Continuum Variability of Active Galactic Nuclei in the Optical–Ultraviolet Range

The Astronomical …, 2001

We present an empirical investigation of the colors of quasars in the Sloan Digital Sky Survey (SDSS) photometric system. The sample studied includes 2625 quasars with SDSS photometry : 1759 quasars found during SDSS spectroscopic commissioning and SDSS follow-up observations on other telescopes, 50 matches to FIRST quasars, 573 matches to quasars from the NASA Extragalactic Database, and 243 quasars from two or more of these sources. The quasars are distributed in a wide stripe centered on 2¡ .5 the celestial equator covering D529 deg2. Positions (accurate to and SDSS magnitudes are given for 0A .2) the 898 quasars known prior to SDSS spectroscopic commissioning. New SDSS quasars, which range in brightness from i* \ 15.39 to the photometric magnitude limit of the survey, represent an increase of over 200% in the number of known quasars in this area of the sky. The ensemble average of the observed colors of quasars in the SDSS passbands are well represented by a power-law continuum with and are close to those predicted by previous simulations. However, the contributions a l \ [0.5 ( f l P la) of the "" small blue (or j3000) bump ÏÏ and other strong emission lines have a signiÐcant e †ect upon the colors. The color-redshift relation exhibits considerable structure, which may be of use in determining photometric redshifts for quasars from their colors alone. The range of colors at a given redshift can generally be accounted for by a range in the optical spectral index with a distribution a l \ [0.5^0.65 ÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈ 2308 COLORS OF 2625 QUASARS 2309 (95% conÐdence), but there is a red tail in the distribution. This tail may be a sign of internal reddening, especially since fainter objects at a given redshift tend to exhibit redder colors than the average. Finally, we show that there is a continuum of properties between quasars and Seyfert galaxies, and we test the validity of the traditional dividing line between the two classes of active galactic nuclei. (M B \ [23)

We have used optical V and R band observations from the Massive Compact Halo Object (MACHO) project on a sample of 59 quasars behind the Magellanic clouds to study their long term optical flux and colour variations. These quasars, lying in the redshift range of 0.2 < z < 2.8 and having apparent V band magnitudes between 16.6 and 20.1 mag, have observations ranging from 49 to 1353 epochs spanning over 7.5 yr with frequency of sampling between 2 to 10 days. All the quasars show variability during the observing period. The normalised excess variance (F var) in V and R bands are in the range 0.2% < F V var < 1.6% and 0.1% < F R var < 1.5% respectively. In a large fraction of the sources, F var is larger in the V band compared to the R band. From the z-transformed discrete cross-correlation function analysis, we find that there is no lag between the V and R band variations. Adopting the Markov Chain Monte Carlo (MCMC) approach, and properly taking into account the correlation between the errors in colours and magnitudes, it is found that the majority of sources show a bluer when brighter trend, while a minor fraction of quasars show the opposite behaviour. This is similar to the results obtained from another two independent algorithms, namely the weighted linear least squares fit (FITEXY) and the bivariate correlated errors and intrinsic scatter regression (BCES). However, the ordinary least squares (OLS) fit, normally used in the colour variability studies of quasars, indicates that all the quasars studied here show a bluer when brighter trend. It is therefore very clear that the OLS algorithm cannot be used for the study of colour variability in quasars.

The dependence of the long-term optical/UV variability on the spectral and fundamental physical parameters for radio-quiet active galactic nuclei (AGNs) is investigated. The multi-epoch-repeated photometric scanning data in the Stripe-82 region of the Sloan Digital Sky Survey (SDSS) are exploited for two comparative AGN samples (mostly quasars) selected therein: a broad-line Seyfert 1 (BLS1) type sample and a narrow-line Seyfert 1 (NLS1) type AGN sample within redshifts 0.3-0.8. Their spectral parameters are derived from the SDSS spectroscopic data. It is found that on rest-frame timescales of several years the NLS1-type AGNs show systematically smaller variability compared to the BLS1-type AGNs. In fact, the variability amplitude is found to correlate, though only moderately, with the eigenvector 1 parameters, i.e., the smaller the Hβ linewidth, the weaker the [O iii] and the stronger the Fe ii emission, the smaller the variability amplitude. Moreover, an interesting inverse correlation is found between the variability and the Eddington ratio, which is perhaps more fundamental. The previously known dependence of the variability on luminosity is not significant, and the dependence on black hole mass-as claimed in recent papers and also present in our data-fades out when controlling for the Eddington ratio in the correlation analysis, though these may be partly due to the limited ranges of luminosity and black hole mass of our samples. Our result strongly supports that an accretion disk is likely to play a major role in producing the optical/UV variability.

New Astronomy Reviews, 2001

The origin of the infrared emission in Active Galactic Nuclei (AGN), whose strength is comparable to the optical/ultra-violet (OUV) emission, is generally thought to be a combination of thermal emission from dust and non-thermal, synchrotron emis-Preliminary results from ISO observations of X-ray selected and high-redshift AGN will be described.

Monthly Notices of the Royal Astronomical Society, 1999

March and 1996 July. A distinctive feature of this survey is its photometric accuracy, , 0X02 V magY achieved through differential photometry with CCD detectors, which allows the detection of faint levels of variability. We find that the relative variability, d saL, observed in the V band is anticorrelated with both luminosity and redshift, although we have no means of discovering the dominant relation, given the strong coupling between luminosity and redshift for the objects in our sample. We introduce a model for the dependence of quasar variability on frequency that is consistent with multiwavelength observations of the nuclear variability of the Seyfert galaxy NGC 4151. We show that correcting the observed variability for this effect slightly increases the significance of the trends of variability with luminosity and redshift. Assuming that variability depends only on the luminosity, we show that the corrected variability is anticorrelated with luminosity and is in good agreement with predictions of a simple Poissonian model. The energy derived for the hypothetical pulses, , 10 50 erg, agrees well with those obtained in other studies. We also find that the radio-loud objects in our sample tend to be more variable than the radio-quiet ones, for all luminosities and redshifts.

Monthly Notices of the Royal Astronomical Society, 1999

We present U BV R photometry of a sample of 73 radio quasars, about 80 per cent complete, with redshifts 0.4-2.8. From these data the shape of the spectral energy distribution (SED) in the rest-frame blue/UV is analysed, using the individual sources as well as through broad-band composite SEDs. The SEDs of the individual sources are generally well fitted with power-laws, with slopes α ranging from 0.4 to-1.7 (S ν ∝ ν α). Two sources with α < −1.6 were excluded from the general study for having very red SEDs, significantly deviating with respect to the remaining sources. The composite SEDs cover the range 1300-4500Å and the only emission feature apparent from the broad-band spectra is the Civλ1549 line, in agreement with expectations from line equivalent width measuremets of radio-loud quasars from the literature. The shape of the composites in the logS ν-logν plane exhibits a break at around 3000Å where the spectrum changes from α blue = 0.11 ± 0.16 at λ > 3000Å to α UV = −0.66 ± 0.15 at λ < 3000Å. Although the broad-band spectral points are expected to include some masked contamination from emission lines/bumps, the break cannot be explained by line/bump emission, and most likely reflects an intrinsic trend in the continuum. The continuum shape is shown to depend on redshift. For the quasars with z < 1.2 we find α blue = 0.21 ± 0.16 and α UV = −0.87 ± 0.20, i.e. a higher steepening. For z > 1.2 α UV is more flat, −0.48 ± 0.12, and there are too few spectral points longward of 3000 A to obtain α blue and analyse the presence of the 3000Å break. A trend similar to that between α UV and z is found between α UV and luminosity at 2400Å, L 2400 , with luminous quasars exhibiting a harder spectrum. The data show an intrinsic correlation between L 2400 and the radio power at 408 MHz, not related to selection effects or independent cosmic evolution. The correlations α UVz, α UV-L 2400 and L 2400-z appear to be consistent with accretion disc models with approximately constant black hole mass and accretion rates decreasing with time. If the trends L 2400-z and P 408-z are predominantly related to a selection bias, rather than cosmic evolution, only one of the correlations α UV-L 2400 or α UV-z need to be intrinsic.

The Astrophysical Journal, 1999

We investigate the relations between the observed emission-line strengths, widths, and continuum properties of a sample of 41 low-redshift (z \ 1) quasars for which contemporaneous IR/soft X-ray spectral energy distributions are available. This includes investigating correlations between optical and UV lines with both the luminosity and the shape of the quasarsÏ continuum, as well as correlations between the various lines. The sample is heterogeneous, primarily selected on the existence of good-quality Einstein X-ray data, and includes 18 radio-loud and 23 radio-quiet quasars. We Ðnd anticorrelations between the equivalent width and various UV luminosities (the Baldwin e †ect) for the Lya and Hb lines and a marginal anticorrelation for C III]. Exclusion of narrow-line, low-luminosity active galactic nuclei reveals a signiÐcant Baldwin e †ect for the C IV and C III] lines. A signiÐcant anticorrelation of EW(C IV) with is also present. We Ðnd no correlations between any lines and the X-ray luminosity or X-ray a ox slope. The Fe II optical multiplet shows no simple relationship with luminosity or continuum slope ; however, there is a tendency for objects with Ñat X-ray spectra and/or strong X-ray luminosities to have weak Fe II.

The Astrophysical Journal

We report on the diversity in quasar spectra from the Baryon Oscillation Spectroscopic Survey. After filtering the spectra to mitigate selection effects and Malmquist bias associated with a nearly flux-limited sample, we create high signal-to-noise ratio composite spectra from 58,656 quasars (2.1 ≤ z ≤ 3.5), binned by luminosity, spectral index, and redshift. With these composite spectra, we confirm the traditional Baldwin effect (i.e. the anti-correlation of C iv equivalent width and luminosity) that follows the relation W λ ∝ L βw with slope β w = -0.35 ± 0.004, -0.35 ± 0.005, and -0.41 ± 0.005 for z = 2.25, 2.46, and 2.84, respectively. In addition to the redshift evolution in the slope of the Baldwin effect, we find redshift evolution in average quasar spectral features at fixed luminosity. The spectroscopic signature of the redshift evolution is correlated at 98% with the signature of varying luminosity, indicating that they arise from the same physical mechanism. At a fixed luminosity, the average C iv FWHM decreases with increasing redshift and is anti-correlated with C iv equivalent width. The spectroscopic signature associated with C iv FWHM suggests that the trends in luminosity and redshift are likely caused by a superposition of effects that are related to black hole mass and Eddington ratio. The redshift evolution is the consequence of a changing balance between these two quantities as quasars evolve toward a population with lower typical accretion rates at a given black hole mass.

Monthly Notices of the Royal Astronomical Society, 2016

We use a sample of 1669 quasars (r < 20.15, 3.6 < z < 4.0) from the Baryon Oscillation Spectroscopic Survey to study the intrinsic shape of their continuum and the Lyman continuum photon escape fraction (f esc,q), estimated as the ratio between the observed flux and the expected intrinsic flux (corrected for the intergalactic medium absorption) in the wavelength range 865-885 Å rest frame. Modelling the intrinsic quasar (QSO) continuum shape with a power law, F λ ∝ λ −γ , we find a median γ = 1.30 (with a dispersion of 0.38, no dependence on the redshift and a mild intrinsic luminosity dependence) and a mean f esc,q = 0.75 (independent of the QSO luminosity and/or redshift). The f esc,q distribution shows a peak around zero and a long tail of higher values, with a resulting dispersion of 0.7. If we assume for the QSO continuum a double power-law shape (also compatible with the data) with a break located at λ br = 1000 Å and a softening γ = 0.72 at wavelengths shorter than λ br , the mean f esc,q rises to 0.82. Combining our γ and f esc,q estimates with the observed evolution of the active galactic nucleus (AGN) luminosity function (LF), we compute the AGN contribution to the UV ionizing background (UVB) as a function of redshift. AGN brighter than one-tenth of the characteristic luminosity of the LF are able to produce most of it up to z ∼ 3, if the present sample is representative of their properties. At higher redshifts, a contribution of the galaxy population is required. Assuming an escape fraction of Lyman continuum photons from galaxies between 5.5 and 7.6 per cent, independent of the galaxy luminosity and/or redshift, a remarkably good fit to the observational UVB data up to z ∼ 6 is obtained. At lower redshift, the extrapolation of our empirical estimate agrees well with recent UVB observations, dispelling the so-called Photon Underproduction Crisis.

arXiv (Cornell University), 2022

We characterize the optical variability properties of eight lobe-dominated radio quasars (QSOs): B2 0709+37, FBQS J095206.3+235245, PG 1004+130, [HB89] 1156+631, [HB89] 1425+267, [HB89] 1503+691, [HB89] 1721+343, 4C +74.26, systematically monitored for a duration of 13 years since 2009. The quasars are radio-loud objects with extended radio lobes that indicate their orientation close to the sky plane. Five of the eight QSOs are classified as giant radio quasars. All quasars showed variability during our monitoring, with magnitude variations between 0.3 and 1 mag for the least variable and the most variable QSO, respectively. We performed both structure function (SF) analysis and power spectrum density (PSD) analysis for the variability characterization and search for characteristic timescales and periodicities. As a result of our analysis, we obtained relatively steep SF slopes (α ranging from 0.49 to 0.75) that are consistent with the derived PSD slopes (∼2-3). All the PSDs show a good fit to single power law forms, indicating a red-noise character of variability between ∼13 years and weeks timescales. We did not measure reliable characteristic timescales of variability from the SF analysis which indicates that the duration of the gathered data is too short to reveal them. The absence of bends in the PSDs (change of slope from ≥1 to ∼0) on longer timescales indicates that optical variations are most likely caused by thermal instabilities in the accretion disk.