Photometric variability of massive young stellar objects. I
  The VVV survey has allowed for an unprecedented number of multi-epoch
observations of the southern Galactic plane. In a recent paper,13 massive young
stellar objects(MYSOs) have already been identified within the highly
variable(\Delta Ks > 1 mag) YSO sample of another published work.This study
aims to understand the general nature of variability in MYSOs.We present the
first systematic study of variability in a large sample of candidate MYSOs.We
examined the data for variability of the putative driving sources of all known
Spitzer EGOs and bright 24 \mu m sources coinciding with the peak of 870 \mu m
detected ATLASGAL clumps, a total of 718 targets. Of these, 190 point sources
(139 EGOs and 51 non-EGOs) displayed variability (IQR > 0.05, \Delta Ks > 0.15
mag). Light-curves(LCs) have been sub-classified into eruptive, dipper, fader,
short-term-variable and long-period variable-YSO categories.Lomb-Scargle
periodogram analysis of periodic LCs was carried out. 1 - 870 \mu m spectral
energy distributions of the variable sources were fitted with YSO models to
obtain representative properties. 41% of the variable sources are represented
by > 4Msun objects, and only 6% by > 8Msun objects.The highest-mass objects are
mostly non-EGOs,deeply embedded.By placing them on the HR diagram we show that
most lower mass,EGO type objects are concentrated on the putative birth-line
position, while the luminous non-EGO type objects group around the ZAMS
track.Some of the most luminous far infrared sources in the massive clumps and
infrared quiet driving sources of EGOs have been missed out by this study owing
to an uniform sample selection method.A high rate of detectable variability in
EGO targets (139 out of 153 searched) implies that near-infrared variability in
MYSOs is closely linked to the accretion phenomenon and outflow activity.
