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1

A Survey Length for AGN Variability Studies

Kozłowski S.

Acta Astronomica

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2021

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Vol. 71, No. 2

103--112

EN

The damped random walk (DRW) process is one of the most commonly used and simplest stochastic models to describe variability of active galactic nuclei (AGN). An AGN light curve can be converted to just two DRW model parameters - the signal decorrelation timescale τ and the asymptotic amplitude SF∞. In principle, these two model parameters may be correlated with the physical parameters of AGN. By simulation means, we have recently shown that in order to measure the decorrelation timescale accurately, the experiment or the light curve length must be at least 10 times the underlying decorrelation timescale. In this paper, we investigate the origin of this requirement and find that typical AGN light curves do not sufficiently represent the intrinsic stationary process. We simulated extremely long (10 000τ) AGN light curves using DRW, and then measured the variance and the mean of short light curves spanning 1-1000τ. We modeled these light curves with DRW to obtain both the signal decorrelation timescale τ and the asymptotic amplitude SF∞. The variance in light curves shorter than ≈30τ is smaller than that of the input process, as estimated by both a simple calculation from the light curve and by DRW modeling. This means that while the simulated stochastic process is intrinsically stationary, short light curves do not adequately represent the stationary process. Since the variance and timescale are correlated, underestimated variances in short light curves lead to underestimated timescales as compared to the input process. It seems, that a simulated AGN light curve does not fully represent the underlying DRW process until its length reaches even ≈30 decorrelation timescales. Modeling short AGN light curves with DRW leads to biases in measured parameters of the model - the amplitude being too small and the timescale being too short.

2

Merger Signatures in Radio Loud and Radio Quiet Quasars

Garofalo D., Webster B., Bishop K.

Acta Astronomica

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2020

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Vol. 70, No. 1

75--85

EN

While the origin of the radio loud/radio quiet dichotomy is still debated, the consensus is that radio loud and radio quiet quasars are both triggered by mergers, yet merger signatures are not evenly distributed among the two groups. Whereas they are detected in radio loud quasars at a rate of 80-100%, the rate is considerably smaller in radio quiet quasars at 20-30%. Because the radio loud/radio quiet dichotomy is a counterrotation/co-rotation accretion dichotomy around spinning black holes in our paradigm, and counterrotation spins black holes down rapidly, radio loud quasars live comparably shorter lifetimes. As a result, they are more likely to be observed when less time elapses from the merger that triggered them. In order to check our model, we work backwards from the observed merger rates to obtain accretion rates for both the jetted and non-jetted quasars in our model. The goal is to check the model prescription that both families of active galaxies accrete at a rate that is compatible with cold, radiatively efficient, thin disks. We find that if average accretion rates in radio loud quasars span the range 2-80% of the Eddington accretion rate, we obtain a match with the observed 80-100% rate of merger signatures. Because radio quiet quasars emerge in different ways in the model with phases whose lifetimes vary by much more, from a few million to a billion years, the rates of merger signatures are more difficult to obtain. Nonetheless, we show that for the radio quiet quasars with longest lifetimes that should dominate the merger signature rates, average accretion rates in the range 5-7.5% of the Eddington limit make theory compatible with observations. Interestingly, these numbers are compatible with the assumption in the model that the relevant radio quiet quasars in the analysis live longer and therefore experience a decline in accretion rate over time.

3

Empirical Conversions of Broad-Band Optical and Infrared Magnitudes to Monochromatic Continuum Luminosities for Active Galactic Nuclei

Kozłowski S.

Acta Astronomica

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2015

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Vol. 65, No. 3

251--265

EN

We use public data for 105 783 quasars from The Sloan Digital Sky Survey (SDSS) Data Release 7 (DR7) that include spectral monochromatic luminosities at 5100 Å, 3000 Å, and 1350 Å, and the corresponding observed broad-band ugriz, VRI (converted), JHK and WISE magnitudes, and derive broad-band-to-monochromatic luminosity ratios independent of a cosmological model. The ratios span the redshift range of z = 0.1 ÷ 4.9 and may serve as a proxy for measuring the bolometric luminosity, broad line region (BLR) radii and/or black hole masses, whenever flux-calibrated spectra are unavailable or the existing spectra have low signal-to-noise ratios. They are provided both in tabular and parametric form.

4

J1145-0033 - The Most Distant Giant Radio Source?

Kuźmicz A., Kuligowska E., Jamrozy M.

Acta Astronomica

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2011

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Vol. 61, No. 1

71--82

EN

We present J1145-0033, a candidate for the most distant (z=2.055) lobe-dominated giant radio quasar, with a projected linear size of 1.34 Mpc. This quasar has both FRII-type radio morphology and broad absorption lines in its optical spectrum. Some physical characteristics (e.g., black hole mass, accretion rate, equipartition magnetic field, energy density and particle density of ambient medium) based on the optical and radio data are provided. We have also found that the quasar has a relatively large central black hole mass and a very small accretion rate in comparison with similar objects.

5

Search for QSO Candidates in OGLE-II Data

Eyer L.

Acta Astronomica

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2002

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Vol. 52, No. 3

241--262

EN

A search for faint slowly variable objects was undertaken in the hope of finding QSO candidates behind the Small and Large Magellanic Clouds (SMC and LMC). This search used the optical variability properties of point sources from the Magellanic Cloud OGLE-II photometric data. Objects bluer than V-I=0.9 mag and within 17 mag < I <20.5 mag were studied. Robust variograms/structure functions have been computed for each time-series and only candidates showing a significantly increasing variability over longer time scales were selected. Several light curves were identified as having probable artifacts and were therefore removed. Stars showing signs of periodicity or small trends in their light curves were also removed and we are left with mostly either Be stars (γ Cas stars) or QSO candidates. We present a list of 25 slowly varying objects for SMC and 155 for LMC, out of 15 000 and 53 000 variable objects respectively. Of these, about 15 objects for the SMC and 118 objects for the LMC are QSO candidates.