We present results of two-colour photometry with high time resolution of the
violently variable BL Lac object AO 0235+164. We have found extreme intranight
variability with amplitudes of ~ 100 % over time scales of 24 hours. Changes of
0.5 magnitudes in both R and V bands were measured within a single night, and
variations up to 1.2 magnitudes occurred from night to night. A complete
outburst with an amplitude ~ 30 % was observed during one of the nights, while
the spectrum remained unchanged. This seems to support an origin based on a
thin relativistic shock propagating in such a way that it changes the viewing
angle, as recently suggested by Kraus et al. (1999) and Qian et al. (2000).Comment: 4 pages, 3 figures, to appear in Astronomy & Astrophysics (Letters

Cellone, S. A.

Combi, J. A.

Romero, G. E.

Astronomy and Astrophysics

English

arXiv

We present results of two-colour photometry with high time resolution of the violently variable BL Lac object AO 0235+164. We have found extreme intranight variability with amplitudes of ~ 100% over time scales of 24 hours. Changes of 0.5 magnitudes in both R and V bands were measured within a single night, and variations up to 1.2 magnitudes occurred from night to night. A complete outburst with an amplitude ~ 30% was observed during one of the nights, while the spectrum remained unchanged. This seems to support an origin based on a thin relativistic shock propagating in such a way that it changes the viewing angle, as recently suggested by Kraus et al. (1999) and Qian et al. (2000).Fil: Combi, Jorge Ariel. Provincia de Buenos Aires. Gobernación. Comisión de Investigaciones Científicas. Instituto Argentino de Radioastronomía. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto Argentino de Radioastronomía; ArgentinaFil: Cellone, Sergio Aldo. Universidad Nacional de la Plata. Facultad de Ciencias Astronómicas y Geofísicas; ArgentinaFil: Romero, Gustavo Esteban. Provincia de Buenos Aires. Gobernación. Comisión de Investigaciones Científicas. Instituto Argentino de Radioastronomía. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto Argentino de Radioastronomía; Argentin

Combi, Jorge Ariel

Cellone, Sergio Aldo

Romero, Gustavo Esteban

CONICET Digital

Astron. Astrophys. 360, L47–L50 (2000) ASTRONOMYANDASTROPHYSICSLetter to the EditorExtreme intranight variability in the BL Lacertae object AO 0235+164G.E. Romero1?,??, S.A. Cellone2,??, and J.A. Combi1,?,??1 Instituto Argentino de Radioastronomı́a, C.C.5, (1894) Villa Elisa, Bs. As., Argentina2 Facultad de Ciencias Astron´ micas y Geof́ısicas, UNLP, Paseo del Bosque, 1900 La Plata, ArgentinaReceived 17 July 2000 / Accepted 27 July 2000Abstract. We present results of two-colour photometry withhigh time resolution of the violently variable BL Lac object AO0235+164. We have found extreme intranight variability withamplitudes of∼ 100% over time scales of 24 hours. Changes of0.5 magnitudes in bothR andV bands were measured withina single night, and variations up to 1.2 magnitudes occurredfrom night to night. A complete outburst with an amplitude∼30% was observed during one of the nights, while the spectrumremained unchanged. This seems to support an origin based on athin relativistic shock propagating in such a way that it changesthe viewing angle, as recently suggested by Kraus et al. (1999)and Qian et al. (2000).Key words: galaxies: BL Lacertae objects: individual: AO0235+164 – galaxies: photometry – radiation mechanisms: non-thermal1. IntroductionThe BL Lac object AO 0235+164 is one of the most intensivelystudied blazars. It is a very compact source (e.g. Jones et al.1984, Chu et al. 1996) which ejects superluminal componentswith apparent velocities up to∼ 30c (see Fan et al. 1996 and ref-erences therein). The object presents emission lines at a redshiftof z = 0.94 and foreground absorption features atz = 0.85 andz = 0.524, which have led several authors to study a gravita-tional microlensing scenario for this source (e.g. Stickel et al.1988, Abraham et al. 1993).The historical lightcurves at different optical frequenciesof AO 0235+164 have been recently compiled by Fan & Lin(2000). This source is one of the most optically variable BL Lacobjects (e.g. Webb et al. 1988). Very rapid changes of its fluxdensity have been reported across the entire electromagneticspectrum. Quirrenbach et al. (1992), Romero et al. (1997), andKraus et al. (1999) have detected intraday radio variability. AtSend offprint requests to: G.E. Romero? Member of CONICET?? Visiting Astronomer, Complejo Astrońomico El Leoncito, oper-ated under agreement between CONICET and the National Universi-ties of La Plata, Ćordoba, and San Juan.optical wavelengths, Rabbette et al. (1996), Heidt & Wagner(1996), and Noble & Miller (1996) have found rapid changesof a few tenths of a magnitude within a single night. Schrammet al. (1994) reported extreme optical microvariability (1.6 magin 48 hours), although not with high time resolution. At highnergies, AO 0235+164 has also displayed significant variability(see Hartman et al. 1999).In this Letter we present the results of two-colour opticalCCD photometry with high time resolution of AO 0235+164carried out with the 2.15-m CASLEO telescope. Our observa-tions revealed one of the most extreme variability events everobserved at optical frequencies in any blazar. The observationalresults can be used to shed some light on the mechanisms thatgenerate the short-term flux changes in this puzzling object.2. Observations and data analysisThe observations were carried out during 6 consecutive nightsin November 1999 with the 2.15-m CASLEO telescope at ElLeoncito, San Juan, Argentina, as part of an extensive programof optical monitoring of gamma-ray blazars. A cryogenically-cooled CCD camera with an excellent cosmetics Tek-1024 chip(with a read-out-noise of 9.6 electrons and a gain of 1.98 elec-trons adu−1) was used. Originally, several blazars were sched-uled for observation with a JohnsonV filter, but when it be-came obvious that A0 0235+164 was undergoing extraordinarybrightness variations we focused only on this source and in-corporated observations with a Kron-CousinsR filter in orderto achieve two-colour photometry and a high time resolution.These conditions were obtained from the third to the last night.Typical integration times were∼ 100 s. The CCD frames werebias subtracted and flat-fielded using dome-flats and twilightframes in order to correct the pixel-to-pixel variations of theCCD. Standard stars selected from Landolt (1992) were alsoobserved each night for magnitude calibration.Each target frame contained, in addition to the BL Lac ob-ject, several stars which were used for comparison and controlpurposes following the procedures described in detail in Romeroet al. (1999). Data reduction was made with the IRAF softwarepackage. The aperture routine APPHOT was applied to performthe differential photometry, and lightcurves were calculated asLETTERL48 G.E. Romero et al.: Extreme variability in AO 0235+164target minus comparison star, as usual in this kind of studies (e.g.Carini et al. 1991). In order to avoid possible spurious variabil-ity introduced by seeing fluctuations that could affect the blazarbut not the point-like stars, we have followed the recommenda-tions of Cellone et al. (2000) and selected an 8-pixel (6.5 arcsec)radius aperture. Checks using plots of the PSF FHWM vs. timeconfirmed that the aperture was correctly selected, and that themagnitude changes were real and not originated in varying lightcontamination from the weak foreground galaxies induced bysmall changes in seeing conditions.The standard deviation (σ) of comparison minus controlstars was adopted as a measurement of the observational er-rors. The average variability errors wereσR ≈ 0.011 mag andσV ≈ 0.012 mag, whereas the individual photometric errorswere typically∼ 0.004 mag and∼ 0.005 mag for theR andVbands, respectively.Absolute calibration to the standard system was performedfor AO 0235+164 and the field stars, tying all nights to the best(photometric) one. The expected zero-point error is0.01 magfor both R andV bands, although internal consistency in theV − R colours should be better than this. The blazar’s stan-dard magnitudes were corrected for Galactic extinction adopt-ing EB−V = 0.08 (Schlegel et al. 1998), and absolute fluxdensities in both bands were calculated using the calibrationsof Bessell (1979). Tables with the resulting values of the fluxdensities of AO 0235+164 from November 3 to November 81999 are available upon request from G.E. Romero1.Spectral indices for the blazar were computed fitting apower-law spectrumF ∝ να and errors adequately propagatedfrom the flux values. Spectral index variability during the ob-servation span was additionally controlled using the scatter ofthe spectral indices obtained for the field stars|σα| ∼ 0.074.3. ResultsThe complete differentialV -band lightcurve of AO 0235+ 164is presented in Fig. 1 along with the corresponding curve forthe stellar comparison. The confidence of the variability is ata 26.3σ level. For theR band the confidence is even higher:30.9σ. In Table 1 we present a summary of the results obtainedfrom the observations; from left to right we list the observingband, the UT date, the variability errorσ obtained from thescatter of the field stars, the timescale of the variability definedastv = ∆F (dF/dt)−1, the confidence levelC of the observedvariability estimated as the ratio between the scatter in the BLLac lightcurve and the observational errors:C = σBl/σ, thevariability amplitudes defined as in Heidt & Wagner (1996):Y = 100[(4F )2 − 2σ2]1/2/ 〈F 〉 (where〈F 〉 is the averagedflux density), the fractional variability index defined asFV =4F/Fmin, and, finally, the average flux density in mJy.The lightcurve shown in Fig. 1 presents one of the mostextreme forms of optical variability reported in the literaturesince the introduction of CCD cameras. Between the fourth andthe fifth nights the source brightness changed by∼ 1.2 mag in1 romero@irma.iar.unlp.edu.ar0 1 2 3 4 5 62.01.81.61.41.21.00.80.60.40.20.0-0.2-0.4   AO 0235+164 Comparison + 1 magDifferential magnitudeJD 2451485+Fig. 1. Differential lightcurve in theV band for AO 0235+164 andcomparison star during 6 consecutive nights in November, 1999.Table 1. Results of optical intranight variability observations of AO0235+164 (entire campaign)Band UT Date σ tv C Y FV 〈F 〉(mag) (hours) (%) mJyR Nov 5-8, 1999 0.011 26.8 30.9 99.3 1.74 0.872V Nov 3-8, 1999 0.012 26.8 26.2 109.4 2.04 0.563about 24 hours. Translated to flux density this means a changeof about 100%: the source doubled its flux density in one day.During the fifth night there was a variation of∼ 0.5 mag insix hours. This extreme behaviour is even more violent than thevery strong outbursts observed in 1990 and 1991 by Noble &Miller (1996) in this same object, when intranight amplitudesof ∼ 0.25 mag were registered.In Fig. 2 we show the spectral index evolution during theobservations (starting on the third night when theR filterwas incorporated). The average index is〈α〉 = −3.04 (i.e.,〈(V − R)0〉 = 0.68), a steep value in agreement with the veryred colour indices quoted by V́eron-Cetty & V́eron (2000).Some day-to-day variability seems to be present in the spec-tral index, but the confidence level (C = 1.65) is too low as tobe conclusive. A Pearson’s correlation analysis yields values ofr = 0.87 for the linear correlation between both bands and thespectral index, whereas it isr ≈ 0.5 for the stars, as expected justfrom correlated observational errors. This could mean a trend inthe sense that the source becomes brighter when the spectrumgets harder, as observed in other blazars (e.g. Romero et al. 2000LETTERG.E. Romero et al.: Extreme variability in AO 0235+164 L492.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0-3.8-3.6-3.4-3.2-3.0-2.8-2.6-2.4-2.2  Spectral indexJD 2451485+Fig. 2. Spectral index during the observations.and references therein). But, we emphasize, the confidence levelis too low as to allow any conclusion in this respect.A correlation analysis using an interpolated correlationfunction (ICF) gives a time lag of just 14.7 minutes for theentire lightcurves, in the sense that variations appear first athigher frequencies. However, the associated error, determinedas in Gaskell & Peterson (1987), is 31.5 minutes, hence thevariations can be considered as simultaneous at both bands.On the night of November 5, a complete outburst was ob-served. The flux density in theV -band changed fromFmin ≈0.46 mJy toFmax ≈ 0.63 mJy at a rate of〈dF/dt〉 ≈ 0.04mJy/hr and returned to its original value in 6 hours. This meansa fluctuation with an amplitudeY ≈ 30.4% in theV band. IntheR band the variation was similar:Y ≈ 29.4%, with identi-cal timescale. The ICF analysis is consistent with no time lag atall: 〈lag〉 = 1.3±5.7 minutes. During the outburst, the spectralindex did not undergo significant changes; its average value was〈α〉 = −3.11, with a scatter ofσ = 0.09 for the entire night.In Fig. 3 we show the lightcurves in both bands for this event aswell as the spectral index behaviour.4. DiscussionSeveral models have been proposed to explain the rapid variabil-ity of AO 0235+164. Interstellar scintillation is only relevant atcm-wavelengths (e.g. Romero et al. 1997, Kraus et al. 1999), butrelativistic shocks and gravitational microlensing can produceimportant changes in the flux density at optical wavelengths.Accretion disk instabilities are also often invoked as a source ofoptical variations in AGNs (e.g. Wiita 1996), but any accretiondisk model would have enormous difficulties to explain out-2.50 2.55 2.60 2.65 2.70 2.75 2.80 2.850.40.50.60.70.80.91.0   V Band R BandFlux density (mJy)2.50 2.55 2.60 2.65 2.70 2.75 2.80 2.85-3.8-3.6-3.4-3.2-3.0-2.8-2.6-2.4  JD 2451485+Spectral indexFig. 3. Outburst observed on November 5, 1999.bursts of about 100% in a few hours. Extremely large and rapidoptical variations require the introduction of relativistic effectsin order to be explained (e.g. Marscher 1992). These effectsare present if an important part of the emission received by theobserver is originated in a superluminal feature, such as a thinrelativistic shock propagating down the jet of the source. Rapidvariability, then, arises when the shock interacts with density in-homogeneities or turbulent features in the flow (e.g. Marscher1990, Qian et al. 1991, Romero 1995). The shock evolution,however, is not achromatic: radiative losses make the sourcebrighter when the spectrum flattens (e.g. Marscher 1997). Thisis not the case for the rapid outburst shown in Fig. 3, althoughthe evolution of the source during the entire observational cam-paign could be consistent with this behaviour (as we mentionedabove the confidence on this point is not too high). The rapidchange observed on November 5 1999, on the contrary, suggestsa geometric origin because of its lack of spectral variability andtime symmetry.Kraus et al. (1999) outlined a precessing jet model for AO0235+164 where the trajectory of the shocked components is de-t rmined by collimation in the magnetic field of the perturbedbeam, which can develop an helical configuration. This modelhas been recently expanded by Qian et al. (2000) in order toi clude aberration effects. If the relativistic source changes theviewing angle due to its propagation through the helical path,LETTERL50 G.E. Romero et al.: Extreme variability in AO 0235+164the Doppler factor should vary, introducing important changesin the observed flux density. For a Lorentz factorΓ = 25 (Qianet al. 2000), a small change from0◦ to 1◦ in the direction of theshock with the line of sight would result in a change from 50 to42 in the Doppler factor, and, consequently, in a flux variationof ∼ 70% for the emitting component at all optical wavebands.If the contribution from this component represents a significantfraction of the total flux density, changes of∼ 30% can be easilyobtained without variations in the spectral index. If aberrationeffects are included, then the helical beam model can also ex-plain the radio behaviour observed by Kraus et al. (1999), as itis shown by Qian et al. (2000), as well as possible rotations ofthe polarization angle (e.g. K̈onigl & Choudhuri 1985, Romeroet al. 1995a).Alternatively to intrinsic models involving shocks, super-luminal gravitational microlensing (e.g. Romero et al. 1995b)could explain the rapid and time symmetric outbursts observedin AO 0235+164. This is also an achromatic phenomenon; smalllenses in the interposed galaxies could amplify the emissionfrom a thin shock producing fast variations superposed to thelonger ones, which are due to the shock propagation. Althoughthis mechanism probably fails at radio frequencies because tem-peratures in excess to the inverse Compton limit are demandedfor the emitting region in the blazar, at optical bands it cannotbe ruled out at present (see Rabbette et al. 1996 for additionaldetails and numerical estimates). The fact that a similar out-burst was observed on the night of November 4 (only in theV band) opens the question of how frequently these outburstsoccur. A large number could pose problems to the microlensinginterpretation because a high density of small lenses would berequired in the interposed galaxy, leading to a “smearing out”of the individual outbursts.Future simultaneous radio-optical observations with hightime resolution, as well as rapid polarimetric observations,would be very helpful to improve our understanding of thisextraordinary blazar.Acknowledgements.The authors acknowledge use of the CCD anddata acquisition system supported under US National Science Foun-dation grant AST-90-15827 to R.M. Rich. They are also very gratefulto the CASLEO staff for their kind assistance during the observationsand to J.H. Fan and S.J. Qian for useful remarks. This work has beensupported by the Argentine agencies CONICET (PIP 0430/98) andANPCT (PICT 98 No. 03-04881), as well as by Fundación Antorchas(through funds granted to GER and JAC).ReferencesAbraham R.G., Crawford C.S., Merrifield M.R., et al., 1993, ApJ 415,101Bessell M.S., 1979, PASP 91, 589Carini M.T., Miller H.R., Noble J.C., et al., 1991, AJ 101, 1196Cellone S.A., Romero G.E., Combi J.A., 2000, AJ 119, 1534Chu H.S., B̊aåth L.B., Rantakyr̈o F.T., et al., 1996, A&A 307, 15Fan J.H., Xie G.Z., Wen S.L., 1996, A&AS 116, 409Fan J.H., Lin R.G., 2000, ApJ 537, 101Gaskell C.M., Peterson B.M., 1987, ApJS 65, 1Hartman R.C., Bertsch D.L., Bloom S.D., et al., 1999, ApJS 123, 79Heidt J., Wagner S.J., 1996, A&A 305, 42Jones D.L., B̊aåth L.B., Davis M.M., et al., 1984, ApJ 284, 60Königl A., Choudhuri A.R., 1985, ApJ 289, 173Kraus A., Quirrenbach A., Lobanov A.P., et al., 1999, A&A 344, 807Landolt A.U., 1992, AJ 104, 340Marscher A.P., 1990, in: Parsec-Scale radio jets, ed. Zensus J.A., Pear-son T.J., Cambridge Univ. Press: Cambridge, p.236Marscher A.P., 1992, in: Physics of Active Galactic Nuclei, eds.Duschls W.J., Wagner S.J., Springer-Verlag, Heidelberg, p.510Marscher A.P., 1997, Perugia University Obs. Publ. 3, 81Noble J.C., Miller H.R., 1996, ASP Conf. Ser. 110, 30Qian S.J., Quirrenbach A., Witzel A., et al., 1991, A&A 241, 15Qian S.J., Kraus A., Witzel A., et al., 2000, A&A 357, 84Quirrenbach A., Witzel A, Krichbaum T.P., et al., 1992, A&A 258, 279Rabbette M., McBreen B., Steel S., et al., 1996, A&A 310, 1Romero G.E., 1995, Ap&SS 234, 49Romero G.E., Combi J.A., Vucetich H., 1995a, Ap&SS 225, 283Romero G.E., Surpi G., Vucetich H., 1995b, A&A 301, 641Romero G.E., Combi J.A., Benaglia P., et al., 1997, A&A 326, 77Romero G.E., Cellone S.A., Combi J.A., 1999, A&AS 135, 477Romero G.E., Cellone S.A., Combi J.A., 2000, AJ, in pressSchlegel D.J., Finkbeiner D.P., Davis M., 1998, ApJ 500, 525Schramm K.-J., Bogeest U., Kühl D., et al., 1994, A&AS 106, 349,1994Stickel M., Fried J.W., K̈uhr H., 1988, A&A 198, L13Véron-Cetty M.-P., V́eron P., 2000, ESO Sci. Rep. 19, 1.Webb J.R., Smith A.G., Leacock R.J., et al., 1988, AJ 95, 374Wiita P.J., 1996, ASP Conf. Ser. 110, 42LETTER

Extreme intranight variability in the BL Lacertae object AO 0235+164

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Extreme intranight variability in the BL Lacertae object AO 0235+164

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