The Origin of the Vanishing Soft X-Ray Excess in the Changing-look Active Galactic Nucleus Mrk 590
Ritesh Ghosh, Sibasish Laha, K. Deshmukh, V. Bhalerao, G. C. Dewangan, Ritaban Chatterjee
Abstract
Abstract We have studied the nature and origin of the soft X-ray excess detected in the interesting changing-look active galactic nucleus (CLAGN) Mrk 590 using two decades of multiwavelength observations from XMM-Newton, Suzaku, Swift, and NuSTAR. In light of vanishing soft excess in this CLAGN, we test two models, “the warm Comptonization” and “the ionized disk reflection,” using extensive UV/X-ray spectral analysis. Our main findings are as follows: (1) the soft X-ray excess emission, last observed in 2004, vanished in 2011 and never reappeared in any of the later observations; (2) we detected a significant variability (∼300%) in the observed optical–UV and power-law flux between observations with the lowest state ( L bol = 4.4 × 10 43 erg s −1 , in 2016) and those with the highest state ( L bol = 1.2 × 10 44 erg s −1 , in 2018); (3) the UV and power-law fluxes follow the same temporal pattern; (4) the photon index showed a significant variation ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi mathvariant="normal">Γ</mml:mi> <mml:mo>=</mml:mo> <mml:msubsup> <mml:mrow> <mml:mn>1.88</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>0.08</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>0.02</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi mathvariant="normal">Γ</mml:mi> <mml:mo>=</mml:mo> <mml:msubsup> <mml:mrow> <mml:mn>1.58</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>0.03</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>0.02</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> in 2002 and 2021, respectively) between observations; (5) no Compton hump was detected in the source spectra, but a narrow Fe K α line is present in all observations; (6) we detected a high-energy cutoff in power-law continuum ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msubsup> <mml:mrow> <mml:mn>92</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>25</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>55</mml:mn> </mml:mrow> </mml:msubsup> <mml:mspace width="0.25em"/> <mml:mi>keV</mml:mi> </mml:math> and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msubsup> <mml:mrow> <mml:mn>60</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>08</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>10</mml:mn> </mml:mrow> </mml:msubsup> <mml:mspace width="0.25em"/> <mml:mi>keV</mml:mi> </mml:math> ) with the latest NuSTAR observations; (7) the warm Comptonization model needs an additional diskbb component to describe the source UV bump; and (8) there is no correlation between the Eddington rate and the soft excess as found in other CLAGNs. We conclude that, given the spectral variability in UV/X-rays, the ionized disk reflection or the warm Comptonization models may not be adequate to describe the vanishing soft excess feature observed in Mrk 590.