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

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 ∼ 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. N uST AR 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 cc ∼ 120 R g , 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.

Spectral and timing analysis of the bursting pulsar GRO J1744-28 with RXTE observations

“…In the timing analysis, we find that the pulsed fraction is positively correlated to both the energy band (at 3-30 keV) and luminosity. This result is well in agreement with the reports by D' Aì et al (2015); Doroshenko et al (2015); Younes et al (2015);D'Aì et al (2016); Cui (1997). In addition, thanks to the stable pulse profiles in different energy bands, we studied the energy-dependent lags with respect to the 16-20 keV band.…”

“…It has been suggested that the hard X-ray lag is due to the Compton reverberation-like process (Lightman et al 1978;Payne 1980;Guilbert et al 1982;D'Aì et al 2016). In a nutshell, in a hot, thermal plasma hard X-rays can be produced by repeated Compton scattering, because the energy of the incident photons can be amplified by a factor of ∼ 1+ 4kT mec 2 for non-relativistic electrons per scattering, where the kT is the temperature of the hot plasma.…”

“…For each energy band, a phase φ1 can be obtained by fitting the folded lightcurves (D'Aì et al 2016). We studied the phase shift (or the time lag) of soft X-rays compared to the hardest energy band (16-20 keV) since pulsations detected at hard X-rays are relatively accurate due to a larger A1.…”