Measuring Light Echos in NGC 4051

TL;DR

This study analyzes X-ray light echoes in NGC 4051, revealing energy-dependent time lags consistent with reverberation from circumnuclear scattering, suggesting a high covering fraction of reprocessing material near the black hole.

Contribution

First detailed analysis of energy-dependent X-ray time lags in NGC 4051 using NuSTAR data, supporting reverberation models involving circumnuclear scattering.

Findings

Hard X-ray bands lag softer bands by at least 1000s.

Soft-band lags up to 400s observed at higher frequencies.

Reverberation delays consistent with circumnuclear scattering, with a high covering fraction.

Abstract

Five archived X-ray observations of NGC 4051, taken using the NuSTAR observatory, have been analysed, revealing lags between flux variations in bands covering a wide range of X-ray photon energy. In all pairs of bands compared, the harder band consistently lags the softer band by at least 1000s, at temporal frequencies ~5E-5 Hz. In addition, soft-band lags up to 400s are measured at frequencies ~2E-4 Hz. Light echos from an excess of soft band emission in the inner accretion disk cannot explain the lags in these data, as they are seen in cross-correlations with energy bands where the softer band is expected to have no contribution from reflection. The basic properties of the time delays have been parameterised by fitting a top hat response function that varies with photon energy, taking fully into account the covariance between measured time lag values. The low-frequency hard-band lags and the transition to soft-band lags are consistent with time lags arising as reverberation delays from circumnuclear scattering of X-rays, although greater model complexity is required to explain the entire spectrum of lags. The scattered fraction increases with increasing photon energy as expected, and the scattered fraction is high, indicating the reprocessor to have a global covering fraction ~50% around the continuum source. Circumnuclear material, possibly associated with a disk wind at a few hundred gravitational radii from the primary X-ray source, may provide suitable reprocessing.