Long-term variability of the optical spectra of NGC 4151

Abstract

Aims. Results of long-term spectral monitoring of the active galactic nucleus of NGC 4151 are presented (11 years, from 1996 to 2006). Methods. High quality spectra (S /N > 50 in the continuum near Hα and Hβ) were obtained in the spectral range ∼4000 to 7500 Å, with a resolution between 5 and 15 Å, using the 6-m and the 1-m SAO's telescopes (Russia), the GHAO's 2.1-m telescope (Cananea, México), and the OAN-SPM's 2.1-m telescope (San-Pedro, México). The observed fluxes of the Hα, Hβ, Hγ, and HeIIλ4686 emission lines and of the continuum at the observed wavelength 5117 Å were corrected for the position angle, the seeing, and the aperture effects. Results. We found that the continuum and line fluxes varied strongly (up to a factor 6) during the monitoring period. The emission was maximum in 1996−1998, and there were two minima in 2001 and in 2005. As a consequence, the spectral type of the nucleus changed from a Sy1.5 in the maximum activity state to a Sy1.8 in the minimum state. The Hα, Hγ, and Heλ4686 fluxes correlated well with the Hβ flux. The line profiles were strongly variable, showing changes of the blue and red asymmetry. The flux ratios of the blue/red wings and of the blue (or red) wing/core of Hα and Hβ varied differently. We considered three characteristic periods during which the Hβ and Hα profiles were similar : 1996−1999, 2000−2001, and 2002−2006. The line-to-continuum flux ratios were different; in particular during the first period (1996)(1997)(1998)(1999), the lines were not correlated with the continuum and saturated at high fluxes. In the second and third periods (2000)(2001)(2002)(2003)(2004)(2005)(2006), where the continuum flux was small, the Hα and Hβ fluxes were well correlated to the continuum flux, meaning that the ionizing continuum was a good extrapolation of the optical continuum. The CCFs are often asymmetrical and the time lags between the lines and the continuum are badly defined, indicating the presence of a complex BLR, with dimensions from 1 to 50 light-days. Conclusions. We discuss the different responses of Hβ and Hα to the continuum during the monitoring period.

Long-Term Monitoring of the Broad-Line Region Properties in a Selected Sample of AGN

“…0.42 0.05 ± 0.03 1.6 × 10 8 Shapovalova et al, 2008Shapovalova et al, , 2010bRakić et al, 2017Rakić et al, (1996Rakić et al, -2006 6,110 Shapovalova et al, 2001Shapovalova et al, , 2010aJovanović et al, 2010;Popović et al, 2011Popović et al, (1995Popović et al, -2007 11,900 Shapovalova et al, 2016;Kovačević et al, 2017Kovačević et al, (1990Kovačević et al, -2014 5,610…”

“…This is in support of the prediction that the photoionization by the central continuum source is mainly responsible for the heating and line production, which is one of the assumptions needed when calculating the BLR radius from reverberation mapping. However, if considering a single object, the correlation between the line and continuum emission can be violated, which is seen in the case of NGC 4151 and Ark 564 (see for details Shapovalova et al, 2008Shapovalova et al, , 2012. Another correlation that is present and significant is the intrinsic FIGURE 2 | Hβ line luminosity for all 7 objects vs. the continuum luminosity at 5100 Å (dots).…”

“…All spectral data for each objects have been first presented and analyzed separately in Shapovalova et al (2001Shapovalova et al ( , 2004Shapovalova et al ( , 2008Shapovalova et al ( , 2010aShapovalova et al ( , 2012Shapovalova et al ( , 2013Shapovalova et al ( , 2016Shapovalova et al ( , 2017. However, since the obtained spectra are observed, reduced and analyzed in the same manner, the final result is a uniform set of data, which can be compared and discussed.…”

Oscillatory patterns in the light curves of five long-term monitored type 1 active galactic nuclei

“…In addition, as time coverage of observations of NGC 4151 continuum is shorter then time coverage of observations of the Hα line, our study did not investigate how the weak (or even absent in some time periods) correlation between the continuum and emission line fluxes of this object (see details in Shapovalova et al 2008) would affect the detection of correlation of their oscillatory patterns. Finally, to investigate the periodic characteristics of the sample, we correlated logarithmic values of obtained periods with AGN optical luminosities and with their BLR sizes.…”

Evidence for Periodicity in 43 Year-Long Monitoring of NGC 5548

“…All the results presented below are based on the assumption that the luminosity of the [O III] 5007 line, and all other narrow emission lines, do not change in time (see for example Peterson et al 2002;Shapovalova et al 2008). We assumed that the slit width and position angles used in the various campaigns were not affected with the O III region size in the NGC 5548, which appeared to be very compact (see Schmitt et al 2003).…”