Optical Spectral Variability of the Very-High-Energy Gamma-Ray Blazar 1ES 1011+496
M. Boettcher, B. Hivick, J. Dashti, K. Fultz, S. Gupta, C. Gusbar, M. Joshi, A. Lamerato, T. Peery, D. Principe, A. Rajasingam, P.Roustazadeh, J. Shields
aa r X i v : . [ a s t r o - ph . H E ] O c t Accepted for publication in
The Astrophysical Journal
Optical Spectral Variability of the Very-High-Energy Gamma-RayBlazar 1ES 1011+496
M. B¨ottcher , B. Hivick , J. Dashti , K. Fultz , S. Gupta , C. Gusbar , M. Joshi , , A.Lamerato , T. Peery , D. Principe , , A. Rajasingam , P. Roustazadeh , J. Shields ABSTRACT
We present results of five years of optical (UBVRI) observations of the very-high-energy gamma-ray blazar 1ES 1011+496 at the MDM Observatory. We cali-brated UBVRI magnitudes of five comparison stars in the field of the object. Mostof our observations were done during moderately faint states of 1ES 1011+496with R & .
0. The light curves exhibit moderate, closely correlated variabil-ity in all optical wavebands on time scales of a few days. A cross-correlationanalysis between optical bands does not show significant evidence for time lags.We find a positive correlation (Pearson’s r = 0 .
57; probability of non-correlation P ( > r ) ≈ × − ) between the R-band magnitude and the B - R color in-dex, indicating a bluer-when-brighter trend. Snap-shot optical spectral energydistributions (SEDs) exhibit a peak within the optical regime, typically betweenthe V and B bands. We find a strong ( r = 0 .
78; probability of non-correlation P ( > r ) ≈ − ) positive correlation between the νF ν peak flux and the peakfrequency, best fit by a relation νF pk ν ∝ ν k pk with k = 2 . ± .
17. Such a cor-relation is consistent with the optical (synchrotron) variability of 1ES 1011+496being primarily driven by changes in the magnetic field.
Subject headings: galaxies: active — BL Lacertae objects: individual (1ES1011+496) — radiation mechanisms: non-thermal Astrophysical Institute, Department of Physics and Astronomy,Clippinger 251B, Ohio University, Athens, OH 45701, USA Institute for Astrophysical Research, Boston University,725 Commonwealth Ave., Boston, MA 02215, USA Rochester Institute of Technology, 84 Lomb Memorial Drive,Rochester, NY 14623, USA
1. Introduction
Blazars are the most violent class of active galactic nuclei, consisting of flat-spectrumradio quasars (FSRQs) and BL Lac objects (named after their historical prototype, BL Lacer-tae). They exhibit rapid variability down to time scales as short as a few minutes (Aharonian et al.2007; Albert et al. 2007a). Their observed flux is dominated by a non-thermal continuumexhibiting two broad spectral bumps: A low-frequency bump from radio to UV – X-rayfrequencies, and a high-frequency component from X-ray to γ -rays. In the framework ofrelativistic jet models, the low-frequency (radio – optical/UV) emission from blazars is in-terpreted as synchrotron emission from nonthermal electrons in a relativistic jet. The high-frequency (X-ray – γ -ray) emission could either be produced via Compton upscattering oflow frequency radiation by the same electrons responsible for the synchrotron emission (lep-tonic jet models; for a recent review see, e.g., B¨ottcher 2007), or due to hadronic processesinitiated by relativistic protons co-accelerated with the electrons (hadronic models, for arecent discussion see, e.g., M¨ucke & Protheroe 2001; M¨ucke et al. 2003).To date, about 30 blazars have been detected in very high energy (VHE, >
100 GeV) γ -rays with ground-based air ˇCerenkov telescope facilities . Most of these TeV blazars belongto the sub-class of high-frequency peaked BL Lac objects (HBLs). They are characterized bya synchrotron spectrum peaking at frequencies ν pksy & Hz, i.e., in the UV or X-ray range,and γ -ray peaks at ν pk γ & Hz, i.e., typically beyond the Fermi energy range (Abdo et al.2010). VHE γ -rays from sources at cosmological distances can be absorbed by the extra-galactic background light (EBL) due to γγ pair production (e.g., Dwek & Krennrich 2005;Stecker et al. 2006; Franceschini et al. 2008; Gilmore et al. 2009; Finke et al. 2010). Hence,the EBL absorption of distant VHE γ -ray sources may provide a probe of the spectrum andcosmological evolution of the EBL, which is notoriously difficult to measure directly due tobright foregrounds.The BL Lac object 1ES 1011+496 was detected as a VHE γ -ray emitter by MAGIC in thespring of 2007 (Albert et al. 2007b). Follow-up optical spectroscopy at the MMT confirmedthe previously uncertain redshift of z = 0 . ± .
002 for this source (Albert et al. 2007b).At the time of its VHE detection, 1ES 1011+496 was the most distant VHE γ -ray sourceknown with a well-determined redshift, and to date it still ranks among the top five. It mighttherefore offer a prime opportunity for studying the EBL through its absorption signature atVHE γ -rays. However, in order to exploit this opportunity, a thorough understanding of theintrinsic spectral energy distribution (SED) of the source, constrained through observationsat lower (radio through GeV γ -ray) frequencies, is essential. For this purpose, we present For a complete list of VHE γ -ray sources see http://tevcat.uchicago.edu γ -ray detection of 1ES 1011+496 was triggered by a large optical outburstof the source in 2007 March (Albert et al. 2007b). This object is regularly monitored in theoptical R-band through the Turku blazar monitoring program led by Kari Nilsson, with the1.03 m telescope of the Tuorla Observatory in Finland, as well as the 35 cm telescope of theKVA Observatory on La Palma, Canary Islands, Spain . Apart from this, 1ES 1011+496 hasso far received rather little attention by optical observers, and its optical spectral variabilityhas remained unexplored. The object has been observed in X-rays by Einstein (Elvis et al.1992) and more recently, Swift/XRT (Abdo et al. 2010). In all observations, it shows a steepX-ray spectrum, indicating the dominance of synchrotron emission in the X-ray regime.1ES 1011+496 may also be associated with the EGRET γ -ray source 3EG J1009+4855(Hartman et al. 1999), although this association is uncertain (Sowards-Emmerd et al. 2003).The object is clearly detected by Fermi and listed in the Fermi 3-month catalogue as thesource 0FGL J1015.2+4927. The Fermi data reveal a rising νF ν spectrum (i.e., photon indexΓ <
2) in the 100 MeV – 30 GeV energy range (Abdo et al. 2010).Optical/UV observations were performed by Swift/UVOT in May 2008 (Abdo et al.2010), during the rising phase of an optical outburst similar to the one triggering the MAGICdiscovery observations in 2007. During those observations, Swift/UVOT measured a rising νF ν optical/UV continuum spectrum. This, together with the hard Fermi spectrum, justifiedthe classification of 1ES 1011+496 as an HBL.Over the course of ∼ § §
3. We performed a cross-correlation analysis between the variabilitypatterns in different optical bands, which we describe in §
4. We summarize and discuss ourresults in § Daily light curves are posted at http://users.utu.fi/kani/1m/index.html
N W8.49’
Fig. 1.— Finding chart (R band) of the field around 1ES 1011+496 with 5 comparison stars 5 –
2. Observations, data reduction, and light curves
Optical (UBVRI) data were collected at the 1.3m McGraw-Hill Telescope of the MDMObservatory on the south-west ridge of Kitt Peak, Arizona, during 9 ∼ phot routine within the IRAF package DAOPHOT. Following standardprocedures for IRAF photometric calibrations (Massey & Davis 1992), we used the routine fitparams to solve the transformation equations to evaluate the calibrated, physical magni-tudes of our standard stars. The resulting calibrated magnitudes are listed in Table 1.After calibration of our comparison stars, we extracted instrumental magnitudes of thecomparison stars and 1ES 1011+496 using the phot routine within the DAOPHOT packageof IRAF, and converted instrumental to physical magnitudes assuming that the differencebetween instrumental and physical magnitudes is the same for the object and all comparisonstars. The resulting light curves for all observing runs combined are displayed in Fig. 2.The figure illustrates that for most of our runs, the object was in a rather faint optical statewith R & .
0, and shows very moderate variability.Table 1. Calibrated Magnitudes of Comparison Stars in Fig. 1
Star U B V R I1 15 . ± .
010 14 . ± .
002 13 . ± .
002 13 . ± .
002 12 . ± . . ± .
050 15 . ± .
003 14 . ± .
002 14 . ± .
002 13 . ± . . ± .
043 16 . ± .
007 15 . ± .
003 15 . ± .
005 15 . ± . . ± .
018 14 . ± .
002 14 . ± .
002 14 . ± .
002 13 . ± . . ± .
018 16 . ± .
008 15 . ± .
003 15 . ± .
005 15 . ± . U M a g B M a g V M a g R M a g I M a g Fig. 2.— Multi-band (UBVRI) light curves of 1ES 1011+496 from our MDM observations 7 – U M a g B M a g V M a g R M a g I M a g Fig. 3.— Multi-band (UBVRI) light curves of 1ES 1011+496 during the high activity statein April – May 2008 8 –However, we did observe substantial variability during our two observing runs in 2008April 9 – 14 and May 10 – 13. This was during the rising phase of a major outburst thatpeaked later that year. Unfortunately, to our knowledge, there are no observations availablecovering the peak of that outburst. During our observations we found a maximum R-bandbrightness of R ∼ .
8. The object later exceeded R peak < . . Fig. 3 shows the multi-band light curves from our 2008 April and May runs. They exhibit variability on time scalesof a few days, but no evidence for intraday variability. The variability in all optical bandsappears well correlated. This correlation will be investigated in more detail in §
3. Optical spectral variability
In order to test whether the variability discussed in the previous section is associatedwith spectral changes, we first calculated B - R color indices for any pair of B and R mag-nitudes measured within 15 minutes of each other. The resulting color-magnitude diagram(R magnitude vs. B - R color) is shown in Figure 4. Error bars on B - R are calculated viastandard error progatation, i.e., σ B − R = p σ R + σ B . The data clearly indicate color variabil-ity. A fit of a constant B - R color as a function of R magnitude results in χ ν = 2 .
86. A fitof a linear correlation results in a marginally acceptable χ ν = 1 .
36. A correlation analysis ofthe color-magnitude data set yields a Pearson’s correlation coefficient of r = 0 .
57, which isgenerally interpreted as a weak positive correlation between color and magnitude. In orderto quantify the probability of such a correlation coefficient resulting from an uncorrelateddata set, we performed Monte-Carlo simulations of 1 billion randomly produced, uncorre-lated data sets, extending over similar spreads of values, and with the same number of datapoints as our observational data set. The Pearson’s correlation coefficient for each set wasevaluated, and from the entire ensemble, the probability of a correlation coefficient | r | > x for values of 0 < x < r = 0 .
57 in a data setwith the characteristics of our R vs. B - R data is P ( | r | ≥ . ≈ × − , indicating, infact, a highly significant correlation. The observed correlation corresponds to a bluer-when-brighter trend, as observed in most BL Lac objects. This is likely to reflect the dynamics ofthe non-thermal synchrotron emission from the jet dominating in the optical regime.In order to investigate spectral changes in the optical continuum in more detail, weextracted UBVRI SEDs for all sequences of magnitudes measured within 15 minutes of eachother. For this purpose, the magnitudes were de-reddened using the Galactic extinction http://users.utu.fi/kani/1m/index.html B - R χ ν = . χ = 2.86 Fig. 4.— Color-magnitude diagram for 1ES 1011+496. The data show significant colorvariability. A positive linear correlation (bluer when brighter) is indicated by a Pearson’scorrelation coefficient of r = 0 .
57, with a probability for non-correlation of P ( > r ) ≈ × − . 10 – ν [Hz] ν F ν [ J y H z ] Fig. 5.— Snap-shot UBVRI SEDs of 1ES 1011+496. The SEDs reveal a νF ν peak typicallybetween the B and V band and suggest a positive correlation between νF ν peak flux andpeak frequency. 11 –coefficients as given in the NASA Extragalactic Database and converted to νF ν fluxes. Arepresentative sample of the resulting SEDs is plotted in Figure 5. The SEDs all exhibita νF ν peak in the optical regime, typically between the V and B bands. They suggest apositive trend of increasing νF ν peak flux with increasing peak frequency, in accordance withthe weak B - R vs. R correlation found above. We tested this hypothesis further by fittingall optical SEDs with a simple parabolic shape to determine the peak frequency, ν peak , andthe peak flux, νF pk ν . The best fit values for our entire data set are plotted in Fig. 6.The νF ν peak flux and peak frequencies are clearly correlated, with Pearson’s r = 0 . P ( > r ) ≈ − . The best linear regression fit tothe logarithms of the νF pk ν and ν pk values yields a power-law correlation νF pk ν ∝ ν k pk with k = 2 . ± .
17. A possible interpretation of this synchrotron peak shift will be discussed in §
5. A visual inspection of Figure 6 seems to suggest a steepening of the peak flux vs. peakfrequency dependence towards high peak frequencies (and peak fluxes). However, whenrestricting the regression to high frequencies (e.g., ν pk & . × Hz), the correlationbetween peak flux and peak frequency vanishes, therefore not allowing for a quantificationof a possible change of the correlation slope towards high frequencies.
4. Cross-correlation analysis
A visual inspection of the light curves in Fig. 2 and 3 suggests that the variability inall wavebands is closely correlated. In order to corroborate this finding, we performed aDiscrete Correlation Function (DCF, Edelson & Krolik 1988) analysis among all our lightcurves. Figure 7 shows a typical example of the resulting DCF between the B and R bandlight curves. The DCFs between the light curves of all bands peak at values near 1, indicatinga close correlation between all optical bands. This is expected if the optical continuum isdominated by nonthermal synchrotron emission of the same relativistic electron population.Time lags between different frequency bands could potentially serve as a diagnostic of themagnetic field strength in the emitting region (modulo the Doppler factor, see, e.g., B¨ottcher2007). We therefore fitted the DCFs with an asymmetric Gaussian to determine possibleinter-band delays through the fitted peak of the DCF. However, any lags indicated by ourDCFs are all either consistent with 0 or at the ∼ σ level. Furthermore, our sequentialdata-taking process introduces an artificial “lag” of up to ∼
10 min, and none of the lagsfound through the DCF analysis are larger than that. Therefore, we conclude that we did
12 – ν pk [Hz]1.2e121.4e121.6e121.8e122.0e12 ν F ν pk [ J y H z ] ν pk2 Fig. 6.— Best-fit peak frequency vs. peak νF ν flux for the entire data set. The data show asignificant correlation ( r = 0 . P ( > r ) ≈ − ), best fit by a dependence νF pk ν ∝ ν k pk with k = 2 . ± .
17. The dashed line indicates a putative correlation with index k = 2. 13 – -2 -1 0 1 2 τ [hr]00.51 D C F Fig. 7.— Discrete Correlation Function between the B and R bands. A positive τ wouldindicate a hard lag. The DCF has been fitted with an asymmetric Gaussian. The best-fitpeak delay is τ pk = ( − . ± .
2) min. 14 –not detect any inter-band time lags.
5. Summary and Discussion
We have presented an analysis of data from 5 years of observations of the BL Lac object1ES 1011+496 at the 1.3m McGraw-Hill Telescope of the MDM Observatory. We foundmoderate variability on a time scale of several days throughout most of our observations.The variability at all (UBVRI) optical bands is well correlated with no detectable time lagsbetween them. The snap-shot SEDs during our observations showed a synchrotron peakwithin the optical range, typically between the V and B bands. The B - R color is correlated( r = 0 . P uncorr ( > r ) ≈ × − ) with the R-band magnitude, indicating a bluer-when-brighter trend. Such a trend is observed in many BL Lac objects, where the optical emissionis strongly dominated by synchrotron emission from the jet. We note that the oppositebehaviour has been found in several quasar-type blazars, where a slowly variable Big BlueBump, signaling a contribution due to a luminous accretion disk, dilutes the continuumvariability at the blue end of the optical spectrum (e.g., Raiteri et al. 2008).An analysis of the location of the synchrotron peak within the optical regime reveals apeak shift characterized by νF pk ν ∝ ν k pk with k = 2 . ± .
17, consistent with a ν scaling.There is a range of possible causes for the optical (and multi-wavelength) variabilityof blazar emission. These include changes in the Doppler factor (e.g., caused by a bendingjet), injection of a new relativistic particle population into the jet (plausibly caused bya shock), a changing acceleration efficiency (changing characteristic Lorentz factors of theradiating electrons and possibly a change of the spectral index of the non-thermal electrondistribution), and/or a change of the magnetic field. In a realistic scenario, several of theseeffects might be at work at the same time to produce the observed blazar variability. However,one can make simple predictions concerning the shift (in frequency and νF ν peak flux) of thesynchrotron peak for at least three cases: A changing Doppler factor, a changing magneticfield, and a change of the characteristic (peak) electron Lorentz factor (leaving all otherparameters of the emission region unchanged).The peak frequency of the synchrotron spectrum is related to a peak in the electronspectrum at a characteristic Lorentz factor γ p , the magnetic field B and the Doppler factor D = (Γ [1 − β Γ cos θ obs ]) − , where Γ = (1 − β ) − / is the bulk Lorentz factor of the emissionregion and θ obs is the angle between the line of sight towards Earth and the direction ofmotion (the jet axis), through 15 – ν pk ∝ γ p B D (1)The νF ν flux at the synchrotron peak is related to those quantities through νF pk ν ∝ γ p B D (2)From equations 1 and 2, we see that if the variability is dominated by a changing Dopplerfactor, one would expect a synchrotron peak shift as νF pk ν ∝ ν . A change solely in thecharacteristic electron Lorentz factor, γ p , would result in a synchrotron peak shift as νF pk ν ∝ ν pk , while a change in only the magnetic field yields the behaviour νF pk ν ∝ ν .Therefore, we conclude that the synchrotron peak shift found in our data set is consistentwith the variability being dominated by a changing magnetic field. However, as pointed outabove, we need to caution that such a change in the magnetic field might realistically alsoimpact the shape of the electron distribution, primarily through a changing synchrotroncooling time scale. Clearly, more sophisticated analyses of this synchrotron peak shift areneeded, but are beyond the scope of this paper.We note that the shape of the optical SEDs found in all of our observations contradictsthe optical-UV spectrum observed by Swift/UVOT on 2008 May 2 and 8, which indicatesa rising slope throughout the optical regime (Abdo et al. 2010). However, most of ourobservations were taken during moderately faint states of the source, while the Swift/UVOTspectrum corresponds to a bright state, similar to the major optical flare that triggered theMAGIC detection in 2007. Given the trend of the synchrotron peak shift which we foundin our data, it is conceivable that the Swift/UVOT observations correspond to an extremecase of a high synchrotron peak frequency, in accord with a very high optical flux.This work was supported by NASA through Chandra Guest Observer Program awardGO8-9100X, XMM-Newton Guest Observer Program awards NNX08AD67G and NNX09AV45G,and Fermi Guest Investigator Program award NNX09AT82G. REFERENCES
Abdo, A. A., et al., 2010, ApJ, 716, 30Aharonian, F., et al., 2007, ApJ, 664, L71Albert, J., et al., 2007a, ApJ, 669, 862 16 –Albert, J., et al., 2007b, ApJ, 667, L21B¨ottcher, M., 2007, ApSS, 309, 95Dwek, E., & Krennrich, F., 2005, apJ, 618, 657Edelson, R. A., & Krolik, J. H., 1988, ApJ, 333, 646Elvis, M., Plummer, D., Schachter, J., & Fabbiano, G., 1992, ApJS, 80, 257Finke, J. D., Razzaque, S., & Dermer, C. D., 2010, ApJ, 712, 238Franceschini, A., Rodighiero, G., & Vaccari, M., 2008, A&A, 487, 837Gilmore, R. C., et al., 2009, MNRAS, 399, 1694Hartman, R. C., et al., 1999, ApJS, 129, 79Landoldt, A. U., 1992, AJ, 104, 340Massey, P., & Davis, L. E., 1992, “A User’s Guide to Stellar CCD Photometry with IRAF”, http://iraf.net/irafdocs/daophot2/
M¨ucke, A., & Protheroe, R. J., 2001, Astropart. Phys., 15, 121M¨ucke, A., Protheroe, R. J., Engel, R., Rachen, J. P., & Stanev, T., 2003, Astropart. Phys.,18, 593Raiteri, C. M., et al., 2008, A&A, 491, 755Sowards-Emmerd, D., Roman, R. W., & Michelson, P. F., 2003, ApJ, 590, 109Stecker, F. W., Malkan, M. A., & Scully, S. T., 2006, ApJ, 648, 774