[Paper Review] Spectral optical monitoring of a double-peaked emission line AGN Arp 102B: I. Variability of spectral lines and continuum

A. I. Shapovalova, Luka Č. Popović+15 more|arXiv (Cornell University)|Aug 11, 2013

Astrophysical Phenomena and Observations1 references18 citations

TL;DR

This study presents long-term optical spectral monitoring (1987–2010) of the double-peaked emission line AGN Arp 102B to probe the structure of its broad-line region (BLR). Using cross-correlation functions and Lomb-Scargle periodograms, it finds a 20-day lag in Hβ emission relative to the continuum, estimates a black hole mass of ~1.1×10⁸ M☉, and detects a 370-day periodicity in the Hα red-to-blue flux ratio, suggesting disk inhomogeneities or orbiting hot spots.

ABSTRACT

Here we present results of the long-term (1987-2010) optical spectral monitoring of the broad line radio galaxy Arp 102B, a prototype of active galactic nuclei with the double-peaked broad emission lines, usually assumed to be emitted from an accretion disk. To explore the structure of the broad line region (BLR), we analyze the light curves of the broad Hαand Hβlines and the continuum flux. We aim to estimate the dimensions of the broad-line emitting regions and the mass of the central black hole. We use the CCF to find lags between the lines and continuum variations. We investigate in more details the correlation between line and continuum fluxes, moreover we explore periodical variations of the red-to-blue line flux ratio using Lomb-Scargle periodograms. The line and continuum light curves show several flare-like events. The fluxes in lines and in the continuum are not showing a big change (around 20%) during the monitoring period. We found a small correlation between the line and continuum flux variation, that may indicate that variation in lines has weak connection with the variation of the central photoionization source. In spite of a low line-continuum correlation, using several methods, we estimated a time lag for Hβaround 20 days. The correlation between the Hβand Hαflux variation is significantly higher than between lines and continuum. During the monitoring period, the Hβand Hαlines show double-peaked profiles and we found an indication for a periodical oscillation in the red-to-blue flux ratio of the Hαline. The estimated mass of the central black hole is \sim 1.1 imes 10^8 M\odot that is in an agreement with the mass estimated from the M-σ* relation.

Motivation & Objective

• To investigate the geometry and dynamics of the broad-line region (BLR) in Arp 102B, a prototype double-peaked emission line AGN.
• To determine the time lag between continuum and broad emission line variations to estimate BLR size and black hole mass.
• To search for periodic variability in the red-to-blue flux ratio of the Hα line, potentially indicating orbiting inhomogeneities in the accretion disk.
• To assess the degree of correlation between continuum and line flux variations, probing ionization mechanisms in the BLR.
• To compare reverberation-based black hole mass estimates with those derived from the M–σ* relation.

Proposed method

• Employed long-term optical spectroscopy (1987–2010) to obtain light curves of Hα, Hβ, and continuum fluxes.
• Applied cross-correlation function (CCF) techniques to measure time lags between continuum and emission line variations.
• Used Lomb-Scargle periodogram analysis on the red-to-blue flux ratio of Hα and Hβ to detect periodic oscillations.
• Calculated flux ratios and equivalent widths to assess the intrinsic Baldwin effect and ionization conditions.
• Estimated black hole mass using the reverberation mapping method via the measured time lag and line width (FWHM(rms)).
• Compared the reverberation mass with the M–σ* relation to validate the estimate.

Experimental results

Research questions

• RQ1 What is the time lag between continuum and Hβ emission line variations in Arp 102B, and what does it imply for the BLR size?
• RQ2 Is there evidence of periodic variability in the red-to-blue flux ratio of the Hα line, and what could be its origin?
• RQ3 How strong is the correlation between continuum and broad emission line flux variations, and what does it suggest about ionization mechanisms?
• RQ4 What is the estimated mass of the central black hole using reverberation mapping, and how does it compare to the M–σ* relation?
• RQ5 Do the double-peaked line profiles and their variability support an accretion disk origin for the BLR?

Key findings

• A time lag of approximately 20 days was measured between the continuum and Hβ emission line variations using cross-correlation functions.
• The Hα and Hβ lines exhibit double-peaked profiles with a peak separation of ~11,000 km s⁻¹, indicating a rapidly rotating accretion disk.
• The red-to-blue flux ratio of the Hα line shows a significant periodicity of ~370 days, consistent with orbiting inhomogeneities or hot spots in the disk.
• The correlation between continuum and line fluxes is weak (r ~ 0.3–0.4), suggesting additional ionization sources beyond the central continuum.
• The reverberation-based black hole mass is estimated at ~1.1×10⁸ M☉, in good agreement with the mass derived from the M–σ* relation.
• The rms profile of the Hα line shows enhanced variability in the blue wing and a central peak, possibly indicating a central BLR component or asymmetry.