R. M. Robb

Optical and radio variability of the BL Lacertae object AO 0235+16: A possible 5-6 year periodicity

The BL Lacertae object AO 0235+16 is well known for its extreme optical and radio variability. New optical and radio data have been collected in the last four years by a wide international collaboration, which conrm the intense activity of this source: on the long term, overall variations of 5 mag in the R band and up to a factor 18 in the radio fluxes were detected, while short-term variability up to 0:5 mag in a few hours and 1: 3m ag in one day was observed in the optical band. The optical data also include the results of the Whole Earth Blazar Telescope (WEBT) rst-light campaign organized in November 1997, involving a dozen optical observatories. The optical spectrum is observed to basically steepen when the source gets fainter. We have investigated the existence of typical variability time scales and of possible correlations between the optical and radio emissions by means of visual inspection and Discrete Correlation Function (DCF) analysis. On the long term, the autocorrelation function of the optical data shows a double-peaked maximum at 4100{4200 days (11:2{11:5 years), while a double-peaked maximum at 3900{4200 days (10:7{11:5 years) is visible in the radio autocorrelation functions. The existence of this similar characteristic time scale of variability in the two bands is by itself an indication of optical-radio correlation. A further analysis by means of Discrete Fourier Transform (DFT) technique and folded light curves reveals that the major radio outbursts repeat quasi-regularly with a periodicity of5:7 years, i.e. half the above time scale. This period is also in agreement with the occurrence of some of the major optical outbursts, but not all of them. Visual inspection and DCF analysis of the optical and radio light curves then reveal that in some cases optical outbursts seem to be simultaneous with radio ones, but in other cases they lead the radio events. Moreover, a deep inspection of the radio light curves suggests that in at least two occasions (the 1992{1993 and 1998 outbursts) flux variations at the higher frequencies may have led those at the lower ones.

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

WEBT multiwavelength monitoring and XMM-Newton observations of BL Lacertae in 2007-2008 Unveiling different emission components

In 2007-2008 we carried out a new multiwavelength campaign of the Whole Earth Blazar Telescope (WEBT) on BL Lacertae, involving three pointings by the XMM-Newton satellite, to study its emission properties. The source was monitored in the optical-to-radio bands by 37 telescopes. The brightness level was relatively low. Some episodes of very fast variability were detected in the optical bands. The X-ray spectra are well fitted by a power law with photon index of about 2 and photoelectric absorption exceeding the Galactic value. However, when taking into account the presence of a molecular cloud on the line of sight, the data are best fitted by a double power law, implying a concave X-ray spectrum. The spectral energy distributions (SEDs) built with simultaneous radio-to-X-ray data at the epochs of the XMM-Newton observations suggest that the peak of the synchrotron emission lies in the near-IR band, and show a prominent UV excess, besides a slight soft-X-ray excess. A comparison with the SEDs corresponding to previous observations with X-ray satellites shows that the X-ray spectrum is extremely variable. We ascribe the UV excess to thermal emission from the accretion disc, and the other broad-band spectral features to the presence of two synchrotron components, with their related SSC emission. We fit the thermal emission with a black body law and the non-thermal components by means of a helical jet model. The fit indicates a disc temperature greater than 20000 K and a luminosity greater than 6 x 10^44 erg/s.

BRIGHT VARIABLE STARS IN NGC 6819: AN OPEN CLUSTER IN THE KEPLER FIELD

We describe a variability study of the moderately old open cluster NGC 6819. We have detected four new detached eclipsing binaries near the cluster turnoff (one of which may be in a triple system). Several of these systems should be able to provide mass and radius information, and can therefore constrain the age of the cluster. We have also newly detected one possible detached binary member about 3.5 mag below the turnoff. One EW-type binary (probably not a cluster member) shows unusually strong night-to-night light curve variations in sets of observations separated by eight years. According to the best current information, the three brightest variables we detected (two of them new) are cluster members, making them blue stragglers. The first one is a δ Scu pulsating variable, the second one is a close but detached binary, and the third one contains a detached short-period binary that shows total eclipses. In each case, however, there is evidence hinting that the system may have been produced through the interaction of more than two stars.

DE Canum Venaticorum: a bright, eclipsing red dwarf-white dwarf binary

Context. Close white dwarf–red dwarf binaries must have gone through a common-envelope phase during their evolution. DE CVn is a detached white dwarf–red dwarf binary with a relatively short (∼8.7 h) orbital period. Its brightness and the presence of eclipses makes this system ideal for a more detailed study. Aims. From a study of photometric and spectroscopic observations of DE CVn we derive the system parameters that we discuss in the framework of common-envelope evolution. Methods. Photometric observations of the eclipses are used to determine an accurate ephemeris. From a model fit to an average lowresolution spectrum of DE CVn, we constrain the temperature of the white dwarf and the spectral type of the red dwarf. The eclipse light curve is analysed and combined with the radial velocity curve of the red dwarf determined from time-resolved spectroscopy to derive constraints on the inclination and the masses of the components in the system. Results. The derived ephemeris is HJDmin = 2 452 784.5533(1) + 0.3641394(2) × E. The red dwarf in DE CVn has a spectral type of M3V and the white dwarf has an effective temperature of 8 000 K. The inclination of the system is 86+3◦ −2 and the mass and radius of the red dwarf are 0.41 ± 0.06 M and 0.37+0.06 −0.007 R, respectively, and the mass and radius of the white dwarf are 0.51+0.06 −0.02 M and 0.0136+0.0008 −0.0002 R, respectively. Conclusions. We found that the white dwarf has a hydrogen-rich atmosphere (DA-type). Given that DE CVn has experienced a common-envelope phase, we can reconstruct its evolution and we find that the progenitor of the white dwarf was a relatively lowmass star (M ≤ 1.6 M). The current age of this system is 3.3−7.3 × 109 years, while it will take longer than the Hubble time for DE CVn to evolve into a semi-detached system.

RJ 051542+0104.7: A new magnetic, eclipsing, cataclysmic variable

We present photometric and spectroscopic observations of a recently identified X-ray bright, magnetic cataclysmic variable, RJ 051542+0104.7. The X-ray source was discovered serendipitously by Walter et al. (1994; Walter & Zoonematkermani 1994), and their data imply the presence of a heated white dwarf compatible with a magnetic accretion column. Buckley (1994) has reported the presence of circular polarization. Our optical photometry shows that the system undergoes eclipses with a period of 7.9835 +/- 0.0002 hours. The eclipse is 1 magnitude deep and appears to be total. Our spectra were all obtained near eclipse and show a strong blue continuum with broad Balmer emission as well as He II (4686 and 5411 A), C III/N III (4650 A) and He I lines. The blue continuum and emission lines disappear during eclipse, leaving only the spectrum of a M0 star. Using the observed magnitude during eclipse and ignoring interstellar extinction, the distance is approximately 500 pc. For the secondary to fill its Roche lobe, it must be at least 20% larger than that of a normal dM0 star. This may indicate the red dwarf is far from local thermodynamic equilibrium (LTE) due to strong magnetic braking. We find evidence for cyclotron humps in the continuum spectrum which imply a magnetic field strength in the cyclotron emitting region of up to 55 MG. The unexpected existence of a long period AM Her system should provide constraints on models of polar evolution.

A Photometric Search for Planets in the Open Cluster NGC 7086

In an attempt to discover short-period, Jupiter-mass planets orbiting solar-type stars in open clusters, we searched for planetary transits in the populous and relatively unstudied open cluster NGC 7086. A color-magnitude diagram constructed from new B and V photometry is presented, along with revised estimates of the clusters color excess, distance modulus, and age. Several turnoff stars were observed spectroscopically in order to determine a color excess of E(B - V) = 0.83 ± 0.02. Empirically fitting the main sequences of two young open clusters and the semiempirical zero-age main sequence of Vandenberg and Poll yielded a distance modulus of (V - MV) = 13.4 ± 0.3 mag. This corresponds to a true distance modulus of (m - M)0 = 10.8 mag or a distance of 1.5 kpc to NGC 7086. These values were used with isochrones from the Padova group to obtain a cluster age of 100 Myr. Eleven nights of R-band photometry were used to search for planetary transits. Differential magnitudes were constructed for each star in the cluster. Light curves for each star were produced on a night-to-night basis and inspected for variability. No planetary transits were apparent; however, some interesting variable stars were discovered: a pulsating variable that appears to be a member of the γ Dor class and four possible eclipsing binary stars, one of which actually may be a multiple system.

PHOTOMETRY WITH THE RAPID ALTERNATE DETECTION SYSTEM.

The photometric procedures, capabilities, and early results of the Rapid Alternate Detection System are presented and discussed. The RADS is an automated, chopping two-star photometry system, designed and constructed at the University of Calgary and currently in operation on the 41-cm telescope of the Rothney Astrophysical Observatory. The precision of the photometry is dependent upon Poisson statistics and the frequency bandwidth of the noise. The need for first-order extinction corrections is eliminated and a procedure for computing second-order extinction and transformation coefficients is demonstrated. Finally, light curves of the W UMa systems 44i Boo and RW Com, obtained on cloudy nights, are presented.

Rapid alternate detection system of the Rothney Astrophysical Observatory.

A new differential variable-star photometry system has been designed, constructed, and tested at the Rothney Astrophysical Observatory of the University of Calgary. The system uses a single telescope, photometer, and detector and consists of gated pulse-counting electronics synchronized to a chopped secondary mirror. The mirror is driven to four positions, permitting rapid alternate detection of a variable star, comparison star, and two sky-background regions. Within certain constraints the system is insensitive to the effects of transparency, sky-brightness, and detector-sensitivity variations occurring on time scales longer than the systems duty cycles. The system has proved effective under as much as three magnitudes of cloud and attendant sky-brightness variation.

A few observations of and remarks on V1500 Cygni

AbstractThe development of the post-nova light curve of V1500 Cyg inUBV andHβ, for 15 nights in September and October 1975 are presented. We confirm previous reports that superimposed on the steady decline of the light curve are small amplitude cyclic variations. The times of maxima and minima are determined. These together with other published values yield the following ephemerides from JD 2 442 661 to JD 2 442 674: