THE ASTROPHYSICAL JOURNAL SUPPLEMENT SERIES 1999 .
Copyright by the IOP PUBLISHING LTD. José Francisco Salgado et al. 1999. "14-year program monitoring the flux densities of33 radio sources at low frequencies," ApJS 120 77 doi:10.1086/313169THE ASTROPHYSICAL JOURNAL SUPPLEMENT SERIES, 120 : 77È93, 1999 January( 1999. All rights reserved. Printed in U.S.A.14-YEAR PROGRAM MONITORING THE FLUX DENSITIES OF 33 RADIO SOURCES ATLOW FREQUENCIESJOSE FRANCISCO SALGADO,1 DANIEL R. ALTSCHULER,2 TAPASI GHOSH,2 BRIAN K. DENNISON,3KENNETH J. MITCHELL,4 AND HARRY E. PAYNE5Received 1997 December 23 ; accepted 1998 August 19ABSTRACTWe present the results of a low-frequency Ñux density monitoring program of 33 extragalactic radiosources. The light curves at 318 and 430 MHz over a 14 yr period are presented. The measurementswere made with the NAIC Arecibo 305 m radio telescope at approximately bimonthly intervals between1980 January and 1989 February and at less regular intervals between 1989 October and 1993 October,for a total of 64 observing sessions. In addition, we provide a Ðrst discussion of the results, pointing outseveral source properties and interesting objects.Subject headings : ISM : general È quasars : general È radio continuum : galaxies1.INTRODUCTIONproperties, which led to the VLBI measurements of a subsample of our sources. These results are the subject of aseparate study (Altschuler et al. 1995).The radio intensity variability of active galactic nuclei(AGNs) has been the subject of extensive study since itsdiscovery by Dent (1965) and Sholomitskii (1965). The shorttimescales of variability of a few months at centimetricwavelengths implied such high brightness temperatures inthese compact sources that bulk relativistic motions of thesynchrotron-emitting plasma at angles close to the line ofsight had to be invoked (for a review see Altschuler 1989).To date, the most successful models for explaining suchphenomena involve relativistic shocks propagating in radiojets (Ko nigl & Choudhuri 1985 ; Hughes, Aller, & Aller1989 ; Marscher 1992). The subsequent discoveries of lowfrequency variability (\1 GHz) and intraday variabilityhave pushed these models to their extremes. The currentscenarios suggest that low-frequency variability is mostlycaused by refractive interstellar scintillation (RISS, for areview, see Rickett 1986 and references therein), while forsources showing intraday variability, the observer appearsto be probing the very inner jet, where the Ñuid is likely tobe highly relativistic (Wagner et al. 1996).Between 1980 and 1993, we monitored 33 compact extragalactic radio sources at 318 and 430 MHz using the NAICArecibo 305 m radio telescope. Results for 1980 January to1984 December have been reported in Mitchell et al. (1994,hereafter Paper I), and discussions of selected sources canbe found in Altschuler et al. (1984), Dennison et al. (1984),and OÏDell et al. (1988). Here we present the data observedsubsequent to 1984. In 2 we describe the observationalprocedure and data reduction for the sessions between 1984and 1993. In 3È5 we present ““ light curves ÏÏ with all themeasurements since 1980 January and provide a Ðrst discussion of the results, pointing out several source propertiesand interesting objects. The theory of RISS predicts adependence between source structure and variability2.SAMPLE AND OBSERVATIONSWe selected 33 compact extragalactic sources that wereknown to be, or suspected of being, variable at low frequencies (Condon et al. 1979 ; Dennison et al. 1981). Theseare listed in Table 1. In this table, columns (3)È(6) give thesource positions in equatorial and Galactic coordinates.The redshifts, where known, are listed in column (7).Optical identiÐcations are presented in column (8), where Gstands for galaxy, Q for quasar, B for BL Lac object, HPQfor quasars with high ([3%) optical polarization, and Sey 2for type 2 Seyfert galaxy.In addition to these sources, eight standard Arecibo Ñuxdensity calibrators were observed in identical fashion to thesample sources. These are listed in the lower section ofTable 1.We used the Arecibo 305 m radio telescope in drift-scanmode at approximately bimonthly intervals between 1980and 1989 and at less regular intervals between 1989 and1993, for a total of 64 observing sessions. Internal calibration was achieved by switching on a signal of constantstrength for a short interval at the beginning of each driftscan. Each source was always observed at the same siderealtime (hour angle) in order to reproduce the confusion contribution to the scan and to minimize the residual errorsdue to the zenith angle-gain calibration. A single polarization was recorded at each frequency, linear at 318 MHz andcircular at 430 MHz.To estimate the Ñux densities, we Ðtted Gaussians to thedrift scans of the source. Scans were inspected visually andcorrections to the baseline were made as required. Care wastaken to ensure consistency in the measurements of a givensource at a given frequency over the 14 yr. Scans withserious problems, such as those containing strongterrestrial/solar interference or ionospheric/interplanetaryscintillations, were rejected.The heights of the Ðtted Gaussians were normalized bythe internal noise calibration and corrected for antennagain. For the 305 m antenna, the gain depends upon thezenith angle (ZA) of the observation, and to a much smallerextent on the feed platform elevation, which varies becauseof thermal expansion of the support cables. Finally, the1 Department of Astronomy, University of Michigan, Ann Arbor, MI481091-1090 ; salgado astro.lsa.umich.edu.2 National Astronomy and Ionosphere Center-Arecibo Observatory,H3 Box 53995, Arecibo, PR 00612 ; daniel naic.edu, tghosh naic.edu.3 Department of Physics, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061 ; dennison astro.phys.vt.edu.4 General Sciences Corporation, 4600 Power Mill Rd, Suite 400,Bettsville, MD, 20705-2675 ; mitchell stars.dnet.nasa.gov.5 Space Telescope Science Institute, 3700 San Martin Drive, Baltimore,MD 21218 ; payne stsci.edu.77
78J. F. SALGADO ET AL.Vol. 120TABLE 1MONITORED SOURCESGALACTIC COORDINATESID(1)SOURCE(2)1 .2 .3 .4 .5 .6 .7 .8 .9 .10 . . . . . .11 . . . . . .12 . . . . . .13 . . . . . .14 . . . . . .15 . . . . . .16 . . . . . .17 . . . . . .18 . . . . . .19 . . . . . .20 . . . . . .21 . . . . . .22 . . . . . .23 . . . . . .24 . . . . . .25 . . . . . .26 . . . . . .27 . . . . . .28 . . . . . .29 . . . . . .30 . . . . . .31 . . . . . .32 . . . . . .33 . . . . . 251]1582319]272a (1950.0)a(3)d 0.552.865.370.267.542.240.846.442.311.87.9[ PQB.HPQ.Sey 235418Calibration Sources34 . . . . . .35 . . . . . .36 . . . . . .37 . . . . . .38 . . . . . .39 . . . . . .40 . . . . . .41 . . . . . .556.539.384.9Q.Q.QQ.QNOTE.ÈUnits of right ascension are hours, minutes, and seconds, and units of declination are degrees,arcminutes, and arcseconds.a Coordinates, redshifts and types from Ve ron-Cetty & Ve ron (1993), except the optical identiÐcation of2050]364, from Biretta, Schneider, & Gunn (1985).calibration sources were used to convert the gain-correctedresponses into janskys. The Ñux density scale is based uponthat of Baars et al. (1977). Details of the data reduction anderror estimation procedures are described in Paper I andare not repeated here.In practice, a slightly di†erent data reduction procedurewas used after the observing session of 1984 December.Following this session, the main modiÐcation was that amore rigorous calibration step height and baseline determination procedure was established. When we reanalyzed the430 MHz data as a check, it led to a slight di†erence forsome of the Ñux density values for the session of 1989December of Paper I. Figure 1 shows a comparison wherethe values of Paper 1 and of the present paper are represented by Sa and Sb, respectively. The solid and dashedlines represent the mean and median values of Sb/Sa,0.982 0.009, and 0.998, respectively. The dotted lines represent the 3 p conÐdence limits of the mean.3.PRELIMINARY ANALYSISThe light curves for the 33 sources monitored are displayed in Figure 2, where the individual Ñux densities havebeen normalized by the weighted mean over the entire monitoring period. Table 2 presents the results of our prelimi-
No. 1, 1999LOW-FREQUENCY FLUX DENSITY MONITORING PROGRAM79In Figure 3 we present curves of the expected modulationindices due to noise as a function of Ñux density using equation (2), and the two p-values in Table 3. Also drawn arecurves of 2MI and 3MI versus Ñux density. The MI valuesccactually measured (see Table 2) are superimposed. Sourcesfalling above the 2MI curve were clearly variable duringcthe monitoring period. However, we note that any longterm systematic change (often clearly correlated at bothfrequencies) may result in sources falling below the 2MIccurve in Figure 3. In such cases, the above criterion fails todistinguish a variable source.3.2. Structure-Function AnalysisAnother technique for analyzing time series, which hasbeen applied to low-frequency variablity studies by variousauthors (e.g., Spangler et al. 1993), is structure-functionanalysis. We computed the normalized structure function(&) for all sample sources using the relation&(q) \FIG. 1.ÈComparison of Ñux density values determined by Mitchell etal. (1994), Sa, and Salgado et al. (this paper), Sb. The solid and dashed linesrepresent the mean and median values of Sb/Sa, 0.982 0.009 and 0.998,respectively. The dotted lines represent the 3 p conÐdence limits of themean.nary analysis of this data set. In columns (3) and (10) we listthe mean Ñux densities (S1 ) at 318 and 430 MHz, respectively. For each source, we have also calculated modulationindex (MI) and structure function (&) at both frequencies, aswell as two-frequency cross-correlation functions.3.1. Modulation IndicesFor each source, modulation indices at both 318 and 430MHz were calculated using MI \ p /S1 , where p is the rmsssÑux density Ñuctuation over the entiremonitoringperiod.These values are listed in columns (4) and (11) of Table 2.In order to estimate the contribution of measurementerrors to the values of modulation index for each source, weundertook the following error analysis. The total measurement error (p ) at any epoch is considered to be the quadratic sum of twot independent factors :p2 \ p2 ] S1 2p2 ,(1)tcgwhere the Ñux densityÈindependent term, p , is mostly dueto confusion and interference, while thec Ñux densityÈdependent error, p , is due to pointing errors as well as anygerror in gain estimation.From straight-line Ðts to the plot of average values of p2versus S1 2 of all sources, we estimated the values of p and pTseparately for the 318 and 430 MHz systems. For cthe linegfeed systems of the Arecibo Telescope, these errors fell intotwo setsÈsources at low and at high zenith angles. The ZAvalues that divided the two sets were also frequencydependent owing to the di†erent designs of the two feeds.Table 3 summarizes the adopted values of p and p in thecgtwo ZA ranges at the two observing frequencies.The modulation index due to measurement uncertaintiesalone was then computed for each source usingMI \ p /S1 \ Jp2/S1 2 ] p2 .ctcg(2)1, S[S(t) [ S(t ] q)]2T2p2(3)where p2 is the variance of the Ñux density. We Ðnd that thenormalized structure functions of these sources can be classiÐed into four basic types, represented schematically inFigure 4. For sources with a type A structure function, asimple estimate of the timescale of variation has been madeusing the lag for which & rises to 0.77. Structure functions oftypes B and C are more complicated, and we believe thatthese are cases where two or more source components arevarying independently. For nonvariable (NV) sources, thestructure function rises to its saturation value of 1 withinthe Ðrst lag, while for slowly varying sources (type D) thestructure function does not reach saturation even for themaximum computed lag of 7 yr. The structure functions of afew sources show a more complex nature that could not beclassiÐed according to the above-mentioned scheme. Theseare marked as U in Table 2.Structure-function types for each source at both frequencies are entered in columns (5) and (2)1 of Table 2. Forstructure functions of type A, estimates of timescales arealso listed in columns (6) and (13). Lower limits for thetimescaled structure functions of type D are also given.3.3. Correlated V ariations at 318 and 430 MHzWe have also calculated cross-correlation functionsbetween the 318 and 430 MHz data. If variations are intrinsic, an increasing time delay toward lower frequency couldbe expected, while for RISS no time delay is predicted.More detailed analysis involving source structures and propagation models will be presented in a future paper. Herewe note only that most sources vary in a correlated fashionat these two frequencies. Exceptions are sources 5, 18, 19,and 31, which are variable at both frequencies but do notshow any correlation. We note that source 22, 1611]343 isthe most variable source in our sample, and the light curvesat the two frequencies are highly correlated.4.DISCUSSIONIn Table 2, we indicated by crosses in the V , V , and VL M on the&columns sources that can be identiÐed as variablebasis of a visual inspection of the light curves, modulationindex studies, and structure-function analysis, respectively.In many cases, these three determinations are consistent.
FIG. 2.È14-year radio ““ light curves ÏÏ of the sources monitored at 318 and 430 MHz. Abscissae : date (yr) ; ordinate : Ñux density normalized by theweighted mean over the entire monitoring period ; parameter : frequency (MHz). The weighted-mean Ñux density is given in the upper right-hand corner ofeach plot.80
FIG. 2.ÈContinued81
FIG. 2.ÈContinued82
FIG. 2.ÈContinued83
FIG. 2.ÈContinued84
FIG. 2.ÈContinued85
FIG. 2.ÈContinued86
FIG. 2.ÈContinued87
FIG. 2.ÈContinued88
FIG. 2.ÈContinued89
FIG. 2.ÈContinued
LOW-FREQUENCY FLUX DENSITY MONITORING PROGRAM91TABLE 2FLUX DENSITIES, MODULATION INDICES, AND STRUCTURE FUNCTION CLASSIFICATION318 MHzID(1)SOURCE(2)Sa(3)MI(4)&(5)1 .2 .3 .4 .5 .6 .7 .8 .9 .10 . . . . . .11 . . . . . .12 . . . . . .13 . . . . . .14 . . . . . .15 . . . . . .16 . . . . . .17 . . . . . .18 . . . . . .19 . . . . . .20 . . . . . .21 . . . . . .22 . . . . . .23 . . . . . .24 . . . . . .25 . . . . . .26 . . . . . .27 . . . . . .28 . . . . . .29 . . . . . .30 . . . . . .31 . . . . . .32 . . . . . .33 . . . . . CNVCCC34 . . . . . .35 . . . . . .36 . . . . . .37 . . . . . .38 . . . . . .39 . . . . . .40 . . . . . .41 . . . . . 620.0110.0140.0190.0060.0100.0100.0100.015430 MHzV ]]]]]]]]]]]]]Calibration 0.0130.0280.0250.0180.042q(13)[80[68.2414306.1688V ]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]a Weighted mean of Ñux densities (Jy). For calibration sources, the adopted Ñux densities (Jy).b Considered variable by one or more of the following determinations : visual inspection of the light curves (V ), MI [ 2MI (V ), and/or byLc Mstructure-function analysis (V ).&c Undetermined.TABLE 3ADOPTED p VALUESZA\15¡ . . . . . .[15¡ . . . . . .pp0.06360.08020.01990.0191c318 MHzg430 MHz\9¡ . . . . . . .[9¡ . . . . . . .0.08940.13730.02180.0198However, we note that the modulation index criterion failsto identify sources 4, 6, 10, 11, 12, 14, 16, 20, 21, 25, 27, 28,29, 30, and 33 as variable at 430 MHz, and sources 10, 27,29, and 33 as variable at 318 MHz. An inspection of thelight curves of all these sources reveals that they have largermeasurement errors, but shows clear, often correlated,variability and classiÐable structure-function types. At 318MHz, source 2, 18, and 19 are found to be nonvariable bythe structure-function criterion, whereas their modulationindices are greater than 2MI . This is due to single episodesc 1988.2, and 1984.7 in theseof variation at epochs 1985.5,sources. Such single episodes of deviant Ñux density are
92J. F. SALGADO ET AL.Vol. 120FIG. 3.ÈDerived modulation indices (MI) of all sources as a function of their weighted-mean Ñux densities. The lowest curve represents the errorcontribution computed using eq. (2). Additionally, 2MI and 3MI vs. Ñux density curves have been drawn.ccmore likely to cause a high modulation index value than avariable sourcelike appearance in the structure-functionanalysis.According to all three criteria, we found that sources0038]328 (No. 1), 0116]319 (No. 3), and 1039]029 (No.15) are nonvariable at both frequencies during the entiremonitoring span. Source 2144]092 (No. 27) is variable at430 MHz according to the structure-function analysis.However, at 318 MHz, both the modulation index and thestructure-function analysis fail to identify it as a variable,despite the source undergoing a variable phase before 1984.This phase is correlated with the one at 430 MHz duringthat time. Sources 1422]202 (No. 19), 1922]333 (No. 25),2050]364 (No. 26), and 2223]210 (No. 30) are nonvariable at 318 MHz. Among these, 2050]364 shows a highlystriking behavior at 430 MHz, with indications of anextreme scattering event as observed for a few other sourcesat higher frequencies by Fiedler et al. (1987). Th
frequency variability (\1 GHz) and intraday variability have pushed these models to their extremes. The current scenarios suggest that low-frequency variability is mostly caused by refractive interstellar scintillation (RISS, for a review, see Rickett 1986 and references therein), while for sources showing intraday variability, the observer appears
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