  We present results on a systematic study of flux variability on hourly
time-scales in a large sample of active galactic nuclei (AGN) in the 3-79 keV
band using data from Nuclear Spectroscopic Telescope Array. Our sample consists
of 4 BL Lac objects (BL Lacs), 3 flat spectrum radio quasars (FSRQs) 24 Seyfert
1, 42 Seyfert 2 and 8 narrow line Seyfert 1 (NLSy1) galaxies. We find that in
the 3-79 keV band, about 65% of the sources in our sample show significant
variations on hourly time scales. Using Mann-Whitney U-test and
Kolmogorov-Smirnov test, we find no difference in the variability behaviour
between Seyfert 1 and 2 galaxies. The blazar sources (FSRQs and BL Lacs) in our
sample, are more variable than Seyfert galaxies that include Seyfert 1 and
Seyfert 2 in the soft (3-10 keV), hard (10-79 keV) and total (3-79 keV) bands.
NLSy1 galaxies show the highest duty cycle of variability (87%), followed by BL
Lacs (82%), Seyfert galaxies (56%) and FSRQs (23%). We obtained flux
doubling/halving time in the hard X-ray band less than 10 min in 11 sources.
The flux variations between the hard and soft bands in all the sources in our
sample are consistent with zero lag.
