X-ray flaring in PDS 456 observed in a high-flux state

TL;DR

This study analyzes a 2007 Suzaku observation of quasar PDS 456, revealing X-ray flares and spectral variability dominated by soft and hard components, with spectral softening during flares likely due to Compton cooling.

Contribution

First detailed analysis of X-ray flares in PDS 456 showing spectral variability and ruling out absorption as the cause, highlighting coronal cooling effects.

Findings

Detected two pairs of X-ray flares on ~50 ks timescales.

Spectral softening during flares linked to Compton cooling of the corona.

Ruled out partial covering absorption as the variability mechanism.

Abstract

We present an analysis of a $190$\,ks (net exposure) \textit{Suzaku} observation, carried out in 2007, of the nearby ($z=0.184$) luminous (L$_{\rm bol}\sim10^{47}$\,erg\,s$^{-1}$) quasar PDS\,456. In this observation, the intrinsically steep bare continuum is revealed compared to subsequent observations, carried out in 2011 and 2013, where the source is fainter, harder and more absorbed. We detected two pairs of prominent hard and soft flares, restricted to the first and second half of the observation respectively. The flares occur on timescales of the order of $\sim50$\,ks, which is equivalent to a light-crossing distance of $\sim10\,R_{\rm g}$ in PDS\,456. From the spectral variability observed during the flares, we find that the continuum changes appear to be dominated by two components: (i) a variable soft component ($<2$\,keV), which may be related to the Comptonized tail of the disc emission, and (ii) a variable hard power-law component ($>2$\,keV). The photon index of the latter power-law component appears to respond to changes in the soft band flux, increasing during the soft X-ray flares. Here the softening of the spectra, observed during the flares, may be due to Compton cooling of the disc corona induced by the increased soft X-ray photon seed flux. In contrast, we rule out partial covering absorption as the physical mechanism behind the observed short timescale spectral variability, as the timescales are likely too short to be accounted for by absorption variability.