Search

We discuss the derivation and behaviour of temperatures and densities of nova ejecta during the nebular stage. A systematic comparison is made between fairly complete sets of abundances for 3 C/0 and 2 Neon novae. The abundances in the ejecta might influence the formation and composition of circumstellar dust in novae.

We present results for IUE, optical and IR observations of Nova Muscae 1983, from early outburst to January 1986 obtained by the European IUE Target of Opportunity Team. A detailed description of the data will appear elsewhere (Hassall et al., 1989), but here we summarise the most important results. The outburst lightcurve initially indicated a fast speed class for this nova, but was later characterised by a rather slow optical decline with two or more secondary outbursts with sudden doubling of the bolometric flux. In Figure 1 , we show the contributions of X-ray, UV, optical and IR to the total luminosity for 1200 days following outburst, assuming a distance of 4.3kpc and an interstellar extinction E(B-V)=0.5. In the absence of dust formation, first the UV and later the X-ray flux (Ogelman et al, 1984) dominate the radiative energy late into the nebular phase. There was a plateau stage lasting about 500 days, with a bolometric luminosity of ~ 10 38 ergs s −1 near the Eddington limit. The secondary outbursts were thus super-Eddington. We estimate a total outburst energy (including kinetic and gravitational potential energy of the ejecta) of ~ 5·l0 46 ergs, corresponding to a mass of ~ 4·10 −6 M o of hydrogen burnt in the thermonuclear runaway.

Une analyse en composantes principales est effectuee sur les spectres IUE de la galaxie Seyfert NGC 4151 en recherchant le nombre et la forme de spectres lineairement independants qui sont necessaires pour reconstruire les donnees dans les limites d'erreur

We present results on the variability of the absorption lines, the emission lines and the X-ray to optical continuum of NGC 4151. The ultraviolet and optical continua vary together and the emission lines follow these continuum changes with delays depending on the line studied. The strength of the absorption lines depends on the continuum strength too but the variability pattern is rather complicated. When the ultraviolet/optical continuum is bright there is no correlation with the X-rays. When this continuum is faint the X-ray flux and the optical/ultraviolet continuum are strictly correlated. Results from Principle Component Analysis of the ME spectra are discussed.

An 8 month monitoring campaign on the Seyfert 1 galaxy Fairall 9 has been conducted with the International Ultraviolet Explorer in an attempt to obtain reliable estimates of continuum-continuum and continuum-emission-line delays for a high-luminosity active galactic nucleus (AGN). While the results of this campaign are more ambiguous than those of previous monitoring campaigns on lower luminosity sources, we find general agreement with the earlier results: (1) there is no measurable lag between ultraviolet continuum bands, and (2) the measured emission-line time lags are very short. It is especially notable that the Lyα + N v emission-line lag is about 1 order of magnitude smaller than determined from a previous campaign by Clavel, Wamsteker, & Glass (1989) when Fairall 9 was in a more luminous state. In other well-monitored sources, specifically NGC 5548 and NGC 3783, the highest ionization lines are found to respond to continuum variations more rapidly than the lower ionization lines, which suggests a radially ionization-stratified broad-line region. In this case, the results are less certain, since none of the emission-line lags are very well determined. The best-determined emission line lag is Lyα + N v, for which we find that the centroid of the continuum-emission-line cross-correlation function is τcent ≈ 14-20 days. We measure a lag τcent ≲ 4 days for He II λ1640; this result is consistent with the ionization-stratification pattern seen in lower luminosity sources, but the relatively large uncertainties in the emission-line lags measured here cannot rule out similar lags for Lyα + N v and He II λ1640 at a high level of significance. We are unable to determine a reliable lag for C IV λ1550, but we note that the profiles of the variable parts of Lyα and C IV λ1550 are not the same, which does not support the hypothesis that the strongest variations in these two lines arise in the same region.

We present the results of an intensive ultraviolet monitoring campaign on the Seyfert 1 galaxy NGC 4151, as part of an effort to study its short time-scale variability over a broad range in wavelength. The nucleus of NGC 4151 was observed continuously with the {\it International Ultraviolet Explorer} (IUE) for 9.3 days, yielding a pair of LWP and SWP spectra every $\sim$70 minutes, and during four-hour periods for 4 days prior to and 5 days after the continuous monitoring period. The sampling frequency of the observations is an order of magnitude higher than that of any previous UV monitoring campaign on a Seyfert galaxy. The continuum fluxes in bands from 1275 \AA\ to 2688 \AA\ went through four significant and well-defined ``events'' of duration 2 -- 3 days during the continuous monitoring period. We find that the amplitudes of the continuum variations decrease with increasing wavelength, which extends a general trend for this and other Seyfert galaxies to smaller time scales (i.e., a few days). The continuum variations in all of the UV bands are {\it simultaneous} to within an accuracy of about 0.15 days, providing a strict constraint on continuum models. The emission-line light curves show only one major event during the continuous monitoring (a slow rise followed by a shallow dip), and do not correlate well with continuum light curves over the (short) duration of the campaign, because the time scale for continuum variations is apparently smaller than the response times of the emission lines., Comment: 39 pages, LaTeX, including 7 PostScript figures; To appear in the ApJ (October 20, 1996) Vol. 470

We present the data and initial results from a combined Hubble Space Telescope (HST)/IUE/ground-based spectroscopic monitoring campaign on the Seyfert 1 galaxy NGC 5548 that was undertaken in order to address questions that require both higher temporal resolution and higher signal-to-noise ratios than were obtained in our previous multiwavelength monitoring of this galaxy in 1988-1989. IUE spectra were obtained once every 2 days for a period of 74 days beginning on 1993 March 14. During the last 39 days of this campaign, spectroscopic observations were also made with the HST Faint Object Spectrograph (FOS) on a daily basis. Ground-based observations, consisting of 165 optical spectra and 77 photometric observations (both CCD imaging and aperture photometry), are reported for the period 1992 October-1993 September, although many of the data are concentrated around the time of the satellite-based program. These data constitute a fifth year of intensive optical monitoring of this galaxy. In this contribution we describe the acquisition and reduction of all of the satellite and ground-based data obtained in this program. We describe in detail various photometric problems with the FOS and explain how we identified and corrected for various anomalies.

We report on the results of intensive ultraviolet spectral monitoring of the Seyfert 1 galaxy NGC 3783. The nucleus of NGC 3783 was observed with the International Ultraviolet Explorer satellite on a regular basis for a total of 7 months, once every 4 days for the first 172 days and once every other day for the final 50 days. Significant variability was observed in both continuum and emission-line fluxes. The light curves for the continuum fluxes exhibited two well-defined local minima or 'dips,' the first lasting is less than or approximately 20 days and the second is less than or approximately 4 days, with additional episodes of relatively rapid flickering of approximately the same amplitude. As in the case of NGC 5548 (the only other Seyfert galaxy that has been the subject of such an intensive, sustained monitoring effort), the largest continuum variations were seen at the shortest wavelengths, so that the continuum became 'harder' when brighter. The variations in the continuum occurred simultaneously at all wavelengths (delta(t) is less than 2 days). Generally, the amplitude of variability of the emission lines was lower than (or comparable to) that of the continuum. Apart from Mg II (which varied little) and N V (which is relatively weak and badly blended with Ly(alpha), the light curves of the emission lines are very similar to the continuum light curves, in each case with a small systematic delay or 'lag.' As for NGC 5548, the highest ionization lines seem to respond with shorter lags than the lower ionization lines. The lags found for NGC 3783 are considerably shorter than those obtained for NGC 5548, with values of (formally) approximately 0 days for He II + O III), and approximately 4 days for Ly(alpha) and C IV. The data further suggest lags of approximately 4 days for Si IV + O IV) and 8-30 days for Si III + C III). Mg II lagged the 1460 A continuum by approximately 9 days, although this result depends on the method of measuring the line flux and may in fact be due to variability of the underlying Fe II lines. Correlation analysis further shows that the power density spectrum contains substantial unresolved power over timescales of is less than or approximately 2 days, and that the character of the continuum variability may change with time.

From the examination of all the low-dispersion IUE spectra of NGC 4151 taken since the launch of IUE in 1978, including the latest data taken in 1990, we follow the evolution of this active nucleus on time scales ranging from days to 10 years. We concentrate our analysis on the campaigns of observations during which the nucleus was not continuously in a minimum state.

Analyse detaillee du spectre d'absorption UV (λλ 1150-3200 A) du noyau de NGC 4151. On a identifie plus de 30 raies d'absorption UV

This paper examines the Mg II 2795-2802 A doublet in the Seyfert galaxy NGC 4151 at a higher resolution than has previously been used, searching for velocity systems in absorption and emission. Evidence is presented for a new, narrow, outflowing absorption system in Mg II having a velocity of 825 km/s relative to the sun, and -165 km/s relative to the systemic velocity of NGC 4151. This feature is not present in Ly-alpha or C IV and possible explanations for this are considered. For the Mg II and C IV lines, a model decomposition of the line profile is presented.

Le spectre revele la presence de bandes d'emission Fe II larges et intenses semblables a celles de nombreux objets a raies larges. Les observations confirment la recente decouverte d'un spectre optique du type s de Seyfert dans NGC 1068, faite par Antonucci et Miller (Astr. J., 1985, 297, 621)

Observations of the hot interstellar gas (T ≫ 104 K) are numerous and imply that the temperatures of substantial amounts of the disk gas to be 2–3 × 105 K and of the halo gas to be 8 × 104 K, but the theoretical models to explain the properties of both the disk and the halo hot gas seem to be in need of some refinement. Observations of the quasar narrow line absorption features suggest that they arise in gas with properties very similar to those of the galactic coronal gas.

On presente les resultats du programme d'observation photographique de la galaxie de Seyfert NGC 4151 pour la periode allant de fevrier 1968 a mars 1983. On a construit une courbe de lumiere optique qui couvre l'epoque pendant laquelle la galaxie a ete observee dans l'UV, 1978-83

A report on the initial performance of the IUE satellite, launched in January, 1978, is given. Attention is given to optical performance, camera performance, background corrections, photometric performance, and data reduction. Samples of high-dispersion and low-dispersion spectra are shown. A remaining problem of proper separation of high dispersion orders in data reduction is discussed.

The paper describes the general design, instrumentation, and ground control system of the International Ultraviolet Explorer (IUE) satellite, which will serve as an astronomical observatory providing both high and low resolution UV spectra of sources other than the sun. Basic data are given on the telescope, the UV-visible converter, the spectrograph optics, the camera response, and the communications systems.

Results from a preliminary study of interstellar absorption lines in the spectra of the three stars HD149757, HD93521 and HD153919, observed by the IUE during its commissioning phase are presented. They demonstrate the capabilities of the IUE for interstellar research. Results from a preliminary study of the emission line spectrum of the planetary nebula NGC 7027 are outlined and used to study its interstellar reddening.

The classical nova outburst is the second most violent explosion that occurs in a galaxy and its violence is exceeded only by a supernova explosion. However, novae are relatively nearby and a nova outburst occurs much more frequently than a supernova outburst so that there are more than 200 outbursts tabulated and discussed by Payne-Gaposchkin (1957) and McLaughlin (1960). The first ultraviolet (hereafter: UV) study of a nova was that of the 1970 outburst of FH Ser (Gallagher and Code, 1974) and that was broad band photometry done by the OAO—A2 satellite. The first UV spectroscopic study was that of V1500 Cyg 1975 done with Copernicus (Jenkins et al., 1977) but it was such a fast nova that the only line visible when the spectrum was taken was Mg II λ2800. Nova Cyg was also studied with the ANS satellite (Wu and Kester, 1975).

Results from observations of the Seyfert galaxy NGC 4151 obtained with Exosat (0.1-25 keV) and IUE (1200-3000 A) during November 1983, and between December 1984 and January 1985, are presented. The stability of the flux during large-scale variations in the UV and X-ray regions suggests that the soft X-ray excess noted by Holt et al. (1980) represents a new spectral component whose temperature is estimated to be 1.8 million K, and luminosity to be 1.7 x 10 to the 42nd erg/s, if it is thermal emission from a hot gas. The observed variable UV and X-ray continua are well correlated down to a time scale of days, and the standard model of a power law with a uniform cold absorber gives a bad fit at low energies in the 1.2-25 keV range. The spectral index of the power law is found to correlate closely with the flux in the 2-10 keV range, with a shallower slope for lower intensity, and the iron fluorescence line at 6.4 keV is seen to respond very slowly to changes in the continuum.

Fitting technics are applied to study the variations of the 1550 line profile and adjacent features in the IUE spectrum of the Seyfert galaxy NGC 4151, from 1978 up to 1983. New important results are found concerning the kinematics and physics of the emission line gas at a few light-days from the continuum source.

The nucleus of the Seyfert galaxy NGC 4151 has been extensively monitored with the International Ultraviolet Explorer (IUE) in the wave- length range 1200–3000 A, a region of the spectrum where the contamination by the stellar component of the nuclear light is negligible. A first detailed account of the data collected from February 1978 to January 1979 is given in Penston et al (1981), and three more papers devoted to the continuum, the emission and the absorption lines based on all IUE data up to May 1980 are in preparation. This note is a summary of the paper on the continuum, which includes also optical and X-ray observations (Perola et al, 1981).

The 1985 outburst of the bright, recurrent nova RS Oph was almost simultaneously observed at X-ray, UV, optical, IR and radio frequencies at many epochs. The abundances in the ejected shell and the development of the bolometric luminosity as a function of time suggest that the cause of the outburst is a nuclear runaway on a massive white dwarf.

The Seyfert galaxy NGC4151 observed when the nucleus is at a minimum has two emission lines of full width at half maximum less than 7 and 16 A, respectively, and varying in intensity by a factor of three in 10 days. These lines are too narrow to be emitted by the whole broad-line region and must arise instead from two localized regions which have a special excitation mechanism, possibly a two-sided jet.

NGC 4151 was observed four times in Nov. 83. The results indicate that: (1) there exists a correlation between the X-ray and UV fluxes on the long term; (2) the soft X-ray excess between 0.1 and 1 keV is probably steeper than expected from the leaky absorber model by Holt et al. (1980); (3) the spectral fit to the ME data, after correction for a soft component, yields α = 1.73±0.27, NH = (15.2±2.2)×1022cm-2, E.W.(Fe line) = 0.208±0.084 keV, and does not require a strong overabundance of Fe in the absorber. The relationship between NH and the strength of the broad emission lines is commented.

During the commissioning phase of IUE several extragalactic objects were observed spectrally at low dispersion in the UV range lambda lambda 1150-3200: the Seyfert galaxies NGC4151 and NGC1068, the QSO 3C273, the BL Lacertae object B2 1101+38, the giant elliptical galaxy M87 and the spiral galaxy M81. The results obtained are presented and a preliminary analysis given for all six objects, discussing the continuous spectrum, extinction, emission line spectrum and absorption line spectrum, where possible for each case. Several new or confirmatory astrophysical results are obtained.

The peculiar galaxy NGC1275 = Per A = 3C84 remains a problem in the classification of extragalactic sources despite intensive study in all available wavebands. Included in Seyfert's original list1, it was described as a Seyfert 2 by Khachikian and Weedman2 on the basis of observed emission line profiles. Later Weedman3, however, declined to classify NGC1275 and most recently Veron4 has suggested that on the basis of its emission line spectrum, variability and radio and polarimetric properties we should regard NGC1275 as a (rather anomalous) BL Lac object. We present here results of IUE spectrophotometry of NGC1275 which show an absorption feature identified with Lyα at a velocity of 8,200 km s−1. We attribute the origin of this feature to the intervening galaxy responsible for the high velocity emission filaments seen around NGC1275. Profile fitting yields a neutral hydrogen column density in the absorbing medium NH ∼ 1.8 × 1020 atoms cm−2. Combining this result with available 21-cm absorption data yields a spin temperature Ts ∼ 52 K for the absorbing gas, typical of that in interstellar H I clouds seen in our Galaxy. The implications of these results are discussed.

The 1450 - 1720 Å spectral region of NGC 4151 is analyzed by means of Gaussian decomposition for 69 different epochs, from early 1978 to the end of 1983. The C IV λ1550 emission profile is well represented by a core, whose full width at half-maximum varies between 2600 and 5500 km s-1, and an ultrabroad component (14,600 km s-1). The variations of the ultrabroad feature suggest that it originates in a region whose "radius" is ≡5 lt-day. The analysis of lines from He II, C III], N IV] and O III] indicates the presence of two additional discrete broad line subregions. A study of the velocity dispersions indicates that the motion of the gas is Keplerian and yields a value of (3.7±0.5)×107M_sun; for the central mass. A correlation is found between the wavelength of the ultrabroad C IV λ1550 component and its intensity. It strongly suggests the existence of a decelerated outflow whose velocity is 4000 km s-1 at 5 lt-day from the nucleus.

Non-LTE calculations for the ultraviolet C I and Si I continuous opacity show that LTE results overestimate the importance of these sources of opacity and underestimate the emergent flux in Alpha Lyr. The largest errors occur between 1100 and 1160 A, where the predicted flux in non-LTE is as much as 50 times larger than in LTE, in reasonable accord with Copernicus observations. The discrepancy between LTE models and observations has been interpreted to result from the existence of a chromosphere. Until a self-consistent non-LTE model atmosphere becomes available, such an interpretation is premature.

Equivalents widths of He II lines in the series n=2,3,4 and 5 are compiled and compared with predictions from plane-parallel, static model atmospheres using a non-LTE theory of line formation. The agreement between observation and prediction for a (50,000,4.0) model atmosphere is good for the upper members of the n=3 and the n=5 series, but the two lines of the n=2 series which are observed and the upper members of the n=4 series (4$Yields$15,4$Yields$17, etc.) are stronger than predicted. Well-determined profiles of lines from the n=3 series indicate v sin i=200 km s$sup -1$. Profiles of the higher members of the n=4 series, however, do not match the predictions, the observed line cores being deeper than predicted. The n=4 level appears to be more overpopulated at moderate depths in the atmosphere than the non-LTE calculations with plane-parallel layers indicate. This may be due to an overlap of the H and He II lines in the even-even series caused by macroturbulent velocities of the hydrogen atoms and helium atoms. (AIP)

A bibliography of atomic-line-identification lists is presented to supplement the material contained in the Ultraviolet and Revised Multiplet Tables and in the finding list by Kelly and Palumbo (1973). The list covers the wavelength range from 911 A to 8205 A.