Color Variability of AGNs

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.

Arxiv preprint astro-ph/ …, 2006

We report on the relative optical variability of the three brightest nearby quasars, 3C 273, PDS 456, and PHL 1811. All three have comparable absolute magnitudes, but PDS 456 and PHL 1811 are radio quiet. PDS 456 is a broadline object, but PHL 1811 could be classified as a high-luminosity Narrow-Line Seyfert 1 (NLS1). Both of the radio-quiet quasars show significant variability on a timescale of a few days. The seasonal rms V-band variability amplitudes of 3C 273 and PDS 456 are indistinguishable, and the seasonal rms variability amplitude of PHL 1811 was only exceeded by 3C 273 once in 30 years of monitoring. We find no evidence that the optical variability of 3C 273 is greater than or more rapid than the variability of the comparably-bright, radio-quiet quasars. This suggests that not only do radio-loud and radio-quiet AGNs have similar spectral energy distributions, but that the variability mechanisms are also similar. The optical variability of 3C 273 is not dominated by a "blazar" component.

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)

Astronomy and Astrophysics, 2004

Photometry of the Q2237+0305gravitational lens in VRI spectral bands with the 1.5-m telescope of the high-altitude Maidanak observatory in 1995-2000 is presented. Monitoring of Q2237+0305 in July-October 2000, made at nearly daily basis, did not reveal rapid (night-tonight and intranight) variations of brightness of the components during this time period. Rather slow changes of magnitudes of the components were observed, such as 0.08m fading of B and C components and 0.05m brightening of D in R band during July 23-October 7, 2000. By good luck three nights of observation in 1999 were almost at the time of the strong brightness peak of image C, and approximately in the middle of the ascending slope of the image A brightness peak. The C component was the most blue one in the system in 1998 and 1999, having changed its (V-I) color from 0.56m to 0.12m since August 1997, while its brightness increased almost 1.2m during this time period. The A component behaved similarly between August 1998 and August 2000, having become 0.47m brighter in R, and at the same time, 0.15m bluer. A correlation between the color variations and variations of magnitudes of the components is demonstrated to be significant and reaches 0.75, with a regression line slope of 0.33. A color (V-I) vrs color (V-R) plot shows the components settled in a cluster, stretched along a line with a slope of 1.31. Both slopes are noticeably smaller than those expected if a standard galactic interstellar reddening law were responsible for the differences between the colors of images and their variations over time. We attribute the brightness and color changes to microlensing of the quasar's structure, which we conclude is more compact at shorter wavelengths, as predicted by most quasar models featuring an energizing central source.

The Astronomical Journal, 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)

2022

TotalDat.fits.gz: A FITS table storing information for each of the quasars used in the sample. The names, formats, and contents of each of the columns in this table are described in Table 1. All time-series data (MJD_x, MAG_x, MAG_ERR_x), structure function data (DT_REST_x, SF_x, SF_ERR_x), and PSD data (REST_FREQ_x, CARMA_PSD_x, CARMA_PSD_ERR_L_x, CARMA_PSD_ERR_U_x) are stored as arrays. EnsDat.fits.gz: A FITS table storing information for the ensemble analysis conducted on different subsets of the total sample. The names, formats, and contents of each of the columns in this table are described in Table 2. Similar to the previous file, time-series, structure function, and PSD data are stored as arrays. It should be noted that for each quasar/ensemble, each array will be the same length to conform to the FITS file standards. Therefore, to force each array to be the same shape, arrays shorter than the largest array will be filled with either NaNs or empty strings until they reach thi...

2010

We assess the effects of simulated active galactic nuclei (AGNs) on the colour and morphology measurements of their host galaxies. To test the morphology measurements, we select a sample of galaxies not known to host AGNs and add a series of point sources scaled to represent specified fractions of the observed V band light detected from the resulting systems; we then compare morphology measurements of the simulated systems to measurements of the original galaxies. AGN contributions > ∼ 20 per cent bias most of the morphology measurements tested, though the extent of the apparent bias depends on the morphological characteristics of the original galaxies. We test colour measurements by adding to non-AGN galaxy spectra a quasar spectrum scaled to contribute specified fractions of the rest-frame B band light detected from the resulting systems. A quasar fraction of 5 per cent can move the NUV−r colour of an elliptical galaxy from the UV-optical red sequence to the green valley, and 20 per cent can move it into the blue cloud. Combining the colour and morphology results, we find that a galaxy/AGN system with an AGN contribution > ∼ 20 per cent may appear bluer and more bulge-dominated than the underlying galaxy. We conclude that (1) bulge-dominated, E/S0/Sa, and early-type morphology classifications are accurate for red AGN host galaxies and may be accurate for blue host galaxies, unless the AGN manifests itself as a well-defined point source; and (2) although highly unobscured AGNs, such as the quasar used for our experiments, can significantly bias the measured colours of AGN host galaxies, it is possible to identify such systems by examining optical images of the hosts for the presence of a point source and/or measuring the level of nuclear obscuration.

The Astrophysical Journal, 2015

We conduct a pilot investigation to determine the optimal combination of color and variability information to identify quasars in current and future multi-epoch optical surveys. We use a Bayesian quasar selection algorithm (Richards et al. 2004) to identify 35,820 type 1 quasar candidates in a 239 deg 2 field of the Sloan Digital Sky Survey (SDSS) Stripe 82, using a combination of optical photometry and variability. Color analysis is performed on 5-band single-and multi-epoch SDSS optical photometry to a depth of r ∼ 22.4. From these data, variability parameters are calculated by fitting the structure function of each object in each band with a power law model using 10 to > 100 observations over timescales from ∼ 1 day to ∼ 8 years. Selection was based on a training sample of 13,221 spectroscopically-confirmed type-1 quasars, largely from the SDSS. Using variability alone, colors alone, and combining variability and colors we achieve 91%, 93%, and 97% quasar completeness and 98%, 98%, and 97% efficiency respectively, with particular improvement in the selection of quasars at 2.7 < z < 3.5 where quasars and stars have similar optical colors. The 22,867 quasar candidates that are not spectroscopically confirmed reach a depth of i ∼ 22.0; 21,876 (95.7%) are dimmer than coadded i-band magnitude of 19.9, the cut off for spectroscopic follow-up for SDSS on Stripe 82. Brighter than 19.9, we find 5.7% more quasar candidates without confirming spectra in sky regions otherwise considered complete. The resulting quasar sample has sufficient purity (and statistically correctable incompleteness) to produce a luminosity function comparable to those determined by spectroscopic investigations. We discuss improvements that can be made to the process in preparation for performing similar photometric selection and science on data from post-SDSS sky surveys.

Astronomy and Astrophysics, 1997

The continuum and emission lines properties of a sample of 73 blazars is studied, investigating differences and similarities among normal low-polarization quasars (LPQ), radio-loud high polarized quasars (HPQ), and BL Lacertae objects (BLL).

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.