This artifact falls under the heading of flare / ghost image. It is caused by internal reflections. The camera lens is a complex array of multiple polished glass lens elements. Some elements are cemented together, some are air-spaced. Each has two polished surfaces that reflect away about 2% of the light that otherwise would traverse the lens. The 2% that is reflected hits other polished surfaces and reflects. The bottom line is, there is a fair quantity of unwanted reflections going on inside the camera.

Most of this stray light bathes the film or sensor with a uniform level of stray light, and this reduces image contrast. What you are seeing is a ghost image of the iris (aperture). This is a common phenomenon caused when there is a bright light source just outside the edges of the image.

You could have likely mitigated this artifact if you had mounted a lens hood. This is a funnel-like attachment that mounts surrounding the front of your camera lens. The lens hood shields the lens from seeing bright sources that are just outside the camera’s field of view. You might consider buying one, they are not too expensive. If due to bright light sources within the vista, nothing works to mitigate.

In the past, such artifacts and ghost images were more evil. Glass lenses actually reflect away about 4% however, nowadays they have a coat or multiple coats of minerals that reduce reflections to about 2%.