Jose Joaquin Garcia Clavel

Multiwavelength observations of short-timescale variability in NGC 4151 .3. X-ray and gammma-ray observations

A series of ROSAT, ASCA and Compton Gamma-Ray Observatory (CGRO) observations of the Seyfert galaxy NGC 4151 were carried out during the period 1993 November 30 to December 13 as part of an intensive campaign to study the multiwavelength spectral characteristics of its short time-scale variability. In the softest X-ray bands monitored by ROSAT (0.1–0.4 keV, 0.5–1.0 keV) the source flux remained constant throughout the observing period. However, in an adjacent band (1.0–2.0 keV) significant variability was evident, the most obvious feature being a marked increase (factor 1.45) in the count rate over a timescale of ∼ 2 days commencing roughly 3 days into the monitoring period. In contrast, only a low amplitude of variability (<∼ 10%) was measured in the four ASCA observations in the 2-10 keV band (but note that the first ASCA observation was performed somewhat after the onset of the flux increase seen by ROSAT). The count rates recorded by the Oriented Scintillation Spectrometer Experiment (OSSE) on CGRO are consistent with ±15% variations in the 50–150 keV gamma-ray band but there is no direct correspondence between the gamma-ray and soft X-ray light curves. The 0.1 to ∼ 300 keV spectrum of NGC 4151 is dominated by a hard power-law continuum which is cut-off at both high (∼ 90 keV) and low (∼ 4 keV) energy. A high energy cut-off is characteristic of a continuum generated by the process of thermal Comptonization whereas that at low energy arises from absorption in line-of-sight gas. In NGC 4151 this gas may be partially photoionized by the continuum source but still retains significant opacity below 1 keV. The observed soft X-ray variability may be the result of changes in the level of the underlying soft-hard X-ray continuum or changes in the line-of-sight absorption. The data marginally favour the former, in which case the difference between the soft X-ray and gammaray light curves implies a steepening of the continuum as the source brightens, consistent with earlier observations. As noted in earlier studies there is a soft excess below 1 keV which probably arises from more than one scattered and/or thermal components. The 1–2 keV soft X-ray and the ultraviolet continuum light curves (e.g. near 1440Å ) show reasonably good correspondence, although the relative amplitude of the variations is much higher in the X-ray data. The observed ultraviolet to X-ray correlation has a similar slope to that established in earlier studies, although a significant residual ultraviolet flux is evident in the recent observations. A possible interpretation is that the X-ray to gamma-ray continuum is produced in a patchy dissipative corona above the surface of an accretion disk and that the correlated ultraviolet flux results from the reprocessing of part of this continuum by the disk. The residual ultraviolet flux may then arise from the reprocessing and/or the viscous heating of the disk.A series of ROSAT, ASCA and Compton Gamma-Ray Observatory (CGRO) observations of the Seyfert galaxy NGC 4151 were carried out during the period 1993 November 30 to December 13 as part of an intensive campaign to study the multiwavelength spectral characteristics of its short time-scale variability. In the softest X-ray bands monitored by ROSAT (0.1–0.4 keV, 0.5–1.0 keV) the source flux remained constant throughout the observing period. However, in an adjacent band (1.0–2.0 keV) significant variability was evident, the most obvious feature being a marked increase (factor 1.45) in the count rate over a timescale of ∼ 2 days commencing roughly 3 days into the monitoring period. In contrast, only a low amplitude of variability (<∼ 10%) was measured in the four ASCA observations in the 2-10 keV band (but note that the first ASCA observation was performed somewhat after the onset of the flux increase seen by ROSAT). The count rates recorded by the Oriented Scintillation Spectrometer Experiment (OSSE) on CGRO are consistent with ±15% variations in the 50–150 keV gamma-ray band but there is no direct correspondence between the gamma-ray and soft X-ray light curves. The 0.1 to ∼ 300 keV spectrum of NGC 4151 is dominated by a hard power-law continuum which is cut-off at both high (∼ 90 keV) and low (∼ 4 keV) energy. A high energy cut-off is characteristic of a continuum generated by the process of thermal Comptonization whereas that at low energy arises from absorption in line-of-sight gas. In NGC 4151 this gas may be partially photoionized by the continuum source but still retains significant opacity below 1 keV. The observed soft X-ray variability may be the result of changes in the level of the underlying soft-hard X-ray continuum or changes in the line-of-sight absorption. The data marginally favour the former, in which case the difference between the soft X-ray and gammaray light curves implies a steepening of the continuum as the source brightens, consistent with earlier observations. As noted in earlier studies there is a soft excess below 1 keV which probably arises from more than one scattered and/or thermal components. The 1–2 keV soft X-ray and the ultraviolet continuum light curves (e.g. near 1440A ) show reasonably good correspondence, although the relative amplitude of the variations is much higher in the X-ray data. The observed ultraviolet to X-ray correlation has a similar slope to that established in earlier studies, although a significant residual ultraviolet flux is evident in the recent observations. A possible interpretation is that the X-ray to gamma-ray continuum is produced in a patchy dissipative corona above the surface of an accretion disk and that the correlated ultraviolet flux results from the reprocessing of part of this continuum by the disk. The residual ultraviolet flux may then arise from the reprocessing and/or the viscous heating of the disk.

Multiwavelength Observations of Short-Timescale Variability in NGC 4151. I. Ultraviolet Observations

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

ISO Observations of Symbiotic Stars

A number of symbiotic stars have been observed with ISO. In addition to a number of emission lines, SWS observations of the symbiotic novae RR Tel and V1016 Cyg reveal prominent, broad 10 & 18 µm silicate dust features. The 10 µm, features are similar to the crystalline silicate profiles seen in classical novae. There is some evidence that the silicate brightness in V1016 Cyg varies with Mira-component phase. However, the silicate feature in RR Tel also showed some variation even though observations were made at very similar Mira-component phases. PHT observations of S-type symbiotic stars show the IR emission to be dominated by the red-giant component. However, an excess in the PHT-P filters from 10 to 15 µm is evident in all the stars, and there may be a broad 3.2 µm absorption feature or a broad 3.8 µm emission feature. At this time we have no adequate physical explanations for any of these features.

ISO Observations of Classical Novae

We describe the ISO programme to observe classical novae. The programme includes observations of novae during and shortly after eruption, and old novae. ISO observations of far infrared fine structure lines are providing us with information about physical conditions in nova ejecta which complement and extend knowledge obtained from ground-based observations. Surprisingly, we are getting little information about dust in nova systems, despite the fact that many novae are prolific dust-producers.

The Lyα Forest towards HS1700+6416 at High Resolution

We have analyzed high resolution (b ≈ 15 km/s) optical spectra of the QSO HS1700+6416 (z em = 2.72). The high S/N ratio in the Lyα forest allows a reasonable determination of the neutral hydrogen column density as well as of the velocity dispersion of the clouds. A weak, but significant, trend for stronger lines to be broader suggests that their column densities (N HI) and Doppler parameters (b) are related. A simplified model of very flattened, highly ionized clouds, gravitationally confined by dark matter, gives a good representation of the data, where the observed N HI−b correlation is due to orientation effects.

Metal Line Absorptions Toward the QSO HS1700+6416

We have analyzed high resolution (b ≈ 15 km/s) and low resolution (R ≈ 3A) optical spectra of the QSO HS1700+6416 (z em = 2.72) taken with the ISIS spectrograph at the WHT. We are able to identify more than 50 metal lines.

High-Resolution Spectroscopy of the Bright Quasar HS 1946+7658

The optical spectrum of the QSO HS 1946+7658 (z em = 3.05) has been observed at high resolution (b ~ 15km/s) in the wavelength interval 4115 – 4970A. The signal-to-noise ratio (≥ 10) in the Lyα forest allows the determination of the neutral hydrogen column density as well the velocity dispersion of the intervening clouds by fitting Voigt profiles. The trend for stronger lines to be broader suggests that their column densities (N HI) and Doppler parameters (b) are related. It is difficult to explain this observed log N HI − b diagram (specially the deficit of strong lines with low b values) as a selection effect.