Optical flux behaviour of a sample of Fermi blazars
E. J. Marchesini, I. Andruchow, S. A. Cellone, J. A. Combi, L. Zibecchi, J. Martí, G. E. Romero, A. J. Muñoz-Arjonilla, P. Luque-Escamilla, J. R. Sánchez-Sutil
aa r X i v : . [ a s t r o - ph . GA ] M a y Astronomy & Astrophysics manuscript no. MarchesiniEJetal c (cid:13)
ESO 2018October 11, 2018
Optical flux behaviour of a sample of Fermi blazars
E. J. Marchesini , , I. Andruchow , , S. A. Cellone , , J. A. Combi , , L. Zibecchi , , J. Mart´ı , , G. E.Romero , , A. J. Mu˜noz-Arjonilla , P. Luque-Escamilla , , and J. R. S´anchez-Sutil Facultad de Ciencias Astron´omicas y Geof´ısicas, Universidad Nacional de La Plata, Paseo del Bosque, B1900FWALa Plata, Argentina. IALP, CONICET-UNLP, CCT La Plata, Paseo del Bosque, B1900FWA - La Plata - Argentina. IAR, CONICET, CCT La Plata, C.C. No. 5 (1894) Villa Elisa - Buenos Aires - Argentina. Departamento de F´ısica (EPS), Universidad de Ja´en, Campus Las Lagunillas s/n, A3, 23071 - Ja´en - Spain. Dept. de Ingenier´ıa Mec´anica y Minera, EPS de Ja´en, Universidad de Ja´en, Campus Las Lagunillas s/n, A3-402,23071 Ja´en, Spain. Grupo de Investigaci´on FQM-322, Universidad de Ja´en, Campus Las Lagunillas s/n, A3-065, 23071 Ja´en, Spain.Preprint online version: October 11, 2018
ABSTRACT
Aims.
We aim at investigating the time-behaviour of a sample of gamma-ray blazars. We present the results from a 13month-long optical photometry monitoring campaign of the blazars PKS 0048 − − − Methods.
We analyse the variability of each object, focusing on different time-scales (long term, short term, andmicrovariability), in an attempt to achieve a statistical comparison of the results.
Results.
After applying a geometric model to explain the variability results, we found that it is possible that a slightchange in the direction of the jet generates the variations detected in some objects during this campaign.
Key words.
BL Lacertae objects: individual: PKS 0048 − − −
1. Introduction
Blazars are a sub-type of active galactic nuclei (AGN),whose jets axes are extremely close to the line of sight;the electromagnetic (non-thermal) emission from the jet isthus relativistically boosted and dominates along the wholespectrum. These objects often show a flat radio spectrumand apparent superluminal motions at VLBI scales (Urry,1999), while both short- and long-term variations are de-tected in optical flux (e.g. Miller et al., 1989; Carini et al.,1990, 1992; Romero et al., 2002). Most blazars present mi-crovariability, e.g.: variations over intranight timescales.These changes in brightness can also be detected at otherwavelengths, from radio to gamma rays.Blazars are subclassified as flat spectrum radio quasars(FSRQ) and BL Lac objects (BL Lacs). In the first case,strong emission lines are present in their optical spectrawhile, in the second, these lines are weak or non-existent.A (possibly) more physically based classification takes intoaccount the position of the synchrotron peak (SP) in thespectral energy distribution (SED): in this way, blazarsare classified as high- (HSP), intermediate- (ISP) and low-synchrotron peak (LSP) objects, according to the frequencyof the low-energy peak (Abdo et al., 2010b). In most cases,BL Lacs are identified as HSP, while most FSRQs are LSP.However, this differentiation is not clear-cut since there areseveral transition objects.Recently, Giommi et al. (2012) studied the difference inthe emission properties between these sub-types of blazars.They found that FSRQs maintain their LSP classification,which is the same as Fanaroff-Riley II (FRII) radio-galaxies, according to their ionisation degree. On the other hand,BL Lac objects are a mixture of two intrinsically differentsources: FR I objects, with weak or non-existent emissionlines, and FR II objects whose lines were diluted by severaleffects, mainly because of a strong thermal (accretion disk)component. In this sense, the SP classification might bedriven by selection effects, while the FR I–FR II division isa classification based on physical properties.A detailed knowledge of the optical flux properties inblazars is thus relevant to get a clearer picture of the com-plex interrelations between emission properties at differentfrequencies. In particular, flux variability studies are im-portant to test the size and location of the emitting re-gion at a given frequency, as well as to give clues on themechanisms which originate the emission. Variability at theshortest measurable timescales is supposed to provide in-formation on emitting regions at the smallest spatial scales,which can be useful to test any relation with the emissionat higher frequencies.In this work we analyse the optical flux variabilityin a sample of nine blazars. All these sources were de-tected at high frequencies by different satellites in X-rays(e.g. Giommi et al., 2012; Cusumano et al., 2010), and by
Fermi
LAT (Fermi Large Area Telescope) at GeV ener-gies (Abdo et al., 2010a). Some of them were also detectedat TeV energies by experiments like HESS and MAGIC.According to Abdo et al. (2010c), all the blazars in oursample show a SED of the LSP class, with the exception ofPKS 0048 −
1. J. Marchesini et al.: Optical flux behaviour of a sample of Fermi blazars
Table 1: Sample
Object Type α J2000 . δ J2000 . m NED z PKS 0048 −
090 BL-Lac 00:50:41.3 − −
055 FSRQ (TeV) 12:56:11.1 − −
089 QSO (HP+TeV) 15:12:50.5 − Column 1 is the source name; column 2, the type; position is given in columns 3 and 4; visual magnitude and redshift(as found in NASA Extragalactic Database - NED) are presented in columns 5 and 6, respectively.
2. Observations and data reduction
A sample of nine gamma-ray blazars was photometricallyfollowed with high temporal resolution ( < V and R Johnson-Kron-Cousins fil-ters with the no longer operating 1.52 m telescope at theEstaci´on de Observaci´on de Calar Alto, part of the NationalAstronomical Observatory (OAN) of Spain. This telescopewas equipped with a 1024 × ′ . × ′ . − scale). Full width at half maximum (FWHM) values variedfrom 2 . . reduction package. The IRAF apphot package was used to extract the photometry for all the im-ages, with aperture diameters of 4 arcsec for all objects withthe exception of PKS 2230+114 (3 . . F -test. For each light curve, we estimatedthe F parameter, defined as the ratio of the varianceof the object − comparison light curve ( σ − c ) to the vari-ance of the control − comparison light curve ( σ − c ). If F = σ − c /σ − c ≥ F αn , the object is said to be variable, being F αn a critical value. This critical value is calculated for a set of n = N − N is the number ofpoints in the light-curve, while α is chosen to determine thedesired level of confidence. If α = 0 .
01, then the F test hasa 99% confidence level.In this process, a statistical weight Γ, as introduced byHowell et al. (1988), was adopted for the F test to con-sider effects caused by the difference in magnitude of the IRAF is distributed by the National Optical AstronomyObservatories, which are operated by the Association ofUniversities for Research in Astronomy, Inc., under cooperativeagreement with the National Science Foundation. source and the stars. The Γ factor can be derived from mea-surable quantities from a given observation, such as sky–substracted counts from the object, sky photons, and theread–out noise.As a result of taking this weight into account, F = σ − c / (cid:0) Γ σ − c (cid:1) . The importance of this weighting to avoidspurious variability results in differential photometry stud-ies of blazars has been underscored by Cellone et al. (2007).
3. Results
In this section we present a brief description of each objectand the optical flux variability results that we found.
PKS 0048 − is a strongly variable BL-Lac object. Itsredshift is highly uncertain since its spectrum is basicallyflat; Stickel et al. (1993) derived a lower limit of z ≥ .
2. Atentative value of z ≈ .
63 was estimated (Rector & Stocke,2001) and then confirmed at z = 0 .
635 (Landoni et al.,2012). This BL-Lac object, as most of its kind, shows vari-ability at almost every wavelength range, from radio to op-tical bands (Pica et al., 1988). It has also been detected inX-rays (Brinkmann et al., 2000). On the other hand, sev-eral works have found variability in optical polarization,reporting a value of polarization degree h P V i = 10 . ∼
450 days for the first half of the 80s, and a periodof almost 600 days for late 80s and early 90s (these periodswere determined with a very high confidence level). Theseauthors also reported a change of about 90 ◦ in the directionof the jet during its propagation from 1995 to 2002. This be-haviour implies a strong structural variability, which woulddeserve further studies. We used the magnitudes of the fieldstars published in Villata et al. (1998) to estimate the meanstandard magnitude for PKS 0048 −
097 in each filter dur-ing our observations; resulting in 15 . ± .
05 mag in V ,15 . ± .
04 in R band the first night, and 15 . ± .
05 magin V , 15 . ± .
05 in R band the second night. These magni-tudes are in agreement with those presented by Fan & Lin(2000), who found V = 15 .
86 and R = 15 .
40. In the presentwork, this object showed small amplitude variability in the R band.
2. J. Marchesini et al.: Optical flux behaviour of a sample of Fermi blazars
PKS 0754 + is a highly polarized and variableLSP blazar (Ghosh & Soundararajaperumal, 1995;Ghosh et al., 2000) of the BL-Lac type (Tapia et al.,1977). Its long-term variability at optical bands can betraced back to 1980 (e.g. Baumert, 1980; Pica et al., 1988;Sillanpaa et al., 1991; Katajainen et al., 2000). Infraredvariations were detected by Fan & Lin (1999), as well as aradio flare by Nieppola et al. (2009). This object shows afeatureless spectrum and its host galaxy can be resolvedin the images (Abraham et al., 1991; Falomo, 1996), butit is rather faint compared to the AGN. Nilsson et al.(2003) suggest a host magnitude of R = 18 .
55 mag and r eff = 1 . z = 0 . ± .
001 based on the identification of two faintemission lines. We followed this source during the nightof March 24, 2006, in which, the F test results in novariability neither in the V nor in the R bands. Using thecomparison stars and standard magnitudes published byFiorucci et al. (1998), the mean standard magnitudes forPKS 0754+100 are 16 . ± .
05 mag in V and 16 . ± . R bands. Compared with Fan & Lin (2000), whopresent mean values of V = 15 .
40 and R = 14 .
27, ourobservations clearly correspond to a minimum activitystate. [HB89] 0827 + is classified as a FSRQ (Healey et al.,2007). It is also a gamma-ray bright quasar, with a red-shift z = 0 .
939 (Hewett & Wild, 2010). Variability on aday-long scale has been detected at B , V , and R opti-cal bands (Villata et al., 1997; Raiteri et al., 1998), as wellas long-term variability in the near-infrared J , H , and K ′ bands (Enya et al., 2002). The jet emerging from the coreof 0827+243 has been detected both at X-ray and radiobands, showing an apparent superluminal motion of over20 c (Jorstad et al., 2001; Piner et al., 2006). This jet has ahighly unusual morphology bending almost 90 ◦ at the X-ray band, while in radio only the external 90 ◦ -bended sec-tion is visible. A swinging-nozzle model has been proposedto explain this unusual jet behaviour (Jorstad & Marscher,2004); this model proposes a scenario in which the jet flowhas always been straight, but the AGN smoothly changedits direction, leaving traces of energised matter towards itsformer direction, and hence the difference both in struc-ture and in emission can be explained. Our data show nointranight variability (both in V and in R bands) duringeach of the three nights [HB89] 0827+243 was observed.Regarding an internight time-scale, the light-curves analy-sis detects significant variability. PKS 0851 + (also known as OJ 287) is a well knownAGN. Its redshift is well-determined at z = 0 . ∼ R =18 . r e = 1 . M R > − .
0. The complex radio structure shown by thejet of this source was reported by Jorstad et al. (2001) andJorstad et al. (2005). We found this source to display strongactivity, resulting variable in both filters ( R and V ), ineach night and when considering two consecutive nights.With the magnitudes given by Fiorucci & Tosti (1996) andGonz´alez-P´erez et al. (2001), we obtained mean standardestimates of 14 . ± .
01 in V and 14 . ± .
01 in R the firstnight, and 14 . ± .
01 in V , 14 . ± .
01 in R the secondnight. Fan & Lin (2000) propose a periodic light curve forPKS 0851+202, with a period of 12 years. Our results arein agreement with the magnitudes expected for this peri-odic variability and with the values observed by Bach et al.(2007). PKS 1253 − (also known as 3C 279), at a redshift of z = 0 . γ -rays bythe EGRET telescope (Hartman et al., 1992), as well asthe first superluminal blazar ever detected (Whitney et al.,1971). It is very well known for its variability at every wave-length and also for its γ -ray flares (see Grandi et al., 1996;Wehrle et al., 2001; B¨ottcher et al., 2007; Giuliani et al.,2009). It seems that some periods of extreme vari-ability could be present in this source, for example,the optical outburst detected during 2001–2002 reportedby Kartaltepe & Balonek (2007). Andruchow et al. (2003)found strong microvariability in both optical polarizationdegree and its position angle, and consequently proposeda jet rotation model to explain the behaviour of the po-sition angle. On the other hand, Nilsson et al. (2009) re-solved the host galaxy, reporting an apparent magnitude m = 18 . ± . I band, an effective radius r eff = 2 . ± . M R = − . γ flare, after which its flux be-gan to decrease at all optical wavelengths. Our observa-tions started less than a month after the end of thatcampaign. Using the field star magnitudes published byGonz´alez-P´erez et al. (2001), we obtained mean standardmagnitudes of V = 15 . ± .
03 and R = 14 . ± .
02 mag( h V − R i ≃ . V / ∆ t ≈ .
15 and ∆ R/ ∆ t ≈ .
10 mag per day) at theend of the 2005-2006 WEBT campaign on 3C 279.Our results show that the blazar continued its de-creasing flux trend, although either with a smaller slope,or the flux stabilised sometime before the start of ourobservations. We found some low amplitude activity inboth bands during the first night, but no further significant The Whole Earth Blazar Telescope( ) 3. J. Marchesini et al.: Optical flux behaviour of a sample of Fermi blazars
Fig. 1: Examples of differential light curves from our sample. m BL , m c , m k are the instrumental magnitudes for the blazar,the comparison star and the control star, respectively. From top to bottom, left to right: PKS 1253 − V filter are shown in green, data for the R filter in red. Upper panels:object − comparison star differential light curves; lower panels: control star − comparison star differential light curves.Note that both panels are shown with the same scale for a given blazar.intranight variability. However, we found strong internightvariability in both bands. PKS 1510 − at a redshift z = 0 . ± . P and in position angle θ (Andruchow et al., 2005), as well as a rapid variation in γ -rays (D’Ammando et al., 2009). Simultaneously with this γ -ray flare, these authors also found intense variability atoptical and radio wavelengths. Its jet is detected primarily in radio, with superluminal motion (Homan et al., 2001),and also in X-rays (Sambruna et al., 2004) and at infraredwavelengths (Rantakyro et al., 1998), and is described asa rather extended, diffuse, and bended jet (Jorstad et al.,2001). Although there was no detectable activity duringthe two nights we followed this object, and no internightvariability in the R band, we found clear internight vari-ability in the V band. Again, derived from the magnitudesin Gonz´alez-P´erez et al. (2001), we found mean standardmagnitudes of 16 . ± .
01 in V , 16 . ± .
01 in R the firstnight, 16 . ± .
02 in V and 16 . ± .
01 in R the secondnight. PKS 1749 + is a BL-Lac object with a redshift z = 0 . γ -ray frequencies
4. J. Marchesini et al.: Optical flux behaviour of a sample of Fermi blazars (Abdo et al., 2010a). The optical and infrared polariza-tions also show variability (Brindle et al., 1986). We foundsmall amplitude activity for this object during one of thenights in the V band. However, at internight scales, thissource showed the largest amplitude variability for thewhole sample, reaching a change in its differential mag-nitude of ∼ . . ± .
04 mag in V and 17 . ± .
05 magin R the first night to 17 . ± .
04 in V and 16 . ± .
04 in R the second night. According to the light curve given inFan & Lin (2000), these values correspond to a minimumof activity. PKS 2230 + (also known as CTA 102) is a blazar at z = 1 . c , derived from VLBI observations (see, Jorstad et al.,2001; Rantakyr¨o et al., 2003; Jorstad et al., 2005). Its his-tory of optical variability can be tracked back to 1973(Pica et al., 1988), with sporadic detections afterwards(see, for example, Wilkes et al., 1994; Ghosh et al., 2000;V´eron-Cetty & V´eron, 2001; Romero et al., 2002). We re-port here a clear internight variability in both R and V bands. We also detected one night (over four) with activ-ity, in the V band. PKS 2251 + (also known as 3C 454.3) with z = 0 . γ -rays. In December12, the flux increased ∼ . . ∼ . ∼ AGILE /Swift data (Vercellone et al., 2008). Anotheroutburst was detected in 2009, at optical, X-ray and gammafrequencies. This flare was studied by Striani et al. (2010),and its polarization was studied by Sasada et al. (2012);these authors report two distinct rotation events. Becauseof its frequent outbursts and extreme variability, this ob-ject is usually used as an example when studying the va-lidity of the SP classification (Gopal-Krishna et al., 2011;Ghisellini et al., 2011). We report a strong internight vari-ability in the R and V bands. Furthermore, we found mi-crovariability in the R band within one out of three nightswe followed this blazar. Taking mean values and using themagnitudes published in Gonz´alez-P´erez et al. (2001) andin Fiorucci et al. (1998), we found mean standard magni-tudes of 16 . ± .
09 in V , 16 . ± .
09 in R (first night),16 . ± .
09 in V , 15 . ± .
09 in R (second night), and16 . ± .
09 in V , 15 . ± .
09 in R (third night). Thesevalues are in agreement with those given in Raiteri et al.(2007), as with the general trend of increasing magnitudes. The variability results are summarised in Table 2 .Column 1 gives the object name with the date, while thefollowing columns give, respectively for V and R : the ob-servational error, σ , obtained from the standard deviationof the control − comparison differential light-curve for eachfilter; the variability results; the confidence parameter ( F );the gamma corrective factor (Γ); the number of points inthe light curves; and the corresponding critical value, F t for n = N − − m BL , m c , m k are the instrumental magnitudes forthe blazar, the comparison star and the control star, re-spectively. Observations in the R band are represented inred, while observations in the V band are shown in green.As a check, we re-calculated the variability state of eachlight curve after exchanging the stars, using the comparisonas a control star and vice versa. The results did not change,except for four light curves which changed their variabilitystate. This is an indication that in most cases we were ableto choose well behaved stars as comparison and control.Another indirect evidence of this is the fact that Γ ≈ σ values were below 0 .
02 mag withthe exception of PKS 0048 − σ = 0 .
4. Discussion
Internight variability was detected in most of our sample.In particular, results for PKS 1749+096, PKS 1253 − m ∼ . F test results, the largest short-scale variations were detected for PKS 2230+114 andPKS 2251+158, one in each band, with an amplitude of0.05 mag in the first case and of 0.02 mag in the second.However, given that the other band shows no signs of vari-ability and that there are few data points, we suggest tak-ing the F -test results with caution (see Zibecchi et al. inpreparation, for a critical evaluation of the F–test and otherstatistics as variability indicators).Different models have been proposed to explain (mi-cro)variability in blazars. In particular, we want to testwhether our variability results can be explained by aswinging jet scenario (e.g. Gopal-Krishna & Wiita, 1992;Romero et al., 1995; Bachev et al., 2012, and referencestherein). This means that variability is a consequence ofslight deviations of the direction of motion of the emittingparticle populations along the relativistic jet, which is closeto the line of sight, thus leading to a change in the asso-ciated Doppler factor. This, in turn, produces a significantchange in the observed magnitude. This is a consequence ofrelativistic effects in the jet plasma playing an important Photometry results summarised in Table 2 are onlyavailable in electronic form at the CDS via anony-mous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/ 5. J. Marchesini et al.: Optical flux behaviour of a sample of Fermi blazars
Table 2: Statistical results for the differential light curves.
UT Date σ (mag) Variable? F Γ N F t [m/d/y] V R V R V R V R V R V R
PKS 0048-090
PKS 0754+100
HB89 ] PKS 0851+202
PKS 1253-055
PKS 1510-089
PKS 1749+096
PKS 2230+114
PKS 2251+158 role by magnifying any perturbation. Since PKS 1749+096presented the strongest variation measured during our cam-paign, we show and discuss the application of this model toour results for this specific object. We find a similar resultfor the remaining objects in our sample.Following Nesci et al. (2002) the expected temporalchange in the observed magnitude of the blazar is relatedto the temporal change in the jet viewing angle d θ/ d t jet bythe expressiond m/ d t obs = 1 .
086 (3+ α ) β γ δ sin( θ ) (1+ z ) − (d θ/ d t jet ) , (1)where α is the spectral index ( F ν ∝ ν − α ), β the bulkplasma velocity in the jet in terms of the speed of light, γ is the Lorentz factor, δ is the Doppler factor, and wehave included the redshift correction factor. We note that the time interval on the left hand corresponds to the ob-server’s frame, while that on the right hand is in the jet’sreference frame.To calculate d θ/ d t jet , we used the parameters reportedby Hovatta et al. (2009): a Lorentz factor γ = 7 .
5, aDoppler factor δ = 12 .
0, and a jet angle to the line ofsight θ = 3 . ◦ , while we adopted the optical spectral index α V R = 2 .
85 from Ojha et al. (2009).From the data here reported, we estimated a mean valueof d m/ d t obs = − .
16 mag per day. Calculating the (intrin-sic) velocity, β = p − /γ = 0 . θ/ d t jet = − . × − radians per day, i.e.,d θ/ d t jet = − .
6. J. Marchesini et al.: Optical flux behaviour of a sample of Fermi blazars
This procedure was also applied to another blazar notincluded in this campaign, AO 0235+164 ( z = 0 . m/ d t obs = 2 .
67 mag per day), andanother night with a lower amplitude (d m/ d t obs = 1 . γ = 12 . δ = 24 . θ = 0 . ◦ (Hovatta et al., 2009), and α V R = 2 .
15, from Cellone et al. (2007). The results wered θ/ d t jet = 1 . × − rad per day, i.e., d θ/ d t jet = 65 . θ/ d t jet = 1 . × − rad per day, i.e., d θ/ d t jet = 48 . θ ,such as that reported by Hovatta et al. (2009), the relevantequations rapidly tend to a strongly non-linear behaviour.Thus, we also used the set of parameters inferred from amore realistic SED modelling that was given by Ackermannet al. (2012): γ = δ = 20, θ = 2 . ◦ . These resulted ind θ/ d t jet = 2 . × − radians per day, i.e., d θ/ d t jet = 9 . θ/ d t jet = 2 . × − radians per day, i.e., d θ/ d t jet = 7 . δ factor from Eq. 1. While, in this frame, rates of changein the viewing angle for AO 0235+164 become implausiblyhigh, for PKS 1749+096 we obtain d θ/ d t gal = − .
5. Conclusions
We presented optical differential photometry data for asample of nine blazars. Only four of them show statisti-cally significant variability: PKS 1253 − − ∼ . F V = 1902 .
39 and F R = 1411 . ∼
10 arcmin per day.
Acknowledgements.
E. J. M. would like to thank FCAGLP and UNLPfor offering the opportunity to study this career at no cost, as wellas for the resources given to accomplish this work. S. A. C. and I. A.thank ANPCyT for funding (PICT2008-0627). The whole team wouldalso like to thank Dr. Nicola Masetti for his suggestions, as well as theanonymous referee for the clear and helpful advice given. This workwas supported by the Consejer´ıa de Econom´ıa, Innovaci´on, Cienciay Empleo of Junta de Andaluc´ıa under excellence grant FQM-1343and research group FQM-322, as well as FEDER funds. G. E. R. issupported by Grant AYA2013-47447-C3-1-P (Spain).
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7. J. Marchesini et al.: Optical flux behaviour of a sample of Fermi blazars