FUV Variable Sources in M31
  The Ultraviolet Imaging Telescope (UVIT) onboard the AstroSat observatory has
imaged the Andromeda Galaxy (M31) from 2017 to 2019 in FUV and NUV with the
high spatial resolution of ~1". The survey covered the large sky area of M31
with a set of observations (Fields) each 28 arcminutes in diameter. Field 1 was
observed in two epochs with the F148W filter, separated by 1133 days (~3.10
years). The 6.4 kpc diameter Field 1 (at the distance of M31) includes a
substantial part of the inner spiral arms of the galaxy. We identify UVIT
sources in both epochs of Field 1 and obtain catalogs of sources that are
variable in FUV at > 3 sigma and > 5 sigma confidence level. The fraction of
FUV variable sources is higher for brighter sources, and the fraction is higher
in the two main spiral arms compared to other areas. This is evidence that a
significant fraction of the FUV variables are associated with hot young stars.
Source counterparts are found for 42 of the 86 > 5 sigma FUV variables using
existing catalogs. The counterparts include 10 star clusters, 6 HII regions, 5
regular or semiregular variables, 6 other variables and 6 nova or nova
candidates. The UVIT FUV-NUV and FUV-FUV color-magnitude diagrams confirm the
association of most of the FUV variables with hot young stars. A catalog of
UVIT photometry for the variable sources is presented.
