Henner H. Fink

ROSAT monitoring of persistent giant and rapid variability in the narrow-line Seyfert 1 galaxy IRAS 13224-3809

We report evidence for persistent giant and rapid X-ray variability in the radio-quiet, ultrasoft, strong Fe II, narrow-line Seyfert 1 galaxy IRAS 13224-3809. Within a 30 day ROSAT High Resolution Imager (HRI) monitoring observation at least five giant amplitude count rate variations are visible, with the maximum observed amplitude of variability being about a factor of 60. We detect a rise by a factor of about 57 in just two days. IRAS 13224-3809 appears to be the most X-ray variable Seyfert known, and its variability is probably nonlinear. We carefully check the identification of the highly variable X-ray source with the distant galaxy, and it appears to be secure. We examine possible explanations for the giant variability. Unusually strong relativistic effects and partial covering by occulting structures on an accretion disc can provide plausible explanations of the X-ray data, and we explore these two scenarios. Relativistic boosting effects may be relevant to understanding the strong X-ray variability of some steep spectrum Seyferts more generally.

The unusual X-ray and optical properties of the ultrasoft active galactic nucleus Zwicky 159.034 (RE J1237+264)

Zwicky 159.034, one of the Seyfert galaxies identified with EUV sources detected during the ROSAT Wide Field Camera (WFC) all-sky survey, has unusual properties. The ROSAT Position Sensitive Proportional Counter (PSPC) 0.1--2.5 keV X-ray spectrum, obtained simultaneously with the WFC survey, appears extremely steep. Subsequent deeper pointed observations with ROSAT revealed that its 0.1--2.5 keV count rate had decreased by an extremely large amount (a factor of about 70). This is comparable to the variability amplitude seen in another ultrasoft Seyfert, E1615+061. There appears to be about 10 times as much flux in the soft component as in any expected hard tail suggesting that, in the absence of partial covering of the hard flux, the soft flux cannot arise via reprocessing of the hard tail. Its peculiar optical spectrum has permitted lines with widths about 1200--1500 km s^{-1}, and forbidden lines are weak or absent. Its optical spectrum shows evidence for variability, and might be used to study the role reprocessed X-rays play in the formation of optical lines.

Multiwavelength monitoring of the BL Lacertae object PKS 2155-304. I - The IUE campaign

Daily monitoring of PKS 2155-304 with the IUE satellite throughout November 1991 revealed dramatic large-amplitude rapid variations in the UV flux of this BL Lac object. Many smaller, rapid flares are superposed on a general doubling of the intensity. During the five-day period when sampling was roughly continuous, the rapid flaring had an apparent quasi-periodic nature, with peaks repeating every 0.7 day. The short- and long-wavelength UV light curves are well correlated with each other, and with the optical light curve deduced from the Fine Error Sensor on IUE. The formal lag is zero, but the cross-correlation is asymmetric in the sense that the shorter wavelength emission leads the longer. The UV spectral shape varies a small but significant amount. The correlation between spectral shape and intensity is complex. The sign of the correlation is consistent with the nonthermal acceleration processes expected in relativistic plasmas, so that the present results are consistent with relativistic jet models, which can also account for quasi-periodic flaring.

Soft X-Ray Properties of Seyfert Galaxies in the ROSAT All-Sky Survey

We present the results of ROSAT All-Sky Survey observations of Seyfert and IR luminous galaxies from the extended 12 μm galaxy sample and the optically selected CfA sample. Detections are available for 80% (44/55) of the Seyfert 1s and 34% (23/67) of the Seyfert 2s in the 12 μm sample, and for 76% (26/34) of the Seyfert 1s and 38% (6/16) of the Seyfert 2s in the CfA Sample. Roughly half of the Seyfert galaxies (mostly Seyfert 1s) have been fitted to an absorbed power-law model, yielding an average photon index of Γ = 2.26 ± 0.11 for 43 Seyfert 1s and Γ = 2.45 ± 0.18 for 10 Seyfert 2s, with both types having a median value of 2.3. The soft X-ray luminosity corelates with the 12 μm luminosity, with Seyfert 1s, having relatively more soft X-ray emission than Seyfert 2s of similar mid-IR luminosities by factor of 1.6 ± 0.3. Several physical interpretaions of these results are discused, including the standard unified model for Seyfert galaxies. Infrared luminous non-Seyferts are shown to have similar disributions of soft X-ray luminosity and X-ray-to-IR slope as Seyfert 2s suggesting that some of them may harbor obscured active nuclei (as has already ben shown to be true for several objects) and/or that the soft X-rays from some Seyfert 2s may be nonnuclear. A soft X-ray luminosity function (XLF) is calculated for the 12 μm sample, which is described well by a single power law with a slope of −1.75. The normalization of this XLF agrees well with that of a hard X-ray selected sample. Several of our results, related to the XLF and the X-ray-to-IR relation, are shown to be consistent with the hard x-ray observations of the 12 μm sample by barcons et al.

Multiwavelength monitoring of the BL Lacertae object PKS 2155-304. 4: Multiwavelength analysis

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 trough optical wave bands were also strongly correlated, with no measurable lag down to limiting timescales of approximately less than 1-2 hr. This strong correlation extends to the near-infrared, but the less intensive sampling precludes measurement of any lag beyomnd an upper limit of approximately less than 1 day. These lags and limits of the order of hours are much shorter than 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 Q(exp 2) 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 (approximately less than 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 produced closer in, has problems explaining the magnitude of the ultraviolet/X-ray lag, because the X-ray-emitting electrons have very short lifetimes (t(sub 1/2) 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.

Multiwavelength Monitoring of the BL Lacertae Object PKS 2155–304 in 1994 May. II. The IUE Campaign

PKS 2155-304, the brightest BL Lac object in the ultraviolet sky, was monitored with the International Ultraviolet Explorer satellite at ~1 hr time resolution for 10 nearly uninterrupted days in 1994 May. The campaign, which was coordinated with Extreme Ultraviolet Explorer, ROSAT, and ASCA monitoring, along with optical and radio observations from the ground, yielded the largest set of spectra and the richest short-timescale variability information ever gathered for a blazar at UV wavelengths. The source flared dramatically during the first day, with an increase by a factor of ~2.2 in an hour and a half. In subsequent days, the flux maintained a nearly constant level for ~5 days, then flared with ~35% amplitude for 2 days. The same variability was seen in both short- and long-wavelength IUE light curves, with zero formal lag (2 hr), except during the rapid initial flare, when the variations were not resolved. Spectral index variations were small and not clearly correlated with flux. The flux variability observed in the present monitoring is so rapid that, for the first time, based on the UV emission alone, the traditional ΔL/Δt limit indicating relativistic beaming is exceeded. The most rapid variations, under the likely assumption of synchrotron radiation, lead to a lower limit of 1 G on the magnetic field strength in the UV-emitting region. These results are compared with earlier intensive monitoring of PKS 2155-304 with IUE in 1991 November, when the UV flux variations had completely different characteristics.

ROSAT soft X-ray properties of the Large Bright Quasar Survey: modelling of stacked X-ray spectra

We develop and apply a novel method of analysis to study the X-ray spectral properties of 908 QSOs in the Large Bright Quasar Survey (LBQS) that were observed during the soft X-ray ROSAT All-Sky Survey (RASS). Due to the relatively short ( ::;; 600 s) RASS exposure times, only 10 per cent of the QSOs are detected in X-rays, so X-ray spectral model fits for individual QSOs are precluded by poor photon statistics. Spectral stacking provides effectively much more sensitive X-ray observations for an average QSO in bins of redshift, and for several classes of QSOs. We model the stacked X-ray spectra in a way that obviates both the distinction between detections and non-detections, and the need to stack together only those objects observed through similar Galactic column densities N H. In application to the LBQSIRASS sample, we marginally confirm a flattening of the X-ray spectral index of QSOs toward higher redshifts. Radio-loud QSOs show flatter X-ray photon indices, in agreement with previous work. We show that the apparent flattening of the photon index with redshift is not due to an increasing fractional contribution from the radio-loud class of QSOs.

Three new BL Lacertae objects in the Palomar-Green Survey

We have identified three BL Lacertae objects in the Palomar-Green Survey which were previously misclassified as DC white dwarfs, namely PG 1246+586, PG 1424+240, and PG 1437+398. Our reclassification is based on the detection of these objects as x-ray sources in the ROSAT all-sky survey and upon our subsequent detection of intrinsic linearly polarized and variable optical emission from these sources. As a result of the ROSAT survey, the number of identified BL Lac objects in the Palomar-Green catalog of UV excess objects has been doubled. Corrected optical positions are presented for PG 1246+586 and PG 1437+398.

ROSAT observations of the blazar PKS 0537-441

PKS 0537-441, an object with properties intermediate between BL Lacertae objects and OVVs, was observed with ROSAT in 1991 April. The X-ray state F 1keV =0.79±0.05 μJy is brighter than those detected by the Einstein Observatory (1979-1980), and EXOSAT (1985) satellites, while the spectral slope α ph =2.1±0.4 is steeper. The hydrogen column density N H =(3.0±1.3)×10 20 cm −2 is consistent with that deduced from 21 cm observations. Optical spectrophotometry of 1991 February indicates a moderately high state of the source. The observations are discussed also in the light of the newly discovered gamma-ray emission

Simultaneous ROSAT, Ginga, VLA, IUE, and optical observations of the bright quasar H1821+643

We report on the first simultaneous optical, UV, and X-ray spectroscopy of the low-redshift (z = 0.297), high-luminosity QSO H1821+643. This multiwavelength campaign was complemented by optical imaging and radio mapping with the VLA. Although there was no significant variability during the 40 day monitoring period in either the UV or X-ray bands, a number of new results were obtained. There is significant variability in the optical, UV, and X-ray bands on time scales of years, both in flux level and spectral shape. In the co-added ultraviolet spectrum we detected a Lyman-α absorption line at z = 0.225. In the radio map the QSO is extended in the north-south direction at 20 cm, and can be resolved into two separate components at 6 cm. In the ROSAT data we separated (both spatially and spectroscopically) the emission originating in H1821+643 from the nearby hot white dwarf Kl-16, and thus we isolated the intrinsic soft X-ray excess in the QSO. The IUE and ROSAT data were combined with new optical spectroscopy and hard X-ray data from Ginga, to reveal a strong optical/UV bump with a flat slope (α ~ -0.96, v∞ v α) in the UV, a steep soft X-ray excess (α ~ -4) between ~ 0.15 keV and ~ 0.5 keV, and a typical hard X-ray slope of α ~ -0.8 extending up to 10 keV While the optical/UV bump and the soft X-ray excess can each be fit satisfactorily with standard accretion disk models, a joint fit to the optical/UV X-ray bump cannot be achieved. This suggests that either the optical/UV or the soft X-ray emission do not arise in an accretion disk, or that the standard bare disk model should be modified (possibly by inclusion of electron scattering) to account for both the strong, flat UV emission and the steep, soft X-ray excess.