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Träfflista för sökning "WFRF:(Santos Lleo M.) "

Sökning: WFRF:(Santos Lleo M.)

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  • Crenshaw, D. M., et al. (författare)
  • Multiwavelength observations of short-timescale variability in NGC 4151. I. Ultraviolet observations
  • 1996
  • Ingår i: Astrophysical Journal. - 0004-637X .- 1538-4357. ; 470:1, s. 322-335335
  • Tidskriftsartikel (refereegranskat)abstract
    • <p>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</p>
  • 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. - 0004-637X .- 1538-4357. ; 470:1, s. 364-377377
  • Tidskriftsartikel (refereegranskat)abstract
    • <p>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)</p>
  • 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
    • <p>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</p>
  • 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. - 0004-637X .- 1538-4357. ; 366:1, s. 64-8181
  • Tidskriftsartikel (refereegranskat)abstract
    • <p>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</p>
  • 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
    • <p>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</p>
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