  At present, the main challenge to the interpretation of variations in
gravity-sensitive line strengths as driven by a non-universal initial mass
function (IMF), lies in understanding the effect of other parameters describing
unresolved stellar populations, such as elemental abundance ratios. We combine
various TiO-based, IMF-sensitive indicators in the optical and NIR spectral
windows, along with the FeH-based Wing-Ford band to break this degeneracy. We
obtain a significant radial trend of the IMF slope in XSG1, a massive
early-type galaxy (ETG), with velocity dispersion sigma~300km/s, observed with
the VLT/X-SHOOTER instrument. In addition, we constrain both the shape and
normalization of the IMF based only on a stellar population analysis. We
robustly rule out a single power-law to describe the IMF, whereas a power law
tapered off to a constant value at low masses (defined as a bimodal IMF) is
consistent with all the observational spectroscopic data and with the stellar
M/L constraints based on the Jeans Anisotropic Modelling method. The IMF in
XSG1 is bottom-heavy in the central regions (corresponding to a bimodal IMF
slope Gb~3, or a mass normalization mismatch parameter alpha~2), changing
towards a standard Milky-Way like IMF (Gb~1.3; alpha~1) around half of the
effective radius. This result, combined with previous observations of local IMF
variations in massive ETGs, reflects the varying processes underlying the
formation of this type of galaxies, between the central core and the outer
regions.
