  We report the analysis of simultaneous multi-wavelength data of the high
energy peaked blazar RGB J0710+591 from the LAXPC, SXT and UVIT instruments
on-board AstroSat. The wide band X-ray spectrum (0.35 -- 30 keV) is modelled as
synchrotron emission from a non-thermal distribution of high energy electrons.
The spectrum is unusually curved, with a curvature parameter $\beta_p \sim 6.4$
for a log parabola particle distribution, or a high energy spectral index $p_2
> 4.5$ for a broken power-law distribution. The spectrum shows more curvature
than an earlier quasi-simultaneous analysis of Swift-XRT/NuSTAR data where the
parameters were $\beta_p \sim 2.2$ or $p_2 \sim 4$. It has long been known that
a power-law electron distribution can be produced from a region where particles
are accelerated under Fermi process and the radiative losses in acceleration
site decide the maximum attainable Lorentz factor, $\gamma_{max}$.
Consequently, this quantity decides the energy at which the spectrum curves
steeply. We show that such a distribution provides a more natural explanation
for the AstroSat data as well as the earlier XRT/NuSTAR observation, making
this as the first well constrained determination of the photon energy
corresponding to $\gamma_{max}$. This in turn provides an estimate of the
acceleration time-scale as a function of magnetic field and Doppler factor. The
UVIT observations are consistent with earlier optical/UV measurements and
reconfirm that they plausibly correspond to a different radiative component
than the one responsible for the X-ray emission.
