The subject of your question is flash synchronization. A little background information will help you understand.
Your camera sports a “focal plane” shutter design. This mechanism uses a curtain, not unlike a window shade to cover the entire image sensor chip. Thus its normal state is closed and since it is opaque, it prevents light from the lens from playing on the surface of the image sensor.
When you press the “go” button, the shutter curtain begins its journey. It travels the span of the image area. Now the curtain contains a slit. This slit uncovers a portion of the imaging sensor. That portion is the actual width of the slit. When the shutter is set high, say 1/1000 of a second, the silt width is tiny. The image sensor only sees light from the lens through this slit. The slit travels the entire distance exposing all entire image area but the travel time is longer than 1/1000 of a second. The shutter speed is the time it takes for the slit to travel just its width.
As you change shutter speeds, you are actually changing the slit width. Each time you slow down the shutter you are actually enlarging the slit width. On your camera at 1/200 of a second the slit width is just wide enough to expose the entire image area to the light from the lens.
When using an electronic flash, the duration of the flash is super quick. If your shutter is set faster than 1/200 of a second, the blitz of the flash catches an image of slit. Sorry to report that if you choose a shutter speed greater than 1/200, the result will be partially uncovered image area.
By the way, we call the setting you need “X” synchronization. In the early days of flash bulb photography, a voltage was applied to the flash bulb in advance of the opening of the shutter. This setting was called “M” synchronization. The “M” is for medium delay which was 20 milliseconds in advance of the shutter reaching full opening. Since electronic flash requires no delay, this zero delay was labeled “X”. Check your camera manual, you will find that your camera works with electronic flash at the “X” synchronization setting of 1/200 of a second shutter speed or slower. These slower shutter speeds provide a slit width that uncovers the entire image area.
