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Hi Reemul,
At that late hour at that part of the sky, extremely likely to be a geosynchronous
satellite or an upper stage booster associated with same.
Being in such high orbits, these objects are still bathed in sunlight even late
at night. From here in Australia, there are a very large number of them
along the band in the sky in which they are parked.
An observing colleague and I have observed them ourselves on numerous
occasions and we have see them brighten and dim like a lighthouse in
the way you describe as they tumble.
When you throw, say, 20" of aperture at them you can sometimes detect
fainter brightening of some facets of the object as it rotates that you would
not normally see naked eye.
Were you sure it was moving in an Az/Alt sense? Through magnification such
as binoculars or a telescope, one can get the illusion that a geosynchronous
object is moving as the Earth rotates and the background stars appear to
move. On a telescope with a drive, you can turn the drive off and the satellite
will just stay steady in the FOV as the stars drift across the field.
As the sun, satellite, viewer angle slowly changes, the object will
appear to fade over time despite the fact that it is at the same azimuth and altitude
with respect the observer.
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