Structure function of the UV variability of Q0957+561

Abstract

We present a detailed structure function analysis of the UV variability of Q0957+561. From optical observations in the 2005−2007 seasons, we constructed normalized structure functions of the quasar luminosity at restframe wavelengths λ ∼ 2100 Å and λ ∼ 2600 Å. Old optical records (1995−1996 seasons) also allow the structure function to be obtained at λ ∼ 2100 Å, but 10 years ago in the observer's frame. These three structure functions are then compared to predictions of both simple and relatively sophisticated (incorporating two independent variable components) Poissonian models. We do not find clear evidence of a chromatic mechanism of variability. From the recent data, ∼100-d time-symmetric and ∼170-d time-asymmetric flares are produced at both restframe wavelengths. Taking into account measurements of time delays and the existence of an EUV/radio jet, reverberation is probably the main mechanism of variability. Thus, two types of EUV/X-ray fluctuations would be generated within or close to the jet and later reprocessed by the disc gas in the two emission rings, at λ ∼ 2100 Å and λ ∼ 2600 Å. The ∼100-d time-symmetric shots are also responsible for most of the ∼2100 Å variability detected in the old experiment (10 years ago). However, there is no evidence of asymmetric shots in the old UV variability. If reverberation is the involved mechanism of variability, this could mean an intermittent production of high-energy asymmetric fluctuations. The old records are also consistent with the presence of very short-lifetime (∼10 d) symmetric flares, which may represent additional evidence of time evolution. Despite these exciting findings, we cannot rule out the possibility of unfortunate gaps in the old light curves (or a relatively short monitoring period) and very short-timescale systematic noise. We also discuss the quasar structure that emerges from the variability scenario.

The Mechanism of Intrinsic Variability in Bright Gravitationally Lensed Quasars at 1 less than z less than 2

“…Differences between time coverages, gaps and artifacts in the g ( ~ 2100 Å) and r ( ~ 2600 Å) bands are the most plausible causes of subtle differences between both growths [7]. Apart from this achromaticity, [7] showed that ~ 0.7, so a reverberation scenario could explain the variability of Q0957+561. Initial slopes 0.5 and ~ 1 are not favoured by the LQLM observations (see the dashed line in Fig.…”

“…This showed a steep growth, so here we want to re-discuss the variability structure and to compare it with the = 0.7 law. The structure function SF(L) at restframe lag is given by (e.g., [7]) (1), are computed as the standard deviations of the means (averaged sums) for the different time lag bins. Thus, the uncertainties are likely underestimated because not all pairs of data in a given bin are independent [3].…”

“…(1) to the luminosity variance and take the square-root. This normalized structure function was introduced in [7]. Moreover, we only consider rest-frame lags substantially below the rest-frame duration of the A+B combined record, i.e., P / 4(1+ z) , where P~ 500 d is the observer-frame duration of the combined light curve.…”

“…Accretion disc instabilities were described through the cellular-automaton model [2,5]. However, cellular-automaton simulations neglect hydrodynamical effects, which seem to lead to another kind of flares [6] and a different initial slope of the structure function [7]. Second, the observed structure function included observational noise.…”

“…While brightness records of non-lensed distant quasars may contain unrecognized extrinsic variations (caused by, e.g., moving compact objects within the intergalactic medium [4]), we can assess the extrinsic variability of certain GLQs and determine their intrinsic fluctuations (and the corresponding structure functions). Thus, GLQs allow us to fairly study the nature of violent phenomena in distant sources, which very likely arise in the vicinity of a supermassive black hole, e.g., [7,8,22,23]. In this paper, we concentrate on some bright GLQs at 1 < z < 2 that were observed with the 2-m Liverpool Robotic Telescope (Liverpool Quasar Lens Monitoring -LQLM, e.g., [8,23]) and the 1.3-m Warsaw Telescope (Optical Gravitational Lensing Experiment -OGLE, e.g., [24]).…”