On the evidence for narrow, relativistically shifted X-ray lines

TL;DR

This paper reviews the statistical significance of narrow, relativistically shifted X-ray lines in active galaxies, suggesting many detections may be false positives due to publication bias and emphasizing the need for higher quality, uniform data analysis.

Contribution

It critically assesses the evidence for shifted X-ray lines, highlighting the influence of publication bias and advocating for standardized analysis of better data.

Findings

Reported line strength correlates with uncertainty, indicating many lines may be false detections.

Better observations tend to show smaller lines, consistent with random fluctuations.

The prevalence of these lines remains uncertain without uniform high-quality analysis.

Abstract

In recent years there have been many reported detections of highly redshifted or blueshifted narrow spectral lines (both emission or absorption) in the X-ray spectra of active galaxies, but these are all modest detections in terms of their statistical significance. The aim of this paper is to review the issue of the significance of these detections and, in particular, take account of publication bias. A literature search revealed 38 reported detections of narrow, strongly shifted (v/c >= 0.05) X-ray lines in the 1.5-20 keV spectra of Seyfert galaxies and quasars. These published data show a close, linear relationship between the estimated line strength and its uncertainty, in the sense that better observations (with smaller uncertainties) only ever show the smallest lines. This result is consistent with many of the reported lines being false detections resulting from random fluctuations, drawn from a large body of data and filtered by publication bias such that only the most `significant' fluctuations are ever reported. The reality of many of these features, and certainly their prevalence in the population at large, therefore remains an open question that is best settled though uniform analysis (and reporting) of higher quality observations.