Simultaneous X-ray, ultraviolet, optical, infrared, and radio monitoring data were used to test and constrain models of continuum emission from the BL Lacertae object PKS 2155-304. Intensively sampled ultraviolet and soft X-ray light curves showed a clear temporal correlation, with the X-rays leading the ultraviolet by 2-3 hr. This lag was found to be significantly different from zero after an exhaustive comparison of four different techniques for measuring temporal correlations. Variations in the ultraviolet through optical wave bands were also all strongly correlated, with no measurable lag down to limiting timescales of less-than-or-similar-to 1-2 hr. This strong correlation extends to the near-infrared, but the less intensive sampling precludes measurement of any lag beyond an upper limit of less-than-or-similar-to 1 day. These lags and limits of the order of hours are much shorter than the most rapid observed single-band variations. Because of the very sparse radio sampling, it was not possible to measure quantitatively the correlation and lag with shorter wavelengths, but the data do suggest that the radio may lag the optical/ultraviolet by approximately 1 week, with longer delays and weaker variations to longer radio wavelengths. The epoch-folding Q2 statistic was used to test for periodicity, and no evidence for strict or quasi-periodicity was found in any of the light curves.Because they lead the lower frequencies, the soft X-rays (less-than-or-similar-to 1 keV) cannot arise from synchrotron self-Compton scattering. These results also rule out the accretion disk model, which predicts a measurable lag between ultraviolet/optical wavelength bands and a correlation between hardness and brightness, neither of which were seen. They are consistent with the entire radio through X-ray continuum arising from direct synchrotron emission from a relativistic jet. However, the tapered jet model, in which the X-ray emission is prochrotron emission from a relativistic jet. However, the tapered jet model, in which the X-ray emission is produced closer in, has problems explaining the magnitude of the ultraviolet/X-ray lag, because the X-ray-emitting electrons have very short lifetimes (t1/2 much-less-than 1 s). The result that the lag is much smaller than the variability timescale suggests instead that the radiation may be produced in a flattened region such as a shock front.

We present in this work the results of a five-month monitoring campaign of the active galactic nucleus in NGC 3227. Both the Balmer Hbeta and Halpha emission lines, as well as the optical continuum have been analysed. A deblending procedure has been applied which allows to separate the contributions to the line emission from the narrow line region (NLR) on one side and from the broad line region (BLR) on the other side. The stellar contribution to the observed continuum within a 1.5 x 4 arcsec2 slit is found to be in the range 24-40% when the AGN is in the lowest state of activity observed during this monitoring period. The largest source of uncertainty in deriving the light-curves of the BLR emission and of the AGN continuum emission comes from differences in the seeing conditions prevailing at the various observational epochs. We have applied a correction factor for this effect. The optical AGN continuum and the BLR emission in NGC 3227 appear to vary by about 40% on a timescale of 1.5 month. We have followed a burst with maximum around early March and minimum by mid-April 1990. The levels of the broad line emission and the lambda6750 continuum are well correlated, a fact consistent with the classical assumption that the BLR material is photoionized by the central continuum source. Cross-correlation analyses between the light-curves of the lambda6750 AGN continuum and the BLR Halpha emission indicate that the lag of the BLR emission with respect to the continuum variations is 17 +/- 7 d. This result suggests that the BLR clouds which are affected by changes in the flux level of the central ionizing source lie about 17 light-days away. The variable part of the Halpha line shows a flat-topped profile with FWHM approximately 2900 km s-1. Within the observed range of continuum fluctuations (approximately 40%), the broad wings of the Halpha emission line do not vary in a noticeable way. This implies either that the BLR clouds building up the wings remain fully ionized during the continuum variations or that the time sampling should be increased if one wished to measure such an effect. Introduced together with other AGN in the plane {Balmer lag, luminosity}, NGC 3227 contributes to define a relationship slightly less steep than the BLR size scaling as the square root of the absolute luminosity.

view Abstract Citations (502) References (43) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS 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 Clavel, J. ; Reichert, G. A. ; Alloin, D. ; Crenshaw, D. M. ; Kriss, G. ; Krolik, J. H. ; Malkan, M. A. ; Netzer, H. ; Peterson, B. M. ; Wamsteker, W. ; Altamore, A. ; Baribaud, T. ; Barr, P. ; Beck, S. ; Binette, L. ; Bromage, G. E. ; Brosch, N. ; Diaz, A. I. ; Filippenko, A. V. ; Fricke, K. ; Gaskell, C. M. ; Giommi, P. ; Glass, I. S. ; Gondhalekar, P. ; Hackney, R. L. ; Halpern, J. P. ; Hutter, D. J. ; Joersaeter, S. ; Kinney, A. L. ; Kollatschny, W. ; Koratkar, A. ; Korista, K. T. ; Laor, A. ; Lasota, J. -P. ; Leibowitz, E. ; Maoz, D. ; Martin, P. G. ; Mazeh, T. ; Meurs, E. J. A. ; Nair, A. D. ; O'Brien, P. ; Pelat, D. ; Perez, E. ; Perola, G. C. ; Ptak, R. L. ; Rodriguez-Pascual, P. ; Rosenblatt, E. I. ; Sadun, A. C. ; Santos-Lleo, M. ; Shaw, R. A. and 7 more Abstract We 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 ratio of maximum to minimum flux was ~ 4.5 for the continuum at 1350 A, and the continuum was significantly bluer when it was brighter. The high ionization emission lines showed the strongest variations, with N V λ1240 and He II λ1640 exhibiting maximum-to-minimum flux ratios as high as those of the continuum. Intermediate-ionization lines, including Lyα λ1216, C IV λ1549, and C II] λ1909, had maximum- to-minimum amplitudes of ~ 2, and Mg II λ2798, the lowest ionization line, exhibited the smallest amplitude fluctuations, ~ 1.3. The great majority of all variations were well resolved in time. Apart from Mg II λ2798, the emission-line variations correlate extremely well with those of the 1350 A continuum if allowance is made for a systematic delay, lending qualitative support to the view that photoionization by the nuclear continuum is responsible for driving the emission lines. The delay of a given line seems to depend on the degree of ionization of its species. The He II λ1640 and N V λ1240 features exhibit the shortest delay. {DELTA}t ~ 4-10 days, while the Lyα λ1216 and C IV λ1549 lines yield 8-16 days. The Si IV + O IV] λ1402 feature and the C III] λ1909 line exhibit significantly larger delays, between 12 and 34 days. In the case of Mg II λ2798, the cross-correlation is broad and shallow, so that the delay is not only loosely constrained. {DELTA}t ~ 34-72 days. Publication: The Astrophysical Journal Pub Date: January 1991 DOI: 10.1086/169540 Bibcode: 1991ApJ...366...64C Keywords: Active Galactic Nuclei; Astronomical Spectroscopy; Galactic Structure; Iue; Seyfert Galaxies; Emission Spectra; Time Series Analysis; Ultraviolet Spectra; Variability; Astrophysics; GALAXIES: INDIVIDUAL NGC NUMBER: NGC 5548; GALAXIES: NUCLEI; GALAXIES: SEYFERT; ULTRAVIOLET: SPECTRA full text sources ADS | data products SIMBAD (1) NED (1) MAST (1) INES (1) Related Materials (14) Part 2: 1991ApJ...368..119P Part 3: 1992ApJ...392..470P Part 4: 1993ApJ...408..416D Part 5: 1994ApJ...425..582R Part 6: 1994ApJ...425..609S Part 7: 1994ApJ...425..622P Part 8: 1995ApJS...97..285K Part 9: 1997ApJS..110....9R Part 10: 1997ApJS..112..271S Part 11: 1997ApJS..113...69W Part 12: 1998ApJS..115..185D Part 13: 1998ApJ...509..163O Part 14: 1998ApJ...500..162C Part 15: 1999ApJ...510..659P