  Using numerical simulations of helical inflationary magnetogenesis in a low
reheating temperature scenario, we show that the magnetic energy spectrum is
strongly peaked at a particular wavenumber that depends on the reheating
temperature. Gravitational waves (GWs) are produced at frequencies between 3
nHz and 50 mHz for reheating temperatures between 150 MeV and 3x10^5 GeV,
respectively. At and below the peak frequency, the stress spectrum is always
found to be that of white noise. This implies a linear increase of GW energy
per logarithmic wavenumber interval, instead of a cubic one, as previously
thought. Both in the helical and nonhelical cases, the GW spectrum is followed
by a sharp drop for frequencies above the respective peak frequency. In this
magnetogenesis scenario, the presence of a helical term extends the peak of the
GW spectrum and therefore also the position of the aforementioned drop toward
larger frequencies compared to the case without helicity. This might make a
difference in it being detectable with space interferometers. The efficiency of
GW production is found to be almost the same as in the nonhelical case, and
independent of the reheating temperature, provided the electromagnetic energy
at the end of reheating is fixed to be a certain fraction of the radiation
energy density. Also, contrary to the case without helicity, the electric
energy is now less than the magnetic energy during reheating. The fractional
circular polarization is found to be nearly hundred per cent in a certain range
below the peak frequency range.
