X-ray/UV variability and the origin of soft X-ray excess emission from II Zw 177

TL;DR

This study analyzes X-ray and UV variability in the Seyfert galaxy II Zw 177, revealing that soft X-ray excess emission is well described by blurred reflection and Comptonization, with variability driven mainly by intrinsic coronal changes.

Contribution

It provides detailed time-resolved spectroscopy showing the correlation between soft excess and powerlaw components, and suggests intrinsic coronal variability as the main driver of X-ray changes, challenging light-bending models.

Findings

Soft X-ray excess contributes up to 77% in 2012.

Soft excess well modeled by blurred reflection and Comptonization.

X-ray variability driven by intrinsic coronal changes, not light bending.

Abstract

We study X-ray and UV emission from the narrow-line Seyfert 1 galaxy II~Zw~177 using a $137\ks$ long and another $13\ks$ short \xmm{} observation performed in 2012 and 2001, respectively. Both observations show soft X-ray excess emission contributing $76.9\pm4.9\%$ in 2012 and $58.8\pm10.2\%$ in 2001 in the $0.3-2\kev$ band. We find that both blurred reflection from an ionized disc and Comptonized disc emission describe the observed soft excess well. Time-resolved spectroscopy on scales of $\sim20\ks$ reveals strong correlation between the soft excess and the powerlaw components. The fractional variability amplitude $F_{var}$ derived from EPIC-pn lightcurves at different energy bands is nearly constant ($F_{var} \sim20\%$). This is in contrast to other AGNs where the lack of short term variation in soft X-ray excess emission has been attributed to intense light bending in the framework of the "lamppost" model. Thus, the variations in powerlaw emission are most likely intrinsic to corona rather than just due to the changes of height of compact corona. The variable UV emission ($F_{var} \sim 1\%$) is uncorrelated to any of the X-ray components on short timescales suggesting that the UV emission is not dominated by the reprocessed emission. The gradual observed decline in the UV emission in 2012 may be related to the secular decline due to the changes in the accretion rate. In this case, the short term X-ray variability is not due to the changes in the seed photons but intrinsic to the hot corona.