Nongravitational motion of 3I/ATLAS far from the Sun: robust detection, weak constraints on the activity law

3I/ATLAS provides an unprecedented astrometric dataset for studying nongravitational (NG) effects in an interstellar object, but existing orbital solutions are highly sensitive to assumptions regarding the heliocentric activity law and the treatment of the astrometric data. We investigate the extent to which astrometric observations constrain the heliocentric dependence of the NG activity function, focusing on the identifiability of different functional forms of g(r) within standard NG orbit-determination frameworks. Astrometric observations obtained at heliocentric distances larger than 3.49 au before perihelion and 2.67 au after perihelion were analysed. Purely gravitational and NG orbital solutions were determined using several activity laws, including the standard water-ice sublimation law and flatter more volatile ice prescriptions. The resulting residual distributions were evaluated using normality tests and analyses of temporal trends in the observed-minus-calculated residuals. The distant pre-perihelion observations are well described by a purely gravitational (GR) orbit, whereas the distant post-perihelion data exhibit a statistically significant non-Gaussian residual distribution under the GR model. When the distant pre- and post-perihelion observations are combined, NG acceleration becomes clearly detectable through a significant reduction in the residual rms and the disappearance of systematic residual trends. However, the astrometric data do not uniquely distinguish between the tested activity laws: standard water-ice, more-volatile, and extreme-volatile prescriptions produce statistically comparable orbital fits despite their markedly different heliocentric dependences. The existence of NG acceleration is therefore considerably better constrained by astrometry than its physical origin or heliocentric dependence.