The non-constant slope of the CIV Baldwin effect in NGC 4151

Abstract

The relationship between the emission-line equivalent width and the continuum luminosity, so called the Baldwin effect, plays an important role in studying the physics of the broad line region of AGNs. Using the archived ultraviolet spectra obtained by IUE, HST, and HUT in 1978-2002, we investigated the intrinsic CIV Baldwin effect of a well-studied Seyfert 1 galaxy NGC 4151. Both its continuum flux and CIV emission-line flux varied about two orders of magnitude in more than two decades, making it one of the best targets for studying the slope variation of the Baldwin effect. We fitted the CIV line profile of the 490 archived UV spectra of NGC 4151 with the same model consisting of a few Gaussian components, and derived the slope in the log-log plot for the total flux of CIV emission line against the UV continuum flux in different observation epochs.We found that the slope is not constant for NGC 4151, as it varies from 0.58 in the highest flux epoch to 0.83 in the lowest flux epoch. The slope evidently decreases as the continuum flux increases, which reinforces the previous findings of the non-constant slope in the H_beta Baldwin effect of NGC 5548 and the CIV Baldwin effect of Fairall 9. Our result suggests that such a non-constant slope may not be unusual for AGNs. Its physical origin is probably related to the different non-linear responses of the emission line to the variable ionizing continuum caused by the different accretion modes at different luminosity levels. We briefly discuss the effects of various absorption components in the CIV line profile of NGC 4151 and argue that the slope variation is not driven mainly by the absorption effect. Intensive, long-term and high resolution spectral observations of strongly variable AGNs are needed in the future to confirm our result.

arXiv comment8 pages, 5 figures, references added and typo corrected, accepted for publication in A&A

References in corpus (18)

• Reverberation Measurements for 17 Quasars and the Size-Mass-Luminosity Relations in Active Galactic Nuclei
• Central Masses and Broad-Line Region Sizes of Active Galactic Nuclei. II. A Homogeneous Analysis of a Large Reverberation-Mapping Database
• A Composite HST Spectrum of Quasars
• Intrinsic Absorption Lines in Seyfert 1 Galaxies. I. Ultraviolet Spectra from the Hubble Space Telescope
• Steps Toward Determination of the Size and Structure of the Broad-Line Region in Active Galactic Nuclei. XVI. A Thirteen-Year Study of Spectral Variability in NGC 5548
• Multiwavelength observations of short time-scale variability in NGC 4151. IV. Analysis of multiwavelength continuum variability
• Average UV Quasar Spectra in the Context of Eigenvector 1: A Baldwin Effect Governed by Eddington Ratio?
• Active Galactic Nuclei Emission-Line Properties Versus Eddington Ratio
• On the origin of the C IV Baldwin effect in AGN
• Simultaneous Ultraviolet and X-ray Observations of the Seyfert Galaxy NGC 4151. I. Physical Conditions in the X-ray Absorbers
• Quasars as Cosmological Probes: The Ionizing Continuum, Gas Metallicity and the EW-L Relation
• The Baldwin Effect and Black Hole Accretion: A Spectral Principal Component Analysis of a Complete QSO Sample
• Multiwavelength observations of short time-scale variability in NGC 4151. I. Ultraviolet observations
• STIS Echelle Observations of the Seyfert Galaxy NGC 4151: Physical Conditions in the Ultraviolet Absorbers
• An Intrinsic Baldwin Effect in the H-beta Broad Emission Line in the Spectrum of NGC 5548
• GHRS Monitoring of the Outflowing Material in NGC 4151
• STIS Echelle Observations of NGC 4151: Variable Ionization of the Intrinsic UV Absorbers
• Is the slope of the intrinsic Baldwin effect constant?

Cited by in corpus (9)

• Interpreting broad emission-line variations I : Factors influencing the emission-line response
• The CIV Baldwin effect in QSOs from Seventh Data Release of the Sloan Digital Sky Survey
• The intrinsic Baldwin effect in broad Balmer lines of six long-term monitored AGNs
• Extreme Variability Quasars in Their Various States. I: The sample selection and composite SDSS spectra
• Interpreting broad emission-line variations II: Tensions between luminosity, characteristic size and responsivity
• The environment of weak emission-line quasars
• Relation between the variations in the MgII $\lambda 2798$ emission-line and the 3000 Å continuum
• Physical properties and the variability mechanism of the He I outflow in NGC 4151
• Extreme Variability Quasars in Their Various States. II: Spectral Variation Revealed with Multi-epoch Spectra