017]

If I receive an email with Gmail on my Sony Xperia X Compact I hear a notification sound on my second device, a Sony Xperia J without SIM card. 

I like to know how these phones are connected even though the Xperia J has no SIM card. I have already tried if the phones are communicate with NFC. But I cannot see any connection if I lay the two phones on top of each other.

The display of the Xperia J is no more working (black). Therefore I can no more see what happens on this phone when I get an email.

Thank you for help.

Edited: I think I found a solution myself. Because the Xperia J doesn't support NFC the notifications stem probable from the Gmail App on this phone. It must be still connected to WiFi.