Can a PhD student publish a paper outside PhD topic without supervisor approval only by its own?
There is no general reason why this should not be possible/allowed.

You will have to check the regulations of your department/university for PhD students: They may require you to seek approval of your advisor prior to submission of papers. Potentially a journal could have a similar rule in place, but I have never encountered this.

That said, I do not believe this to be advisable: If your relationship with your PhD advisor is so bad that do you not feel you can ask them about your intention to submit this paper, you should probably seek a new advisor. If you are so close to graduating that your prefer sitting it out, then rocking the boat is probably a bad idea.