Elia M. Leibowitz

Whole earth telescope observations of the DBV white dwarf GD 358

We report on the analysis of 154 hours of early continuous high-speed photometry on the pulsating DB white dwarf (DBV) GD 358, obtained during the Whole Earth Telescope (WET) run of 1990 May. The power spectrum of the light curve is dominated by power in the range from 1000 to 2400 microHz with more than 180 significant peaks in the total spectrum. We identify all of the triplet frequencies as degree l = 1, and from the details of their spacings we derive the total stellar mass as 0.61 + or - 0.03 solar mass, the mass of the outer helium envelope as 2.0 + or - 1.0 x 10(exp -6) M(sub *), the absolute luminosity as 0.050 + or - 0.012 solar luminosity and the distance as 42 + or - 3 pc. We find strong evidence for differential rotation in the radial direction -- the outer envelope is rotating at least 1.8 times faster than the core -- and we detect the presence of a weak magnetic field with a strength of 1300 + or - 300 G. We also find a significant power at the sums and differences of the dominant frequencies, indicating nonlinear processes are significant, but they have a richness and complexity that rules out resonant mode coupling as a major cause.

Understanding the Cool DA White Dwarf Pulsator, G29-38

The white dwarfs are promising laboratories for the study of cosmochronology and stellar evolution. Through observations of the pulsating white dwarfs, we can measure their internal structures and compositions, critical to understanding post main sequence evolution, along with their cooling rates, allowing us to calibrate their ages directly. The most important set of white dwarf variables to measure are the oldest of the pulsators, the cool DAVs, which have not previously been explored through asteroseismology due to their complexity and instability. Through a time-series photometry data set spanning ten years, we explore the pulsation spectrum of the cool DAV, G29-38 and find an underlying structure of 19 (not including multiplet components) normal-mode, probably l=1 pulsations amidst an abundance of time variability and linear combination modes. Modelling results are incomplete, but we suggest possible starting directions and discuss probable values for the stellar mass and hydrogen layer size. For the first time, we have made sense out of the complicated power spectra of a large-amplitude DA pulsator. We have shown its seemingly erratic set of observed frequencies can be understood in terms of a recurring set of normal-mode pulsations and their linear combinations. With this result, we have opened the interior secrets of the DAVs to future asteroseismological modelling, thereby joining the rest of the known white dwarf pulsators.

High-rate active galaxy monitoring at the wise observatory. III, The broad-line region of NGC 4151

Results of an AGN monitoring program for the Seyfert galaxy NGC 4151 are presented. The space and velocity distribution of the broad large region (BLR) gas in this AGN are analyzed. Through cross-correlation, it is found that both Hα and Hβ lag the continuum variations by 9 ± 12 days. Through Fourier and maximun entropy deconvolutions of the continuum and emission-line curves, the approximate transfert function of the BLR in this object is recovered.

X-ray spectroscopy of the 2006 outburst of RS Ophiuchi

We present X-ray grating spectra of the recurrent nova RS Ophiuchi during its 2006 outburst, obtained with XMM-Newton and Chandra. For the first month after optical maximum, the X-ray spectrum was hard and dominated by emission lines of H-like and He-like ions. The X-ray luminosity was 2.4 × 1036 ergs s−1 in the 0.33-10 keV range. The spectra indicate a collisionally dominated plasma with a broad range of temperatures and an energy-dependent velocity structure. During an observation obtained in week 4, a soft X-ray flare occurred in which a new system of soft, higher velocity emission lines appeared in the spectrum. Then, during weeks 6-10, the supersoft continuum of the hot white dwarf atmosphere was the dominant emission component. The X-ray luminosity reached at least 9 × 1037 ergs s−1 in the 0.2-1 keV range, while the intrinsic nebular absorption decreased by a factor of 5 since the first observation. Preliminary model fitting indicates a white dwarf temperature of ~800,000 K, and a mass of at least 1.2 M☉. Therefore, RS Oph may be an important Type Ia supernova progenitor. We show that the data are consistent with mass loss ending before day 54 of the outburst, and nuclear burning ending around day 69. A rapid decay in X-ray luminosity followed after week 10. The X-ray luminosity 5, 7, and 8 months after optical maximum dropped by more than 2 orders of magnitude. The spectra do not appear to be consistent with emission from an accretion disk.

A detection of the evolutionary time scale of the DA white dwarf G117- B15A with the whole earth telescope

The time rate of change for the main pulsation period of the 13,000 K DA white dwarf G117 - B15A has been detected using the Whole Earth Telescope (WET). The observed rate of period change, P(dot) = (12.0 + or - 3.5) x 10 to the -15th s/s, is somewhat larger than the published theoretical calculations of the rate of period change due to cooling, based on carbon core white dwarf models. Other effects that could contribute to the observed rate of period change are discussed.

High-rate spectroscopic active galactic nucleus monitoring at the Wise Observatory. I. Markarian 279

Spectrophotometric monitoring of a sample of AGNs has achieved a temporal sampling rate of once every 3-4 days, over a period of 5-7 months. By observing each object simultaneously with a nearby field star, in a long-slit spectroscopy mode, accurate flux measurements were obtained even during bad weather and full moon. Results for the Seyfert 1 galaxy MrK 279 are discussed. This object brightened by 20-50 percent in H-alpha, H-beta, and the optical continuum during the period of observation. The high sampling rate and small error allow the determination, through cross-correlation, of the lag in the emission-line response to the continuum brightening. A lag of 12 days is observed. The size of the broadline region (BLR) is estimated to be 12 + or - 3 light days in this object; the results rule out, at the 95 percent significance level, a BLR of 21 light-days or larger. This result is in conflict with the standard photoionization model of AGNs, which predicts a BLR size larger by an order of magnitude. 27 refs.

High-rate spectroscopic active galactic nucleus monitoring at the wise observatory. II: NGC 5548

Les observations spectroscopiques de la galaxie de Seyfert de type 1, NGC 5548, sont analysees. Les fonctions de correlation croisee entre le continuum et les raies Hα et Hβ sont etudiees. La taille de la region de raies larges est determinee

Asteroseismology of a Star Cooled by Neutrino Emission: The Pulsating Pre-White Dwarf PG 0122+200

Observation of g-mode pulsations in the variable pre-white dwarf (GW Virginis) stars provides a unique means to probe their interiors and to study the late stages of stellar evolution. Multisite campaigns have in several cases proved highly successful in decoding pre-white dwarf light curves. Three previous attempts to untangle the pulsation spectrum of the coolest GW Virginis star, PG 0122+200, confirmed the existence of multiple g-modes but left the fundamental period spacing and therefore the stars mass and luminosity in doubt. We present an analysis based on new observations of PG 0122+200 obtained during a Whole Earth Telescope (WET) campaign conducted in the fall of 1996. Although our coverage was, because of bad weather, far poorer than in previous WET campaigns, we confirm the previous result that PG 0122+200 rotates once in 1.6 ± 0.1 days. The most likely period spacing supported by the data implies a mass of 0.69±0.03 M☉. Based on the best seismology we can currently do, the cooling of PG 0122+200 is dominated by neutrino losses. This is not true for all pre-white dwarf stars and makes PG 0122+200 the prime candidate for learning useful physics. Constraints placed on the cooling rate of PG 0122+200 by future measurement of dΠ/dt could provide a unique test of the standard theory of lepton interactions in the (experimentally unexplored) region of phase-space appropriate to pre-white dwarf interiors.

Multiperiodic variations in the last 104-yr light curve of the symbiotic star BF Cyg

We analyse a light curve (LC) of the symbiotic star BF Cyg, covering 114 yr of its photometric history. The star had a major outburst around the year 1894. Since then the mean optical brightness of the system is in steady decline, reaching only in the last few years its pre-outburst value. Superposed on this general decline are some six less intense outbursts of 1‐2 mag and duration of 2000‐5000 d. We find a cycle of 6376 d, or possibly twice this period, in the occurrence of these outbursts. We suggest that the origin of the system outbursts is in some magnetic cycle in the outer layers of the giant star of the system, akin to the less intense 8000-d magnetic cycle of our Sun. We further find, that in addition to its well-known binary period of 757.3 d, BF Cyg possesses also another photometric period of 798.8 d. This could be the rotation period of the giant star of the system. If it is, the beat period of these two periodicities, 14 580 d, is the rotation period of a tidal wave on the surface of the giant. A fourth period of 4436 d, the beat period of the 14 580-d and the 6376-d cycles is possibly also present in the LC. We predict that BF Cyg will be at the peak of its next outburst around the month of May in the year 2007. The newly discovered 798.8-d period explains the disappearance of the orbital modulation at some epochs in the LC. The 757.3-d oscillations will be damped again around the year 2013.

REGULARITIES IN THE LONG-TERM OPTICAL LIGHT CURVE OF THE BLACK HOLE CANDIDATE BINARY A0620 - 00 (V616 MON)

ABSTRACT We have monitored the R and I magnitude of the black hole candidate system A0620-00 (V616 Mon) in the years 1991-1995 at the Wise Observatory. Combining our datawith some additional measurements, we analyze a sparsely covered 7 year light curveof the star. We find that the average R-band magnitude is varying on a time scale ofa 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 theminima between them, vary by a few percent relative to one another, in a seeminglyrandom way. One maximum is on the average higher by 0.05 mag. than the other.The depth of the second minimum is varying with significantly higher amplitude, inclear 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. It deepens whenthe system brightness declines, and it becomes the deeper among the two minima attimes of minimum system light.According to the commonly acceptable phasing of the binary cycle, the system-atically varying minimum corresponds to inferior conjunction of the red dwarf. Wecannot suggest any simple geometrical model for explaining the regularities that wefind in the long term photometric behaviour of the V616 Mon binary system.Key words: binaries: close - stars: individuals: A0620-00 - X-ray: stars