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 $\sim$ 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 $R_{\mathrm{cc}}$ $\sim$ 120 $R_{\mathrm{g}}$, 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.

Accepted for publication in MNRAS Letters on 2016 June 01

References in corpus (7)

• X-ray reverberation around accreting black holes
• Thermal and Bulk Comptonization in Accretion-Powered X-Ray Pulsars
• High-Resolution X-Ray Spectroscopy of the Bursting Pulsar GRO J1744-28
• GRO J1744-28: an intermediate B-field pulsar in a low mass X-ray binary
• Spectral and timing properties of the accreting X-ray millisecond pulsar IGR J17498-2921
• Simultaneous NuSTAR/Chandra observations of the Bursting Pulsar GRO J1744-28 during its third reactivation
• NuSTAR detection of hard X-ray phase lags from the accreting pulsar GS 0834-430

Cited by in corpus (4)

• The Evolution of X-ray Bursts in the "Bursting Pulsar" GRO J1744-28
• Timing of the accreting millisecond pulsar IGR J17591-2342: evidence of spin-down during accretion
• Spectral and timing analysis of the bursting pulsar GRO J1744-28 with RXTE observations
• Recurrent Instability in LMXB Accretion Disks: How Strange is GRS 1915+105?