Constraining black hole spin in PG 1535+547 amidst complex multi-layered absorption

TL;DR

This work uses broadband XMM-Newton and NuSTAR data across three epochs to constrain the spin of the supermassive black hole in PG 1535+547 amidst complex, multi-layer absorption. By applying relativistic reflection models (relxillCp, relxilllpCp) alongside distant reflection (borus) and multi-component absorbers (XSTAR, tbfeo), the authors disentangle continuum, reflection, and absorption to derive a robust spin constraint. They find a rapidly spinning black hole with $a>0.99$ in a lamppost configuration during the 2016 epoch, where a reflection-dominated spectrum and a very compact corona ($h\leq 1.72\,r_g$) are observed, consistent with strong light bending and a flux drop by a factor of ~7. Across epochs, obscuration varies with neutral and ionized absorbers, yet the reflection signature persists, enabling spin inference even in moderately obscured regimes. The results have implications for the growth history of the BH and demonstrate the efficacy of broadband spectroscopy in robust spin measurements under complex absorption.

Abstract

We present a spectroscopic analysis of XMM-Newton and NuSTAR observations of the 'complex' NLS1 PG 1535+547 at redshift $z=0.038$. These observations span three epochs: 2002 and 2006 with XMM-Newton alone, covering the $0.3-10$ keV energy range, and a coordinated XMM-Newton and NuSTAR observation in 2016, covering the $0.3-60$ keV energy range. The X-ray spectra across all epochs exhibit both neutral and ionized absorption, along with reflection features from the accretion disc, including a prominent Compton hump in the broadband data. Notably, the spectral shape varies across epochs. Our analysis suggests this variability is attributed to changes in both line-of-sight absorption and the intrinsic emission from PG 1535+547. The source is obscured by multiple layers of partially and/or fully covering neutral and ionized absorbers, with neutral column densities ranging from undetectable levels in the least obscured phase to $\sim0.3-5\times10^{23}\mathrm{cm^{-2}}$ in the most obscured phase. A clear warm absorber is revealed during the least obscured phase. The continuum remains fairly consistent ($Γ\approx 2.2\pm0.1$) during the first two observations, followed by a substantial flux decrease (by a factor of $\sim7$ in the $2-10$ keV band) in 2016 compared to 2006. The 2016 data indicates the source is in a reflection-dominated state during this epoch, with a reflection fraction of $R>7$ and an X-ray source located at a height $\leq 1.72r_g$. Simultaneous fitting of the multi-epoch data suggests a rapidly rotating black hole with a spin parameter, $a>0.99$. These findings imply that strong light-bending effects may account for the observed continuum flux reduction.

Figures (9)

• Figure 1: The spectra of PG 1535+547 for all three epochs, unfolded using a simple power law model. Black and teal represent spectra from epochs 1 and 2, respectively. The orange and blue depicts the broadband data from the coordinated XMM-Newton and NuSTAR observations respectively, during epoch 3.
• Figure 2: The top panel displays the model fit obtained from the simultaneous fits for epochs 1, 2, and 3. The bottom three panels show the residuals in terms of sigma for each epoch, shown sequentially from top to bottom for epochs 1, 2, and 3. Data points are presented in the same colors as in Figure \ref{['fig-1']}, with the corresponding solid lines indicating the total model fit.
• Figure 3: Spectral model from Model 1, highlighting contributions from individual components to the overall spectrum of PG $1535+547$. The top and the bottom panels show the spectral model contributions with and without absorption, respectively. The black line shows the resultant model fitting the X-ray data, while the blue, cyan, purple, red and orange lines represent the contributions from the scattered powerlaw, diffuse plasma emission from mekal, distant reflection from borus, and the intrinsic continuum and inner disc reflection components from relxillCp, respectively.
• Figure 4: Spectral model from Model 2, highlighting contributions from individual components to the overall spectrum of PG $1535+547$. The top and the bottom panels show the spectral model contributions with and without absorption, respectively. The black line shows the resultant model fitting the X-ray data, while the blue, cyan, purple, red and orange lines represent the contributions from the scattered powerlaw, diffuse plasma emission from mekal, distant reflection from borus, and the intrinsic continuum and inner disc reflection components from relxilllpCp, respectively.
• Figure 5: Left: Contours for spin versus source height from the lamppost model during epoch 3, with source height expressed in unita of vertical horizon radius ($r_{\rm h}$). Right: Contours for spin versus inclination from the lamppost model. Confidence levels are shown at 90% (black), 95% (red), and 99% (green). The blue marker in both plots indicates the best-fit values obtained from the spectral fits.