Discovery of hard phase lags in the pulsed emission of GRO J1744-28

We report on the discovery and energy dependence of hard phase lags in the 2.14 Hz pulsed profiles of GRO J1744-28. We used data from XMM-Newton and NuSTAR. We were able to well constrain the lag spectrum with respect to the softest (0.3-2.3 keV) band: the delay shows increasing lag values reaching a maximum delay of ~12 ms, between 6 and 6.4 keV. After this maximum, the value of the hard lag drops to ~7 ms, followed by a recovery to a plateau at ~9 ms for energies above 8 keV. NuSTAR data confirm this trend up to 30 keV, but the measurements are statistically poorer, and therefore, less constraining. The lag-energy pattern up to the discontinuity is well described by a logarithmic function. Assuming this is due to a Compton reverberation mechanism, we derive a size for the Compton cloud Rcc ~ 120 Rg, consistent with previous estimates on the magnetospheric radius. In this scenario, the sharp discontinuity at ~6.5 keV appears difficult to interpret and suggests the possible influence of the reflected component in this energy range. We therefore propose the possible coexistence of both Compton and disc reverberation to explain the scale of the lags and its energy dependence.