Unusual optical quiescence of the classical BL Lac object PKS 0735+178 on intranight time scale
Arti Goyal, Gopal-Krishna, G. C. Anupama, D. K. Sahu, R. Sagar, S. Britzen, M. Karouzos, M. F. Aller, H. D. Aller
aa r X i v : . [ a s t r o - ph . C O ] J u l Mon. Not. R. Astron. Soc. , 1– ?? (2008) Printed 24 June 2018 (MN LaTEX style file v2.2) Unusual optical quiescence of the classical BL Lacob ject PKS 0735+178 on intranight time scale
Arti Goyal ⋆ , Gopal-Krishna , G. C. Anupama , D. K. Sahu , R. Sagar ,S. Britzen , M. Karouzos † , M. F. Aller , H. D. Aller Aryabhatta Research Institute of observational sciencES (ARIES), Manora Peak, Naini Tal 263 129, India NCRA.TIFR, Pune University Campus, Pune 411 007, India Indian Institute of Astrophysics (IIA) Bangalore 560 034, India Max-Planck-Institut f¨ur Radioastronomie, Auf dem H¨ugel 69, 53121 Bonn, Germany Astronomy Department, University of Michigan, Dennison Building, Ann Arbor, MI 48109-1090, USA
Released 2008 Xxxxx XX
ABSTRACT
We present the result of our extensive intranight optical monitoring ofthe well known low-energy peaked BL Lac (LBL) object PKS 0735+178. Thislong-term follow-up consists of R -band monitoring for a minimum duration of ∼ > & ∼ c (cid:13) Goyal et al.
Key words:
BL Lac objects – general - individual : PKS 0735+178 – vari-ability – intranight – optical: Galaxies - active – Galaxies - jets
Characterized by extreme faintness or absence of broad emission lines in their optical/UVspectra, BL Lac objects are a subset of the blazar population for which the dominant sourceof emission is believed to be a relativistic jet of non-thermal radiation (e.g., Blandford &Rees 1978; Urry & Padovani 1995). A key optical property of blazars is large and frequentlyoccuring rapid flux variability (also termed as INOV), which is not exhibited by other radio-loud or radio-quiet active galactic nuclei (AGN). Thus, a classical, radio-selected BL lacobject (RBL), when monitored continuously for more than about 4 hours, is quite likely toshow optical variability at the level of a few percent, on hour-like time scale (e.g., Miller,Carini & Goodrich 1989; Wagner & Witzel 1995; Romero et al. 2002). More specifically,INOV amplitude ψ >
3% is expected to occur in such observations, with a probability (dutycycle: DC) of ∼
53% (Gopal-Krishna et al. 2003; Stalin et al. 2005). A similar estimate (DC ∼ ψ > ⋆ E-mail: [email protected] † Member of the International Max Planck Research School (IMPRS) for Astronomy and Astrophysics at the Universities ofBonn and Cologne c (cid:13) , 1– ?? NOV of PKS0735+178 ψ < ∼ R -band differential light curves (DLCs)for a total of 17 nights, between 1998 and 2008. Thus, the DLCs are available for minimumone night every winter since 1998, except for 2002. Each DLC is longer than ∼ −
2% level. In thispaper, we report these new data and discuss our entire dataset in the broader context of thepublished multi-wavelength observations of this well known BL Lac object.PKS 0735+178 is among the first sources to be designated as “classical BL Lac” (Carswellet al. 1974). Its host galaxy still remains unresolved (e.g., Pursimo et al. 2002) but anabsorption feature in the optical spectrum, identified as Mg-II, has yielded a lower redshiftlimit of z > .
424 (e.g., Rector & Stocke 2001). Hartman et al. (1999) have reported γ − raydetection of this blazar using EGRET. On the basis of its spectral energy distributionpeaking at 10 − Hz, it can be confidently classified as a ‘low energy peaked’ BL Lac(LBL) (Padovani et al. 2006; Nieppola, Tornikoski & Valtaoja 2006; Ghisellini, Tavecchio& Chiaberge 2005). Whereas in the optical and even in the radio band PKS 0735+178 hasshown strong variability (Webb et al. 1988; Ciprini et al. 2007; Qian & Tao 2004; Fig. 1;Sect. 4) characteristic of blazars, its X-ray and γ − ray emission was found to be quite steady(Bregman et al. 1984; Madejski & Schwarz 1988; Nolan et al. 2003). Specifically, based ontheir analysis of the Einstein Observatory (IPC) observations from April, 1979 till March,1981, Madejski & Schwarz (1988) found no deviation from the mean flux density at ∼ c (cid:13) , 1– ?? Goyal et al. which was interpreted as the superposition of many homogeneous components of incoherentsynchrotron radiation, so called “cosmic conspiracy” (Cotton et al. 1980).
The photometric observations were carried out using the 104-cm Sampurnanand telescope(ST) located at Aryabhatta Research Institute of observational sciencES (ARIES), Naini Tal(India), except on one night when the 201-cm Himalayan Chandra telescope (HCT) of IAOat Hanle (India) was used. ST has a Ritchey-Chretien (RC) optics with a f /
13 beam (Sagar1999). The detector was a cryogenically cooled 2048 × − /pixel and a gain of 10 e − / Analog to DigitalUnit (ADU) in an usually employed slow readout mode. Each pixel has a dimension of24 µ m which corresponds to 0.37 arcsec on the sky, covering a total field of 13 ′ × ′ .Observations were carried out in 2 × / N ratio. The seeingmostly ranged between ∼ ′′ to ∼ ′′ , as determined using three moderately bright starsrecorded along with the blazar on the same CCD frame (Fig. 2).The HCT is located at the Indian Astronomical Observatory (IAO), Hanle (Ladakh) innorthern India. It is also of the RC design with a f / . Thedetector was a cryogenically cooled 2048 × × µ m so that the image scale of 0.29 arcsec / pixel coversan area of 10 ′ × ′ on the sky. The readout noise of CCD is 4.87 e − /pixel and the gain is1.22 e − / ADU. The CCD was used in an unbinned mode.All the observations were carried out using R filter for which the CCDs used have max-imum sensitivity. The exposure time was typically 12-30 minutes for the ST and about3 minutes for the HCT observations. The field positioning was adjusted so as to also in-clude within the CCD frame at least 2-3 comparison stars within about a magnitude ofthe blazar, in order to minimize the possibility of spurious variability detection (see, e.g.,Cellone, Romero & Araudo 2007). For both the telescopes, the bias frames were taken in-termittently and twilight sky flats were obtained. Table 1 gives the log of our observationsof this blazar, including those already reported in Sagar et al. (2004). The data providedinclude for each observation the date, the telescope used, number of data points in the DLC, ∼ iao c (cid:13) , 1– ?? NOV of PKS0735+178 eff and the amplitude ψ (Sect.3) as well as a remark on the INOV status.The preprocessing of the CCD images (i.e. bias subtraction, flat-fielding and cosmic-rayremoval) was done following the standard procedures in IRAF and MIDAS packages. Theinstrumental magnitudes of the blazar and the stars in the image frames were determinedby aperture photometry, using DAOPHOT II (Stetson 1987). The magnitudes of the blazarwere measured relative to the nearby comparison stars present on the same CCD frame inorder to account for the extinction of blazar’s light due to the earth’s atmosphere. This way,Differential Light Curves (DLCs) of the blazar were generated relative to three comparisonstars. Likewise, DLCs were also generated for each comparison star relative to the othertwo comparison stars. For each night, the selection of the optimum aperture radius forphotometry was done on the basis of the observed dispersions in the star-star DLCs generatedusing different aperture radii, starting from the median seeing (FWHM) value on that nightto 4 times that value. The aperture selected was the one which showed minimum scatter forthe steadiest DLC found for the various pairs of the comparison stars. The selected apertureradius was then used to generate DLCs for the target blazar relative to the comparisonstars, as well as for the individual comparison stars, by pairing them with the remaining twocomparison stars. The ‘seeing’ was monitored throughout the night using three moderatelybright stars recorded in each CCD frame. Additional details of the data reduction procedurecan be found in Stalin et al. (2004, 2005). Figure 2 shows our newly obtained intranight DLCs for PKS 0735+178 and the comparisonstars, along with the plots of ‘seeing’, as described above. By combining these with theDLCs for the four nights, published in Sagar et al. (2004), we have generated ‘long-term(differential) light curve’, relative to the same set of comparison stars as used by Sagar etal. (2004), thus maintaining continuity in the long-term optical DLC (Fig. 3).
The opticalfield of PKS 0735+178, marking the blazar and the comparison stars used by us,is shown in Figure 4.
The long-term DLC exhibits a peak-to-peak variation of ∼ Image Reduction and Analysis Facility Munich Image and Data Analysis System Dominion Astrophysical Observatory Photometry softwarec (cid:13) , 1– ?? Goyal et al. by computing the average instrumental magnitude for each of those nights and calibratingthose values using at least two standard stars available on the CCD frames, after ensuringthat they had remained unsaturated throughout the monitoring. The standard stars usedare: C7 and D (as given by Ciprini et al. 2007). The calibrated LTOV data points are plottedin Fig. 1a.Based on the intranight DLCs (Fig. 2), we have determined the INOV status and theINOV amplitude ( ψ ) for each night, as given in Table 1. Table 2 lists the positions andapparent magnitudes of the comparison stars used in producing the INOV and LTOV dif-ferential light curves (Figs. 2 & 3). The INOV classification ‘variable’ (V) or ‘non-variable’(N) was decided using a computed parameter C eff , basically following the criteria of Jang& Miller (1997). We define C for a given DLC as the ratio of its standard deviation, σ T and ησ err , where σ err is the average of the rms errors of its individual data points and η wasestimated to be 1.5 (Stalin et al. 2004, 2005; Gopal-Krishna et al. 2003; Sagar et al. 2004).However, our analysis for the present dataset yields η = 1 . eff for a given observing session, using the C values (as defined above)determined for the DLCs of the blazar relative to different comparison stars monitored dur-ing that session (for details, see Sagar et al. 2004). This procedure has the advantage ofutilizing the multiple DLCs of an AGN available during a single session (i.e., relative todifferent comparison stars). The source is termed ‘V’ if C eff > eff is found to bein range of 1.95 to 2.57, corresponding to a confidence level between 95% to 99%. Finally,the peak-to-peak INOV amplitude ( ψ ) was calculated using the definition (Romero, Cellone& Combi 1999): ψ = q ( D max − D min ) − σ (1)with D max = maximum in the AGN differential light curve D min = minimum in the AGN differential light curve σ = η h σ err i The INOV duty cycle (DC) for PKS 0735+178 was computed using our entire datasetof 17 nights, following the definition of Romero, Cellone & Combi (1999) (see, also, Stalinet al. 2004): c (cid:13) , 1– ?? NOV of PKS0735+178 DC = 100 P ni =1 N i (1 / ∆ t i ) P ni =1 (1 / ∆ t i ) % (2)where ∆ t i = ∆ t i,obs (1 + z ) − is the duration of monitoring session of a source on the i thnight, corrected for the blazar’s cosmological redshift z . Note that since the source was notmonitored for identical duration on each night, the computation has been weighted by theactual duration of monitoring ∆ t i . N i was set equal to 1 if INOV was detected, otherwise N i = 0.DC is found to be ∼ ∼
42% if the two cases of probable INOVare also included. The key result, however, is that large INOV ( ψ > ) was consistentlyabsent on all the 17 nights , even though the data quality remained adequate throughout. In this section we summarize the complex flux variability patterns exhibited by this blazar,in order to focus attention on both its normal and anomalous aspects. Such a backgroundperspective is important for appreciating its rather surprising INOV behavior established inthis work.
Recently, Ciprini et al. (2007) have published a long-term B -band light curve of PKS0735+178, spanning almost 100 years (1906 - 2004) (see, also Fan et al. 1997). Of this,the best sampled segment covers the last 33 years (867 nights, 1970 onwards). In Fig. 1bwe reproduce the light curve , taking median of the data binned into successive one-yearintervals. Starting from Feb., 1993, the data given in Ciprini et al. (2007) are based on CCDmonitoring ( BV RI ), with the densest sampling attained in the R -band. Again, we havetaken the medians for successive 1-year bins and augmented those data with our own R -band measurements for the period 1998-2008, after converting the calibrated magnitudes toflux densities and averaging over each night (Sect. 3). The composite R -band light curve forthe period 1993-2008 is shown in Fig. 1a. It is found that the blazar’s optical flux droppedclose to the historical minimum (occuring in early 1997), at the epoch 2007.0 from where itdoubled by the end of 2007 and then dropped back in 2008 to a level close to the historical Data points were retrieved from Fig. 4 [lower panel] of Ciprini et al. (2007) using the standalone version of a programme
Dexter (http://vo.uni-hd.de/dexter/ui/ui/custom) available over SAO/NASA ADS by Demleitner et al. (2001).c (cid:13) , 1– ?? Goyal et al. minimum. Considerably more pronounced optical variability was recorded during the pre-vious 7 years (Fig. 1a). Fig. 2 of Ciprini et al. (2007) shows typical variations of about 1mag on time intervals smaller than 1/2 year (see below), which together with the dominantradio core (see Sect. 1) is consistent with its being a LBL type BL Lac (e.g., Lister & Smith2000).The 100-year optical light curve of PKS 0735+178 shows five optical outbursts, withthe historical maximum (B ∼ ∼ ∼ ∼ ∼ ∼
12 yr. The shortest ofthese characteristic time scales had earlier been noted by Webb et al. (1988) and by Smith& Nair (1995), whereas the intermediate time scale was inferred previously by Qian & Tao(2004). Ciprini et al. (2007) have suggested that these three characteristic time scales maywell be harmonic signatures of one fundamental component of about 4 years. Note that theoptical spectral index ( α o ; defined as S ν ∝ ν − α ) given by Ciprini et al. (2007) for the data set1993-2004 did not show any correlation with the fluctuations in the light curve (upper panelof their Fig. 5) and has remained steady with an average value of 1 . ± .
15, indicatingan essentially achromatic optical variability. However, a rather weak positive correlationbetween the color index and the flux was noticed in the study conducted by Gu et al. (2006)covering a total of 50 nights (between Sept. 2003 - Feb. 2004), when source became bluerwith increasing brightness (see, also, Hu et al. 2006).
Since around 1979, PKS 0735+178 has been regularly monitored at 5, 8 and 15 GHz usingthe 26-metre Michigan dish. The observing technique, calibration procedures and instru-mentation are described elsewhere (see, Aller et al. 1985; 1999). The UMRAO light curvesat 15 and 5 GHz are plotted in Figures 1c & 1d. Figure 1e shows the run of spectral index(defined as S ν ∝ ν a ) calculated by a linear regression analysis of the flux values at the threefrequencies (Note that measurements at these frequencies were treated as simultaneous andhence combined only provided they are separated by no more than q = 0 .
04 year). Figure1f shows the variation of the percentage (linear) polarization at 15 GHz. The monitoring ofthis blazar at still higher frequencies of 22 GHz and 37 GHz has been carried out using the c (cid:13) , 1– ?? NOV of PKS0735+178 ∼ ∼ ∼ ∼ . α r ).Although a general increase in optical variability was noticed during the large radiooutburst of 1987-1997 (Fig. 1b), no clear optical counterpart to that radio outburst is evidentfrom the data (Hanski et al. 2002; Tornikoski et al. 1994; Clements et al. 1995). In contrast,correlated flaring at optical and radio bands is known to be much more common for BL Lacsthan for flat-spectrum radio quasars (FSRQs), suggesting that synchrotron opacity of thenuclear jet may be modest for the BL Lacs (Clements et al. 1995). Characteristic of BL Lac objects, PKS 0735+178 has shown a large long-term variabilityof optical (linear) polarization, from about 1% to more than 30% (Tommasi et al. 2001).However, on day-like or shorter time scales, covered in their polarimetric observations onfour nights ( Dec. 10, 11, 12 and 14, 1999), optical polarization remained steady at ∼ ∼ c (cid:13) , 1– ?? Goyal et al. typical maximum value < γ − ray flares are correlated with the emergence ofnew VLBI components, which is usually explained in terms of the standard model wherethe VLBI components are the manifestations of a relativistic shock propagating through anunderlying relativistic outflow (e.g., Marscher et al. 2008; see, also, Krichbaum et al. 1995). Typical of blazars, the dominant radio core of PKS 0735+178 is surrounded by a ∼ ′′ radio‘halo’ (Cassaro et al. 1999). Its (unbeamed) luminosity is at least an order-of-magnitudeabove the Fanaroff-Riley transition, placing it in the Fanaroff-Riley class II (Fanaroff &Riley 1974). A steep-spectrum radio jet of length ∼ ′′ is seen to extend from the core atposition angle (PA) ∼ ◦ (Tingay et al. 1998). The core has been the target of many VLBIcampaigns (e.g., Agudo et al. 2006 and references therein). Multi-epoch VLBI imaging at8, 22 and 43 GHz at several epochs from mid-1996 to mid-1998 revealed two peculiar sharpbends within the inner 2 milli arcsec ( ∼
11 parsec) of the jet (G`omez et al. 1999; 2001; see,also, Kellermann et al. 1998). The consistency between these images and several previouslyreported, lower-resolution VLBI images taken during mid-1995, suggests that the two bendswere present already in mid-1995 (G`omez et al. 2001 and references therein). Intriguingly,this feature was absent in the VLBI images taken during the several years preceding mid-1992. The jet was then found to be rectilinear extending at PA ∼ ◦ (B˚a˚ath & Zhang1991; Gabuzda, Wardle & Roberts 1989) and some of its knots exhibited large superluminalspeeds (7 c − c ) (e.g., Gabuzda et al. 1994; G`omez et al. 2001 and references therein).Thus, sometime between mid-1992 and mid-1995, the blazar appears to have undergone achange into a regime characterized by quasi-stationary VLBI knots in the jet (G`omez et al.2001; Agudo et al. 2006). Note that the “transition epoch” coincided with a huge decline inthe radio flux that continued till 1998 (Fig. 1c & d). The sharp bends in the VLBI jet wereprobably caused by gas pressure gradients on 10-parsec scale within the nuclear region, since c (cid:13) , 1– ?? NOV of PKS0735+178
MOJAVE
VLBI image (taken on 23November 2002, Lister & Homan 2005) found the jet to be fairly rectilinear and no longerexhibiting the double-bend witnessed during 1995 - 2000 within 3 milli arcsecond from core(e.g., see the 5 and 43 GHz VLBI images made by Agudo et al. 2006; also, Kellermannet al. 2004). The ‘straightened’ VLBI jet with PA ∼ − ◦ , as inferred from the allfive components found within 11 parsec from the core, is extremely mis-aligned from thekilo-parsec scale jet that extends at PA ∼ ◦ (Tingay et al. 1998). Large misalignmentbetween the jets on parsec and kilo-parsec scales is indeed quite common for LBL type BLLac objects (Britzen et al. 2007 and references therein). In December 1998 when we embarked on the intranight optical monitoring of PKS 0735+178,the R -band flux of this BL Lac had risen three-fold during the preceding year, from thehistorical minimum of 0.6 mJy to ∼ ∼ ∼ c (cid:13) , 1– ?? Goyal et al. slightly over 1.7 mag (Fig. 3). Our data (Table 1) provide a hint that the nights of INOVdetection coincide not with the extrema but with gradients in optical flux, consistent withthe trend noted by Webb et al. (1998) and Howard et al. (2004). However, we caution thatsuch a correlation is weak, at best, since the INOV detections were marginal.The key result from our observations is that not even on one of the 17 nights wasthe INOV amplitude of this BL Lac found to be > ∼
50% in any single monitoring sessionlonger than ∼ < . × − . This suggests that PKS 0735+178 has persisted in an INOV quiescent statesince 1998, despite other indications of its returning to an active state during this period.As summarized in Sect. 4, the indicators of renewed activity include (a) the large variationin its optical synchrotron flux on month/year-like time scale and (b) the return of its VLBIjet to the ‘normal’ rectilinear shape. Other indicators of a typical blazar state are the fairlyhigh degrees of optical and radio polarization (Sect. 4.3) and the persistence of its flat radiospectrum (Fig. 1e). On the other hand, a rather uncharacteristic behaviour is echoed by thefact that during the past two decades this blazar has undergone large radio outburst justonce (Sect. 4.2; also, Hovatta et al. 2007) and that too without a clear optical counterpart(Sect. 4.3). Furthermore, its X-ray/ γ − ray emission has been fairly steady (Sect. 1).Here we note that recently from their 4-hour long R -band monitoring of this blazar onJan. 11, 2007 at Yunnan Observatory, Gupta et al. (2008) have reported an INOV detection.However, since their published DLC (Fig. 6 of their paper) is essentially flat and containsno significant structure on hour-like time scale, the INOV claimed by them can at best be of‘flicker’ type, with a time scale much shorter than the hour-like time scale we have consideredhere.In order to augment the present study we now turn attention to the (unpublished)optical monitoring data reported in Tables 3.1 and 4.1 of Dr. John Noble’s PhD dissertation(1995). During Jan. 25 - 31, 1992 he monitored PKS 0735+178 on 6 nights in R -band,using the 1.07-metre Lowell telescope. On 5 of the nights, the monitoring duration wassufficiently long to meet our criterion (6.7, 3.9, 4.2, 5.0 and 8.2 hours) and so also was thesensitivity of the DLCs ( σ V -band monitoring c (cid:13) , 1– ?? NOV of PKS0735+178 σ = 1%). All these results place on astronger footing the finding already emerging from our 17 nights’ monitoring during 1998-2008 (Table 1), namely that this bona-fide radio selected BL Lac is quite exceptional forits propensity to remain in a state of intranight optical quiescence (i.e., ψ Since substantial optical variability on wide ranging time scales, which is typ-ical of blazars, is commonly associated with shocks forming and interacting withinhomogeneities in the Doppler boosted synchrotron jets (e.g., Marscher, Gear& Travis 1992), it is tempting to ask if the prolonged uncharacteristically sub-dued INOV level of the classical blazar PKS 0735+178 is a manifestation of someunusual property of its jet. One model that explicitly attempts to address thisquestion, particularly in the context of hour-like or shorter time scales of vari-ability, invokes interaction of relativistic shocks in the jet with sub-parsec scaleirregularities (Romero 1995). This scenario has been developed specifically forthe two-fluid model of AGN jets originally proposed by Sol et al. (1989), whereinan extremely light and narrow beam of relativistic pair-plasma responsible forthe apparent superluminal motion in the nuclear jets, is enveloped by a widerjet comprised of a much denser non-relativistic electron-proton plasma whichcarries most of the kinetic energy and terminates in kiloparsec sized hot spots(Pelletier & Roland 1989; Henri & Pelletier 1991). It has been argued that sub-parsec scale irregularities (needed for INOV) could arise in the beam of such jetsdue to classical macroscopic Kelvin-Helmholtz instabilties, in case the axial mag-netic field in the beam, B z , is below a critical value B c = [4 πn b m e c ( γ b − / γ − b ,where n b and γ b are the electron number density and bulk Lorentz factor of thebeam, respectively and m e is the electron rest mass (Romero 1995). Applyingthis model to the case of the well studied intraday variable blazar 0917+624, for c (cid:13) , 1– ?? Goyal et al. which estimates of the beam’s density and magnetic field are available, Romerohas shown that for a highly supersonic beam flow and equipartition magneticfields, the (fastest growing) reflection modes would cross over into the unstableregime and a rapid transition to a turbulent jet would occur, yielding the basicsetting for INOV. Although estimates for the basic physical parameters for thepresent LBL PKS 0735+178 are not available, it is conceivable that its beam isfairly stable to the K-H instabilities (i.e., B z > B c , see above). Such a prospectis indeed supported by the polarimetric VLBI data revealing that the magneticfield in the nuclear jet of this LBL is predominently axial (Agudo et al. 2001),unlike the norm for such BL Lacs (e.g., Lister & Homan 2005 ; Kharb, Gabuzda& Shastri 2008). It is clear that in order to develop a proper understanding of the anomalous INOV be-haviour of PKS 0735+178, clues will have to be gleaned by relating its multi-band variabilitypatterns (of which some are distinctly typical of BL Lacs, while the others are less so, seeabove) to the VLBI imaging and polarimetry of its nuclear jet. The VLBI observations havealready revealed enigmatic multiple twists in the nuclear jet which are transitory, like thesuperluminal motion of its radio knots (Sect. 4). Could the processes underlying this pecu-liar behaviour have kept the inner jet (where INOV presumably originates) hidden from ourview ? These aspects will be examined by us elsewhere (
Britzen et al. 2009, in prep. ),based on an available sequence of VLBI images of this blazar taken at more than 20 epochsover the past two decades.
ACKNOWLEDGMENTS
The authors are thankful to the anonymous referee for helpful suggestions and to Dr. S.Ciprini for providing the R -band data. AG would like to thank Dr. Marcus Demleitner forproviding the alpha version of Dexter’s standalone version before it was released over ADS.M. Karouzos was supported for this research through a stipend from the International MaxPlanck Research School (IMPRS) for Astronomy and Astrophysics. UMRAO is funded by aseries of grants from the NSF, most recently AST-0607523, and by funds from the Universityof Michigan. The 201-cm HCT is operated by the Indian Institute of Astrophysics (IIA).The authors wish to acknowledge the support rendered by staff of IAO and CREST (IIA). c (cid:13) , 1– ?? NOV of PKS0735+178 REFERENCES
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The observation log and INOV results for PKS 0735+178.Date Telescopes used Number Duration ψ C eff Status ‡ Referencesdd.mm.yy of points (hours) (%)26.12.98 ST 49 7.8 1.8 1.13 N Sagar et al. (2004)30.12.99 ST 64 7.4 1.0 0.61 N Sagar et al. (2004)25.12.00 ST 42 6.0 1.6 1.02 N Sagar et al. (2004)25.12.01 ST 43 7.3 1.0 2.8 V Sagar et al. (2004)20.12.03 HCT 36 5.8 1.0 1.78 N present work10.12.04 ST 28 5.8 1.3 3.00 V present work23.12.04 ST 11 5.0 1.2 3.10 V present work02.01.05 ST 20 4.9 0.8 0.97 N present work05.01.05 ST 23 5.8 1.0 2.25 PV present work09.01.05 ST 28 6.7 1.3 3.20 V present work09.11.05 ST 17 3.8 0.7 2.00 PV present work16.11.06 ST 19 4.5 1.1 0.95 N present work29.11.06 ST 26 5.8 1.0 0.83 N present work17.12.06 ST 24 5.6 0.9 1.06 N present work15.12.07 ST 28 6.6 1.9 3.53 V present work16.12.07 ST 27 6.6 1.0 1.45 N present work22.11.08 ST 27 5.6 0.8 0.33 N present work ‡ V = variable; N = non-variable; PV = probable variable
Table 2.
Positions and apparent magnitudes of the comparison stars used in the present study (taken from United StatesNaval Observatory-B catalogue; Monet et al. 2003)Star RA(J2000) Dec(J2000)
B R B-R (mag) (mag) (mag)S1 07 h m s .21 +17 ◦ ′ ′′ .4 16.05 14.86 1.19S2 07 h m s .15 +17 ◦ ′ ′′ .3 15.62 15.25 0.37S3 07 h m s .88 +17 ◦ ′ ′′ .4 16.23 15.26 0.97S4 07 h m s .00 +17 ◦ ′ ′′ .5 16.15 15.55 0.60S5 07 h m s .82 +17 ◦ ′ ′′ .8 15.29 14.68 0.61S6 07 h m s .35 +17 ◦ ′ ′′ .3 15.94 15.59 0.35SS1 07 h m s .42 +17 ◦ ′ ′′ .8 16.48 15.89 0.59SS2 07 h m s .08 +17 ◦ ′ ′′ .6 16.29 15.96 0.33SS3 07 h m s .26 +17 ◦ ′ ′′ .8 16.71 16.46 0.25c (cid:13) , 1– ?? Goyal et al.
Figure 1.
Light curves of PKS 0735+178 (Sect. 4); the vertical broken line marks the epoch of its historical maximum inradio brightness. c (cid:13) , 1– ?? NOV of PKS0735+178 Figure 2.
The intranight DLCs of PKS 0735+178. For each night the upper three panels show the DLCs of the blazar relativeto three steady comparison stars while the lower three panels show the star-star DLCs. The bottom panel gives the plots ofseeing variation for the night, based on three stars. For each night, the date, duration of monitoring and the telescope used arementioned near the top.c (cid:13) , 1– ?? Goyal et al.
Figure 2 . continued c (cid:13) , 1–, 1–
Figure 2 . continued c (cid:13) , 1–, 1– ?? NOV of PKS0735+178 Figure 3.
The long-term R -band differential light curves of PKS 0735+178, derived from from our data spanning 11 years(1998-2008). In order to maintain consistency with our already published data (Sagar et al. 2004), we have used the same threecomparison stars for generating these LTOV plots. Each horizontal sedgement of the light curves represents the DLC for thenight corresponding to the date mentioned at the top, along with the telescope used (shown in parentheses).c (cid:13) , 1– ?? Goyal et al.
Figure 4.
DSS POSS2 R -band 12 ′ × ′ field centered on PKS 0735+178 is shown. The positions of target AGN (markedwith T, within double bar) and the comparison stars is shown within circles and marked with S1...S6 and SS1..SS3 notationsused for making INOV & LTOV plots, respectively. c (cid:13) , 1–, 1–