At the rate of 1 bit in 18 minutes (assuming to just coding the signal as off/on every period), that's 34.2 years to transmit 1 MB. Without error correction. If its aliens, they're not in a particular rush to transmit the latest weather data.
But see, you're not thinking as a Vegan. They'd be using higher harmonics to encode much more data than just the one bit. I thought we had all learned this by now! Back to remedial sci-fi for all of you!
The period - 1080 seconds - is interesting by itself. If I were an alien and wanted to broadcast some important number, an ultra stable magnetar would be my top choice.