Optical Variability of Three Extreme TeV Blazars
Ashwani Pandey, Alok C. Gupta, G. Damljanovic, P. J. Wiita, O. Vince, M. D. Jovanovic
aa r X i v : . [ a s t r o - ph . H E ] J un MNRAS , 1–12 (2020) Preprint 5 June 2020 Compiled using MNRAS L A TEX style file v3.0
Optical Variability of Three Extreme TeV Blazars
Ashwani Pandey ⋆ , Alok C. Gupta † , G. Damljanovic , P. J. Wiita , O. Vince ,and M. D. Jovanovic Aryabhatta Research Institute of Observational Sciences (ARIES), Manora Peak, Nainital – 263001, India Astronomical Observatory, Volgina 7, 11060 Belgrade, Serbia Department of Physics, The College of New Jersey, 2000 Pennington Road, Ewing, NJ 08628-0718, USA
Accepted XXX. Received YYY; in original form ZZZ
ABSTRACT
We present the results of optical photometric observations of three extreme TeVblazars, 1ES 0229+200, 1ES 0414+009, and 1ES 2344+514, taken with two telescopes(1.3 m Devasthal Fast Optical Telescope, and 1.04 m Sampuranand Telescope) inIndia and two (1.4 m Milankovi´c telescope and 60 cm Nedeljkovi´c telescope) in Ser-bia during 2013–2019. We investigated their flux and spectral variability on diversetimescales. We examined a total of 36 intraday R − band light curves of these blazarsfor flux variations using the power-enhanced F -test and the nested ANOVA test. Nosignificant intraday variation was detected on 35 nights, and during the one positivedetection the amplitude of variability was only 2.26 per cent. On yearly timescales, allthree blazars showed clear flux variations in all optical wavebands. The weighted meanoptical spectral index ( α BR ), calculated using B − R color indices, for 1ES 0229+200was 2.09 ± ± ± F ν ∝ ν − α ) in their optical ( VRI ) spectral energy distributions. Abluer-when-brighter trend was only detected in the blazar 1ES 0414+009. We brieflydiscuss different possible physical mechanisms responsible for the observed flux andspectral changes in these blazars on diverse timescales.
Key words: galaxies: active – BL Lacertae objects: general – BL Lacertae objects:individual (1ES 0229+200, 1ES 0414+009, 1ES 2344+514)
Blazars constitute the most enigmatic class of radio-loudactive galactic nuclei (RLAGNs) and are usually dividedinto BL Lacertae objects (BLLs) and flat-spectrum radioquasars (FSRQs). Blazars exhibit large amplitude flux andspectral variability across the whole electromagnetic (EM)spectrum, and in general have core-dominated radio struc-tures. The EM radiation from blazars is predominantly non-thermal and shows strong linear polarization ( > . ◦ (Urry & Padovani 1995) and the emitted radiation from thejet is affected by relativistic beaming, which implies a short-ening of timescales by a factor δ − , where δ is the Dopplerfactor. ⋆ E-mail: [email protected] † E-mail: [email protected]
Blazars emit significant radiation in the complete EMspectrum which gives observers an opportunity to generatetheir spectral energy distribution (SED) from low-energy ra-dio bands to extreme high energies upto γ − rays. SEDs ofblazars are double-humped, and the first SED hump (i.e. lowenergy) peaks in Infra-red (IR) to X-rays while the secondhump (i.e. high energy) peaks in γ − rays (from GeV up toTeV energies). The low energy part of SED is dominated bysynchrotron emission from the relativistic jet while the highenergy one is produced by inverse-Compton (IC) radiationwithin the frequently accepted leptonic scenario. However,the origin of the high energy hump is still under some de-bate, and hadronic as well as lepto-hadronic models havebeen proposed to explain it (e.g. B¨ottcher 2007). Based onthe SED first hump peak frequency, i.e., the synchrotronpeak frequency ν syn , blazars have often been classified intothree subclasses: LSP (low synchrotron frequency peaked) ν syn ≤ Hz, ISP (intermediate synchrotron frequencypeaked) 10 < ν syn < Hz, and HSP (high synchrotron c (cid:13) A. Pandey et al. frequency peaked) ν syn ≥ Hz (Abdo et al. 2010). In arecent study with a very large sample of blazars, a slightlymodified classification scheme of blazars was suggested byFan et al. (2016). According to this revised classificationthese subclasses of blazars are defined as: LSP still having ν syn ≤ Hz, but ISP being in the range 10 < ν syn < . Hz, and so HSPs have ν syn ≥ . Hz. Another sub-class of blazar would be the extreme high frequency peakedBL Lac objects (EHBLs) in which the synchrotron peak fre-quency ν syn lies at > > Hz; Costamante et al.2001; Foffano et al. 2019). These EHBLs are a growing sub-class of blazar whose synchrotron emission peaks at mediumto hard X-ray energies and are therefore good candidates forTeV detection, as their upscattered IC photons would haveexceptionally high energies.TeV emitting blazars are so far rather rare and mostlybelong to the HSP class. The first significant TeV emissionfrom a blazar was detected from Mrk 421 at 0.5 TeV with theWhipple observatory γ − ray telescope (Punch et al. 1992).Until 2003, only six TeV blazars were known which werehaving confirmed detection of TeV emission from two differ-ent γ − ray telescopes (e.g., Krawczynski 2004, for a summaryof their properties). The ground and space-based γ − ray fa-cilities developed and made operational in last about one-and-a-half decades e.g. HESS (High Energy StereoscopicSystem),
MAGIC (Major Atmospheric Gamma-ray ImagingCherenkov),
VERITAS (Very Energetic Radiation ImagingTelescope Array System),
Fermi , etc., have made a revolu-tion in TeV γ − ray astronomy and have discovered a sig-nificant number of TeV emitting galactic and extragalacticsources which now include 73 blazars . Out of these, 14 TeVsources have been cataloged as EHBLs (Foffano et al. 2019).Blazars show detectable flux variations on diversetimescales across the EM spectrum. Blazar variabilitytimescales range from a few minutes to years. AGN fluxvariation timescales from a few minutes to a day is vari-ously known as micro-variability (Miller et al. 1989), intra-day variability (IDV) (Wagner & Witzel 1995) or intra-nightvariability (Sagar et al. 1996). Flux variability on timescalesfrom several days a to few months is commonly known asshort timescale variability (STV) and variability timescalesfrom months to several years can be called long timescalevariability (LTV) (e.g. Gupta et al. 2004).Blazar flux variability on these diverse timescales isan important tool to understand the emission mecha-nism. A puzzling issue is the blazar flux variability ob-served on IDV timescale. To try to elucidate the natureof the emission mechanism of blazars through variabil-ity on different timescales, we have run a project overthe past 15 years and have reported our results in se-ries of papers (e.g., Gupta et al. 2008a,b, 2012, 2016, 2017;Gaur et al. 2010, 2012a,b, 2015a,b; Agarwal & Gupta 2015;Agarwal et al. 2015, 2016; Kalita et al. 2015; Pandey et al.2017, 2018, 2019, 2020; Aggrawal et al. 2018; Zhang et al.2019; Sarkar et al. 2019, and references therein). To continuethese variability studies over diverse timescales in opticalbands, we report here on three TeV γ − ray detected EHBLsnamely, 1ES 0229+200, 1ES 0414+009 and 1ES 2344+514(Costamante et al. 2018; Foffano et al. 2019) which we ob- http://tevcat.uchicago.edu/ served from four ground-based optical telescopes (two in In-dia and two in Serbia) during 2013 – 2019.This paper is organized as follows: Section 2 gives anoverview of the telescopes and photometric observations weused and the data reduction procedure. Analysis techniqueswe used to search for flux variability are discussed in Section3. Results of our variability study are given in Section 4. Adiscussion and conclusions of our study are given in Section5. Optical photometric observations of three extreme TeVblazars, 1ES 0229+200, 1ES 0414+009, and 1ES 2344+514,were carried out using the standard Johnson-Cousin
BV RI filters from 2013 September 6 to 2019 November 4 with fourground-based telescopes, two in India and two in Serbia. Thedetails of these four telescopes and the detectors used forour observations are given in Table A1 in Appendix A. Thecomplete observation logs for these blazars are presented in-dividually in Tables B1 − B3 in Appendix B. The exposuretimes range from 280–300 s in B band, 220–250 s in V band,150–200 s in R band, and 80–120 s in I band.We performed optical photometric observations of thesethree TeV blazars during 2016–2018 using two Indian tele-scopes, the 1.3 m Devasthal Fast Optical Telescope (DFOT)and 1.04 m Sampuranand Telescope (ST). Both these tele-scopes have Ritchey − Chretien (RC) Cassegrain optics andare operated by Aryabhatta Research Institute of Observa-tional Sciences (ARIES), Nainital, India. Observations withthe 1.3 m DFOT were taken using Andor 2K CCD camera,while we observed these blazars with 1.04 m ST using a Py-LoN CCD, except on 2016 November 8 and 9 when the 1.04m ST was still equipped with a Tektronics 1K CCD.We also monitored these three blazars with 1.4 m Mi-lankovi´c telescope and 60 cm Nedeljkovi´c telescope, locatedat Astronomical Station Vidojevica (ASV), Serbia. The 1.4m telescope is equipped with am Andor iKon-L CCD, whilean Apogee Alta E47 CCD camera is mounted on the 60 cmtelescope.The standard optical photometric data reduction pro-cedure we employed on all observations taken with the1.3 m DFOT and 1.04 m ST is discussed in detail inPandey et al. (2019, 2020). It involves cleaning the raw im-age frame through bias-subtraction, flat-fielding and cosmic-ray removal using standard routines of IRAF , followed byperforming aperture photometry in DAOPHOT II softwareto get the instrumental magnitudes of all the sources in theframe. The data obtained from Serbian telescopes were pro-cessed following the same reduction steps as in the case ofIndian telescopes but using the MaxIM DL software pack-age. Image Reduction and Analysis Facility (IRAF) is distributedby the National Optical Astronomy Observatory, which is oper-ated by the Association of Universities for Research in Astronomy(AURA) under a cooperative agreement with the National ScienceFoundation. Dominion Astronomical Observatory Photometry https://diffractionlimited.com/help/maximdl/MaxIm-DL.htm MNRAS , 1–12 (2020)
We used the finding charts provided by Landesstern-warte Heidelberg-K¨onigstuhl for our observations of ex-treme TeV blazars. During each observation, we observedtwo or more comparison stars present in the blazar field.We used the one star having brightness and color most com-parable to that of the blazar to get the calibrated magni-tudes of the blazar. Since all the observations of a partic-ular source were calibrated using the same standard starin a particular filter, the cross-calibration for different in-struments doesn’t significantly affect the measured magni-tudes on longer timescales. For intranight variability studieswe performed observations using one single instrument on aparticular night.During our campaign, we performed multiple R bandobservations of these extreme blazars for a total of 41 nights.However, to search for microvariations, we only selectednights with at least 10 data points. By applying this cri-terion, 14 nights of observations of 1ES 0229+200, 8 nightsof those of 1ES 0414+009, and 14 nights of those of 1ES2344+514 are qualified for IDV analysis. To search for microvariations in the R-band differential lightcurves (DLCs) of the blazars 1ES 0229+200, 1ES 0414+009,and 1ES 2344+514, we employed two of the more recentand most reliable statistical tests: the power-enhanced F -test and the nested analysis of variance (ANOVA) test. Boththe tests involve multiple comparison stars in the analysis,hence, are more powerful than the previously frequently used C − test and F − test (de Diego 2014; de Diego et al. 2015).These statistical tests are described in details in our previouspapers (Pandey et al. 2019, 2020, and references therein).Brief descriptions of these tests are given below. F -test In the power-enhanced F -test, we use the brightest compar-ison star as a reference star to generate the DLCs of theblazar and the remaining ( k ) comparison stars. The power-enhanced F -test statistics is given by F enh = s s c , (1)where s is the variance of the blazar DLC and s c is thecombined variance of the DLCs of k comparison stars. Thevalue of s c is estimated as (Pandey et al. 2019) s c = 1( P kj =1 N j ) − k k X j =1 N i X i =1 s j,i , (2)where N j is the number of data points of the j th compar-ison star and s j,i is the scaled square deviation for the j thcomparison star defined as s j,i = ω j ( m j,i − ¯ m j ) , (3)where ω j , m j,i , and ¯ m j are the scaling factor, differentialmagnitude, and the mean magnitude of the j th comparisonstar DLC, respectively. The scaling factor ω j is taken as the ratio of averaged square error of the blazar DLC to theaveraged square error of the j th comparison star (Joshi et al.2011).In the present work, we always observed two or morecomparison stars in the blazar field. The blazar and all thecomparison stars have the same number of observations ( N ).The degrees of freedom in the numerator, ν , and denomi-nator, ν , in the power-enhanced F statistics are N −
1, and k ( N − F c , corresponding to 99 per cent confidence level anddegrees of freedom ν , and ν . The value of F enh is calculatedfrom Equation 1 and is compared with F c . A light curve iscalled variable (V) if F enh ≥ F c ; otherwise, we consider it tobe nonvariable (NV). ANOVA
The nested
ANOVA test analysis involves all the compari-son stars as reference stars to produce a set of DLCs of theblazar. Each of these DLCs is divided into several groupshaving five points in each group. We calculated the values ofmean square due to groups, MS G , and the mean square dueto nested observations in groups, MS O ( G ) , following Equa-tion (4) of de Diego et al. (2015). The value of the F -statisticis then estimated as F = MS G /MS O ( G ) . A light curve isconsidered as variable if the value of F − statistic ≥ F c at 99per cent confidence level, otherwise, we call it nonvariable. The flux variability amplitudes (Amp; in per cent) on IDVand LTV timescales for the extreme TeV blazars were cal-culated in the standard way as (Heidt & Wagner 1996)Amp = 100 × p ( A max − A min ) − σ , (4)where A max , A min , and σ are the maximum magnitude, min-imum magnitude, and the mean measurement error, respec-tively, in the calibrated light curve of the blazar.The results of IDV analyses of these extreme TeVblazars using both the statistical tests are given in Table1. In the table, a light curve is conservatively declared as avariable only if statistically significant variations were foundby both the probes, otherwise, we labeled it as nonvariable.The amplitude of variability is also mentioned in the lastcolumn of Table 1 for the variable light curve. + α = 02 h m . s ; δ = +20 ◦ ′ ′′ )was first detected in X-rays with the Einstein satellite’sImaging Proportional Counter (IPC) (Elvis et al. 1992) andlater classified as a BL Lac object because of its feature-less optical spectrum (Schachter et al. 1993). It was listedas an HBL on the basis of its X-ray to radio flux ra-tio (Giommi et al. 1995). 1ES 0229+200 is hosted by anelliptical galaxy at a redshift of z = 0 . BR ) observations of the blazar were per-formed between 2006 and 2010 with the ATOM telescopein a multiwavelength campaign by Kaufmann et al. (2011). MNRAS000
ANOVA test analysis involves all the compari-son stars as reference stars to produce a set of DLCs of theblazar. Each of these DLCs is divided into several groupshaving five points in each group. We calculated the values ofmean square due to groups, MS G , and the mean square dueto nested observations in groups, MS O ( G ) , following Equa-tion (4) of de Diego et al. (2015). The value of the F -statisticis then estimated as F = MS G /MS O ( G ) . A light curve isconsidered as variable if the value of F − statistic ≥ F c at 99per cent confidence level, otherwise, we call it nonvariable. The flux variability amplitudes (Amp; in per cent) on IDVand LTV timescales for the extreme TeV blazars were cal-culated in the standard way as (Heidt & Wagner 1996)Amp = 100 × p ( A max − A min ) − σ , (4)where A max , A min , and σ are the maximum magnitude, min-imum magnitude, and the mean measurement error, respec-tively, in the calibrated light curve of the blazar.The results of IDV analyses of these extreme TeVblazars using both the statistical tests are given in Table1. In the table, a light curve is conservatively declared as avariable only if statistically significant variations were foundby both the probes, otherwise, we labeled it as nonvariable.The amplitude of variability is also mentioned in the lastcolumn of Table 1 for the variable light curve. + α = 02 h m . s ; δ = +20 ◦ ′ ′′ )was first detected in X-rays with the Einstein satellite’sImaging Proportional Counter (IPC) (Elvis et al. 1992) andlater classified as a BL Lac object because of its feature-less optical spectrum (Schachter et al. 1993). It was listedas an HBL on the basis of its X-ray to radio flux ra-tio (Giommi et al. 1995). 1ES 0229+200 is hosted by anelliptical galaxy at a redshift of z = 0 . BR ) observations of the blazar were per-formed between 2006 and 2010 with the ATOM telescopein a multiwavelength campaign by Kaufmann et al. (2011). MNRAS000 , 1–12 (2020)
A. Pandey et al.
Table 1.
Results of IDV analyses of extreme TeV blazars
Blazar Name Obs. date Obs. start time Band
Power-enhanced F-test Nested ANOVA
Statusdd-mm-yyyy JD DoF( ν , ν ) F enh F c DoF( ν , ν ) F F c JD (2450000.0+) C a li b r a t e d M a g n i t u d e A (B-0.5) B (B-0.5) C (B-0.5) D (B-0.5) A (R) B (R) C (R) D (R)
Figure 1.
LTV optical ( BR ) light curves of 1ES 0229+200 shown in black ( B ) and red ( R ), respectively. The telescopes employed arenoted at the top of the figure. MNRAS , 1–12 (2020) O p t i c a l Sp e c t r a l I n d e x ( α B R ) O p t i c a l Sp e c t r a l I n d e x ( α B R ) Figure 2.
Variation of optical spectral index ( α BR ) of 1ES0229+200 with respect to time (top) and R-magnitude (bottom). Table 2.
Variation of optical spectral index, α BR , with respectto time and R-magnitude of 1ES 0229+200 Parameter m a c a r a p a α BR vs time − . e − ± . e −
05 2.96 -0.42 5.33e-02 α BR vs R-mag 0 . ± .
31 -6.12 0.34 1.27e-01 a m = slope and c = intercept of α BR against time or Rmagnitude; r = Correlation coefficient; p = null hypothesisprobability Over five years of observing, they found no significant vari-ations in the R band; the average magnitudes in the B and R bands were 18 . ± .
02 and 16 . ± .
01, respec-tively. Wierzcholska et al. (2015) monitored the blazar 1ES0229+200 for 184 nights during 2007–2012. They found mod-est variations of ∼ α BR > . B and R wavebands.The complete photometric observation log of 1ES 0229+200is given in Table B1. The calibrated R − band IDV light curves of the extremeTeV blazar 1ES 0229+200 are shown in Figure C1 in Ap-pendix C. Visual inspection of these LCs indicates either novariation or very small fluctuations in the LCs over a fewhours. We statistically examined these LCs for IDV usingthe tests discussed in Sections 3.1 and 3.2. The results ofthe IDV analysis are given in Table 1. No significant IDVwas detected in any of these 14 LCs with enough data takenduring a single night to conduct such a test.The LTV-calibrated LCs of this source in optical B and R bands are shown in Figure 1, where we have plotted dailyaveraged magnitudes with respect to time. We shifted the B band LC by -0.5 magnitude to make the LTV pattern morevisible. The LTV LCs show variations in both the bands.The amplitudes of variability in B , and R bands are 49.2per cent, and 36.3 per cent, respectively. During our en-tire observing period, the blazar 1ES 0229+200 was in thebrightest state of R mag = 16 .
17 on 2018 December 16, whilethe faintest magnitude detected was R mag = 16 .
54 on 2017January 18. The average B magnitude was 17.36 and theaverage R magnitude was 16.39. To study the spectral variations of the TeV HBL 1ES0229+200 on longer timescales, we calculated the B − R color indices for the 22 nights with observations in both B and R bands. In the cases when there was more than oneobservation of the source during the same night, the aver-age values were used. We then estimated the mean spectralindices, h α BR i , of 1ES 0229+200 using the mean values of B − R color indices, h B − R i , as follows (Wierzcholska et al.2015), h α BR i = 0 . h B − R i log( ν B /ν R ) , (5)where ν B and ν R are the effective frequencies of B and R bands, respectively.The value of spectral indices ( α BR ) ranges from 1 . ± .
14 to 2 . ± .
09. The weighted mean of the optical spectralindex, α BR , was 2 . ± .
01. We plotted the spectral indicesof 1ES 0229+200 with respect to time and R − band magni-tude in the top and bottom panels of Figure 2, respectively.To search for any systematic variations in the spectral index,we fitted each panel in Figure 2 with a polynomial of order1. The results of the fits are given in Table 2. No systematictemporal variation is found in the optical spectral index nordoes the spectral index show any significant correlation with R − band magnitude. + The blazar 1ES 0414+009 ( α = 04 h m . s ; δ =+01 ◦ ′ ′′ ) was initially discovered as an X-ray source asso-ciated with a cluster of galaxies (Ulmer et al. 1980) and laterclassified as a BL Lac object (Ulmer et al. 1983). The red-shift of 1ES 0414+009 is z = 0 .
287 (Halpern et al. 1991), de-rived from the weak stellar absorption lines. McHardy et al.(1992) reported R = 16 .
64 and V = 17 .
21 from observationsduring 1986 and 1987. 1ES 0414+009 was monitored from
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A. Pandey et al.
JD (2450000.0+) C a li b r a t e d M a g n i t u d e A (V+0.5) B (V+0.5) A (R) B (R) A (I-0.5) B (I-0.5)
Figure 3.
LTV optical (
V RI ) light curves of 1ES 0414+009; they are shown in black ( V ); red ( R ) and blue ( I ), respectively. Thetelescopes used are written at the top of the figure. log(ν) (Hz) −26.0−25.5−25.0 l o g ( F ν ) ( e r g / c m / s e c / H z ) Figure 4.
Optical SEDs of 1ES 0414+009 in V , R , and I bands. R -mag ranging from 16.18 to 16.40. Kapanadze (2009)observed this BL Lac during 1997 October – 2007 Februaryon 56 nights. No IDV was found and the R brightness variedfrom 15.86 to 16.92 during this period. A similar brightnessvariation of ≤ α BR ∼ . − . V RI ) photometric observationsof this extreme blazar from 2016 December 30 to 2018 De-cember 29 during 12 nights and observed a total of 346 im-age frames. The observation log of 1ES 0414+009 is givenin Table B2.
The calibrated IDV LCs of the TeV blazar 1ES 0414+009in optical R − band are plotted in Figure C2. Using the sta-tistical tests discussed in Sections 3.1 and 3.2, we found noevidence of statistically significant IDV in any of the 8 nightswith 20 or more observations.The LTV LCs of 1ES 0414+009 in V , R , and I op-tical wavebands are shown in Figure 3, where nightly av-eraged calibrated magnitudes are plotted with respect totime. In the figure, the V , and I band LCs are shifted by+0 . − . V , R , and I bands, respectively. During our observing campaign, theblazar 1ES 0414+009 was detected in the brightest state of R mag = 15 .
66 on 2016 December 30, while the faintest mag-nitude observed was R mag = 16 .
61 on 2018 December 29,a range very similar to those reported during earlier mon-itoring of this source (Kapanadze 2009; Wierzcholska et al.2015). The mean magnitudes over the two years of new datapresented here were 16.62, 16.27, and 15.73 in the V , R , and I bands, respectively. To study spectral variations during our observing period,we extracted the optical (
VRI ) SEDs of the blazar 1ES0414+009 for 10 nights in which observations were per-formed in all three wavebands. For this, we first dereddenedthe calibrated V , R , and I magnitudes by subtracting theGalactic extinction, A λ , from them. The values of A λ weretaken from the NASA Extragalactic Database (NED ). Thedereddened calibrated magnitudes in each band were thenconverted into corresponding flux densities, F ν . The optical https://ned.ipac.caltech.edu/ MNRAS , 1–12 (2020)
Table 3.
Results of a first order polynomial fits to optical SEDs of TeV blazars 1ES 0414+009 and 1ES 2344+514.
Blazar name Observation date α a C a r a p a dd-mm-yyyy1ES 0414+009 30-12-2016 0 . ± .
08 -17.79 -0.99 1.08e-0118-01-2017 0 . ± .
05 -18.76 -0.99 7.03e-0219-01-2017 0 . ± .
09 -18.57 -0.98 1.20e-0121-02-2018 0 . ± .
06 -16.81 -1.00 6.29e-0201-11-2018 0 . ± .
02 -15.77 -1.00 1.77e-0202-11-2018 0 . ± .
13 -15.42 -0.98 1.20e-0115-12-2018 0 . ± .
09 -14.27 -0.99 6.92e-0216-12-2018 0 . ± .
09 -14.12 -0.99 6.93e-0228-12-2018 0 . ± .
08 -12.47 -1.00 5.32e-0229-12-2018 0 . ± .
04 -12.87 -1.00 3.09e-021ES 2344+514 06-09-2013 1 . ± .
09 -1.95 -1.00 4.40e-0220-10-2014 1 . ± .
18 -0.63 -0.99 7.69e-0206-11-2015 1 . ± .
22 -1.30 -0.99 9.90e-0212-11-2015 1 . ± .
23 -3.32 -0.98 1.13e-0116-11-2015 1 . ± .
34 -5.58 -0.96 1.85e-0124-10-2016 1 . ± .
07 -0.17 -1.00 3.02e-0225-10-2016 1 . ± .
11 -0.57 -1.00 4.62e-0226-10-2016 1 . ± .
08 -0.62 -1.00 3.38e-0208-11-2016 1 . ± .
13 0.17 -1.00 5.20e-0224-11-2016 1 . ± .
12 -1.78 -1.00 5.15e-0225-11-2016 1 . ± .
09 -1.90 -1.00 3.81e-0229-12-2016 1 . ± .
03 -2.28 -1.00 1.39e-0230-12-2016 1 . ± .
09 -1.34 -1.00 3.88e-0218-01-2017 1 . ± .
12 -3.31 -1.00 5.51e-0219-01-2017 1 . ± .
10 -3.07 -1.00 4.69e-0212-10-2017 1 . ± .
08 -3.74 -1.00 3.99e-0209-08-2018 1 . ± .
20 2.11 -0.99 7.28e-0210-09-2018 1 . ± .
26 -4.28 -0.98 1.23e-0111-09-2018 1 . ± .
12 -4.37 -1.00 6.00e-0212-09-2018 1 . ± .
63 -7.98 -0.86 3.39e-0105-10-2018 1 . ± .
17 -3.94 -0.99 7.76e-0211-10-2018 1 . ± .
10 -3.01 -1.00 4.39e-0215-10-2018 1 . ± .
10 -3.30 -1.00 4.38e-0231-10-2018 1 . ± .
05 -1.87 -1.00 2.23e-0201-11-2018 1 . ± .
10 -3.08 -1.00 4.63e-0202-11-2018 1 . ± .
08 -2.55 -1.00 3.50e-0202-12-2018 1 . ± .
01 -5.04 -1.00 6.46e-0315-12-2018 1 . ± .
12 -3.31 -1.00 5.38e-0216-12-2018 1 . ± .
11 -3.95 -1.00 5.01e-0228-12-2018 1 . ± .
14 -3.45 -1.00 6.02e-0229-12-2018 1 . ± .
14 -4.73 -0.99 6.58e-0206-06-2019 1 . ± .
04 -4.50 -1.00 2.04e-0207-07-2019 1 . ± .
07 -1.68 -1.00 2.98e-0231-08-2019 1 . ± .
08 -2.43 -1.00 3.31e-0223-10-2019 1 . ± .
03 -1.95 -1.00 1.14e-02 a α = spectral index and C = intercept of log( F ν ) against log( ν ); r = Correlation coefficient; p = null hypothesis probability SEDs of 1ES 0414+009, in log( ν ) vs log( F ν ) representation,are plotted in Figure 4.Since a simple power law ( F ν ∝ ν − α , where α is knownas the optical spectral index) provides a better fit to theblazar optical continuum spectra, we fitted each SED of 1ES0414+009 with a first order polynomial of the form log( F ν )= − α log( ν )+ C to get the values of optical spectral index.The results of the fits are given in Table 3. The values of the spectral indices ( α ) range from 0 . ± .
05 to 0 . ± .
08 andtheir weighted mean was 0 . ± .
01. We show the spectralindices of 1ES 0414+009 with respect to time and R − bandmagnitude in the top and bottom panels of Figure 5, respec-tively. We fitted each panel in Figure 5 with a first orderpolynomial to investigate any systematic variations in thespectral index. The results of the fits are given in Table 4.The optical spectral index increases with time and it also MNRAS000
01. We show the spectralindices of 1ES 0414+009 with respect to time and R − bandmagnitude in the top and bottom panels of Figure 5, respec-tively. We fitted each panel in Figure 5 with a first orderpolynomial to investigate any systematic variations in thespectral index. The results of the fits are given in Table 4.The optical spectral index increases with time and it also MNRAS000 , 1–12 (2020)
A. Pandey et al. O p t i c a l Sp e c t r a l I n d e x ( α ) O p t i c a l Sp e c t r a l I n d e x ( α ) Figure 5.
Variation of optical spectral index of 1ES 0414+009with respect to time (top) and R-magnitude (bottom).
Table 4.
Variation of optical spectral index, α , with respect totime and R-magnitude during our observing campaign of 1ES0414+009 and 1ES 2344+514. Blazar Parameter m a c a r a p a α vs time 4 . e − ± . e −
05 -3.31 0.90 3.32e-04 α vs R-mag 0 . ± .
06 -5.53 0.92 2.01e-041ES 2344+514 α vs time 4 . e − ± . e −
05 1.04 0.21 2.37e-01 α vs R-mag − . ± .
26 8.86 -0.33 5.66e-02 a m = slope and c = intercept of α against time or Rmagnitude; r = Correlation coefficient; p = null hypothesisprobability shows significant positive correlations with R − band mag-nitude; however, it is clear that these are not independentcorrelations. + α = 23 h m . s ; δ =+51 ◦ ′ . ′′ ) was identified as a BL Lac object at aredshift of z = 0 .
044 (Perlman et al. 1996). It was discov-ered as a TeV source with the Whipple Observatory γ -raytelescope, making it the third BL Lac object, after Mrk 421and Mrk 501, to be detected at VHE (Catanese et al. 1998).In the X-rays, rapid variability on a timescale of ∼ JD(2457400.0+) R - M a g n i t u d e Figure 6.
The variable optical R − band IDV LC of the extremeTeV blazar 1ES 2344+514. The observation date and the tele-scope code are mentioned at the top. been reported by Giommi et al. (2000). Miller et al. (1999)reported positive detection of microvariability in the opticalLCs of 1ES 2344+514. Xie et al. (2002) and Fan et al.(2004), separately, observed the blazar in 2000, but didn’tfind any significant IDV. A considerable IDV with thevariability amplitude ∆ R = 0 . ± .
16 on a timescale of∆ t = 4738 was claimed to have been detected on 2005December 28 by Ma et al. (2010). Optical monitoring of theTeV BL Lac 1ES 2344+514 were carried out for 19 nightsduring 2009–2010 by Gaur et al. (2012a). They found nosignificant variation on IDV timescales, but a brightnessvariation of ∆ R ∼ .
65 detected on LTV timescales.We observed the blazar 1ES 2344+514 for 38 nightsbetween 2013 September 6 and 2019 October 23. Duringthis period, we collected a total of 861 image frames in V , R , and I bands. The optical photometric observation log of1ES 2344+514 is given in Table B3. We statistically examined 14 optical R − band LCs of theextreme TeV blazar 1ES 2344+514 for intraday variationsusing the power-enhanced F -test and the nested ANOVAtest. The results of the analysis are shown in Table 1. Nostatistically significant IDV was detected by both the testsduring 13 of the 14 nights, the exception being 2018 October10. On that night, the amplitude of variability was a mod-est 2.26 per cent in the R − band LC. We plotted the onlylight curve showing variability in Figure 6 and all other nonvariable LCs in Figure C3 in Appendix C.The LTV LCs of the blazar 1ES 2344+514 for our en-tire monitoring period are shown in Figure 7, where dailyaveraged V , R , and I band calibrated magnitudes are plot-ted with respect to time. The amplitudes of variability in V , R , and I bands are 40.9 per cent, 37.4 per cent, and47.5 per cent, respectively. During our six year long observ-ing program, the blazar 1ES 2344+514 was observed in thebrightest state of R = 14 .
50 on 2016 November 9, while thefaintest magnitude detected was R = 14 .
88 on 2017 January
MNRAS , 1–12 (2020)
JD (2450000.0+) C a li b r a t e d M a g n i t u d e A (V+0.5)B (V+0.5) C (V+0.5)D (V+0.5) A (R)B (R) C (R)D (R) A (I-0.5)B (I-0.5) C (I-0.5)D (I-0.5)
Figure 7.
LTV optical (
V RI ) light curves of 1ES 2344+514; they are shown in black ( V ), red ( R ), and blue ( I ), respectively. Thetelescopes employed are written at the top of the figure.
18. The mean magnitudes were 15.25, 14.74, and 13.96 in V , R , and I bands, respectively. Similar to our analysis of 1ES 0414+009, we extracted the35 optical (
V RI ) SEDs of 1ES 2344+514 and present themin Figure 8. We fitted these SEDs with the logarithm of apower law. The results of the fits are given in Table 3. Theoptical spectral index values ( α ) range from 1 . ± .
63 to1 . ± .
20. The weighted mean optical spectral index was1 . ± .
01. The spectral indices are plotted against time inthe top panel and R − band magnitude in the bottom panelof Figure 9. The results of linear fits to each panel in Figure9 are given in Table 4. The optical spectral index does notshow significant systematic variation with time, nor does itcorrelate with R − band magnitude. In the present work we analyzed the optical photometricdata for three extreme TeV blazars collected using fourground-based telescopes during 2013–2019. In particular,we studied the flux and spectral variability properties ofthese blazars in optical wavebands on both IDV and LTVtimescales. We examined a total of 36 optical R − band IDVLCs of three blazars using two powerful and robust statis-tical methods: the power-enhanced F -test and the nestedANOVA test. Only one of these 36 IDV LCs exhibits statis-tically significant intraday variations: 1ES 2344+514 on 2018October 10, when the amplitude of variability was only 2.26per cent. So from our IDV analysis, we conclude that opticalLCs of these three TeV HBLs are either constant or shownominal variations on IDV timescales.Microvariations in blazar light curves are frequently de-tected and can be explained by the turbulent plasma flowingat relativistic speed in the jet (Marscher 2014; Pollack et al.2016). Different optical IDV behaviors have been observed in the LBLs and HBLs subclasses of blazars, with HBLs be-ing relatively less variable in optical bands on IDV timescales(e.g. Heidt & Wagner 1998; Gopal-Krishna et al. 2011). Ourresults are in good agreement with this conclusion. The pres-ence of a stronger magnetic field in HBLs could be respon-sible for the different optical microvariability behaviors ofLBLs and HBLs (Romero et al. 1999). The stronger mag-netic field in HBLs might prevent the development of smallfeatures (e.g. density inhomogeneities or bends) by Kelvin-Helmholtz instabilities in the bases of jets, which could oth-erwise interact with the shocks in jets to produce microvari-ability if its value is greater than the critical value B c givenby (Romero 1995) B c = (cid:2) πn e m e c (Γ − (cid:3) / Γ − , (6)where n e and m e are the density and rest mass of electron,respectively; Γ is the jet flow’s bulk Lorentz factor.On longer timescales, all three blazars exhibited fluxvariations in all observed optical wavebands, as would be ex-pected. Among these three blazars, 1ES 0414+009, showedthe maximum variation, ∆ R ∼ .
95, during our monitor-ing period, which is in accord with the brightness variationrange of ≤ R ∼ .
37 we detected in 1ES0229+200 is slightly larger than the variation of ∆ R ≃ . R ∼ .
38 during our six years-long monitoring period.This variation is smaller than the variation of ∆ R ∼ . MNRAS000
38 during our six years-long monitoring period.This variation is smaller than the variation of ∆ R ∼ . MNRAS000 , 1–12 (2020) A. Pandey et al. l o g ( F ν ) ( e r g / c m / s e c / H z ) Figure 8.
Optical SEDs of 1ES 2344+514 in V , R , and I bands. Camenzind & Krockenberger 1992; Gopal-Krishna & Wiita1992; Villata & Raiteri 1999).Using the B − R color indices we estimated the spectralindex, α BR , of 1ES 0229+200 and saw that it didn’t showany correlation with time or the R − band mag. Compar-ing our results with those obtained by Wierzcholska et al.(2015), we observed a relatively flat ( α BR ∼ . − .
2) op-tical continuum spectra of TeV blazar 1ES 0229+200. Wealso obtained the values of optical spectral indices for 1ES0414+009, and 1ES 2344+514 by fitting a single power lawin each of their optical (
V RI ) SEDs. The weighted meanvalues of α are 0.67, and 1.37 for 1ES 0414+009, and 1ES2344+514, respectively. The optical spectral index, α , of1ES 0414+009 shows a significant positive correlation with R − mag indicating a bluer (or flatter)-when-brighter (BWBor FWB) trend; however, no such correlation is seen for 1ES2344+514. The BWB trend is usually observed in BL Lac- ertae objects because their radiation is dominated by non-thermal jet emissions, while the redder (or steeper)-when-brighter (RWB or SWB) trend is more likely seen in FSRQs(e.g. Gu et al. 2006; Gaur et al. 2015a). However, no cleartrend of this type has been found by some authors (e.g.,B¨ottcher et al. 2009; Poon et al. 2009). The BWB trend de-tected 1ES 0414+009 can be explained in different ways(Fiorucci et al. 2004). In the two-component model, the op-tical emission of a blazar is produced by one variable andanother constant component where the variable componenthas a relatively flatter slope ( α var < α const ). Within a onecomponent synchrotron model the BWB trend could be ex-plained by the injection of fresh electrons resulting in anincreased flux as the energy distribution of fresh electrons isharder in comparison to the cooled ones (Kirk et al. 1998;Mastichiadis & Kirk 2002).Our optical photometric observations of three extreme MNRAS , 1–12 (2020) O p t i c a l Sp e c t r a l I n d e x ( α ) O p t i c a l Sp e c t r a l I n d e x ( α ) Figure 9.
Variation of optical spectral index of 1ES 2344+514with respect to time (top) and R-magnitude (bottom).
TeV blazars reveal that their light curves are either con-stant or show small fluctuations on IDV timescales. Howeveron longer timescales they exhibit variability in all opticalbands. The extreme blazar 1ES 0414+009 showed the BWBtrend, while the errors in the optical spectral indices of theother two blazars might have restricted us from measuringany trend, if present, in their spectral changes. To makeany firm conclusions about the general nature of the opticalvariability of extreme TeV blazars a much larger sample ofsuch blazars needs to be observed for even longer times, andpreferably in more optical bands.
ACKNOWLEDGEMENT
We are grateful to the anonymous referee for his/her helpfulcomments/suggestions. GD gratefully acknowledges the ob-serving grant support from the Institute of Astronomy andNAO Rozhen, BAS, via bilateral joint research project “GaiaCelestial Reference Frame (CRF) and fast variable astro-nomical objects” (the head is G. Damljanovic). This work isa part of the Projects no. 176011 “Dynamics and kinematicsof celestial bodies and systems”, no. 176004 “Stellar physics”and no. 176021 “Visible and invisible matter in nearby galax-ies: theory and observations” supported by the Ministry of Education, Science and Technological Development of theRepublic of Serbia.
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APPENDIX A: TELESCOPES ANDINSTRUMENTS DETAILSAPPENDIX B: OBSERVATION LOGSAPPENDIX C: R-BAND INTRADAY LIGHTCURVES
This paper has been typeset from a TEX/L A TEX file prepared bythe author. MNRAS , 1–12 (2020) Table A1.
Details of telescopes and instruments used
Code A B C DTelescope 1.3 m DFOT 1.04 m ST 1.4 m ASV 60 cm ASVCCD Model Andor 2K CCD Tektronics 1K CCD PyLoN CCD Andor iKon-L Apogee Alta E47Chip Size (pixels) 2048 × × × × × µm ) 13.5 × ×
24 20 ×
20 13.5 × × ) 18 ×
18 6 × . × . × . × . e − /ADU) 2.0 11.98 4.0 1.0 3.5Read-out Noise ( e − rms) 7.0 6.9 6.4 7.0 10Typical Seeing (arcsec) 1.2–2.0 1.4–2.6 1.2–2.1 1.0-1.5 1.0-1.5 Note. A: 1.3 m Devasthal Fast Optical Telescope (DFOT) at ARIES, Nainital, India. B: 1.04 m Sampuranand Telescope (ST) at ARIES,Nainital, India. C: 1.4 m Milankovi´c telescope at Astronomical Station Vidojevica (ASV), Serbia. D: 60 cm Nedeljkovi´c telescope atAstronomical Station Vidojevica (ASV), Serbia.
Table B1.
Observation log of optical photometric observationsof 1ES 0229+200. See Table A1 for telescope codes.
Obs. date Obs. start time Telescope Data pointsdd − mm − yyyy JD B,V, R,I24 − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − Table B2.
Observation log of optical photometric observationsof 1ES 0414+009. See Table A1 for telescope codes.
Obs. date Obs. start time Telescope Data pointsdd − mm − yyyy JD B,V, R,I30 − − − − − − − − − − − − − − − − − − − − − − − − MNRAS000
Obs. date Obs. start time Telescope Data pointsdd − mm − yyyy JD B,V, R,I30 − − − − − − − − − − − − − − − − − − − − − − − − MNRAS000 , 1–12 (2020) A. Pandey et al.
JD(2457400.0+) C a li b r a t e d R - M a g n i t u d e Figure C1.
Optical R − band IDV LCs of the extreme TeV blazar 1ES 0229+200. The observation date and the telescope code are givenin each plot. MNRAS , 1–12 (2020) JD(2457400.0+) C a li b r a t e d R - M a g n i t u d e Figure C2.
Optical R − band IDV LCs of the extreme TeV blazar 1ES 0414+009. The observation date and the telescope code are givenin each plot.MNRAS , 1–12 (2020) A. Pandey et al.
JD(2457400.0+) C a li b r a t e d R - M a g n i t u d e Figure C3.
Optical R − band IDV LCs of the extreme TeV blazar 1ES 2344+514. The observation date and the telescope code are givenin each plot. MNRAS , 1–12 (2020) Table B3.
Observation log of optical photometric observationsof 1ES 2344+514. See Table A1 for telescope codes.
Obs. date Obs. start time Telescope Data pointsdd − mm − yyyy JD B,V, R,I06 − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − MNRAS000