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We present the spectral analysis of the LMXB GRO J1744-28 using $\sim$29 ksof NuSTAR data taken in 2017 February at a low luminosity of $3.2\times10^{36}$ erg/s (3-50 keV). The continuum spectrum is modeled with an absorbedpower-law with exponential cut-off, and an additional iron line component. Wefind no obvious indications for a CRSF and therefore perform a detailedcyclotron line search using statistical methods on the pulse phase-averaged aswell as phase-resolved spectra. The previously detected Type II X-ray burstsare absent. Clear pulsations at a period of 2.141124(9) Hz are detected. Thepulse profile shows an indication of a secondary peak, which was not seen athigher flux. The 4$\sigma$ upper limit for the strength of a CRSF in the 3-20keV band is 0.07 keV, lower than the strength of the line found at higherluminosity. 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 thepropeller regime claimed in previous studies on this source. The transitioninto the propeller regime in GRO J1744-28 must therefore be below a luminosityof $3.2\times 10^{36}$ erg/s, which implies a surface magnetic field $\lesssim2.9\times 10^{11}$ 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 hasbeen observed in other sources. This result possibly implies a change inemission geometry as function of mass accretion rate to reduce the depth of theline below our detection limit.

NuSTAR observation of GRO J1744-28 at low mass accretion rate