Clear daylight sighting of a bright object which rapidly accelerated and disappeared.My sister and I were standing on the front porch of her office, enjoying the unseasonably warm and clear weather. I noticed the apparent reflection of sunlight off an object in the air Southwest of our position, traveling in a Northerly direction, at what appeared to be roughly the same altitude and speed as commercial airliners.I called it to my sister's attention because the reflection was much, much brighter than I had ever seen from an aircraft at that altitude and it left no contrail as other aircraft were doing that day. My sister said "Wow, that looks like it's vertical." (i.e., oriented perpendicular to the Earth's surface, rather than parallel, as an airliner is.) We watched it for 15-20 seconds, during which time it traveled about the length of my index finger (held at arm's length, maybe 8-10 degrees of arc), then just disappeared.What made this sighting unusual was that after the reflection disappeared there was a brilliant flash of light further along the approximate track of the object. This flash occurred maybe a half-second after the reflection first disappeared, but the gap between disappearance and flash was about the same distance as the object had traveled during the initial (continuous) observation.So, during the gap between disappearance and flash, (using the formula Distance/Time=Distance per UnitofTime) the object seems to have accelerated to somewhere around 400% of it's initial speed in less than a second.