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We report velocity-delay maps for prominent broad emission lines, Ly_alpha, CIV, HeII and H_beta, in the spectrum of NGC5548. The emission-line responses inhabit the interior of a virial envelope. The velocity-delay maps reveal stratified ionization structure. The HeII response inside 5-10 light-days has a broad single-peaked velocity profile. The Ly_alpha, CIV, and H_beta responses peak inside 10 light-days, extend outside 20 light-days, and exhibit a velocity profile with two peaks separated by 5000 km/s in the 10 to 20 light-day delay range. The velocity-delay maps show that the M-shaped lag vs velocity structure found in previous cross-correlation analysis is the signature of a Keplerian disk with a well-defined outer edge at R=20 light-days. The outer wings of the M arise from the virial envelope, and the U-shaped interior of the M is the lower half of an ellipse in the velocity-delay plane. The far-side response is weaker than that from the near side, so that we see clearly the lower half, but only faintly the upper half, of the velocity--delay ellipse. The delay tau=(R/c)(1-sin(i))=5 light-days at line center is from the near edge of the inclined ring, giving the inclination i=45 deg. A black hole mass of M=7x10^7 Msun is consistent with the velocity-delay structure. A barber-pole pattern with stripes moving from red to blue across the CIV and possibly Ly_alpha line profiles suggests the presence of azimuthal structure rotating around the far side of the broad-line region and may be the signature of precession or orbital motion of structures in the inner disk. Further HST observations of NGC 5548 over a multi-year timespan but with a cadence of perhaps 10 days rather than 1 day could help to clarify the nature of this new AGN phenomenon., 19 pages, 9 figures, ApJ in press

We present geometric and dynamical modeling of the broad line region for the multi-wavelength reverberation mapping campaign focused on NGC 5548 in 2014. The dataset includes photometric and spectroscopic monitoring in the optical and ultraviolet, covering the H$\beta$, C IV, and Ly$\alpha$ broad emission lines. We find an extended disk-like H$\beta$ BLR with a mixture of near-circular and outflowing gas trajectories, while the C IV and Ly$\alpha$ BLRs are much less extended and resemble shell-like structures. There is clear radial structure in the BLR, with C IV and Ly$\alpha$ emission arising at smaller radii than the H$\beta$ emission. Using the three lines, we make three independent black hole mass measurements, all of which are consistent. Combining these results gives a joint inference of $\log_{10}(M_{\rm BH}/M_\odot) = 7.64^{+0.21}_{-0.18}$. We examine the effect of using the $V$ band instead of the UV continuum light curve on the results and find a size difference that is consistent with the measured UV-optical time lag, but the other structural and kinematic parameters remain unchanged, suggesting that the $V$ band is a suitable proxy for the ionizing continuum when exploring the BLR structure and kinematics. Finally, we compare the H$\beta$ results to similar models of data obtained in 2008 when the AGN was at a lower luminosity state. We find that the size of the emitting region increased during this time period, but the geometry and black hole mass remain unchanged, which confirms that the BLR kinematics suitably gauge the gravitational field of the central black hole., Comment: 26 pages, 19 figures, 1 table, accepted for publication in ApJ

The astrophysical journal 896(1), 41 - (2020). doi:10.3847/1538-4357/ab910e, The radio source 3C 264, hosted by the giant elliptical galaxy NGC 3862, was observed with VERITAS between February 2017 and May 2019. These deep observations resulted in the discovery of very-high-energy (VHE; E $>100$ GeV) $\gamma$-ray emission from this active galaxy. An analysis of $\sim$57 hours of quality-selected live time yields a detection at the position of the source, corresponding to a statistical significance of 7.8 standard deviations above background. The observed VHE flux is variable on monthly time scales, with an elevated flux seen in 2018 observations. The VHE emission during this elevated state is well-characterized by a power-law spectrum with a photon index $\Gamma = 2.20 \pm 0.27$ and flux F($>315$ GeV) = ($7.6\pm 1.2_{\mathrm stat} \pm 2.3_{\mathrm syst})\times 10^{-13}$ cm$^{-2}$ s$^{-1}$, or approximately 0.7% of the Crab Nebula flux above the same threshold. 3C 264 ($z = 0.0217$) is the most distant radio galaxy detected at VHE, and the elevated state is thought to be similar to that of the famously outbursting jet in M 87. Consequently, extensive contemporaneous multi-wavelength data were acquired in 2018 at the time of the VHE high state. An analysis of these data, including VLBA, VLA, HST, Chandra and Swift observations in addition to the VERITAS data, is presented, along with a discussion of the resulting spectral energy distribution., Published by Univ.8743, Chicago, Ill. [u.a.]

We model the ultraviolet spectra of the Seyfert 1 galaxy NGC~5548 obtained with the Hubble Space Telescope during the 6-month reverberation-mapping campaign in 2014. Our model of the emission from NGC 5548 corrects for overlying absorption and deblends the individual emission lines. Using the modeled spectra, we measure the response to continuum variations for the deblended and absorption-corrected individual broad emission lines, the velocity-dependent profiles of Ly$\alpha$ and C IV, and the narrow and broad intrinsic absorption features. We find that the time lags for the corrected emission lines are comparable to those for the original data. The velocity-binned lag profiles of Ly$\alpha$ and C IV have a double-peaked structure indicative of a truncated Keplerian disk. The narrow absorption lines show delayed response to continuum variations corresponding to recombination in gas with a density of $\sim 10^5~\rm cm^{-3}$. The high-ionization narrow absorption lines decorrelate from continuum variations during the same period as the broad emission lines. Analyzing the response of these absorption lines during this period shows that the ionizing flux is diminished in strength relative to the far-ultraviolet continuum. The broad absorption lines associated with the X-ray obscurer decrease in strength during this same time interval. The appearance of X-ray obscuration in $\sim\,2012$ corresponds with an increase in the luminosity of NGC 5548 following an extended low state. We suggest that the obscurer is a disk wind triggered by the brightening of NGC 5548 following the decrease in size of the broad-line region during the preceding low-luminosity state., Comment: 50 pages, 30 figures, uses aastex62.cls. Accepted for publication in ApJ, 07/06/2019. High-level products page in MAST will go live after 7/15/2019. Replaced Figure 4 on 7/12/2019 to be more red/green color-blind friendly

We characterize the variability in nearly continuous optical observations of the bright radio-loud quasar 3C 273 and nine additional active galactic nuclei detected with the Fermi Gamma-ray Space Telescope Large Area Telescope (Fermi-LAT). Optical observations were obtained during the K2 mission with the Kepler spacecraft for periods of 49 to 83 days conducted with ≃1 minute (short) or ≃30 minutes (long) cadences in 2015–2017. 3C 273 was quiescent during the course of the observations, varying by only a factor of 1.02. Three objects, PKS 0047+023, PKS 1216-10, and PKS B2320-035, were active, varying by factors of 1.8–3.4. Six other objects were comparatively quiet, varying by factors of less than 1.4. Power spectral densities (PSDs) were calculated for each object. Overall, the slopes of most PSDs, as well as those we reported in an earlier paper, were in the range −2.0 to −2.7 and are consistent with those produced by turbulence in the relativistic jet, and not by “hot spots” in the disk emission. Mechanisms operating in the jet other than turbulence, such as “mini-jets” or “jet-in-jets”, may also produce the observed range of PSD slopes. Both accretion disk and jet models are plausible origins for the 3C 273 optical variability during the K2 observations.

We present the results of an optical spectroscopic monitoring program targeting NGC 5548 as part of a larger multi-wavelength reverberation mapping campaign. The campaign spanned six months and achieved an almost daily cadence with observations from five ground-based telescopes. The H$\beta$ and He II $\lambda$4686 broad emission-line light curves lag that of the 5100 $\AA$ optical continuum by $4.17^{+0.36}_{-0.36}$ days and $0.79^{+0.35}_{-0.34}$ days, respectively. The H$\beta$ lag relative to the 1158 $\AA$ ultraviolet continuum light curve measured by the Hubble Space Telescope is roughly $\sim$50% longer than that measured against the optical continuum, and the lag difference is consistent with the observed lag between the optical and ultraviolet continua. This suggests that the characteristic radius of the broad-line region is $\sim$50% larger than the value inferred from optical data alone. We also measured velocity-resolved emission-line lags for H$\beta$ and found a complex velocity-lag structure with shorter lags in the line wings, indicative of a broad-line region dominated by Keplerian motion. The responses of both the H$\beta$ and He II $\lambda$4686 emission lines to the driving continuum changed significantly halfway through the campaign, a phenomenon also observed for C IV, Ly $\alpha$, He II(+O III]), and Si IV(+O IV]) during the same monitoring period. Finally, given the optical luminosity of NGC 5548 during our campaign, the measured H$\beta$ lag is a factor of five shorter than the expected value implied by the $R_\mathrm{BLR} - L_\mathrm{AGN}$ relation based on the past behavior of NGC 5548., Comment: 22 pages, 13 figures, accepted to ApJ

The BL Lac object 1ES 1011+496 was discovered at Very High Energy gamma-rays by MAGIC in spring 2007. Before that the source was little studied in different wavelengths. Therefore a multi-wavelength (MWL) campaign was organized in spring 2008. Along MAGIC, the MWL campaign included the Metsahovi radio observatory, Bell and KVA optical telescopes and the Swift and AGILE satellites. MAGIC observations span from March to May, 2008 for a total of 27.9 hours, of which 19.4 hours remained after quality cuts. The light curve showed no significant variability. The differential VHE spectrum could be described with a power-law function. Both results were similar to those obtained during the discovery. Swift XRT observations revealed an X-ray flare, characterized by a harder when brighter trend, as is typical for high synchrotron peak BL Lac objects (HBL). Strong optical variability was found during the campaign, but no conclusion on the connection between the optical and VHE gamma-ray bands could be drawn. The contemporaneous SED shows a synchrotron dominated source, unlike concluded in previous work based on nonsimultaneous data, and is well described by a standard one zone synchrotron self Compton model. We also performed a study on the source classification. While the optical and X-ray data taken during our campaign show typical characteristics of an HBL, we suggest, based on archival data, that 1ES 1011+496 is actually a borderline case between intermediate and high synchrotron peak frequency BL Lac objects., 13 pages, accepted for publication in MNRAS

We conduct a multiwavelength continuum variability study of the Seyfert 1 galaxy NGC 5548 to investigate the temperature structure of its accretion disk. The 19 overlapping continuum light curves (1158 to 9157 angstroms) combine simultaneous HST , Swift , and ground-based observations over a 180 day period from 2014 January to July. Light-curve variability is interpreted as the reverberation response of the accretion disk to irradiation by a central time-varying point source. Our model yields the disk inclination, i, temperature T1 at 1 light day from the black hole, and a temperature-radius slope, alpha. We also infer the driving light curve and find that it correlates poorly with both the hard and soft X-ray light curves, suggesting that the X-rays alone may not drive the ultraviolet and optical variability over the observing period. We also decompose the light curves into bright, faint, and mean accretion-disk spectra. These spectra lie below that expected for a standard blackbody accretion disk accreting at L/LEdd = 0.1, Comment: V2: Oops wrong title! V1: Accepted for publication in ApJ, 20 Pages, 11 Figures

We gauge the impact of spacecraft-induced effects on the inferred variability properties of the light curve of the Seyfert 1 AGN Zw 229-15 observed by \Kepler. We compare the light curve of Zw 229-15 obtained from the Kepler MAST database with a re-processed light curve constructed from raw pixel data (Williams & Carini, 2015). We use the first-order structure function, $SF(\delta t)$, to fit both light curves to the damped power-law PSD of Kasliwal, Vogeley & Richards, 2015. On short timescales, we find a steeper log-PSD slope ($\gamma = 2.90$ to within $10$ percent) for the re-processed light curve as compared to the light curve found on MAST ($\gamma = 2.65$ to within $10$ percent)---both inconsistent with a damped random walk which requires $\gamma = 2$. The log-PSD slope inferred for the re-processed light curve is consistent with previous results (Carini & Ryle, 2012, Williams & Carini, 2015) that study the same re-processed light curve. The turnover timescale is almost identical for both light curves ($27.1$ and $27.5$~d for the reprocessed and MAST database light curves). Based on the obvious visual difference between the two versions of the light curve and on the PSD model fits, we conclude that there remain significant levels of spacecraft-induced effects in the standard pipeline reduction of the Kepler data. Re-processing the light curves will change the model inferenced from the data but is unlikely to change the overall scientific conclusion reached by Kasliwal et al. 2015---not all AGN light curves are consistent with the DRW., Comment: 7 pages, 3 figures

We report the results of a study of optical microvariability in radio-quiet and radio-intermediate quasars. Observations were obtained on a total of seven objects: five radio-quiet quasars and two radio-intermediate quasars. No microvariability was detected in either the radio-quiet or radio-intermediate objects in our sample, despite intensive monitoring for several consecutive nights. In one object, PG 1257+346, evidence for interday variability was detected. We examined a sample of 117 radio-quiet objects found in the literature that have been studied for microvariability. This sample is discussed in terms of classification, redshift distribution, R (the ratio of the radio [5 GHz] flux to the optical [4400 A] flux), optical magnitude, luminosity, and observing strategy. Objects with 10 > R > 1 were found to show a higher instance of microvariability than those with R < 1. This suggests that R = 1 is the appropriate cutoff between radio-loud and radio-quiet objects. A preference for low-redshift objects to display microvariability more often than higher redshift objects is seen; however, this is explainable via selection effects. We find a dependence in detection probability on observation length similar to that seen in radio-loud objects.

The 1.3-m telescope at Kitt Peak National Observatory was originally commissioned in 1964 as the “Remotely Controlled Telescope”, a pioneering project in unattended, remote observing. Forty years of technological advances later, the 1.3-m has been refurbished, automated and reborn as the “Robotically Controlled Telescope”. The RCT is used to pursue a variety of research programs which incorporate techniques to achieve extremely high precision photometric monitoring of stars and AGN, as well as to search for transits of extrasolar planets. Since spring of 2003 the RCT has been controlled through pre-scheduled scripts, without interactive human control, to collect imaging data. The work on automation of the observatory system is continuing, with the ultimate goal being that the computer will control the telescope through semiintelligent software to make observations as efficiently as a well-trained human astronomer. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We present the results of a long-term monitoring campaign of the H\alpha line 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 monitor ed during 36 nights spread over 6 months in 2001-2002. Coordinated broad-band photometric observations were also obtained for some targets. Conspicuous evidence for variability in H\alpha is found for the stars displaying a feature 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 H\alpha has been found in 2 stars: HD 14134 and HD 42087 (periodic signals are also detected in other stars, but independent confirmation is required). Rotational modulation is suggested from the similarity between the observed recurrence time-scales (in the range 13-25 days) and estimated periods of stellar rotation. 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., Comment: 18 pages, accepted for publication in MNRAS

We present the results of a search for microvariability in a sample of eight Seyfert galaxies. Microvariability (i.e., variations occurring on timescales of tens of minutes to hours) has been conclusively demonstrated to exist in the class of active galactic nuclei (AGNs) known as blazars. Its existence in other classes of AGNs is far less certain. We present the results of a study of eight Seyfert 1 galaxies, which were intensively monitored in order to determine whether such variations exist in these objects. Only one object, Ark 120, displayed any evidence of microvariations. The implications of these results with respect to current models of the mechanisms responsible for the observed emission in Seyfert galaxies are discussed. We compare our results with those obtained from other studies of microvariability in different classes of AGNs.

BL Lac was observed in the V and R bands on 11 nights in 1997 July during a major optical outburst. It varied continuously, exhibiting nightly variations ranging from 0.1 to 0.6 mag. Over the course of the 11 nights, BL Lac was seen to vary by 1.8 R magnitudes. Color variations were also observed, with BL Lac becoming bluer as it brightened. The influence of variable seeing conditions on the observed variations was investigated. No correlation was found between the observed microvariability and the local seeing conditions.

Optical photometric observations of the blazar 3C 371 are presented as part of an ongoing study of variations in blazars occurring on timescales of minutes to hours, which has been termed microvariability. The observations presented here provide further evidence of the classwide nature of this phenomenon. Timescales as short as these provide important clues about the nature of blazars, and a discussion of the types of possible mechanisms responsible for producing these variations is given.

The RCT 1.3-meter telescope, formerly known as the Kitt Peak National Observatory (KPNO) 50-inch telescope, has been refurbished as a fully robotic telescope, using an autonomous scheduler to take full advantage of the observing site without the requirement of a human presence. Here we detail the current configuration of the RCT, and present as a demonstration of its high-priority science goals, the broadband {\it UBVRI} photometric calibration of the optical facility. In summary, we find the linear color transformation and extinction corrections to be consistent with similar optical KPNO facilities, to within a photometric precision of 10% (at $1\sigma$). While there were identified instrumental errors likely adding to the overall uncertainty, associated with since-resolved issues in engineering and maintenance of the robotic facility, a preliminary verification of this calibration gave good indication that the solution is robust, perhaps to a higher precision than this initial calibration implies. The RCT has been executing regular science operations since 2009, and is largely meeting the science requirements set in its acquisition and re-design., Comment: 14 pages, 10 figures. Accepted by the Astronomical Journal

We report on TeV gamma-ray observations of the blazar Mrk 421 (redshift of 0.031) with the VERITAS observatory and the Whipple 10m Cherenkov telescope. The excellent sensitivity of VERITAS allowed us to sample the TeV gamma-ray fluxes and energy spectra with unprecedented accuracy where Mrk 421 was detected in each of the pointings. A total of 47.3 hrs of VERITAS and 96 hrs of Whipple 10m data were acquired between January 2006 and June 2008. We present the results of a study of the TeV gamma-ray energy spectra as a function of time, and for different flux levels. On May 2nd and 3rd, 2008, bright TeV gamma-ray flares were detected with fluxes reaching the level of 10 Crab. The TeV gamma-ray data were complemented with radio, optical, and X-ray observations, with flux variability found in all bands except for the radio waveband. The combination of the RXTE and Swift X-ray data reveal spectral hardening with increasing flux levels, often correlated with an increase of the source activity in TeV gamma-rays. Contemporaneous spectral energy distributions were generated for 18 nights, each of which are reasonably described by a one-zone SSC model., Comment: 30 pages, 12 figures, accepted by ApJ

Light curves of 3C 279 are presented in optical (R-band), X-rays (RXTE/PCA), and gamma rays (CGRO/EGRET) for 1999 Jan-Feb and 2000 Jan-Mar. During both of those epochs the gamma-ray levels were high, and all three observed bands demonstrated substantial variation, on time scales as short as one day. Correlation analyses provided no consistent pattern, although a rather significant optical/gamma-ray correlation was seen in 1999, with a gamma-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 gamma-ray and X-ray light curves during the large gamma ray flare in 1996 Feb and the two gamma-bright weeks leading up to it; the correlation at that time was strong, with a gamma-ray/X-ray offset of no more than 1 day., Comment: 20 pages, including 7 figures; accepted by The Astrophysical Journal

One of the key programs on the 1.3 m Robotically Controlled Telescope (RCT) located at Kitt Peak National Observatory and the 0.6 m telescope at the Bell Observatory operated by Western Kentucky University is a study of the variability of the class of Active Galactic Nuclei (AGN) known as Blazars. Blazars are highly variable on timescales of minutes to decades and this variability is seen across the electromagnetic spectrum. In addition, they display a featureless spectrum, thus continuum variability provides the only diagnostic of these objects. Variability provides information on the size of the emission region responsible for the observed variations and when observations are obtained at multiple wavelengths, it can be used to discriminate between emission models. However, traditional ground based observations are limited in a variety of ways. We will discuss how an automated facility, with time dedicated to this astrophysically interesting problem, can overcome many of these limitations, and we show results from the Bell Observatory as well as some of the first results of Blazar observations from the RCT. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We present preliminary results of a long-term spectroscopic monitoring of a magnitude-limited (V < 7.5) sample of OB-supergiants (07.5-B9) aimed at establishing the incidence of co-rotating, large-scale wind structures. In the optical, this can be achieved by detecting rotationally modulated variability in Hα. Dramatic line-profile variations operating on a daily (and in some cases on a hourly) timescale are observed. Firm conclusions regarding the origin of the variability must, however, await a detailed period analysis. There is no clear evidence for a causal link between photospheric and wind activities.

We report the results of a study of multicolor optical microvariability in the Blazar S5 0716+714. S5 0716+714 was observed in the B and I bands continuously for five consecutive nights in 2003 March. This is the first study to apply color analysis, structure function, and cross-correlation analysis on an optical data set with the temporal coverage and photometric quality that characterizes this data. The source displayed variability on timescales from days to tens of minutes in both bands. Discrete events in the light curves lead to the determination of a range in the size of the emission regions of R ? 1.07 ?1015 cm to R ? 9.8 ?1015 cm, adopting ? = 20 from Nesci et?al. Hysteresis loops were found in plots of the B-band flux (F B ) and the ratio of the B to the I-band flux (F B/I ); these loops were found to be clockwise for dips in the light curves and counterclockwise for bursts. Their directionality and the symmetric nature of the flares are consistent with the flux variations being dominated by the light crossing time of the emission region and not its intrinsic electron cooling or acceleration timescales. The variability between B and I bands is highly correlated with no significant lags between the B and I-band flux variations detected. Significant lags were detected between the flux in the B band (F B ) and the B/I flux ratio (F B/I ). A structure function analysis shows similar slopes in both bands, lying in the range of ?1.0 to ?2.0, indicating that the observed variability is the result of a fractional noise process, consistent with the variations arising from a turbulent process. A differential I-band calibration for comparison star 4 from the sequence of Villata et?al. is also provided.

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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.

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.

Results of photometric monitoring of the BL Lacertae object PKS 2155 - 304 between 1979 and 1990 are presented. Optical variations were observed on time scales ranging from several minutes to several years. The variations detected with time scales on the order of minutes are among the fastest optical variations detected for a BL Lacertae object. These rapid variations provided constraints on the mass of the supermassive black hole, assumed to be present at the center of the object, of the range between 5.5 +/-0.4 x 10 exp 8 and 8.1 +/-1.0 x 10 exp 9 solar masses. The results indicate that the characteristic variability time scale for the optical variability is shorter than the characteristic variability time scale established for the UV of approximately 50 d reported by Treves et al. (1989).

This letter reports the first clear evidence of a rapid change in the ultraviolet flux of a BL Lac object. Sophisticated analysis of 57 closely spaced IUE spectra yielded clear evidence of an about 12 percent drop in the ultraviolet flux of PKS 2155 - 304 over a 2 day period in October 1989 and a strong indication of a similar drop in a 5 hr period some three weeks later. These are near the fastest variations which could have been detected with these data, but they are still much slower (or weaker) variations than have been observed in the X-rays. However, they are orders of magnitude fater than Seyfert 1 ultraviolet variations. This suggests that a 'standard' Seyfert 1 accretion disk may not be responsible for the ultraviolet emission from PKS 2155 - 304.

The BL Lacertae object, AP Librae, has been photometrically monitored in an effort to study the nature of optical variations which may occur on timescales less than a day. Rapid, large amplitude events have been observed with changes as large as 1.0 mag from night to night. The most rapid rate of change detected was 0.06 + or - 0.01 mag/hr. This is one of the most rapid, large amplitude events ever detected for a BL Lacertae object. 23 refs.

Photometric and spectroscopic observations of Lanning 90 (V1776 Cyg) show that it is an eclipsing cataclysmic variable with an orbital period of 3h57m. The shape of the eclipse is variable, and the brightness out of eclipse can change by over a magnitude in less than a day. The broad-band light curves do not show a hump before eclipse, but time-resolved spectroscopy of one orbit suggests that the H-alpha emission-line strength peaks near phase 0.75. Observations of a number of orbits, however, are needed to confirm that the line strength depends on phase. The velocity amplitude of the primary is 144 + or - 20 km/s as measured from the wings of the H-alpha line. The masses of the components, the orbital inclination, and semimajor axis are estimated from the observations, but the derived system parameters are suspect since the spectroscopic phase lags the photometric phase by 70 deg. A similar shift has been observed in DW UMa and is attributed to a disturbance on the disk which contaminates the Balmer line wings. 20 refs.

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 different spectra: in June it was concave with a very hard component above 5-6 keV (alpha_1 ~ 1.6; alpha_2 ~ 0.15); in December it was well fitted by a single power law (alpha ~ 0.6). During the first BeppoSAX observation BL Lac showed an astonishing variability episode: the 0.3 - 2 keV flux doubled in ~ 20 minutes, while the flux above 4 keV was almost constant. This frequency-dependent event is one of the shortest ever recordered for BL Lac objects and places lower limits on the dimension and the magnetic field 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 of ~ 20% in 20 min. We fit the spectral energy distributions with a homogeneous, one-zone model to constrain the emission region in a very simple but effective SSC + external Compton scenario, highlightning 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 gamma-ray Spectral Energy Distributions., 19 pages, 6 figures, accepted by Astronomy & Astrophysics

This paper combines data from the three preceding papers in order to analyze the multi-waveband variability and spectral energy distribution of the Seyfert~1 galaxy NGC~4151 during the December 1993 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 ($\sim$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 $\sim$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~\AA\ to 6900~\AA. These data do not probe extreme ultraviolet (1200~\AA\ to 0.1~keV) or hard X-ray (2--50~keV) variability. The phase differences between variations in different bands were consistent with zero lag, with upper limits of $\ls$0.15~day between 1275~\AA\ and the other ultraviolet bands, $\ls$0.3~day between 1275~\AA\ and 1.5~keV, and $\ls$1~day between 1275~\AA\ and 5125~\AA. These tight limits represent more than an order of magnitude improvement over those determined in previous multi-waveband AGN monitoring campaigns. The ultraviolet fluctuation power spectra showed no evidence for periodicity, but were instead well-fitted with a very steep, red power-law ($ a = -2.5 $). If photons emitted at a ``primary" waveband are absorbed by nearby material and ``reprocessed" to produce emission at a secondary waveband, causality arguments require that variations in the secondary band follow those in the primary band. The tight interband correlation and limits on the ultraviolet and, Comment: 35 pages, LaTeX (including aaspp4), including 7 PostScript figures; To appear in the ApJ (October 20, 1996) Vol. 470

The BL Lacertae objects, OJ 287, has long been known to exhibit large amplitude variations on time scales as short as a few days. The time rate of change in brightness for these variations has been observed to be in excess of 1.0 mag. per day on several occasions. However, the nature of the optical variations on time scales significantly shorter than a day has not been well established, and in many cases the results have been controversial (Carrasco et al, 1985, Lawrence et al 1987, McHardy and Czerny, 1987). In order to address this problem, we have obtained observations of OJ 287 on four different nights during the past two years using the KPNO 0.9 meter telescope equipped with the direct CCD camera. Variations were detected for OJ 287 on each night with ranges of 0.05 mag. to 0.10 mag. The variations detected on 1986 November 9 are typical of those observed on each of the nights and is shown in Figure 1. A power spectrum analysis of the data was performed and no convincing evidence for the presence of any periodicity was found. Based on this analysis, the short term optical variations observed for OJ 287 have a very different character than the rapid x-ray flickering observed for Seyfert galaxies which have been characterized by 1/f - noise. Therefore, these observations would imply sizes for the source region 3 × 10−5 pc. If one assures that the underlying engine is a supermassive blackhole, then the time scales of the variability suggests masses for the blackhole of 107M⊙ – 108M⊙.