  We present the spectral analysis of the LMXB GRO J1744-28 using $\sim$29 ks
of NuSTAR data taken in 2017 February at a low luminosity of $3.2\times
10^{36}$ erg/s (3-50 keV). The continuum spectrum is modeled with an absorbed
power-law with exponential cut-off, and an additional iron line component. We
find no obvious indications for a CRSF and therefore perform a detailed
cyclotron line search using statistical methods on the pulse phase-averaged as
well as phase-resolved spectra. The previously detected Type II X-ray bursts
are absent. Clear pulsations at a period of 2.141124(9) Hz are detected. The
pulse profile shows an indication of a secondary peak, which was not seen at
higher flux. The 4$\sigma$ upper limit for the strength of a CRSF in the 3-20
keV band is 0.07 keV, lower than the strength of the line found at higher
luminosity. The detection of pulsations shows that the source did not enter the
"propeller" regime, even though the source flux of $4.15\times 10^{-10}$
erg/cm$^{2}$/s was almost one order of magnitude below the threshold for the
propeller regime claimed in previous studies on this source. The transition
into the propeller regime in GRO J1744-28 must therefore be below a luminosity
of $3.2\times 10^{36}$ erg/s, which implies a surface magnetic field $\lesssim
2.9\times 10^{11}$ G and mass accretion rate $\lesssim 1.7\times 10^{16}$ g/s.
A change of the CRSF depth as function of luminosity is not unexpected and has
been observed in other sources. This result possibly implies a change in
emission geometry as function of mass accretion rate to reduce the depth of the
line below our detection limit.

High Energy Astrophysical Phenomena;High Energy Physics - Experiment