Michael T. Carini

Multiwavelength Observations of Short-Timescale Variability in NGC 4151. IV. Analysis of Multiwavelength Continuum Variability

For pt.III see ibid., vol.470, no.1, p.349-63 (1996). Combines data from the three preceding papers in order to analyze the multi wave-band variability and spectral energy distribution of the Seyfert 1 galaxy NGC 4151 during the 1993 December monitoring campaign. The source, which was near its peak historical brightness, showed strong, correlated variability at X-ray, ultraviolet, and optical wavelengths. The strongest variations were seen in medium-energy (~1.5 keV) X-rays, with a normalized variability amplitude (NVA) of 24%. Weaker (NVA=6%) variations (uncorrelated with those at lower energies) were seen at soft gamma-ray energies of ~100 keV. No significant variability was seen in softer (0.1-1 keV) X-ray bands. In the ultraviolet/optical regime, the NVA decreased from 9% to 1% as the wavelength increased from 1275 to 6900 Aring. These data do not probe extreme ultraviolet (1200 Aring to 0.1 keV) or hard X-ray (250 keV) variability. The phase differences between variations in different bands were consistent with zero lag, with upper limits of lsim0.15 day between 1275 Aring and the other ultraviolet bands, lsim0.3 day between 1275 Aring and 1.5 keV, and lsim1 day between 1275 and 5125 Aring. These tight limits represent more than an order of magnitude improvement over those determined in previous multi-wave-band AGN monitoring campaigns. The ultraviolet fluctuation power spectra showed no evidence for periodicity, but were instead well fitted with a very steep, red power law (ales-2.5)

Multiepoch multiwavelength spectra and models for blazar 3C 279

Of the blazars detected by EGRET in GeV γ-rays, 3C 279 is not only the best observed by EGRET but also one of the best monitored at lower frequencies. We have assembled 11 spectra, from GHz radio through GeV γ-rays, from the time intervals of EGRET observations. Although some of the data have appeared in previous publications, most are new, including data taken during the high states in early 1999 and early 2000. All of the spectra show substantial γ-ray contribution to the total luminosity of the object; in a high state, the γ-ray luminosity dominates over that at all other frequencies by a factor of more than 10. There is no clear pattern of time correlation; different bands do not always rise and fall together, even in the optical, X-ray, and γ-ray bands. The spectra are modeled using a leptonic jet, with combined synchrotron self-Compton plus external Compton γ-ray production. Spectral variability of 3C 279 is consistent with variations of the bulk Lorentz factor of the jet, accompanied by changes in the spectral shape of the electron distribution. Our modeling results are consistent with the UV spectrum of 3C 279 being dominated by accretion disk radiation during times of low γ-ray intensity.

The timescales of the optical variability of blazars. III - OJ 287 and BL Lacertae

The BL Lacertae objects OJ 287 and BL Lac have been photometrically monitored in an effort to study the nature of optical variations which may occur on timescales ranging from years to less than a day. The results of ten years of photometric monitoring of these two objects show variations which are consistent with those reported by other authors. No strong dependence of color with source brightness was detected, although both sources exhibited a weak tendency to be bluer when brighter. Microvariability was observed for both objects; variations as large as 0.1 mag/hr were observed for BL Lac and changes as large as 0.08 mag/hr were observed for OJ 287. No evidence for a periodicity was found in the observed variations of either object.

BL Lacertae: Complex spectral variability and rapid synchrotron flare detected with BeppoSAX

We report on two BeppoSAX observations of BL Lac (2200+420) performed respectively in June and December 1999, as part of a ToO program to monitor blazars in high states of activity. During both runs the source has been detected up to 100 keV, but it showed quite dierent spectra: in June it was concave with a very hard component above 5{6 keV (1 1:6; 2 0:15); in December it was well tted by a single power law ( 0:6). During the rst BeppoSAX observation BL Lac showed an astonishing variability episode: the 0.3{2 keV flux doubled in20 min, while the flux above 4 keV was almost constant. This frequency{dependent event is one of the shortest ever recorded for BL Lac objects and places lower limits on the dimension and magnetic eld of the emitting region and on the energy of the synchrotron radiating electrons. A similar but less extreme behaviour is detected also in optical light curves, that display non-simultaneous, smaller fluctuations of20% in 20 min. We t the spectral energy distributions with a homogeneous, one-zone model to constrain the emission region in a very simple but eective SSC + external Compton scenario, highlighting the importance of the location of the emitting region with respect to the Broad Line Region and the relative spectral shape dependence. We compare our data with historical radio to -ray Spectral Energy Distributions.

The WEBT campaign to observe AO 0235+16 in the 2003-2004 observing season. Results from radio-to-optical monitoring and XMM-Newton observations

A multiwavelength campaign to observe the BL Lac object AO 0235+16 (z = 0.94) was set up by the Whole Earth Blazar Telescope (WEBT) collaboration during the observing seasons 2003-2004 and 2004-2005, involving radio, near-IR and optical photometric monitoring, VLBA monitoring, optical spectral monitoring, and three pointings by the XMM-Newton satellite. Here we report on the results of the first season, which involved the participation of 24 optical and near-IR telescopes and 4 radio telescopes, as well as the first XMM-Newton pointing, which occurred on January 18-19, 2004. Unpublished data from previous epochs were also collected (from 5 optical-NIR and 3 radio telescopes), in order to fill the gap between the end of the period presented in Raiteri et al. (2001) and the start of the WEBT campaign. The contribution of the southern AGN, 2 arcsec distant from the source, is taken into account. It is found to especially affect the blue part of the optical spectrum when the source is faint. In the optical and near-IR the source has been very active in the last 3 years, although it has been rather faint most of the time, with noticeable variations of more than a magnitude over a few days. In contrast, in the radio bands it appears to have been quiescent since early 2000. The major radio (and optical) outburst predicted to peak around February-March 2004 (with a six month uncertainty) has not occurred yet. When comparing our results with the historical light curves, two different behaviours seem to characterize the optical outbursts: only the major events present a radio counterpart. The X-ray spectra obtained by the three EPIC detectors are well fitted by a power law with extra-absorption at z = 0.524; the energy index in the 0.2-10 keV range is well constrained: a = 0.645 ± 0.028 and the 1 keV flux density is 0.311 ± 0.008 μJy. The analysis of the X-ray light curves reveals that no significant variations occurred during the pointing. In contrast, simultaneous dense radio monitoring with the 100 m telescope at Effelsberg shows a ∼2-3% flux decrease in 6-7 h, which, if intrinsic, would imply a brightness temperature well above the Compton limit and hence a lower limit to the Doppler factor 6 > 46.

Multiwavelength monitoring of the BL Lacertae object PKS 2155-304. 4: Multiwavelength analysis

Simultaneous X-ray, ultraviolet, optical, infrared, and radio monitoring data were used to test and constrain models of continuum emission from the BL Lacertae object PKS 2155-304. Intensively sampled ultraviolet and soft X-ray light curves showed a clear temporal correlation with the X-rays leading the ultraviolet by 2-3 hr. This lag was found to be significantly different from zero after an exhaustive comparison of four different techniques for measuring temporal correlations. Variations in the ultraviolet trough optical wave bands were also strongly correlated, with no measurable lag down to limiting timescales of approximately less than 1-2 hr. This strong correlation extends to the near-infrared, but the less intensive sampling precludes measurement of any lag beyomnd an upper limit of approximately less than 1 day. These lags and limits of the order of hours are much shorter than most rapid observed single-band variations. Because of the very sparse radio sampling, it was not possible to measure quantitatively the correlation and lag with shorter wavelengths, but the data do suggest that the radio may lag the optical/ultraviolet by approximately 1 week, with longer delays and weaker variations to longer radio wavelengths. The epoch-folding Q(exp 2) statistic was used to test for periodicity, and no evidence for strict or quasi-periodicity was found in any of the light curves. Because they lead the lower frequencies, the soft X-rays (approximately less than 1 keV) cannot arise from synchrotron self-Compton scattering. These results also rule out the accretion disk model, which predicts a measurable lag between ultraviolet/optical wavelength bands and a correlation between hardness and brightness, neither of which were seen. They are consistent with the entire radio through X-ray continuum arising from direct synchrotron emission from a relativistic jet. However, the tapered jet model, in which the X-ray emission is produced closer in, has problems explaining the magnitude of the ultraviolet/X-ray lag, because the X-ray-emitting electrons have very short lifetimes (t(sub 1/2) much less than 1 s). The result that the lag is much smaller than the variability timescale suggests instead that the radiation may be produced in a flattened region such as a shock front.

Coordinated Multiwavelength Observations of BL Lacertae in 2000

BL Lacertae (BL Lac) was the target of an extensive multiwavelength monitoring campaign in the second half of 2000. Simultaneous or quasi-simultaneous observations were taken at radio (University of Michigan Radio Astronomy Observatory and Metsahovi Radio Telescope) and optical (Whole Earth Blazar Telescope (WEBT) collaboration) frequencies, in X-rays (BeppoSAX and RXTE), and at very high energy gamma rays (HEGRA). The WEBT optical campaign achieved an unprecedented time coverage, virtually continuous over several 10-20 hr segments. It revealed intraday variability on timescales of � 1.5 hr and evidence for spectral hardening associated with increasing optical flux. During the campaign, BL Lac underwent a major transition from a rather quiescent state prior to 2000 September, to a flaring state for the rest of the year. ThisBL Lacertae (BL Lac) was the target of an extensive multiwavelength monitoring campaign in the second half of 2000. Simultaneous or quasi-simultaneous observations were taken at radio (University of Michigan Radio Astronomy Observatory andMetsähovi Radio Telescope) and optical (Whole Earth Blazar Telescope [WEBT] collaboration) frequencies, in X-rays (BeppoSAX and RXTE), and at very high energy gamma rays (HEGRA). The WEBT optical campaign achieved an unprecedented time coverage, virtually continuous over several 10–20 hr segments. It revealed intraday variability on timescales of 1.5 hr and evidence for spectral hardening associated with increasing optical flux. During the campaign, BL Lac underwent a major transition from a rather quiescent state prior to 2000 September, to a flaring state for the rest of the year. This 36 Department of Chemistry, Physics, and Astronomy, FrancisMarionUniversity, P.O. Box 100547, Florence, SC 29501-0547. 37 Department of Physics and Astronomy, University of SouthamptonHighfield, Southampton SO17 1BJ, UK. 38 Dipartimento di Fisica Generale, Università di Torino, Via P. Giuria 1, I-10125 Turin, Italy. 34 Department of Physics and Astronomy,Western KentuckyUniversity, 1 Big RedWay, Bowling Green, KY 42104. 35 Department of Physics and Astronomy, Georgia State University, Atlanta, GA 30303. 1 Department of Physics and Astronomy, Clippinger 339, Ohio University, Athens, OH 45701. 2 Department of Astronomy, BostonUniversity, 725 Commonwealth Avenue, Boston,MA 02215. 3 Osservatorio Astronomico di Brera, Via Bianchi 46, I-23807Merate, Italy. 4 IstitutoNazionale di Astrofisica (INAF), Osservatorio Astronomico di Torino, Via Osservatorio 20, I-10025 Pino Torinese, Italy. 5 Department of Astronomy, University ofMichigan, 810 Dennison Building, AnnArbor,MI 48109-1090. 6 Metsähovi Radio Observatory, Helsinki University of Technology,Metsähovintie 114, 02540Kylmälä, Finland. 7 Institut für Experimentelle und Angewandte Physik, Universität Kiel, Leibnitzstrasse 15–19, D-24118Kiel, Germany. 8 Max-Planck-Institut für Kernphysik, Postfach 10 39 80, D-69029Heidelberg, Germany. 9 Physics Department,WashingtonUniversity, 1 Brookings Drive, CB 1105, St. Louis, MO 63130. 10 Abastumani Observatory, 383762Abastumani, Georgia. 11 Astrophysikalisches Institute Potsdam, An der Sternwarte 16, D-14482 Potsdam, Germany. 12 Landessternwarte Heidelberg-Königstuhl, Königstuhl 12, D-69117Heidelberg, Germany. 13 Ulugh Beg Astronomical Institute, Uzbek Academy of Sciences, Astronomicheskaya 33, Tashkent 700052, Uzbekistan. 14 IsaacNewton Institute of Chile, Uzbekistan Branch. 15 Physics Department, University of Crete, 710 03Heraklion, Crete, Greece. 16 IESL, Foundation for Research and Technology-Hellas, 711 10Heraklion, Crete, Greece. 17 Astronomical Institute, OsakaKyoikuUniversity, Kashiwara-shi, Osaka 582-8582, Japan. 18 Tuorla Observatory, 21500 Piikkiö, Finland. 19 Osservatorio Astronomico, Università di Perugia, Via B. Bonfigli, I-06126 Perugia, Italy. 20 Osservatorio Astrofisico di Catania, Viale A. Doria 6, I-95125 Catania, Italy. 21 Department of Physics and Astronomy, University of Victoria, BC, Canada. 22 Institute for Astrophysical Research, BostonUniversity, 725 Commonwealth Avenue, Boston,MA 02215. 23 Center for Astrophysics, GuangzhouUniversity, Guangzhou 510400, China. 24 Astronomical Institute, St. Petersburg State University, Bibliotechnaya Pl. 2, Petrodvoretz, 198504 St. Petersburg, Russia. 25 Dipartimento di Fisica, Università La Sapienza, Piazzale A.Moro 2, I-00185Rome, Italy. 26 Department of Physics and Astronomy, University ofMissouri-St. Louis, 8001 Natural Bridge Road, St. Louis,MO 63121. 27 Jet Propulsion Laboratory, California Institute of Technology, 4800 OakGroveDrive, Pasadena, CA 91109. 28 Department of Astronomy, Faculty of Science, KyotoUniversity, Kyoto, Japan. 29 Clarke and Coyote Astrophysical Observatory, P.O. Box 930,Wilton, CA 95693. 30 Instituto de Astronomı́a, UNAM,Apartado Postal 70-264, 04510MexicoDF,Mexico. 31 Nyrölä Observatory, Jyväskylän Sirius ry, Kyllikinkatu 1, 40950 Jyväskylä, Finland. 32 GuadarramaObservatory, C/ San Pablo 5, Villalba 28409,Madrid, Spain. 33 Department of Physics, University of Colorado, P.O. Box 173364, Denver, CO 80217-3364. The Astrophysical Journal, 596:847–859, 2003 October 20 # 2003. The American Astronomical Society. All rights reserved. Printed in U.S.A. E

Day-Scale Variability of 3C?279 and Searches for Correlations in Gamma-Ray, X-Ray, and Optical Bands

Light curves of 3C 279 are presented in optical (R band), X-rays (RXTE/PCA), and γ rays (CGRO/EGRET) for 1999 January-February and 2000 January-March. During both of those epochs the γ-ray levels were high and all three observed bands demonstrated substantial variation, on timescales as short as 1 day. Correlation analyses provided no consistent pattern, although a rather significant optical/γ-ray correlation was seen in 1999, with a γ-ray lag of ~2.5 days, and there are other suggestions of correlations in the light curves. For comparison, correlation analysis is also presented for the γ-ray and X-ray light curves during the large γ ray flare in 1996 February and the two γ-bright weeks leading up to it; the correlation at that time was strong, with a γ-ray/X-ray offset of no more than 1 day.

Multifrequency variability of the blazar AO 0235+164.The WEBT campaign in 2004-2005 and long-term SED analysis

Aims. A huge multiwavelength campaign targeting the blazar AO 0235+164 was organized by the Whole Earth Blazar Telescope (WEBT) in 2003-2005 to study the variability properties of the source. Methods. Monitoring observations were carried out at cm and mm wavelengths, and in the near-IR and optical bands, while three pointings by the XMM-Newton satellite provided information on the X-ray and UV emission. Results. We present the data acquired during the second observing season, 2004-2005, by 27 radio-to-optical telescopes. The ∼2600 data points collected allow us to trace the low-energy behaviour of the source in detail, revealing an increased near-IR and optical activity with respect to the previous season. Increased variability is also found at the higher radio frequencies, down to ∼15 GHz, but not at the lower ones. While the X-ray (and optical) light curves obtained during the XMM-Newton pointings reveal no significant short-term variability, the simultaneous intraday radio observations with the 100 m telescope at Effelsberg show flux-density changes at 10.5 GHz, which are more likely due to a combination of intrinsic and extrinsic processes. Conclusions. The radio (and optical) outburst predicted to peak around February-March 2004 on the basis of the previously observed 5-6 yr quasi-periodicity did not occur. The analysis of the optical light curves reveals now a longer characteristic time scale of variability of ∼8 yr, which is also present in the radio data. The spectral energy distributions corresponding to the XMM-Newton observations performed during the WEBT campaign are compared with those pertaining to previous pointings of X-ray satellites. Bright, soft X-ray spectra can be described in terms of an extra component, which appears also when the source is faint through a hard UV spectrum and a curvature of the X-ray spectrum. Finally, there might be a correlation between the X-ray and optical bright states with a long time delay of about 5 yr, which would require a geometrical interpretation.

Large-scale wind structures in OB supergiants: a search for rotationally modulated Hα variability ⋆

We present the results of a long-term monitoring campaign of the Hline in a sample of bright OB-supergiants (O7.5-B9) that aims at detecting rotationally modulated changes potentially related to the existence of large-scale wind structures. A total of 22 objects were monitored during 36 nights spread over 6 months in 2001-2002. Co- ordinated broad-band photometric observations were also obtained for some targets. Conspicuous evidence for variability in His found for the stars displaying a fea- ture contaminated by wind emission. Most changes take place on a daily time-scale, although hourly variations are also occasionally detected. Convincing evidence for a cyclical pattern of variability in Hhas been found in 2 stars: HD 14134 and HD 42087 (periodic signals are also detected in other stars, but independent confirmation is re- quired). Rotational modulation is suggested from the similarity between the observed recurrence time-scales (in the range 13-25 days) and estimated periods of stellar rota- tion. We call attention to the atypical case of HD 14134 which exhibits a clear 12.8-d periodicity both in the photometric and in the spectroscopic data sets. This places this object among a handful of early-type stars where one may observe a clear link between extended wind structures and photospheric disturbances. Further modelling may test the hypothesis that azimuthally-extended wind streams are responsible for the patterns of spectral variability in our target stars.