Significant X-ray Line Emission in the 5-6 keV band of NGC 4051

TL;DR

This paper reports the detection of a significant, unexplained X-ray emission line at 5.44 keV in NGC 4051, confirmed across multiple observations, suggesting a stable origin possibly related to the accretion disk or Fe spallation.

Contribution

The study presents the first robust detection of a 5.44 keV line in NGC 4051, with statistical validation and discussion of its potential physical origins, including accretion disk features or Fe spallation.

Findings

Line detected at >99.9% confidence level

Line consistent across multiple epochs and instruments

Possible origin in accretion disk or Fe spallation

Abstract

A Suzaku X-ray observation of NGC 4051 taken during 2005 Nov reveals line emission at 5.44 keV in the rest-frame of the galaxy which does not have an obvious origin in known rest-frame atomic transitions. The improvement to the fit statistic when this line is accounted for establishes its reality at >99.9% confidence: we have also verified that the line is detected in the three XIS units independently. Comparison between the data and Monte Carlo simulations shows that the probability of the line being a statistical fluctuation is p < 3.3 x 10^-4. Consideration of three independent line detections in Suzaku data taken at different epochs yields a probability p< 3 x 10^-11 and thus conclusively demonstrates that it cannot be a statistical fluctuation in the data. The new line and a strong component of Fe Ka emission from neutral material are prominent when the source flux is low, during 2005. Spectra from 2008 show evidence for a line consistent with having the same flux and energy as that observed during 2005, but inconsistent with having a constant equivalent width against the observed continuum. The stability of the line flux and energy suggests that it may not arise in transient hotspots, as has been suggested for similar lines in other sources, but could arise from a special location in the reprocessor, such as the inner edge of the accretion disk. Alternatively, the line energy may be explained by spallation of Fe into Cr, as discussed in a companion paper.