Abstract

We present a generalized phenomenological parameterization of the deceleration parameter q(z) that incorporates an effective radiative component (ERC) in addition to a localized late-time contribution. The proposed framework extends previous two-parameter q(z) reconstructions by explicitly regulating the high-redshift behavior while preserving the late-time transition dynamics. We constrain the free parameters (h, q(0), z(c), z(e)) using late-time observational data from cosmic chronometers (CC), Pantheon+ Type Ia supernovae (SNIa), HII galaxies (HIIG), and intermediate-luminosity quasars (QSO). For the full data combination (CC+SNIa+HIIG+QSO), we obtain q(0) = -0.25(-0.04)(+0.04) and a transition redshift z(T) similar or equal to 0.80, indicating a currently accelerating Universe with a transition occurring earlier than in the ACDM model. Within the redshift range probed by the data, the reconstructed q(z) deviates from the ACDM trend, suggesting a possible reduction of the late-time acceleration. Furthermore, the reconstruction favors a relatively high value of the Hubble parameter, h = 0.729 +/- 0.006. The ERC remains weakly constrained by late-time data but ensures a smooth and monotonic evolution of q(z), j (z), and W-eff (z) across a wide redshift range. Within the observed interval, the model effectively reproduces the late-time behavior of the previous parametrization, while providing a controlled extension toward early epochs. Our results show that current low-and intermediate-redshift data are compatible with a reduced late-time acceleration.