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

TL;DR

This paper reports the discovery of energy-dependent hard phase lags in the pulsar GRO J1744-28, revealing complex reverberation effects likely involving Compton and disk reflection mechanisms.

Contribution

It presents the first detailed measurement of phase lag energy dependence in GRO J1744-28, suggesting a coexistence of Compton and disk reverberation effects.

Findings

Maximum lag of ~12 ms between 6 and 6.4 keV

Lag pattern well described by a logarithmic function

Evidence for both Compton and disk reverberation mechanisms

Abstract

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 $\sim$ 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 $R_{\rm{cc}}$ $\sim$ 120 $R_{\rm g}$, consistent with previous estimates on the magnetospheric radius. In this scenario, the sharp discontinuity at $\sim$ 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 disk reverberation to explain the scale of the lags and its energy dependence.