Wise Observatory Research Articles

X-Ray versus Optical Variations in the Seyfert 1 Nucleus NGC 3516: A Puzzling Disconnectedness

We present optical broadband (B and R) observations of the Seyfert 1 nucleus NGC 3516, obtained at Wise Observatory from March 1997 to March 2002, contemporaneously with X-ray 2-10 keV measurements with RXTE. With these data we increase the temporal baseline of this dataset to 5 years, more than triple to the coverage we have previously presented for this object. Analysis of the new data does not confirm the 100-day lag of X-ray behind optical variations, tentatively reported in our previous work. Indeed, excluding the first year's data, which drive the previous result, there is no significant correlation at any Lag between the X-ray and optical bands. We also find no correlation at any lag between optical flux and various X-ray hardness ratios. We conclude that the close relation observed between the bands during the first year of our program was either a fluke, or perhaps the result of the exceptionally bright state of NGC 3516 in 1997, to which it has yet to return. Reviewing the results of published joint X-ray and UV/optical Seyfert monitoring programs, we speculate that there are at least two components or mechanisms contributing to the X-ray continuum emission up to 10 key: a soft component that is correlated with UV/optical variations on timescales approx. greater than 1 day, and whose presence can be detected when the source is observed at low enough energies (approx. 1 keV), is unabsorbed, or is in a sufficiently bright phase; and a hard component whose variations are uncorrelated with the UV/optical.

Hidden subluminous stars among theFAUSTUV sources towards Ophiuchus

We present results of an analysis of a UV image in the direction of Ophiuchus, obtained with the FAUST instrument. The image contains 228 UV sources. Most of these are identified as normal early-type stars through correlations with catalogued objects. For the first time in this project we identify UV sources as such stars by selecting suitable candidates in crowded fields as the bluest objects in colour–colour diagrams using observations from the Wise Observatory. These candidates are then studied using low-resolution spectroscopy, which allows the determination of spectral types to an accuracy of about one-half class, for 60 stars. Synthetic photometry of spectral data is performed in order to predict the expected UV emission, on the basis of the photometric information. These results are used along with the Hipparcos/Tycho information, to search for subluminous stars. The comparison of the predicted emission with the FAUST measured magnitudes allows us to select 12 stars as highly probable evolved hot stars. High signal-to-noise spectra are obtained for nine of these stars, and Balmer line profiles are compared with the prediction of atmosphere models and with the spectrum of real stellar atmospheres. Among the nine candidates, six are classified as previously unrecognized sdB stars, and two as white dwarfs. Our result indicates that indeed more bright subluminous stars are still unrecognized in the existing samples.

Intermediate-band photometry of a rich cluster of galaxies: A2218

This paper presents four color intermediate-band photometry of the cluster A2218 carried out using the Wise Observatory 1 m telescope. A2218 is one of the richest clusters in the Abell catalogue, with richness class 4. We find that A2218 has an unusually low fraction of blue galaxies for its redshift (z=0.171), and is populated mainly by E/S0 galaxies, while starburst systems are very scarce. The few starbursting galaxies present populate the faint end of the cluster luminosity function, close to the limiting magnitude of our observations. The difference between A2218 and other clusters at similar redshifts is very remarkable, underlining the influence of cluster richness on the evolution of its member galaxies.

Quasar Variability in the Framework of Poissonian Models

In this paper we review the basic Poissonian formulation of quasar variability, using it as a mathematical tool to extract relevant parameters such as the energy, rate and lifetimes of the flares through the analysis of observed light curves. It is shown that in this very general framework the well established anti-correlation between variability amplitude and wavelength can only be understood as an effect of an underlying spectral component which remains stable on long time-scales, and is redder than the variable component. The formalism is applied to the B and R light curves of 42 PG quasars collected by the Wise Observatory group. Variability indices for these data are obtained with a Structure Function analysis. The mean number of living flares is constrained to be in the range between 5 and 100, while their rates are found to be of order 1--100 per yr. Monochromatic optical flare energies of 10^{46-48} erg/A and life-times of 0.5 to 3 yr are derived. Lower limits of typically 25% are established for the contribution of a non-variable component in the R band. The substantial diversity in these properties among quasars invalidates simple versions of the Poissonian model in which flare energies, lifetimes and the background contribution are treated as universal invariants. The good correlation between the EW(H_beta) and the long term variability amplitude is interpreted in a scenario where only the variable component participates in the ionization of the line emitting gas. This idea is consistent with the observed trends of the variability amplitude with lambda, EW(HeII) and the X-ray to optical spectral index (abridged).

A Possible 100 Day X-Ray–to–Optical Lag in the Variations of the Seyfert 1 Nucleus NGC 3516

We present optical broadband (B and R) observations of the Seyfert 1 nucleus NGC 3516, obtained at Wise Observatory from 1997 March to 1998 September contemporaneously with the X-ray 2–10 keV measurements of RXTE. The cross-correlation function shows a positive peak when the optical variations lead the X-rays by ~100 days, and anticorrelation peaks at various leads and delays between the X-rays and the optical. We show that the putative correlation signal at 100 days is entirely due to the slow (30 day) components of the light curves. During the first year of this monitoring, smoothed versions of the light curves are nearly identical copies of each other, but scaled in amplitude and shifted in time. However, for the next 200 days, the X-ray and optical variations are clearly different. During the whole period, the more rapidly changing components of the light curves are uncorrelated at any lag. We consider the detection of these lags tentative and the significance of the correlations uncertain. If the 100 day delay is real, however, one interpretation is that the slowly varying part of the X-ray emission is an echo of the optical emission, Compton scattered from a medium located at, or extending, ~50-100 lt-days from the optical source. We point out that a possibly analogous phenomenon, a lag between hard and soft X-rays for a given variability timescale, exists in Galactic stellar-mass accretors. Remarkably, in both cases the lag corresponds to a light-travel distance of the order of 104 gravitational radii. Alternatively, the lag may not represent a physical size, but some other timescale. For example, it may be the manifestation of an instability propagating inward in an accretion flow, appearing first in the optical and then in the X-rays. In any event, we observe no strong correlation at zero lag, or at the small positive lags expected if the optical continuum were produced by reprocessing of X-rays. An energetically significant reprocessed component in the optical emission of NGC 3516 is thus ruled out by our data.

Wise Observatory system of fast CCD photometry

Wise Observatory system of fast CCD photometry

Long-term optical variability properties of the Palomar-Green quasars

We present results from a monitoring program of 42 quasars from the Palomar Green sample. The objects were observed for 7 years at the Wise Observatory, as part of a long term effort to monitor AGN of various types. This is the most extensive program of its kind carried out to date on a well-defined optically selected quasar sample. The typical sampling interval is ~40 days. One third of the quasars were observed at ~60 epochs and the rest at ~30 epochs in two bands (B&R) with photometric accuracy of ~0.01 mag. We present lightcurves for all of the sources and discuss the sample variability properties. All of the quasars in the sample varied during the campaign with intrinsic rms amplitudes of 5%<sigma_B<34% and 4%<sigma_R<26%. The rms amplitude and colour for the entire sample are sigma_B=14%, sigma_R=12%, and sigma_{B-R}=5%. On time scales of 100- 1000 days the power spectra of the sources have a power-law shape with index approximately 2.0 and a spread <0.6. At least half of the quasars, particularly those that are most variable, become bluer when they brighten, and the rest do not show this behaviour. We quantify this phenomenon, which has been observed previously mainly in Seyfert galaxies. The quasars which are most variable tend also to exhibit asymmetry in their variations, in the sense that the brightening phases last longer than the fading phases. We have searched for correlations between the measured variability properties and other parameters of the quasars, such as luminosity, redshift, radio loudness, and X-ray slope. We find several new correlations, and reproduce some of the correlations reported by previous studies.

Mass Transfer Processes in Classical Nova Systems around Their Outburst Events and Beyond

A continuous photometric study of 12 novae was carried out for more than 365 nights during the past 4 years at the Wise Observatory in a motivation to extend our knowledge on these systems. Main results: we found observational evidence for the early presence of the accretion disc in Nova V1974 Cyg 1992 (permanent superhumps) and probably also in Nova V1425 Aql 1995 (classified as an intermediate polar candidate). We also predicted the future of superhumps in classical nova systems, and in particular the permanent superhumps in Nova V1974 Cygni 1992. They will either continue to prevail in its light curve, while the brightness of the nova stays well above its pre-outburst level, or the system will continue to decay and eventually evolve into a regular SU UMa system, with superhumps appearing only during superoutbursts.

The shape of the LoTr 5 planetary nebula

We observed the large and faint planetary nebula (PN) around IN Com in H-alpha and [NII] light with a coronagraphic CCD device on the Wise Observatory reflector blocking the light from the central star. Our goal was to provide a second image of the object with which to confirm the features seen in the only published photograph from the paper reporting the discovery of this object. The nebula is extremely faint, but a combination of images totalling ~one and a half hours of exposure shows it fairly well. A novel image processing algorithm has been applied to the noisy image in order to reveal faint extended details of the images. The algorithm is based on a non-linear self-adaptive filter applied to the wavelet transform of the image. The nebula is not round or elliptical, but shows a two-lobed and possibly three-lobed morphology, as well as a peculiar hole-like feature East of the central star. There is definite East-West and slightly less definite North-South asymmetry.

Testing environmental influences on star formation with a sample of Low Surface Brightness dwarf galaxies in the Vigo cluster

AbstractWe analyze the star formation activity of an homogeneous LSB dwarf galaxy sample in the Virgo cluster, as a function of the radial velocity relative to the cluster mean velocity and the projected distance from the center of the cluster, using CCD images obtained at the Wise Observatory. The localized Hα emission in the HII regions of this sample is compared to that of an isolated gas-rich sample of LSB dwarf galaxies and that of a representative sample of Blue Compact Dwarf (BCD) galaxies in the cluster. We report preliminary results on the LSB dwarf star formation histories obtained from surface color distribution.

Wolf-Rayet galaxies at many radio wavelengths and Hα: radio super-star-clusters

We have observed nine Wolf-Rayet galaxies at 20,6, 3.6 and 2 cm from the VLA and at Hα from the Wise Observatory, and have mapped the radio spectral index in each galaxy on sub-arcsec scales. We find that more than half the WR sample has distinct regions where the radio spectrum rises between 6 and 2 cm. We believe that this is the radio signature of the youngest super-star-clusters which are otherwise invisible, and which require NLyc ≳ 1052, or equivalent to 750 stars of the 07 subtype. We call these sources radio super-star-clusters.

Regularities in the long-term optical light curve of the black hole candidate binary A0620 — 00 (V616 Mon)

We have monitored the R and I magnitudes of the black hole candidate system A0620-00 (V616 Mon) in the years 1991-95 at the Wise Observatory. Combining our data with some additional measurements, we analyze a sparsely covered 7 years light curve of the star. We find that the average R-band magnitude is varying on a time scale of a few hundreds days, with a peak to peak amplitude of 0.3 mag. The two maxima in the well known double hump binary cycle, as well as one of the minima between them, vary by a few percent relative to one another in a seemingly random way. One maximum is on the average higher than the other by 0.05 mag. The depth of the second minimum is varying with significantly higher amplitude, in clear correlation with the long term variability of the mean magnitude of the system. It is shallower than the other minimum at times of maximum light, and it becomes deeper at times of minimum system light. According to the commonly acceptable phasing of the binary cycle, the systematically varying minimum corresponds to inferior conjunction of the red dwarf in the system. We cannot suggest any simple geometrical model for explaining the regularities that we find in the long term photometric behaviour of the star.

Star Formation in Dwarf Galaxies

We explore mechanisms for the regulation of star formation in dwarf galaxies. We concentrate primarily on a sample in the Virgo Cluster, which has H I and blue total photometry, for which we collected Hα data at the Wise Observatory. We find that dwarf galaxies do not show the tight correlation of the surface brightness of Hα (a star formation indicator) with the H I surface density, or with the ratio of this density to a dynamical timescale, as found for large disk or starburst galaxies. On the other hand, we find the strongest correlation to be with the average blue surface brightness, indicating the presence of a mechanism regulating the star formation by the older (up to 1 Gyr) stellar population if present, or by the stellar population already formed in the present burst.

Late-type dwarf galaxies in the Virgo cluster -- I. The samples

We selected samples of late-type dwarf galaxies in the Virgo cluster with H i information. The galaxies were observed at the Wise Observatory using several broad-band and Hα bandpasses. UV measurements were carried out with the IUE Observatory from VILSPA, and with the FAUST shuttle-borne UV telescope. We describe the observations in detail, paying particular attention to the determination of measurement errors, and present the observational results together with published data and far-infrared information from IRAS. The sample will be analysed in subsequent papers, in order to study star formation mechanisms in galaxies.

Galaxy candidates in the Zone of Avoidance

Motivated by recent discoveries of nearby galaxies in the Zone of Avoidance (ZOA), we conducted a pilot study of galaxy candidates at low galactic latitude, near galactic longitude l ∼135°, where the Supergalactic plane is crossed by the Galactic plane. We observed with the 1-m Wise Observatory in the l band 17 of the ‘promising’ candidates identified by visual examination of Palomar red plates by Hau et al. A few candidates were also observed in R or B bands, or had spectroscopic observations performed at the Isaac Newton Telescope and at the Wise Observatory. Our study suggests that there are probably 10 galaxies in this sample. We also identify a probable planetary nebula. The final confirmation of the nature of these sources must await the availability of full spectroscopic information. The success rate of ∼50 per cent in identifying galaxies at galactic latitude ǀbǀ < 5° indicates that the ZOA is a promising region to discover new galaxies.

FAUST observations of ultraviolet sources in the direction of Coma

We analyse an ultraviolet observation toward the Coma cluster by means of the FAUST shuttle-borne telescope. We detect 56 candidate sources to a signal-to-noise ratio of 6.5. Most sources have optical counterparts in existing catalogues and are stars. 10 sources with no listed counterparts were observed at the Wise Observatory. We present identifications based on low-resolution spectrophotometry, and discuss the foreground stars and the galaxies, all in the foreground of the Coma cluster. The FAUST image yields a higher fraction of hot evolved stars than either of the North Galactic Pole or the Virgo regions, analysed previously. We identify these tentatively as cooling white dwarfs in the Coma Berenices (Mel 111) open cluster.

FAUST Observations of Ultraviolet Sources toward the Virgo Cluster

We analyze three UV images covering a 100 square degree field toward the Virgo cluster,obtained by the FAUST space experiment. We detect 191 sources to a signal-to-noise ratio of 4.4 and identify 94% of them. Most sources have optical counterparts in existing catalogs and about half are identified as galaxies. Some sources with no listed counterpart were observed at the Wise Observatory. We present the results of low resolution visible spectrophotometry and discuss the foreground 101 stellar sources and the 76 detected galaxies, both in the cluster and in the fore- or background. We derive conclusions on star-formation properties of galaxies and on the total UV flux from discrete and diffuse sources in the cluster. We test for the presence of intra-cluster dust, determine the clustering properties of UV emitting galaxies, and derive the UV luminosity function of Virgo galaxies.

Permanent superhumps in nova V1974 Cygni (1992)

We observed V1974 Cygni with a CCD camera attached to the 1-m telescope of the Wise Observatory for 32 nights during the years 1994 and 1995. The two periodicities identified in the light curve, 0.0812585 day 1.95 hours and 0.085 d P..1 2.04 hr, are interpreted as the orbital period of the binary system and a superhump period, respectively. These results and the interpretation imply that no later than 2.5 years after the outburst of the nova an accretion disk is present in the nova stellar system. They also imply that the disk is in the state of “permanent superhump”. Using the precessing disk model of the superhump phenomenon and utilizing additional published observational results we estimate a mass of 0.75–1.07 M⊙ for the white dwarf in this nova system.

Optical monitoring of luminous AGN - I. Radio-loud quasars

We describe six years of a Band R monitoring program of a sample of 44 radio-loud quasars. The observations were performed at the Wise Observatory, Cerro Tololo Interamerican Observatory and McDonald Observatory with a mean sampling of about 4-8 times a year and a typical accuracy of 0.02-0.03 mag. We show the B, R and B -R light-curves of all sources and discuss the sample properties in terms of variability amplitude, luminosity, continuum shape, radio structure and variability time-scale. All objects in our sample have varied during the monitoring campaign. In 15 per cent of the sources the variability is smaller than 0.2 mag and in 45 per cent between 0.2 and 0.4 mag. The amplitude of the variations does not depend on redshift or luminosity. Furthermore, there is no difference, in this respect, between core-dominated and lobe-dominated sources. Our sampling is adequate to calculate the autocorrelation function of 34 sources in the sample and to use it to define the typical variability time-scale. We find that the rest-frame variability time is inversely correlated with the source luminosity. However, external effects caused by the sampling period and the frequency can strongly influence this conclusion and we suggest ways to try to eliminate such biases. If correct, the inverse correlation with luminosity can dominate any cosmological (1 +z) dependence.

High-Rate Active Galaxy Monitoring at the Wise Observatory. III. The Broad-Line Region of NGC 4151: Erratum

view Abstract Citations References (1) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS High-Rate Active Galaxy Monitoring at the Wise Observatory. III. The Broad-Line Region of NGC 4151: Erratum Maoz, D. ; Netzer, H. ; Mazeh, T. ; Beck, S. ; Almoznino, E. ; Leibowitz, E. ; Brosch, N. ; Mendelson, H. ; Laor, A. Abstract In the paper "High-Rate Active Galaxy monitoring at the Wise Observatory. III. The Broad-Line Region of NGC 4151" by D. Maoz et al. (ApJ, 367,493 [1991]), a correction should be made to the abstract. The fourth sentence of the abstract should be: "Through cross-correlation, we find that both Hα and Hβ lag the continuum variations by 9 +/- 2 days.", and not 9 +/- 12. Publication: The Astrophysical Journal Pub Date: August 1993 DOI: 10.1086/173014 Bibcode: 1993ApJ...413..445M Keywords: ERRATA; ADDENDA full text sources ADS | Related Materials (3) Original Paper: 1991ApJ...367..493M Part 1: 1990ApJ...351...75M Part 2: 1990ApJ...353..108N