Multiwavelength Variability of the Blazars Mrk 421 and 3C 454.3 in the High-State
aa r X i v : . [ a s t r o - ph . C O ] D ec ; Submitted to AJ
Preprint typeset using L A TEX style emulateapj v. 5/2/11
MULTIWAVELENGTH VARIABILITY OF THE BLAZARS MRK 421 AND 3C 454.3 IN THE HIGH-STATE
Haritma Gaur , Alok C. Gupta , Paul J. Wiita Submitted to AJ
ABSTRACTWe report the results of photometric observations of the blazars Mrk 421 and 3C 454.3 designed tosearch for intraday variability (IDV) and short-term variability (STV). Optical photometric observa-tions were spread over eighteen nights for Mrk 421 and seven nights for 3C 454.3 during our observingrun in 2009-2010 at the 1.04 m telescope at ARIES, India. Genuine IDV is found for the source 3C454.3 but not for Mrk 421. Genuine STV is found for both sources. Mrk 421 was revealed by theMAXI X-ray detector on the International Space Station to be in an exceptionally high flux state in2010 January - February. We performed a correlation between the X-ray and optical bands to searchfor time delays and found a weak correlation with higher frequencies leading the lower frequenciesby about ten days. The blazar 3C 454.3 was found to be in high flux state in November-December2009. We performed correlations in optical observations made at three telescopes, along with X-raydata from the MAXI satellite and public release γ -ray data from the Fermi space telescope. We foundstrong correlations between the γ -ray and optical bands at a time lag of about four days but the X-rayflux is not correlated with either. We briefly discuss the possible reasons for the time delays betweenthese bands within the framework of existing models for X-ray and γ -ray emission mechanisms. Subject headings: galaxies: active — BL Lacertae objects: general — BL Lacertae objects: individual(Mrk 421) — quasars: individual 3C 454.3) INTRODUCTION
Blazars are the radio − loud Active Galactic Nuclei(AGN) that are classified as BL Lacertae objects (BLLacs) if they have largely featureless optical spectra oras flat spectrum radio quasars (FSRQs) if they haveprominent emission lines. All blazars are characterizedby broadband non-thermal emission extending over thecomplete electromagnetic spectrum, strong polarizationfrom radio to optical wavelengths, and displays of violentvariability on timescales that can extend from a fractionof an hour to many years. In the usual orientation-basedunified model of radio − loud AGN, blazar jets make anangle of ≤ ◦ from the line of sight and the emissionfrom these jets is thus Doppler boosted and dominateswhat we observe (Urry & Padovani 1995).Blazar variability timescales have often been arbitrar-ily divided into three classes: timescales from a few tensof minutes to less than a day are called intraday variabil-ity (IDV) (Wagner & Witzel 1995) or micro-variabilityor intranight variability, those from several days to afew months are short timescale variability (STV), whilelong timescale variability (LTV) covers changes from sev-eral months to many years (Gupta et al. 2004). Thespectral energy distribution (SED) of blazars have twopeaks (e.g., Giommi, Ansari & Micol 1995; Fossati etal. 1998). The locations of those peaks can be used toclassify blazars into LBLs (Low Energy Peaked Blazars)with the first hump in the near infrared (NIR) or opticalband and the second hump usually peaking at GeV γ -ray [email protected] Aryabhatta Research Institute of Observational Sciences(ARIES), Manora Peak, Nainital - 263129, India Department of Physics, DDU Gorakhpur University,Gorakhpur - 273009, India Department of Physics, The College of New Jersey, P.O. Box7718, Ewing, NJ 08628, USA energies, while HBLs (High Energy Peaked Blazars) arethose with first peak in the UV or X-ray band and thesecond peak located at up to TeV energies (e.g., Padovani& Giommi 1995). The high polarization of the radio tooptical emission suggests that the lower energy peak isproduced via the synchrotron process but the high energyemission mechanism in blazars is not yet fully under-stood, though it is probably due to the inverse Compton(IC) mechanism. 1.1.
Mrk 421
With its redshift z = 0 . α . =11h 04m 27.2s and δ . =+38 ◦ ′ ′′ ) isamong the closest blazars, at a distance of 134 Mpc(H = 71 km s − Mpc − , Ω m = 0.27, Ω λ = 0.73). Itis classified as an HBL because the energy of its syn-chrotron peak is higher than 0.1 keV. It is the bright-est TeV γ − ray emitting blazar in northern hemisphere.Mrk 421 was first noted to be an object with a blue excesswhich turned out to be an elliptical galaxy with a brightpoint like nucleus (Ulrich et al. 1975). The object showedoptical polarization and the spectrum of the nucleus wasseen to be featureless and so it was classified as a BL Lac.Mrk 421 was detected in the GeV band by the EGRETinstrument on the Compton Gamma − ray Observatory(CGRO) (Lin et al. 1992; Michelson et al. 1992). It wasthe first known extragalactic TeV γ − ray emitter (Punchet al. 1992), and has been repeatedly confirmed as a TeVsource by ground-based γ -ray telescopes (Aleksic et al.2011; Acciari et al. 2011 and references therein).Mrk 421 has been extensively observed at all wave-lengths and some noticeable studies in low-energy bandsinclude an exhaustive compilation of radio data at 22and 37 GHz over about 25 years (Ter¨asranta et al. 2004,2005). The source is characterized by strong variabilityin the optical region (e.g., Miller 1975; Liu et al. 1997) in-cluding LTV of ∼ ∼
85 mCrab in the 2.0–10.0 keVband, with the corresponding first peak of the SED of-ten occurring at >
10 keV (Tramacere et al. 2009). Ahard X-ray flare from Mrk 421 was detected by Super-AGILE on 2008 June 10 (Costa et al. 2008) which wasfollowed by detection in γ − rays (Pittori et al. 2008)by the AGILE/GRID (Gamma-ray Imaging Detector).Ushio et al. (2009) presented the observations of X-rayvariability of Mrk 421 with Suzaku. Recently, strong X-ray flares were detected from the source in 2010 Januaryand February with the Monitor of All-sky X − ray Image(MAXI) instrument on the International Space Station(ISS). The February 2010 flare reached 164 mCrab andis the strongest among those reported from the object(Isobe et al. 2010).The time-average energy spectrum of Mrk 421 dur-ing the flaring stage has been measured at high energieswith HESS using large-zenith-angle observations (Aharo-nian et al. 2005) and with MAGIC (Albert et al. 2007).X − ray and TeV flares were observed around May 16,1994 (Takahashi et al. 1994; Kerrick et al. 1995) andaround April 25, 1995 (Takahashi et al. 1995). Theobserved correlated variability between X-rays and TeV γ − rays (Maraschi et al. 1999; Fossati et al. 2008) can beexplained in the synchrotron-self Compton (SSC) frame-work (Ghisellini et al. 1998), whereas the external Comp-ton (EC) scenario is unlikely to apply in HBLs due to thelow density of ambient photons. There was a detection ofa rapid variability timescale of TeV γ − ray emission fromMrk 421 ( ∼
10 min; Gaidos et al. 1996) that may require avery large Doppler factor δ ≥
50 (Ghisellini & Tavecchio2008). Mrk 421 has been a target of several simultane-ous multi-wavelength monitoring campaigns (Takahashiet al. 2000; Rebillot et al. 2006; Fossati et al. 2008; Lichtiet al. 2008). Despite all the studies of this source, wefound that there have been a very few investigations ofIDV and STV of Mrk 421 in optical bands. So we decidedto pursue the present study which addresses both theIDV and STV of Mrk 421. Our optical observations aresynchronized with the X − ray observations from MAXI.The present optical observations give simultaneous in-formation in an additional spectral window and make itpossible to search for the correlations and time delays be-tween optical and X − ray bands. This work could give ususeful input for multi − wavelength modelling of blazarsand lead to a better understanding of the cause of theirvariability. 1.2.
3C 454.3 α . =22h 53m 57.75s δ . =+16 ◦ ′ . ′′ ) is a well known flat spectrum ra-dio quasar (FSRQ) at redshift z=0.859. It has displayed pronounced variability at all wavelengths and has beenextensively observed over the years in most energy bands,from radio (e.g., Bennett 1962) through microwave (Ben-nett et al. 2003), optical (e.g., Sandage 1966; Raiteri etal. 1998), X − ray (e.g., Worrall et al. 1987; Tavecchio etal. 2002), low energy γ − ray (Blom et al. 1995; Zhang etal. 2005) and high energy γ − ray (Hartmann et al. 1993,1999).3C 454.3 entered a bright phase starting at 2000,and has shown remarkable activity in the past decade(Fuhrmann et al. 2006; Villata et al. 2006, 2007, Raiteriet al. 2007, 2008). In spring 2005 it underwent majoroutbursts, reaching an R-band magnitude of 12.0 thatwas the largest apparent optical luminosity ever recordedfrom this blazar (Villata et al. 2006). A Whole EarthBlazar Telescope monitoring effort continued after theoptical outburst and followed the subsequent radio ac-tivity (Villata et al. 2007) and then the faint state inthe 2006-2007 observing season (Raiteri et al. 2007). Inthis last period, the relatively low contribution of thesynchrotron emission from the jet meant that the “littleblue bump”, believed to arise from Fe-line emission fromthe broad line region, as well as the “big blue bump”,due to the nearly thermal emission from the accretiondisc were recognizable. An increase in activity occurredat X-ray and radio wavelengths as well, with the 230 GHzradio variations having a delay of ∼ γ -rays. It was subse-quently monitored by AGILE in 2007-2009 and showedrepeated flares that usually coincide with periods of in-tense optical and enhanced X-ray activity (Chen et al.2007, Donnarumma et al. 2009). During this time span,very bright γ − ray emission was detected (Tosti et al.2008), with an excellent correlation between the γ rayand NIR/optical variations (Bonning et al. 2009).The source is listed as 1FGL J2253.9+1608 in theFirst FERMI-LAT active galactic nucleus (AGN) cat-alog (Abdo et al. 2010). 3C454.3 is the first source forwhich daily resolved broadband spectral energy distribu-tion (SEDs) with GeV data have been obtained (Abdoet al. 2009). Strong Ly α radiation has been seen from3C454.3 (Bonnoli et al. 2010), indicating the presenceof an external photon source for the Compton scatteringaside from torus emission (Sikora et al. 2009). Modelingof SEDs has been performed by Finke & Dermer (2010),and Pacciani et al. (2010).This FSRQ showed strong activity at optical frequen-cies in 2008-2009. (Villata et al. 2008; Sasada et al. 2009;Gupta et al. 2009). Foschini et al. (2010) claim γ − rayvariability from 3C454.3 on timescales as short as a fewhours from the LAT data. The continuous monitoringby the Fermi-LAT showed that the source activity fadedconsiderably in early 2009 and then rose back up fromJune onward. It underwent an exceptional outburst in2009 November − γ − ray source in the sky for over a weak. (Striani et al.2009, 2010; Escande & Tanaka 2009).Here we present our optical observations from ARIES,India during the outburst flare in November-December2009, along with data from the optical monitoring pro-gram during this period with the Small and Moder-ate Aperture Research Telescope System (SMARTS) inChile as well as optical data from the 1.5 m telescope ofKANATA observatory in Japan. In order to see if thereis related variability of the blazar 3C 454.3 in optical,X-ray and gamma bands, we correlate the above opticaldata with the X-ray data (in 2-10 KeV) obtained fromMAXI as well as with 1-300 Gev fluxes made public bythe Fermi Science Support Center. These results shouldbe useful for understanding the γ -ray and X-ray emissionmechanisms and could also offer a test of the existingmodels (leptonic and hadronic) for the γ -ray emission.The paper is structured as follows. In Section 2, wegive brief descriptions of observations and data reductionmethods. In Section 3, we discuss the techniques we usedto search for variability properties and we provide theresults in Section 4. A discussion and our conclusionsare given in Section 5. OBSERVATIONS AND DATAREDUCTIONS
ARIES observations and data reduction
Our optical photometric observations were carried outin the B, V, R and I pass bands between November 2009and June 2010 using the 104-cm Sampurnanand tele-scope (ST) located at the Aryabhatta Research Instituteof Observational Sciences (ARIES) in Nainital, India. Ithas RitcheyChretien (RC) optics with a f/13 beam. Thedetector was a cryogenically cooled 2048 × − pixel − and a gain of 10 e − ADU − in the employed slow readout mode. Each pixel has adimension of 24 µ m , corresponding to 0.37 arc-sec onthe sky, thereby covering a total field of 13 ′ × ′ . Wecarried out observations in a 2 × ∼ . ′′ ∼ . ′′
0. The detailed obser-vation logs of the blazars Mrk 421 and 3C 454.3 are givenin Table 1.Image processing was done using the standard routinesin Image Reduction and Analysis Facility (IRAF) .Data processing to provide the instrumental magni-tudes of the stars and the target source was done us-ing the Dominion Astronomical Observatory Photome-try (DAOPHOT II) software to perform the concentriccircular aperture photometric technique (Stetson 1987,1992). We observed local standard stars in the field ofblazars. For Mrk 421, we observed three local standardstars, labeled 1, 2 and 3 while we had nine local standardstars for 3C 454.3. Aperture photometry was carried out IRAF is distributed by the National Optical Astronomy Ob-servatories, which are operated by the Association of Universitiesfor Research in Astronomy, Inc., under cooperative agreement withthe National Science Foundation. with four concentric aperture radii, i.e., ∼ × FWHM,2 × FWHM, 3 × FWHM and 4 × FWHM. On comparingthe photometric results, we found that aperture radii of2 × FWHM almost always provided the best S/N for bothblazars and the standard stars, so we adopted those aper-tures for our final data reductions. The flux from thenucleus of Mrk 421 is contaminated by the emission ofthe host galaxy. To remove this constant component, weused the measurements of Nilsson et al. (2007) to esti-mate the host galaxy emission in the R-band. This flux isused to obtain the corresponding contributions for the Band V bands (Fukugita et al. 1995) and we corrected forGalactic extinction using the extinction map of Schlegel,Finkbeiner & Davis (1998).Two standard stars in each blazar field, Stars 1 and 2for Mrk 421 and Stars C and D for 3C 454.3, were used tocheck that the standard stars were mutually non-variableand finally one standard star each, Star 1 and Star D,were used to calibrate the instrumental magnitudes ofMrk 421 and 3C 454.3, respectively.2.2.
Optical Data from KANATA
The KANATA 1.5 m telescope performed long termmonitoring of the source 3C 454.3 in the R-band witha cadence of one-day. This data have been reported inPacciani et al. (2010) and the details of the observationsare given there. The data were kindly provided to us byM. Uemura and cover the range from 5 November 2009to 3 January 2010.2.3.
Optical Data from SMARTS
The SMARTS photometric data and light curves(LCs) for 3C 454.3 are publicly available on the web(Bailyn 1999). Monitoring of the sources are carried outon the 1.3 m telescope located at Cerro Tololo Inter-American Observatory (CTIO) with the ANDICAM in-strument. ANDICAM is a dual-channel imager with adichroic that feeds an optical CCD and an IR imager,which can obtain simultaneous data from 0.4 to 2.2 µ m(Bailyn 1999). We have taken data from the SMARTSweb archive for 3C 454.3 from 5 November 2009 to 13December 2010 in the B, V and R bands.2.4. X-ray archival data from MAXI
MAXI is the first astronomical payload to be installedin the Japanese Experiment Module − Exposed Facility(JEM − EF or Kibo − EF) on the ISS and has high sensi-tivity as an all-sky X-ray monitor. It started operation in2009 August (Matsuoka et al. 2009) and has two typesof X-ray slit cameras with wide fields of view and twotypes of X-ray detectors. Although MAXI has two X-rayinstruments, the Gas Slit Camera (GSC) and the SolidState Slit Camera (SSC; Tsunemi et al. 2010), we haveanalyzed only GSC data because it has both higher sen-sitivity and larger sky coverage. The GSC consists of 12one-dimensional position sensitive proportional countershaving 5350 cm detection area in total that are sensi-tive to X-ray photons with energies from 2-20 keV, whilethe SSC is composed of 32 X-ray CCD cameras with anenergy range of 0.15-12 keV. The MAXI GSC signals forthe sources were integrated orbit by orbit within a 3 ◦ × ◦ square aligned to the scan direction centered on thesource; the background was evaluated from two squaresoffset by ± ◦ along the scan direction in the sky, each ofthe same size as that of the source region. The extractedcounts were normalized by dividing by a total exposure(in units of cm sec) obtained with a time integration ofthe collimator effective area. The data from all the ac-tivated counters are summed up. We downloaded 1 dayaverage X-ray fluxes from MAXI for both Mrk 421 and3C 454.3. This data cover the period November 2009to June 2010 for Mrk 421 and the period November–December 2009 for 3C 454.3.2.5. Fermi γ -ray data The Fermi Space Telescope’s Large Area Telescope(LAT) is designed to measure the cosmic gamma-ray fluxup to ∼
300 GeV. It is an imaging, wide field-of-view high-energy pair conversion telescope with energy range from ∼
20 MeV to ≥
300 GeV (Michelson 2007). As a service tothe community, the LAT Instrument Science OperationsCenter provides daily and weekly averaged fluxes for anumber of blazars. Fluxes and 1 σ uncertainties for 1–300GeV band, using preliminary instrument response func-tions and calibrations, are made available on-line. Weobtained the FERMI-LAT data for 3C 454.3 (on dailybasis synchronized with optical data). Daily fluxes arenot available for the source Mrk 421 but weekly values areavailable. However, this sparse data would be insufficientto provide adequate information on any correlated vari-ability between the γ -ray and optical and X-ray bands,so we do not consider it further. METHODS3.1.
Variability Detection Criterion
Variability of the sources Mrk 421 and 3C 454.3 wasinvestigated by computing the commonly used quantity C (Romero et al. 1999) that is defined as the average ofC and C : C = σ ( BL − StarA ) σ ( StarA − StarB ) & C = σ ( BL − StarB ) σ ( StarA − StarB ) . (1)Using aperture photometry of the source and standardstars in the field, we determined the differential instru-mental magnitude of the blazar and standard star A,blazar and standard star B and standard star A vs. stan-dard star B. Then, we determined observational scatters σ (BL − Star) and σ (Star A − Star B). If
C > .
57, thenominal confidence limit of the presence of variability is99%; however, C is not a true statistic and this confi-dence level is usually too conservative (de Diego 2010).As discussed above, we used Star 1 and Star 2 for Mrk421 and Star C and Star D for 3C 454.3 as Star A andStar B, respectively, in the above expression.We also test any claims of variability using a properstatistic that is reasonable to employ for differential pho-tometry, the F -test (de Diego 2010). Given two samplevariances such as s Q for the blazar instrumental LC mea-surements and s ∗ for those of the standard star, then F = s Q s ∗ . (2) http://maxi.riken.jp/top/ The number of degrees of freedom for each sample, ν Q and ν ∗ , will be the same and equal to the number ofmeasurements N minus 1 ( ν = N − F value isthen compared with the F ( α ) ν Q ,ν ∗ critical value, where α isthe significance level set for the test. The smaller the α value, the more improbable that the result is producedby chance. If F is larger than the critical value, the nullhypothesis (no variability) is discarded. We have per-formed the F -test at two significance levels (0.1% and1%) which correspond to 3 σ and 2.6 σ detections, respec-tively.The percentage variation in the LCs is calculated byusing the variability amplitude parameter A , introducedby Heidt & Wagner (1996) and defined as A = 100 < A > × p ( A max − A min ) − σ (%) , (3)where A max and A min are the maximum and minimumfluxes in the calibrated LCs of the blazar, < A > is theirmean, and the average measurement error of the blazarLC is σ .The calculated F statistics, C “statistics” and vari-ability amplitude ( A ) values are listed in Tables 2 and3. 3.2. Structure Function
The structure function (SF) is a technique that canprovide some information on the nature of the physicalprocess causing any observed variability. The SF is freefrom any constant offset in the time series (Rutman 1978;Simonetti et al. 1985; Paltani et al. 1997). For detailsabout the SF as we have employed it, see Gaur et al.(2010).We have carried out the SF analysis of all of those LCswhich satisfy the variability detection criteria. Recently,Emmanoulopoulos et al. (2010) have discussed the weak-nesses of the SF method, including spurious indicationsof timescales and periodicities. So, we have cross checkedthe SF results by the DCF method to look for any hintsof periodicity.3.3.
Discrete Correlation Function Analysis
The Discrete Correlation Function (DCF) was first in-troduced by Edelson & Krolik (1988) and was generalizedby Hufnagel & Bregman (1992) to include a better errorestimate. For details about the DCF see Tonnikoski etal. (1994), Hovatta et al. (2007) and references therein.For two different data trains, any strong peak in the DCFcan indicate the possible time lag. RESULTS
Intra − Day Variability of blazars in theR-bandMrk 421
We intensively observed the blazar Mrk 421using a R filter during nine nights from 21 November2009 to 9 April 2010. The complete observing log ofthe blazar is in Table 1. The LC of the blazar Mrk 421(calibrated) and the differential instrumental magnitude(of Star 1-Star 2) are displayed in Fig. 1 for those ninenights. We have performed both C and F tests on thosenine nights; however, no genuine intra-day variability wasfound during any of them. The C and F values are givenin Table 2 and they never exceed the formal significancecriteria.
3C 454.3
We observed the blazar 3C 454.3 through anR filter on seven nights from 22 November 2009 through21 December 2009. The LC of 3C 454.3 and the differ-ential instrumental magnitude (StarC - StarD) are dis-played in Fig. 2. The complete observing log for thisblazar is given in Table 1. The C , F and A values forthis IDV are listed in Table 2. We found that the C val-ues and results of the F -test both show significant valuesfor four nights (22 Nov, 13 Dec, 15 Dec and 20 Dec 2009)so it is clear that the source has shown IDV during fournights of our observations. We have carried out the SFand DCF analysis of those four LCs satisfying the vari-ability detection criteria and these are shown in Fig. 3;however, no significant variability timescale was detectedin any of those LCs.4.2. Short − Term Flux and Color Variability
Mrk 421
The nightly LCs of Mrk 421 (calibrated magnitude)in B, V, R, (B − V), (V − R) and (B − R) are plotted inthe different panels in Fig. 4. Here we estimate the 99%confidence detection level of short-term variability usingthe detection tests described in Section 3.1 and calculatethe variability amplitude using Eq. (3).
B pass-band:
The short-term LC of Mrk 421 in theB-band is displayed in the upper left panel of Fig. 4.The maximum variation noticed in the source is 0.70mag (between its brightest level at 14.26 mag on JD2455187.52290 and the faintest level at 14.96 mag on JD2455296.10980). The values of the C and F -tests supportthe existence of short-term variations in the source in B -band observations. We calculated short-term variabilityamplitude using Eq. (3) and found that the source hasvaried ∼ V pass-band:
The short-term LC of Mrk 421 inthe V-band is shown in the middle left panel of Fig. 4.The maximum variation noticed in the source is 0.59mag (between its brightest level at 13.59 mag on JD2455207.59130 and the faintest level at 14.18 mag on JD2455296.10703). The values of the C and F -test also sup-port the existence of short-term variation in the sourcein these V-band observations. The short-term variabilityamplitude is ∼ R pass-band:
The corresponding LC of Mrk 421 inthe R-band is in the lower left panel of Fig. 4. The maxi-mum variation noticed in the source is 0.49 mag (betweenits brightest level at 12.64 mag on JD 2455187.50831 andthe faintest level at 13.16 mag on JD 2455158.44563).Again, the C and F -tests both indicate short-term R-band variations are present with an amplitude of ∼ (B − V) color:
The short-term LC of Mrk 421 in the(B − V) color is shown in the lower right panel of Fig. 4.The maximum variation noticed in the source is 0.17 mag(between its color range 0.39 mag at JD 2455271.11511and 0.56 mag at JD 2455358.12050). However, neitherthe C - nor F -test provide support for the existence ofsignificant (B − V) color variations in our observations. (V − R) color:
The short-term LC of Mrk 421 for(V − R) is displayed in the upper right panel of Fig. 4.The maximum variation noticed in the source is 0.13 magbetween the color range 0.25 mag at JD 2455207.59130 and 0.38 mag at JD 2455296.11602. Again, no significant(V − R) color variations are seen in our observations.4.2.2.
3C 454.3
The nightly LCs of 3C 454.3 (calibrated magnitude)in B, V, I, (B − V) and (V − R) are plotted in Fig. 2 andthat for the R band is plotted in Fig. 5. Here, C - and F -tests could not be performed on the entire large data-sets because only the nominal calibrated magnitudes of3C 454.3 are available on SMARTS site (for B, V and Rbands) and without the unavailable data for comparisonstars we cannot compute those quantities. The same isthe case for the KANATA data (which is only for the Rband). Therefore we performed C- and F-tests on theARIES data only and those values are quoted in Table3. However, when computing the amplitude of varia-tion, we have used the whole data-set including data fromSMARTS as well as KANATA. B pass-band:
The short-term LC of 3C 454.3 in theB-band is displayed in the top of the middle bottom panelof Fig. 2. The maximum variation noticed in the sourceis 1.593 mag (between its brightest level at 14.937 magon JD 2455179.05072 and the faintest level at 16.53 magon JD 2455143.58696). We performed C- and F-tests onthe ARIES data and found the variations to be highlysignificant. We calculated the STV amplitude of wholedataset using Eq. (3) and found that the source has var-ied by ∼ V pass-band:
The short-term LC of 3C 454.3 in theV-band is given in the middle of the middle bottom panelof Fig. 2. The maximum variation noticed in the sourceis 1.581 mag (between its brightest level at 14.331 magon JD 2455179.05491 and the faintest level at 15.912 magon JD 2455141.57627). Our C- and F-tests performed onthe ARIES data yielded highly significant values. Theamplitude of the variation of whole data-set is ∼ R pass-band:
The short-term LC of 3C 454.3 in theR-band is shown in Fig. 5. In this plot we have com-bined our data with those provided by the SMARTS andKANATA telescopes. Again, C- and F-tests are per-formed only on ARIES data and values are much higherthan 0.999% significance. The maximum variation no-ticed in the source is 1.578 mag (between its brightestlevel at 13.709 mag on JD 2455167.91 and the faintestlevel at 15.505 mag on JD 2455141.5774). From the fig-ure, it is clear that there are two flares with first flareof 0.44 mag peaking near JD 2455152 and second flarewith 1.31 mag peaking near JD 2455171. The amplitudeof total variation in the STV light curve is ∼ I pass-band:
The short-term LC of 3C 454.3 in theI-band is displayed in the lower portion of the lower mid-dle panel of Fig. 2. The C- and F-tests performed onARIES data still yield high significance for the varia-tions. The maximum variation noticed in the source is0.656 mag (between its brightest level at 13.105 mag onJD 2455179.04786 and the faintest level at 13.761 magon JD 2455186.08768). As SMARTS data is not avail-able in the I band, we have many fewer data points andso the observed STV amplitude is only ∼ Correlated variations between color and magni-tude:
Color–magnitude plots of 3C 454.3 are displayedin bottom right panel of Fig. 2. The upper and lowersub-panels respectively show the (V − R) and (B − R) col-ors plotted with respect to V magnitude. The straightlines shown are the best linear fit for each of the color in-dices, Y , against magnitude, X , for each of the sources: Y = mX + c . For (V − R), the fitted value for the slopeof the curve, m = − c = 1 . r = − . p = 5 . × − , thus indicating a verystrong correlation. Similarly, for (B − V), the fitted val-ues for the slope of the curve is m = − .
05 and thatfor the constants is c = 1 .
30. The linear Pearson cor-relation coefficient is, r = − .
48 and the correspond-ing p = 0 . Correlated Variability
Mrk 421
Fig. 6 displays the X − ray and optical LCs of the 2009–2010 observing season. We can see from the X − ray LCthat a brightness increase, apparently corresponding to amodest flare, occurred around JD=2455197. Meanwhile,the optical observations in R-band show an increase inbrightness around JD=2455177, which peaks at 12.93mag and a decline after at JD=2455197 (though we don’thave data for that entire interval and we probably havemissed the actual peak). Still we can say that the flaresin the X − ray and optical bands are seen in the samegeneral time span.In the X − ray LCs, there is an even bigger flare peak-ing at JD=2455243. Unfortunately, we could not ob-tain any optical data between 21 January and 13 March2010, so we appear to have missed this flare in the opti-cal. Therefore we have performed the DCF analysis onlyin the temporal region containing the first flare regionfrom the beginning of the data train at JD=2455150 toJD=2455230 (shown by a vertical line in Fig. 6). In theX − ray data-set, we found that the source flux countswere given as negative on a few days of observations andsuch days were omitted in our analysis. The DCF be-tween the X − ray is displayed in the top right panel ofFig. 6. The distribution of points have two significantmaximas, ∼ ∼
3C 454.3
For this source we are able to cross-correlate the vari-ability across γ -ray, X-ray and optical bands. A promi-nent peak at around JD 2455167 and a short flare nearJD 2455170 are seen in both the optical LC and the γ -rayLC (Fig. 7). However, these features are not seen in theX-ray LCs also shown in Fig. 7. We performed a DCFbetween the γ -ray (1-300 GeV) flux and the LCs in theoptical R-band. This DCF shows a large peak correlationamplitude of ∼ τ = 4 . ± γ -ray light curves withlow order polynomials to extract those trends. From the de-trended LCs, we obtained the underlying probabilitydistributions of the fluctuations. Using this probabilitydistribution, we performed random sampling and therebygenerated 3000 realizations of a random LC with the un-derlying statistical properties of the original LC for boththe optical and γ -ray bands. Next, taking each combi-nation of these randomly generated LCs, we determinedtheir DCFs. We checked the ability of DCF to find thereal time lags. Since the simulated DCFs also gave timelags of 4.5 days, we examined the p -values at a lag of4.5 days. The null hypothesis examined here is that thehighest correlation value at the given lag of the actualdata is higher than the simulated light curve’s DCF atthe same time lag. We found correlation value of 0.70 tobe at a p -value of 0.99 at the time lag of 4.5 days. Hence,our observed correlation amplitude of 0.90 indicates sig-nificance well above 0.99.4.4. X-ray Hardness − Ratio Analysis
We can crudely study the spectral variability of thesource through the hardness ratio. It is defined as eitherthe ratio of counts ( b/a ) or the ratio of the difference andsum of the counts ( b − a ) / ( b + a ), (Zhang et al. 2006).We use the former definition. In Fig. 6, the top left panelshows the hardness ratio, 4-10 keV/2-4 keV, as a functionof intensity for Mrk 421, while the middle right panel inFig. 7 shows the same quantity for 3C 454.3. For Mrk421, there is no correlation ( r = − .
17 with p = 0 . r =0 .
40 with p = 0 .
1) between source hardness ratio and itsintensity. DISCUSSION AND CONCLUSIONS
During our observation period of November 2009 toJune 2010, we monitored Mrk 421 for IDV in 9 nights,but genuine IDV in any of the B, V and R pass-bandswas not detected in any night as both C and F valueswere always less than the 99% significance levels. We no-ticed the existence of significant short-term variability inthis blazar from our observations and the source showeda maximum variation in the B band of 0.70 magnitudes.The total short-term variation detected in our observa-tions in the B, V, and R bands are ∼ ∼
53% and ∼ − V) and (V − R) colors.We monitored 3C 454.3 for IDV during 7 nights duringNovember and December 2009. We performed C and F -tests and found that 4 of these 7 nights showed genuineIDV. To search for any variability timescale, we com-puted the SF and DCF, but no significant variabilitytimescales were detected. We noticed the existence ofsignificant short-term variability in this FSRQ blazar butcould not perform C or F -tests on the whole data-setsas we had only calibrated data from the Chilean andJapanese telescopes and not the needed data for com-parison stars. So we performed C - and F -tests only onthe ARIES data but we calculated variability amplitudesbased on the full data-set. We found short-term vari-ability amplitudes in the B, V, R and I pass-bands to be ∼ ∼ ∼ ∼ − ray LC, there are two strong flares, with thefirst flare essentially coinciding with the flare in the op-tical band. Flares from the source was reported by Isobeet al. (2010) in their 2010 January-February observationsfrom MAXI GSC. The maximum 2-10 KeV flux in theJanuary and February flares were 120 ±
10 mCrab and164 ±
17 mCrab, respectively, and the latter maximum isthe highest among those reported from Mrk 421 so far.Also, the MAXI GSC spectrum around the maximum ofthe flares was found to be consistent with a spectral indexindicative of synchrotron radiation. We found that therewas a correlation between the optical and X-ray bands(
DCF = 0 .
88) at a negative lag of 9.5 ± c asthey lose energy via both synchrotron and IC processes(Marscher & Gear 1985). The highest energy electronswill suffer the most severe radiative losses so that theyonly maintain these high energies and produce high en-ergy photons over short distances. Hence, the highest frequency radiation can be emitted only within a thinsheet behind the shock front. The thickness of the sheetincreases as the frequency decreases until the frequencyis so low that radiative losses are negligible across the en-tire shocked region. Thus, in this standard model a flarecaused by a shock spreads across multiple wavebands,but the time-scale of variability can be much shorter athigher frequencies. The X-ray flux in a shock-inducedflare should therefore peak first, followed by the opticaland then lower frequencies, whereas the γ -ray flux, if pro-duced by IC scattering from the X-ray (or EUV) photonscould then lead the optical, while if produced from theoptical or IR photons, would lag the optical.The blazar 3C 454.3 was also found in flaring stateduring November-December 2009. The present obser-vations confirmed the presence of fairly significant colorvariations that support the presence of thermal emissionbeneath the dominant non-thermal jet. Raiteri et al.(2007) previously found good evidence for big and lit-tle “blue bumps” in the SED of 3C 454.3 during periodsof low emission. Prominent peaks are seen nearly si-multaneously in the optical and γ -ray bands but thesefeatures are not seen in the X-ray band. The DCF ofthe γ and optical bands shows a peak correlation am-plitude of ∼ τ =4.5 ± γ -rays leadingthe optical rays. Similar behavior was found by Bon-ning et al. (2009) for the flare in July 2008. The cor-related optical/ γ -ray variability supports the externalCompton model in which relativistic electrons in the jetradiate radio through UV synchrotron photons and in-verse Compton scatter IR/optical photons to hard X-rayand γ -ray energies (Dermer & Schlickeiser 1993; Sikora etal. 1994). The lack of correlation seen in the DCF for X-rays with respect to optical/gamma bands can be reason-ably understood if the X-rays are coming from low-energyelectrons inverse Compton scattering external UV pho-tons, rather than higher energy electrons producing syn-chrotron photons. These lower energy electrons wouldvary more slowly and thus plausibly give rise to the rel-atively stable X-ray emission. The modest quantity ofour data, which allows for the likely identification of justone flare between the optical and γ -ray bands, precludesour attempting to produce more detailed models.We gratefully acknowledge Kanata team for observa-tions and Prof. M. Uemura for providing us publishedKanata data on the blazar 3C 454.3. HG is thankfulto Dr. K. Nilsson for a discussion about host galaxycontributions to observed flux. We thank the refereefor several very helpful suggestions. This research hasmade use of MAXI data provided by RIXEN, JAXA andthe MAXI team. The acquisition and analysis of theSMARTS data are supported by Fermi GI grants 011283and 31155 (PI C. Bailyn). This research has made use ofthe NASA/IPAC Extragalactic Database (NED) whichis operated by Jet Propulsion Laboratory, California In-stitute of Technology, under contract with the NationalAeronautics and Space Administration. REFERENCESAbdo, A. A., et al. 2009, ApJ, 699, 817Abdo, A. A., et al. 2010, ApJ, 715, 429 Acciari, V. A., et al. 2011, arXiv:1106.1210Aharonian, F., et al. 2005, A&A, 437, 95
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Fig. 1.—
R band light curves (LCs) of Mrk 421. Upper curves are the calibrated LCs of Mkr 421 (w.r.t Star 1). Lower curves are thedifferential instrumental magnitudes of Star 1 & Star 2 with arbitrary offsets. Fig. 2.—
LCs and color indices of 3C 454.3. The first seven panels contain R-band LCs. Upper curves are the calibrated LCs of 3C 454.3(w.r.t. star D). Lower curves are the differential instrumental magnitudes of stars C and D with arbitrary offsets. The eighth panel showsB, V and I band light curves of 3C 454.3 (SMARTS data in starred symbols and ARIES data in open circles); the last panel shows the(V-R) and (B-V) colors against V magnitude. Fig. 3.—
IDV LCs in the R-band for 3C 454.3 with their respective structure function (SF) and discrete correlation function (DCF) ineach row from left to right, respectively.
150 200 250 300 35013.21312.812.614.21413.813.61514.814.614.414.2 BVRJD (2455000+)
Fig. 4.—
Short-term variability LCs and color indices of Mrk 421 in the B, V and R bands and (V-R) and (B-V) colors during the2009–2010 season. Fig. 5.—
Short-term variability LC of 3C 454.3 in R band. Open circles represent data from KANATA observatory, Japan; open trianglesrepresent the data from ARIES, Nainital; open squares represents the data from SMARTS. -20 -10 0 10 20-0.500.5100.10.20.30.4150 200 250 300 35013.613.413.21312.8150 200 250 300 35000.050.10.150.20.2500.050.10.150.20.25 0.05 0.1 0.150.511.52 JD (2455000+) JD (2455000+)Time lag (in days)(2-10 KeV) Fig. 6.— X − ray LCs for Mrk 421: 2-4 KeV (lower left panel); 4-10 KeV (middle left panel) and 2-10 KeV (middle right panel). AlsoR-band LC (lower right panel), hardness intensity plot (upper left panel) and DCF (upper right panel) performed on the optical vs X − raydata in the first flare region (lasting until the vertical line). Fig. 7.—
Gamma, X-ray and optical LCs of 3C 454.3 (upper panels); X − ray LCs for 3C 454.3 in 2–4 keV, 4–10 keV and hardnessintensity plot (middle panels); DCF between gamma vs. optical (horizontal line indicates 99% significance level), γ -ray vs. X-ray and X-rayvs. optical (in lower panels). Table 1. Observation log of optical photometric observations of Mrk 421 and 3C 454.3
Source Date of Filters Dataname observation pointsMrk421 2009 Nov 21 B,V,R 1,1,362009 Nov 22 B,V,R 1,1,222009 Dec 11 B,V,R 1,1,22009 Dec 13 B,V,R 2,2,22009 Dec 15 B,V,R 2,2,22009 Dec 20 B,V,R 2,2,22009 Dec 21 B,V,R 2,2,22010 Jan 10 B,V,R 1,1,842010 Jan 11 B,V,R 1,1,872010 Jan 20 B,V,R 1,1,802010 Mar 15 B,V,R 1,1,702010 Mar 16 B,V,R 1,1,702010 Mar 22 B,V,R 1,1,852010 Apr 09 B,V,R 1,1,802010 Apr 15 B,V,R 1,1,12010 Apr 16 B,V,R 1,1,12010 Apr 20 B,V,R 0,1,12010 Jun 10 B,V,R 1,1,13C454.3 2009 Nov 22 B,V,R,I 1,1,62,02009 Dec 11 B,V,R,I 1,1,46,12009 Dec 13 B,V,R,I 1,1,39,12009 Dec 15 B,V,R,I 1,1,46,12009 Dec 18 B,V,R,I 1,1,33,12009 Dec 20 B,V,R,I 1,0,41,12009 Dec 21 B,V,R,I 1,1,75,1
Table 2. IDV observations of Mrk 421 and 3C 454.3
Source Name Date N C-Test F-Test Variable A (%)(dd.mm.yy) C , C F , F , F c (0 . , F c (0 . Table 3. STV observations of Mrk 421 and 3C 454.3
Source Name Band N C-Test F-Test Variable A (%) C , C F , F , F c (0 . , F c (0 . ∗∗