| 1. |
- 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
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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
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| 2. |
- 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)
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| 3. |
- 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
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| 4. |
- 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
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| 5. |
- 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
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