Tracing Fe K X-ray reverberation lag in the energy-resolved spectra of Narrow-line Seyfert 1 galaxy Ton S180

Abstract

We report the Fe K relativistic reverberation feature for the first time in the Narrow-line Seyfert\,1 galaxy Ton\,S180. Using a long observation from {\it XMM-Newton} we find that the Fe K emission lag peaks at $117\pm49$ s in the lag energy spectrum computed for frequencies $(0.3-8.5) \times 10^{-4}$ Hz. The lag amplitude drops to $22.85\pm14.20$ s as the frequency increases to $(8.5-30) \times 10^{-4}$ Hz. The time-averaged spectrum of the source shows a relatively narrow Fe K line at $\sim6.4$ keV, resulting in black hole spin to be low ($a=\rm 0.43_{-0.14}^{+0.10}$) found from the reflection modelling. We perform general relativistic transfer function modelling of the lag energy spectra individually. This provides an independent timing-based measure of the spin at $a=0.30_{-0.17}^{+0.34}$, and black hole mass $M_{\rm BH} = 0.29_{-0.16}^{+0.01}\times10^8M_{\odot}$, comparable to the previous measurement, and height of the corona $h = 2.59_{-0.33}^{+5.17}r_{\rm g}$. Further, we observe that the Fe K lag and the black hole mass fit well in the linear lag-mass relation shown by other Seyfert 1 galaxies.

Figures (7)

• Figure 1: XMM-Newton lightcurve of Ton S180 with a 400 s bin size. The lightcurve is extracted from 0.3 to 10 keV.
• Figure 2: Resolving the Fe K emission line profile, produced from the ratio of the X-ray spectrum to a simple powerlaw model. The data is binned to show only a few points for visual purpose.
• Figure 3: Lags as a function of energy for Ton S180 computed for frequencies $(0.3-8.5) \times 10^{-4}$ Hz (left panel) and $(8.5-30) \times 10^{-4}$ Hz (right panel). The lag spectra show a clear Fe K emission feature with peak lags of $117\pm49$ s at frequencies $(0.3-8.5) \times 10^{-4}$ Hz and $22.85\pm14.20$ s at frequencies $(8.5-30) \times 10^{-4}$ Hz. The lags are fitted using the general relativistic transfer function model KYNREVERB. Model lags are represented by the blue steps, whereas the black data points are the computed Fourier lags. Zero lag as a function of energy is shown as the blue dotted line.
• Figure 4: Lag frequency spectrum of Ton S180 computed between the power law-dominated 2--4 keV band and reflection-dominated 4--7 keV band. Positive lags indicate hard lags (4--7 keV hard band lags the 2--4 keV soft band). The lag spectra show a decreasing trend with increasing frequencies, where Fe K emission lag is clear.
• Figure 5: Fe K lag amplitudes versus black hole mass. The plot is reproduced from the published results (shown in black) reported in Table 2 of Kara2016, in which our lag measurement in Ton S180 (shown in green) is overplotted. The blue line indicates a linear model used to fit the data, providing a scaling relation of the lag with the black hole mass as $\tau \propto 0.59~M_{\rm BH}$. The red diagonal lines indicate lag at $1r_g$ and $9r_g$. The Spearman correlation test provides a Spearman rank correlation coefficient of 0.71 at a probability of 0.005.