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1.	Clavel, J., et al. (författare) Steps toward determination of the size and structure of the broad-line region in active galactic nuclei. I. An 8 month campaign of monitoring NGC 5548 with IUE 1991 Ingår i: Astrophysical Journal. - : American Astronomical Society. - 0004-637X .- 1538-4357. ; 366:1, s. 64-8181 Tidskriftsartikel (refereegranskat)abstract The authors present emission-line and ultraviolet continuum observations of a type I Seyfert galaxy in which the time resolution is adequate for describing the character of variability. Using the IUE satellite, the nucleus of NGC 5548 was observed every 4 days for a period of 8 months. Its mean properties-continuum shape, line ratios-are not unusual for type I Seyfert galaxies, but it was found to be strongly variable. The ultraviolet continuum flux and broad emission line fluxes varied significantly, going through three large maxima and three deep minima. The great majority of all variations were well resolved in time. The data lend qualitative support to the view that photoionization by the nuclear continuum is responsible for driving the emission lines
2.	Crenshaw, D. M., et al. (författare) Multiwavelength observations of short-timescale variability in NGC 4151. I. Ultraviolet observations 1996 Ingår i: Astrophysical Journal. - : American Astronomical Society. - 0004-637X .- 1538-4357. ; 470:1, s. 322-335335 Tidskriftsartikel (refereegranskat)abstract Presents the results of an intensive ultraviolet monitoring campaign on the Seyfert 1 galaxy NGC 4151, as part of an effort to study its short-timescale variability over a broad range in wavelength. The nucleus of NGC 4151 was observed continuously with the International Ultraviolet Explorer for 9.3 days, yielding a pair of LWP and SWP spectra every ~70 minutes, and during 4 hr 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 to 2688 Aring went through four significant and well-defined ldquoeventsrdquo of duration 2-3 days during the continuous-monitoring period. The authors 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 timescales (i.e., a few days). The continuum variations in all the UV bands are simultaneous to within an accuracy of ~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 timescale for continuum variations is apparently smaller than the response times of the emission lines
3.	Edelson, R. A., et al. (författare) Multiwavelength observations of short-timescale variability in NGC 4151. IV. Analysis of multiwavelength continuum variability 1996 Ingår i: Astrophysical Journal. - : American Astronomical Society. - 0004-637X .- 1538-4357. ; 470:1, s. 364-377377 Tidskriftsartikel (refereegranskat)abstract For pt.III see ibid., vol.470, no.1, p.349-63 (1996). Combines data from the three preceding papers in order to analyze the multi wave-band variability and spectral energy distribution of the Seyfert 1 galaxy NGC 4151 during the 1993 December 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 (~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 ~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 to 6900 Aring. These data do not probe extreme ultraviolet (1200 Aring to 0.1 keV) or hard X-ray (250 keV) variability. The phase differences between variations in different bands were consistent with zero lag, with upper limits of lsim0.15 day between 1275 Aring and the other ultraviolet bands, lsim0.3 day between 1275 Aring and 1.5 keV, and lsim1 day between 1275 and 5125 Aring. These tight limits represent more than an order of magnitude improvement over those determined in previous multi-wave-band 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 (ales-2.5)
4.	Penston, M. V., et al. (författare) The extended narrow line region of NGC 4151. I. Emission line ratios and their implications 1990 Ingår i: Astronomy and Astrophysics. - 0004-6361 .- 1432-0746. ; 236:1, s. 53-6262 Tidskriftsartikel (refereegranskat)abstract The paper presents the first results from long-slit spectra of the Seyfert galaxy NGC 4151 which give average diagnostic ratios of weak lines in the extended narrow line region (ENLR) of that galaxy and the first direct density measurement in an ENLR. These data confirm that the ENLR is kinematically undisturbed gas in the disc of the galaxy which is illuminated by an ionizing continuum stronger by a factor of 13 than a power law interpolated between observed ultraviolet and X-ray fluxes. Explanations of this apparent excess include a hot thermal continuum, time variations and an anisotropic radiation field. The authors give reasons for favouring anisotropy which might be caused by shadowing by a thick accretion disc or by relativistic beaming. Shadowing by a molecular torus is unlikely, given the absence of an infrared signal from the reradiated flux absorbed by any torus. Anisotropy would have important implications for the bolometric luminosity and nature of active galactic nuclei
5.	Dietrich, M., et al. (författare) Monitoring of active galactic nuclei. IV. The Seyfert 1 galaxy NGC 4593 1994 Ingår i: Astronomy and Astrophysics. - 0004-6361 .- 1432-0746. ; 284:1, s. 33-4343 Tidskriftsartikel (refereegranskat)abstract Results of a five-month campaign of optical monitoring the Seyfert 1 galaxy NGC 4593 are presented. High resolution Halpha and Hbeta spectra and direct images in the Johnson U, B, V, R and I bands were obtained between January and June 1990. The emission lines and the continuum showed strong variations on time scales of weeks to days. The Halpha line varied by more than 30% within only 5 days. Cross-correlating the light curves of the Balmer lines with that of the optical continuum gives a lag of around 4 days. Therefore, this low luminosity Seyfert galaxy may have one of the smallest broad line regions known so far
6.	Erkens, U., et al. (författare) Monitoring of active galactic nuclei. VI. The quasar Mrk 876 1995 Ingår i: Astronomy and Astrophysics. - 0004-6361 .- 1432-0746. ; 296:1, s. 90-9898 Tidskriftsartikel (refereegranskat)abstract For pt. V see ibid., vol. 285, no. 3, p. 857-67 (1994). We present the results of a 6 month monitoring campaign of Mrk 876, a low luminosity quasar with M absap-24. High resolution spectroscopy and direct imaging in the Johnson filters B, V, R, I was carried out in the first half of 1990. Line and continuum light curves are presented. Mrk 876 showed a decrease of 17% in the continuum over the whole observing period whereas the line flux of Hbeta and Halpha stayed constant. This implies, that the radius of the broad line region is larger than 6 light months. Coadded spectra of Hbeta and Halpha are presented which confirm an extreme asymmetry of the line profiles to the red. With respect to much earlier spectra the line profiles changed considerably
7.	Jackson, N., et al. (författare) Monitoring of active galactic nuclei. I. The quasars 1302-102 and 1217+023 1992 Ingår i: Astronomy and Astrophysics. - 0004-6361 .- 1432-0746. ; 262:1, s. 17-2525 Tidskriftsartikel (refereegranskat)abstract Presents the results of a five-month spectroscopic monitoring campaign of two radio-loud quasars PKS 1217+023 and PKS 1302-102. There is a pronounced lack of variability in 1302-102 apart from a possible small change in the continuum level near the end of the monitoring period, but 1217+023 underwent a 20% continuum decrease in the middle of the campaign. No line variability at all was observed. The co-added data represent two of the best spectra yet obtained of any radio-loud quasar other than 3C 273, and the shape of the Hbeta line and the presence underneath it of other contaminating lines are well determined. The Hbeta broad line in 1302-102 is redshifted by 3 Aring with respect to the narrow line region and there is evidence for a separate broad Hbeta feature on top of the main Hbeta emission in both quasars. This feature is stronger in 1217+023 than in 1302-102
8.	Salamanca, I., et al. (författare) Spectroscopic monitoring of active galactic nuclei. III. Size of the broad line region in NGC 3227 1994 Ingår i: Astronomy and Astrophysics. - 0004-6361 .- 1432-0746. ; 282:3, s. 742-752752 Tidskriftsartikel (refereegranskat)abstract For pt.II see ibid., vol.269, no.1-2, p.39-53 (1993). The authors present the results of a five-month monitoring campaign of the active galactic nucleus in NGC 3227. The Hbeta and Halpha emission lines and the optical continuum have been analysed. The optical AGN continuum and the broad-line region (BLR) emission in NGC 3227 appear to vary by about 40% on a timescale of 1.5 month. The levels of the broad-line emission and the 675.0 nm continuum are well correlated, a fact consistent with the assumption that the BLR material is photoionised by the central continuum source. Cross-correlation analyses between the light curves of the 675.0 nm AGN continuum and the BLR H emission indicate that the lag of the BLR emission with respect to the continuum variations is 17plusmn7 days. This result suggests that the BLR clouds that are affected by the changes in the flux of the central ionising source lie about 17 light days away from it
9.	Stirpe, G. M., et al. (författare) Monitoring of active galactic nuclei. V. The Seyfert 1 galaxy Markarian 279 1994 Ingår i: Astronomy and Astrophysics. - 0004-6361 .- 1432-0746. ; 285:3, s. 857-867867 Tidskriftsartikel (refereegranskat)abstract For pt. IV see ibid., vol.284, no.1, p.33-43 (1994). Reports on the Lovers of Active Galaxies' (LAG) monitoring of the Seyfert 1 galaxy Markarian 279 from January to June 1990. The source, which was in a very bright state, gradually weakened after the first month of monitoring: the Halpha and Hbeta flux decreased by 20% and 35% respectively, and the continuum under Halpha by 30%. The luminosity-weighted radius of the broad line region (BLR), as derived from the cross-correlation function, is of the order of 10 light days. This result is very uncertain because the features in the light curves are very shallow, but it is unlikely that the radius of the BLR is more than 1 light month. The profile variations of Halpha confirm that the prevailing motions are not radial. The data of the present campaign and those obtained in previous years, when the source was in a much weaker state, show that the red asymmetry of the Balmer lines correlates positively with the broad line flux. This new effect is briefly discussed
10.	Wanders, I., et al. (författare) Spectroscopic monitoring of active galactic nuclei. II. The Seyfert-1 galaxy NGC 3516 1993 Ingår i: Astronomy and Astrophysics. - 0004-6361 .- 1432-0746. ; 269:1-2, s. 39-5353 Tidskriftsartikel (refereegranskat)abstract The authors present the results of a five month spectroscopic monitoring campaign of the Seyfert-1 galaxy NGC 3516. Using a new calibration method and applying a correction for seeing differences during the various observations they can scale the spectra to each other with a much higher accuracy than hitherto achieved. NGC 3516 shows large amplitude variations on a time scale of several weeks. Asymmetric profile variations occur on the same time scale as the continuum variations. The authors show the presence of a varying dip on the blue wing of the Hbeta profile, which is not present on the Halpha profile, and which corresponds to the absorption features previously seen in the UV emission lines of NGC 3516. From cross-correlation analysis they find a time lag of 14plusmn2 days for the Halpha and 7plusmn3 days for the Hbeta emission-line response to continuum variations
11.	Ronnback, J., et al. (författare) Host galaxies of intermediate redshift radio-loud and radio-quiet quasars 1996 Ingår i: Monthly notices of the Royal Astronomical Society. - 0035-8711 .- 1365-2966. ; 283:1, s. 282-296296 Tidskriftsartikel (refereegranskat)abstract In a search for host galaxies associated with quasars, the authors present results from deep CCD imaging using the European Southern Observatory 3.5-m New Technology Telescope. Altogether 21 targets, 12 radio-loud and nine radio-quiet quasars, were observed in R, and additional V and Gunn i images were collected for a subgroup of these. The quasar redshifts are between 0.4 and 0.8, a range largely unexplored in previous studies. At these redshifts the R band corresponds to a rest-frame wavelength between 4600 and 3600 Aring. Thus, the authors are primarily probing the light from young stellar components of the presumed host galaxies. To separate out the light originating from the host object the authors remove the quasar contribution by scaling the point spread function (PSF). The PSF is a combination of an empirical PSF model, derived from stars lying in the same field as the quasars (used for the core of the PSF), and an analytical model for the wings of the PSF. After PSF subtraction of the QSO image the authors identify extended residual objects in 17 targets (nine in radio-loud and eight in radio-quiet quasars), of which a few are only marginal detections. These objects are large and luminous and their colours are relatively blue. Their V-R colours are consistent with a stellar population typical of late-type spirals and irregular galaxies. The blue colours could be caused by recent star formation events or by scattering of the QSO light
12.	van Genderen, A. M., et al. (författare) A discussion on new VBLUW observations of the X-ray binary Sk. 160=SMC X-1 1981 Ingår i: Astronomy and Astrophysics. - 0004-6361 .- 1432-0746. ; 101:1, s. 101-104104 Tidskriftsartikel (refereegranskat)abstract VBLUW photometry (Walraven system) of Sk. 160=SMC X-1 made in 1973, 1975, and 1977 are discussed. They are compared with those of van Paradijs (1977) made in 1976. Using all available photometric and X-ray mid-eclipse data, a period is determined by a weighted least-squares solution: P=3.89235 dplusmn0.00002 d m.e
13.	van Genderen, A. M., et al. (författare) VBLUW photometry of the association Sco OB1 (containing the open cluster NGC 6231). A discussion on the evolutionary status of the hypergiant zeta 1 Sco (B1 Ia +) 1984 Ingår i: Astronomy and Astrophysics Supplement Series. - 0365-0138 .- 1286-4846. ; 58:3, s. 537-548548 Tidskriftsartikel (refereegranskat)abstract VBLUW photometry of nearly 190 stars in the very young association Sco OB1, including the cluster NGC 6231, is presented and discussed. Temperature and surface gravity are determined with the aid of theoretical colours. Individual reddenings are also obtained. The distance modulus ( m- M) 0=11.00plusmn0.25 is similar to the value found by Shobbrook (1983). It appears that the reddening is not uniformly distributed across the association (~2degtimes1deg), but that areas of low and high reddening, varying from E( B- V) J=0.35 up to 0.67, can be distinguished. Practically all reddening is caused by dust in the local spiral arm up to a distance of at most 700 pc from the Sun. The Sagittarius-Carina arm where Sco OB1 is situated is practically transparent over a distance of ~0.5 kpc. The kinematical age of ~10 6 yr is in reasonable agreement with the average evolutionary age of ~(3.6plusmn0.6)times10 6 yr of the 12 brightest stars. The evolutionary status of the hypergiant zeta 1 Sco (B1 Ia +) is discussed
14.	Wisotzki, L., et al. (författare) Spectroscopy and imaging of QSO host galaxies 2000 Ingår i: Stars, Gas and Dust in Galaxies: Exploring the Links, ASP Conference Proceedings, Vol. 221. - San Francisco: Astronomical Society of the Pacific : Astronomical Society of the Pacific. - 1583810536 ; , s. 225- Konferensbidrag (övrigt vetenskapligt/konstnärligt)
15.	Perry, J. J., et al. (författare) Hysteresis in broad-line regions of active galactic nuclei 1994 Ingår i: Monthly notices of the Royal Astronomical Society. - 0035-8711 .- 1365-2966. ; 271:3, s. 561-572572 Tidskriftsartikel (refereegranskat)abstract The response of emission-line luminosities to changes in the observed continuum in active galaxies is commonly used for reverberation mapping (RM) of the emission-line region. The method assumes regularity in the continuum source and linearity in the line response to the continuum, and that the phase-space distribution of the line-emitting gas is constant in time. The wealth of information contained in the detailed line profiles is under-utilized in current methods of RM. The authors propose a new method to improve this situation. The authors define normalized profiles and show how their use enables one to explore details of the emission-line region that have hitherto eluded detection. Furthermore, the authors investigate the possibility of testing the validity of the assumptions as used in the standard application of RM. The authors present new methods, the hysteresis test and the Q concept - also based on the line profiles - to test the basic assumptions of RM. The test fails when applied to NGC 4151. The authors show that there is good evidence either for significantly extended and complex emission-line regions, or for changes in the mass distribution over time-scales of months or more - i.e. on time-scales comparable to the length of typical observing campaigns
16.	Stirpe, G. M., et al. (författare) Broad emission line variability in the Seyfert 1 galaxies NGC 5548 and NGC 3783 1988 Ingår i: Astronomy and Astrophysics. - 0004-6361 .- 1432-0746. ; 200:1-2, s. 9-1616 Tidskriftsartikel (refereegranskat)abstract High quality spectra are presented for the Seyfert 1 galaxies NGC 5548 and NGC 3783. Halpha and Hbeta were observed at different epochs, with time intervals between 4 and 13 months. The spectra have been scaled to each other by normalizing the flux of the narrow lines. The difference spectra show that the broad emission lines have varied not only in flux, but also in shape. The authors compare their data with those published by Peterson (1987) and Peterson et al. (1987) and conclude that the supermassive binary scenario proposed by the latter is not unique. The observed variations can be interpreted in an accretion disk scenario. The whole data-set also provides evidence against models based on outflowing motion. The He II lambda4686 line discussed together with N III lambda4640 Bowen emission. A limit to the results is given by the long time intervals separating the observations, which do not allow the authors to set as strong constraints on the BLR as high quality, frequent monitoring of Seyfert 1 spectra would
17.	Stirpe, G. M., et al. (författare) Emission line variation in the Seyfert galaxy Fairall 9 and the presence of broad [O III] emission 1989 Ingår i: Astronomy and Astrophysics. - 0004-6361 .- 1432-0746. ; 211:2, s. 310-314314 Tidskriftsartikel (refereegranskat)abstract The authors present two high quality spectra of the Hbeta region in the Seyfert 1 galaxy Fairall 9, separated by a period of 4 years. The variation of the broad lines is discussed, and in particular that of the Fe II lines of multiplet 42. The difference spectrum shows clearly Fe II (42) lambda 5018 and lambda 5169 with equal intensity. There is a strong, broad (4000 km s -1) wing present under the [O III] lambda 5007 line, which varied only by a small amount. The authors show that this variation is entirely due to the variation of Fe II lambda 5018 and that the remainder of the [O III]-wing has not varied. On this basis the authors argue that the wing is not caused by the Fe II (42) lambda 5018 line, nor by any other Fe II lines, but that they are observing high velocity [O III]-emitting material. From the lack of variation in this component a minimum distance to the source can be set of 1.3 pc. Other features of the difference spectrum are also discussed
18.	van Groningen, E. (författare) Accretion disks in Seyfert nuclei : broad line profiles and asymmetries 1983 Ingår i: Astronomy and Astrophysics. - 0004-6361 .- 1432-0746. ; 126:2, s. 363-371371 Tidskriftsartikel (refereegranskat)abstract Keplerian rotation around a massive object in the centre of a Seyfert galaxy is considered as the main broadening mechanism for the broad emission lines. The resultant line profiles are shown to be of power-law form in the wings and are compared with recently published high quality observations. Several possible solutions are given for the [O III]-problem raised by Shields (1978). Asymmetries in the line profiles are explained by anisotropically radiating filaments in two oppositely directed jets emanating from the centre of the disk and by radial motions in or above the disk
19.	van Groningen, E., et al. (författare) An algorithm for the relative scaling of spectra 1992 Ingår i: Publications of the Astronomical Society of the Pacific. - : IOP Publishing. - 0004-6280 .- 1538-3873. ; 104:678, s. 700-703703 Tidskriftsartikel (refereegranskat)abstract A method is described for the automatic relative scaling of spectra in which one or more lines with constant flux are present. The algorithm takes into account differences in spectral resolution, flux factors, and spectral shifts. It was developed for the relative calibration of series of optical spectra obtained in variability studies of active galactic nuclei but it also can be used for other purposes, e.g., the subtraction of an underlying stellar continuum in AGN with a template spectrum. Tests show that a high degree of accuracy is obtained, and that the method is superior to scaling the spectra by eye or measuring narrow-band fluxes
20.	van Groningen, E. (författare) An analysis of the spectra of 3 Seyfert-1 galaxies with strong Ca II emission 1993 Ingår i: Astronomy and Astrophysics. - 0004-6361 .- 1432-0746. ; 272:1, s. 25-3636 Tidskriftsartikel (refereegranskat)abstract High resolution spectroscopy is presented of three Seyfert-1 galaxies-I Zw 1, Akn 564 and Mkn 231. The spectra cover the optical wavelength region and the regions with the Ca II triplet and the [Ca II] lines. The broad lines in I Zw 1 and Akn 564 are relatively narrow (FWHM<1000 km s -1); and in both objects the Ca II triplet lines are considerably broader than the Balmer lines. These lines are formed at large column densities in the line emitting region; electron scattering by thermal electrons in the broad line clouds may explain the larger widths of the Ca II lines. I Zw 1 has three emission line regions. In addition to the broad line region there are two kinematically separated narrow line regions: a high excitation region with relatively broad lines (FWHM~1100 km s -1), blue shifted with regard to the systemic velocity by ~700 km s -1; and a region with very low excitation at the systemic velocity and much narrower lines (FWHM~400 km s -1)
21.	van Groningen, E., et al. (författare) Broad emission line profiles in Seyfert-1 galaxies : [O III]-wings from a transition zone 1989 Ingår i: Astronomy and Astrophysics. - 0004-6361 .- 1432-0746. ; 211:2, s. 293-309309 Tidskriftsartikel (refereegranskat)abstract The authors demonstrate that broad wings are present under the strong [O III] lambda 5007 line in 10 out of 12 observed Seyfert-1 galaxies. The question is addressed whether the wing is due to broad [O III] emission or to some other ion. The alternatives considered are Fe II (42) lambda 5018, He I lambda 5016 and Si II lambdalambda 5041, 5056 or even Hbeta. The authors have searched for a counterpart of the wing in other forbidden and permitted lines. The observations yield a crude estimate of the density in the wing-emitting region. The authors derive a lower limit to the region's size. Because the [O III] emitting gas holds an intermediate position between the broad line region (BRL) and the narrow line region (NLR), both in density and distance, the authors denominate it the transition line region (TLR). They surmise that the outflowing part of the BLR provides the material of the TLR, before slowing down to NLR velocities
22.	van Groningen, E. (författare) Broad emission line profiles in Seyfert-1 galaxies. I. Evidence for a disk and a wind in Mkn 335 1987 Ingår i: Astronomy and Astrophysics. - 0004-6361 .- 1432-0746. ; 186:1-2, s. 103-113113 Tidskriftsartikel (refereegranskat)abstract High resolution spectroscopic observations of the Seyfert-1 galaxy Mkn 335 are presented. This galaxy is exceptional in the sense that its broad emission lines display a strong blue asymmetry. It is shown that the Halpha/Hbeta intensity ratio changes drastically with velocity shift. It is also found that the Hdelta/Hbeta ratio varies only by a small amount from line core to the wings, and is close to the theoretical case B recombination value. It is proposed that these Balmer line ratios are produced by a medium with very high densities and a temperature of about 10 4 K. These observations are interpreted by a model with two dynamically and spatially separate components: a disk which contains the very high density material and emits ~80% of the Balmer lines in a symmetric profile and an outflowing component of much lower density which emits the blue wing and most of Ly alpha and the higher excitation lines (such as C IV lambda1549)
23.	Van Groningen, E., et al. (författare) One-sided jets in extragalactic radiosources 1980 Ingår i: Astronomy and Astrophysics. - 0004-6361 .- 1432-0746. ; 90:1-2, s. L7-L9L9 Tidskriftsartikel (refereegranskat)abstract Some explanations for the existence of one-sided jets in symmetrical extended radio sources are discussed. It is shown that in the case of the quasar 4C32.69, a relativistic Doppler interpretation is improbable. Observational constraints on a model involving anisotropic radiation are also examined. This model cannot be ruled out, although non-relativistic interpretations of one-sided jets are most likely
24.	Wanders, I., et al. (författare) Seeing and aperture effects on [O III]-based flux calibration of AGN spectra. NGC 3516 : a case study 1992 Ingår i: Astronomy and Astrophysics. - 0004-6361 .- 1432-0746. ; 266:1, s. 72-7676 Tidskriftsartikel (refereegranskat)abstract In variability studies of the broad emission lines in the spectra of active galactic nuclei, the narrow emission lines are generally used to flux calibrate spectra taken at different epochs to each other. In objects where the narrow-line region is resolved, seeing affects the ratio of narrow-line flux to broad-line and continuum flux entering the aperture since the broad-line region and the continuum source are unresolved. Thus observing under different seeing conditions introduces artificial correlated variations of the continuum and broad-line fluxes if one assumes the narrow lines to be constant in flux. The authors show that one can correct for this effect by simulating seeing variations using a narrow-band [O III] image, taken in good seeing conditions, of the nearby Seyfert 1 galaxy NGC 3516, in which the narrow-line region is clearly resolved and extends over more than 20". They also show that centring the aperture on the object consistently is very important and miscentring is also a source of artificial variability of the broad-line and continuum fluxes
25.	Örndahl, E., et al. (författare) An optical imaging study of 0.4 <= z <= 0.8 quasar host galaxies. I. Observations and reduction 2003 Ingår i: Astronomy and Astrophysics. - 0004-6361 .- 1432-0746. ; 404, s. 883-899 Tidskriftsartikel (refereegranskat)abstract We have conducted an optical imaging study aimed at resolving the host galaxies of 79 radio-loud and radio-quiet quasars at z=0.4-0.8, extending the number of investigated objects in this redshift range by ~ 45%. Observations were performed mainly in the R band but also in V and I band using the Nordic Optical Telescope on La Palma. In this paper we discuss the sample composition and observations and the reduction techniques used. The quasars were selected in pairs of radio-loud and radio-quiet objects matched in the z-V plane in order to facilitate a statistical comparison. The radio-loud part of the sample contains comparable numbers of flat and steep radio spectrum sources which also are matched in redshift and V magnitude. Point spread function subtraction was performed using one-dimensional luminosity profiles both on the quasar image and on a field star, and subtracted images and luminosity profiles are shown for each quasar field. The detection rate is 60% for the radio-quiet host galaxies and 80% for radio-loud hosts. The host galaxies have magnitudes which make them brighter than an L* galaxy by a factor of 1.5-4 at the low end of the redshift range, which increases by 2-3 times towards the higher end of the redshift range. Both radio-quiet and radio-loud hosts follow the radio galaxy R-z Hubble relation well. Analysis and discussion of colours and morphology is presented in \citet{orn}. Based on observations made with the Nordic Optical Telescope, operated on the island of La Palma jointly by Denmark, Finland, Iceland, Norway, and Sweden, in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofísica de Canarias.

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