We present preliminary results of optical monitoring of sample of FRII-type radio quasars. The optical observations were made with three telescopes, among them one robotic, spanning a time interval longer than two years. Variability in the range of a fraction of a magnitude was observed for all eight targets. We applied the structure function to analyse the brightness changes. The slope of the structure function is only consistent with the disk instability model for two sources; the other sources show values between of the disk instability and starburst models. Finally we argue that such monitoring would be most suitable as a long-term, complementary program for robotic telescopes

Drozdz, M.

Erdem, A.

Jabłeka, Damian

Jamrozy, Marek

Kozieł-Wierzbowska, Dorota

Kuligowska, Elżbieta

Kurpińska-Winiarska, Maria

Kuźmicz, Agnieszka

Ogłoza, W.

Siwak, Michał

Stachowski, G.

Szymański, Tomasz

Winiarski, Maciej

Zoła, Stanisław

Jagiellonian Univeristy Repository

Second Workshop on Robotic Autonomous ObservatoriesASI Conference Series, 2012, Vol. 7, pp 239 – 246Edited by Sergey Guziy, Shashi B. Pandey, Juan C. Tello &Alberto J. Castro-TiradoOptical monitoring of FRII-type radio quasarsS. Zola 1,2∗, A. Kuzmicz1, M. Jamrozy1, M. Winiarski2,T. Szymanski1, D. Koziel-Wierzbowska1, E. Kuligowska1,M. Drozdz2, A. Erdem3, M. Siwak1, G. Stachowski2,M. Kurpinska-Winiarska1, D. Jableka1 and W. Ogloza21Astronomical Observatory, Jagiellonian University, ul. Orla 171, 30-244 Krakow, Poland2Mt. Suhora Observatory, Pedagogical University, ul. Podchorazych 2, 30-084 Krakow, Poland3Astrophysics Research Centre and Observatory, C¸anakkale Onsekiz Mart University,Terziog˘lu Kampu¨su¨, TR-17040, C¸anakkale, TurkeyAbstract. We present preliminary results of optical monitoring of sampleof FRII-type radio quasars. The optical observations were made with threetelescopes, among them one robotic, spanning a time interval longer thantwo years. Variability in the range of a fraction of a magnitude was ob-served for all eight targets. We applied the structure function to analyse thebrightness changes. The slope of the structure function is only consistentwith the disk instability model for two sources; the other sources show val-ues between of the disk instability and starburst models. Finally we arguethat such monitoring would be most suitable as a long-term, complemen-tary program for robotic telescopes.Keywords : galaxies: active – galaxies: jets – galaxies: photometry –quasars: general – quasars: individual: J0713+3656, J0952+2352, J1007+1248, J1158+6254, J1427+2632, J1504+6856, J1723+3417, J2042+75081. IntroductionThe multi-wavelength variability of quasars was reported since their discovery, andmany correlations between variation in quasar luminosity and various physical para-meters have been known for decades (for a summary see e.g. Helfand et al. (2001)).Monitoring of flux changes of quasars helps us to understand the physics of the centralblack hole and the accretion disk. For example, there is growing evidence suggesting∗email: szola@oa.uj.edu.pl240 S. Zola et al.that the variability could be linked to the mass of the central black-hole, (e.g. Wold etal. (2007); Wilhite et al. (2007)).In this study, we are interested particularly in the central black hole and thephysics of the accretion disk, which is associated with long-term variability. There-fore, we have excluded blazars, which are the most violently variable type in theoptical domain, as these objects exhibit intra-night or even shorter period variationsbelieved to be caused by synchroton radiation in the jet. These objects have jets di-rected at very small angles to the line of sight, and the short period variations appearto be due to high Lorentz factors, see (e.g. Villata et al. (2008); Urry & Padovani(1995)) .Several models have been proposed to explain the sources of quasar long-termvariability: the instability of the accretion disk surrounding the supermassive blackhole (SMBH), e.g. Wallinder et al. (1992), Kawaguchi et al. (1998), Pereyra et al.(2006), violent starburts in the innermost regions of the host galaxy, e.g. Artexaga& Terlevich (1994), Artexaga et al. (1997) and microlensing by compact bodies onthe path, e.g. Hawkins (1993), Hawkins (1996), Zackrisson et al. (2003). Thesethree main models – one connected to the AGN itself, one related to the host galaxy’sproperties and one describing geometrical effects – compete to be the factor primarilyresponsible for quasar variability. Distinguishing between these models and identi-fying the one which plays the most significant role in quasar variability can only bedone using data gathered over a long period of time.There have been numerous projects undertaken in the past to study quasar opticalactivity, aimed at explaining the physics behind the observed flux changes. These havebeen based either on analysis of large samples of objects for which data have becomeavailable from recent optical surveys, sometimes supplemented with archival datafrom photographic plates to enlarge the time span to few decades (see e.g. Bauer etal. (2009) and references therein), or on smaller samples of a specific class of targets,e.g. Peterson et al. (1999). While the former approach provides a huge amount ofdata, it inevitably suffers from calibration problems if different sets of data are to becompared; the latter one consists of data of better accuracy but the time span is muchshorter – usually only a few years.In this study, we follow the approach of investigating the light curves of a smallsample of carefully selected objects, chosen to be FRII-type radio quasars (Fanaroff &Riley 1974). Radio-loud quasars with extended radio structures can be used to studydependence of the optical variability on the obscuration by the dusty torus. Assumingthat the disc axis and the jet axis are aligned, one can use the asymmetries in lobeluminosity to infer the information about the disk projection. At high inclinations, thedusty torus present in FRII radio quasars can partially or totally obscure the accretiondisc. Therefore, in high inclination sources any variability due to disc instabilitiesshould disappear. The light curves published in previous studies indicate that theOptical monitoring of FRII-type radio quasars 241J1007+1248                                                1.4 GHz FIRST and R-band DSSLevs = 8.000E-04 * (1, 2, 4, 8, 16, 32, 64) JY/BEAM     DECLINATION (J2000)RIGHT ASCENSION (J2000)10 07 35 30 25 20 1512 51 300050 300049 300048 300047 300046 30J1007+1248                                                1.4 GHz NVSS and R-band DSSLevs = 2.000E-03 * (1, 2, 4, 8, 16, 32, 64, 128) JY/BEAM DECLINATION (J2000)RIGHT ASCENSION (J2000)10 07 35 30 25 20 1512 51 300050 300049 300048 300047 300046 30J1427+2632                                                     1.4 GHz FIRST and R-band DSSLevs = 6.000E-04 * (1, 2, 4, 8, 16, 32) JY/BEAM DECLINATION (J2000)RIGHT ASCENSION (J2000)14 27 50 45 40 35 30 2526 353433323130J1427+2632                                                     1.4 GHz NVSS and R-band DSSLevs = 2.000E-03 * (1, 2, 4, 8, 16, 32) JY/BEAMDECLINATION (J2000)RIGHT ASCENSION (J2000)14 27 50 45 40 35 30 2526 353433323130Figure 1. Radio contours overlayed on an optical DSS image of two targets from our sam-ple. The upper panel shows J1007+1248, which has the smallest linear size (about 370 kpc)among objects in our sample, while the bottom panel shows J1427+2632, with a giant linearsize of 1.16 Mpc. The FIRST and NVSS maps are shown on the left and right panel, respec-tively. Crosses show the location of the parent quasar, and the circles in the upper corners theresolution of the maps.brightness of quasars changes on time scales of months, and that the amplitude istypically a fraction of a magnitude but can sometimes be larger.2. Sample selectionOur sample consists of bright quasars which show strong radio emission. We selectedthem from different catalogs of radio quasars, e.g. Helfand et al. (2001). On the basisof the NVSS (Condon et al. 1998) and FIRST (Becker et al. 1995) radio maps, weselected only those powerful sources which possess extended radio structure (angu-lar size > 20 arcsec) and FRII-type morphology. Based on their lobes’ brightnessasymmetries, the inclination angle (i.e. the angle between the line of sight and the jet242 S. Zola et al. 2.1 2.15 2.2 2.25 2.3 2.35 2.4 2.45 2.5 4800  4900  5000  5100  5200  5300  5400  5500  5600  5700∆  Ma gJD-2450000J0713+3656 2.2 2.3 2.4 2.5 2.6 2.7 4800  4900  5000  5100  5200  5300  5400  5500  5600  5700∆  Ma gJD-2450000J0952+2352 0.8 0.9 1 1.1 1.2 1.3 1.4 4800  4900  5000  5100  5200  5300  5400  5500  5600  5700∆  Ma gJD-2450000J1007+1248 1.2 1.3 1.4 1.5 1.6 1.7 4800  4900  5000  5100  5200  5300  5400  5500  5600  5700∆  Ma gJD-2450000J1158+6254Figure 2. Light curves of four quasars from our sample. The upper panel shows variabilityof J0713+3656 and J0952+2352, while the bottom one that of J1007+1248 and J1158+6254.Squares denote (comparison star − target) measurements and asterisks comparison and checkstar differences.axis) has been estimated by assuming that Doppler boosting is the dominant factor forany observed asymmetry. Our sample comprises radio quasars with inclination anglesranging from 45◦ (J1427+2632) to 90◦ (J0952+2352), required to investigate the roleof a dusty torus in AGNs. Since the proposed optical observations were intended forsmall telescopes located on the northern hemisphere, we applied the additional cri-teria that targets not be fainter than 17th magnitude in the R-band and have positivedeclination (δ > 0). We initially chose eleven sources, but after the first observationsthe 3 faintest targets were dropped. All objects in our sample have known redshifts,covering distances from z = 0.1 to z = 0.97, and having a projected linear size ofthe entire source from 370 kpc to about 1.1 Mpc. We have limited the sample tosuch a small number due to the initial planned use of a manual-mode telescope. Theradio maps and optical images for two of our targets are shown in Fig. 1, while theproperties of all quasars in the sample are listed in Table 1.3. ObservationsObservations of the targets from our sample started in March 2009 using the 50 cmCassegrain telescope of the Astronomical Observatory of the Jagiellonian University.Later, two other 60 cm telescopes joined our campaign, one at Mt. Suhora Obser-Optical monitoring of FRII-type radio quasars 243 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 4800  4900  5000  5100  5200  5300  5400  5500  5600  5700∆  Ma gJD-2450000J1427+2632 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2 2.1 2.2 2.3 4800  4900  5000  5100  5200  5300  5400  5500  5600  5700∆  Ma gJD-2450000J1504+6856 1.7 1.8 1.9 2 2.1 2.2 2.3 2.4 4800  4900  5000  5100  5200  5300  5400  5500  5600  5700∆  Ma gJD-2450000J1723+3417-0.45-0.4-0.35-0.3-0.25-0.2-0.15-0.1-0.05 0 4800  4900  5000  5100  5200  5300  5400  5500  5600  5700∆  Ma gJD-2450000J2042+7508Figure 2. (continued) Light curves of four other quasars: J1427+2632, J1504+6856 (upperpanel), J1723+3417 and J2042+7508 (bottom panel).vatory and the other one at the Tubitak Observatory, Turkey. The latter telescopeworks in a fully robotic mode. All telescopes are equipped with CCD cameras anda set of wide-band, UBVRI (Cousins) filters. The observations were performed bytaking a number of frames (10–12) in the R filter for each target, with (V-R) and (R-I)colors also measured on occasion. Calibration images (bias, dark and flatfield) wereacquired every night. The frames were reduced using the IRAF package, and themagnitude differences between each target and nearby comparison and check starswere then derived through aperture photometry with the CMunipack program. Toensure uniformity of the results, all the reduction was performed by one person. Onseveral occasions, simultaneous observations were performed to exclude/measure anyoffsets between the sites. As a result, we derived a mean point per night, the accuracyof which was not worse than 0.01–0.02 magnitude. Only on a few dates, when ob-servations were made at very high zenith distance, was the accuracy slightly worse.The light curves of our targets are shown in Fig. 2 where squares denote the mag-nitude difference between comparison stars and targets, while asterisks that betweencomparison and check stars.4. Preliminary results and conclusionsAfter two years of observations we have found all of the chosen targets to be variablein the optical wavelength. The amplitude of changes ranges from 0.1m (J0952+2352)244 S. Zola et al.-2-1.8-1.6-1.4-1.2-1-0.8-0.6-0.4-2 -1.5 -1 -0.5  0  0.5l og ( S)  [ ma g]log(t) [years]J1007+1248a = 0.74-1.8-1.6-1.4-1.2-1-0.8-0.6-0.4-2 -1.5 -1 -0.5  0  0.5l og ( S)  [ ma g]log(t) [years]J1427+2632a = 0.58-1.6-1.5-1.4-1.3-1.2-1.1-1-0.9-0.8-0.7-0.6-0.5-1.6 -1.4 -1.2 -1 -0.8 -0.6 -0.4 -0.2  0  0.2  0.4l og ( S)  [ ma g]log(t) [years]J1723+3417a = 0.56-1.7-1.6-1.5-1.4-1.3-1.2-1.1-1-0.9-0.8-0.7-2 -1.5 -1 -0.5  0  0.5l og ( S)  [ ma g]log(t) [years]J2042+7508a = 0.39Figure 3. Structure functions of four targets: J11007+1248, J1427+2632, J1723+3417 andJ2042+7508.to 0.5m (J1723+3417). The amplitude decreases with increasing inclination as ex-pected if torus obscures the accretion disk and the disk is the source of variability.There is, however, one exception: J1504+6856 exhibits brightness variations of about0.4m despite its high (81deg) inclination angle. We can speculate that this might bedue to the absence of a torus in this quasar, or that the torus is misaligned relative tothe accretion disk.Several tools have been used to extract information about the nature of the AGNvariability, including Fourier analysis, the autocorrelation function or the structurefunction (S). The structure function has been widely used to derive information fromthe light curve of quasars, e.g. Cid Fernandez et al. (1997), Hawkins (2002), Baueret al. (2009). We have also applied the structure function method to quantitativelycharacterize the light curves of six targets with the best data coverage. The structurefunctions for J1007+1248, J1427+2632, J1723+3417 and J2042+7508 are presentedin Fig. 3. The coefficients a of the flat part of the structure functions have been calcu-lated and are listed in Table 1. The theoretically predicted slopes of the structure func-tions are equal to 0.44, 0.83 and 0.25 for disk instability, starbursts and microlensing,respectively (Hawkins 1996). Considering slopes of the structure functions derivedfrom our two year-long light curves, the properties of two targets (J0713+3656 andJ2042+7508) are consistent with the prediction that disk instability is the mechanismof their variability. The others are in between the values expected for the disk insta-bility and starbust mechanisms, but neither is consistent with the microlensing model.Optical monitoring of FRII-type radio quasars 245Table 1. Sample of radio quasars chosen for optical monitoring.name Da ib zc ∆md aeJ0713+3656 414 80 0.49 0.2 0.48J0952+2352 702 90 0.97 0.1 -J1007+1248 370 67 0.24 0.4 0.74J1158+6254 385 75 0.59 0.2 -J1427+2632 1158 45 0.36 0.4 0.58J1504+6856 867 81 0.32 0.4 0.56J1723+3417 760 51 0.21 0.5 0.56J2042+7508 1138 61 0.10 0.2 0.39Notes: (a) linear size in kpc; (b) inclination in degrees;(c) redshift; (d) amplitude of variation; (e) slope of thestructure functionFinally, we conclude that such a long-term program could be very well suited forsmall robotic telescopes. 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Optical monitoring of FRII-type radio quasars

Optical monitoring of FRII-type radio quasars

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