W. Brinkmann

XMM-Newton EPIC observations of 21 low-redshift PG quasars

We present an X-ray spectral analysis of 21 low redshift quasars observed with XMM-Newton EPIC. All the sources are Palomar Green quasars with redshifts between 0.05 and 0.4 and have low Galactic absorption along the line-of-sight. A large majority of quasars in the sample (19/21) exhibit a significant soft excess below ∼1-1.5 keV, whilst two objects (PG 1114+445 and I Zw1) show a deficit of soft X-ray flux due to the presence of a strong warm absorber. Indeed, contrary to previous studies with ASCA and ROSAT, we find that the presence of absorption features near 0.6-1.0 keV is common in our sample. At least half of the objects appear to harbor a warm absorber, as found previously in Seyfert 1 galaxies. We find significant detections of Fe Kα emission lines in at least twelve objects, whilst there is evidence for some broadening of the line profile, compared to the EPIC-pn resolution, in five of these quasars. The determination of the nature of this broadening (e.g., Keplerian motion, a blend of lines, relativistic effects) is not possible with the present data and requires either higher S/N or higher resolution spectra. In seven objects the line is located between 6.7-7 keV, corresponding to highly ionized iron, whereas in the other five objects the line energy is consistent with 6.4 keV, i.e. corresponding to near neutral iron. The ionized lines tend to be found in the quasars with the steepest X-ray spectra. We also find a correlation between the continuum power law index Γ and the optical Hβ width, in both the soft and hard X-ray bands, whereby the steepest X-ray spectra are found in objects with narrow Hβ widths, which confirms previous ROSAT and ASCA results. The soft and hard band X-ray photon indices are also strongly correlated, i.e. the steepest soft X-ray spectra correspond the steepest hard X-ray spectra. We propose that a high accretion rate and a smaller black hole mass is likely to be the physical driver responsible for these trends, with the steep spectrum objects likely to have smaller black hole masses accreting near the Eddington rate.

The RGB Sample of Intermediate BL Lacertae Objects

Combining newly identified and previously known BL Lacertae objects from the ROSAT All-Sky Survey-Green Bank (RGB) catalog, we present a sample of 127 BL Lacertae objects, the largest ever derived from a single uniform survey. A complete sample of 33 objects brighter than O = 18.0 mag is also presented. These samples are compared to other known BL Lac samples and are generally found to exhibit properties intermediate between those of the previously disparate classes of high- and low-energy-peaked BL Lacertae objects (HBLs and LBLs, respectively). This result is most dramatic in the distribution of the X-ray to radio logarithmic flux ratios, where the RGB BL Lacertae objects are shown to peak precisely where the sharp dichotomy between the two subclasses was previously seen. The αro versus αox diagram also shows the RGB sample smoothly bridges the gap between the previously distinct subclasses of LBLs and HBLs. The range of broadband spectral energy distributions (SEDs) exhibited by the RGB objects also shows that, contrary to prior claims, searches based on relatively deep surveys cannot limit follow-up spectroscopy to targets with a narrow range of SEDs since BL Lacertae objects clearly constitute a homogeneous population with a wide range of SEDs. As in results based on the Einstein Extended Medium Sensitivity Survey (EMSS) and 1 Jy BL Lac samples, we find a weak but statistically significant correlation between the composite X-ray spectral index αxox and the radio-optical spectral index αro. This implies that the more LBL-like RGB BL Lacertae objects have secondary sources of X-ray emission, possibly from inverse Compton components. This result, in addition to other characteristics of the RGB sample, indicates that the simple unified scheme, which postulates that HBLs and LBLs differ solely by orientation, may be in need of revision. We also present both the X-ray and radio log N- log S distributions for which the competing HBL/LBL unification scenarios have differing predictions. The unknown effects of the triple flux limit inherent in the RGB Complete sample makes quantitative analysis uncertain, but the characteristics of the RGB sample compare well both with results obtained from previous samples and with general theoretical predictions based on a simple Monte Carlo simulation. Our analysis indicates that the unimodal distribution of BL Lac properties found in the RGB sample likely reliably reflects the underlying population, while the bimodal distribution found in earlier studies arose primarily from observational selection effects. The presence of not only intermediate but also extreme HBL and LBL objects is the RGB surveys unique strength and offers clear avenues for future studies that can undoubtedly address the question of how HBLs and LBLs are related.

Asca and rosat x-ray spectra of high-redshift radio-loud quasars

Results are presented on the X-ray properties of 9 high-redshift (1.2 < z < 3.4) radio-loud quasars (RLQs) observed by the Advanced Satellite for Cosmology and Astrophysics (ASCA; 10 observations) and ROSAT (11 observations, for a subset of six quasars). New ASCA observations of S5 0014+81 (z = 3.38) and S5 0836+71 (z = 2.17) and ROSAT observations of PKS 2126-158 for which results were never presented elsewhere are included. A simple model consisting of a power law plus cold, uniform absorption gives acceptable fits to the spectra of all sources. The ASCA spectra of the six brightest objects show evidence for absorption in excess of the Galactic value at a 99% confidence level. Comparison with the ROSAT data suggests that absorption has significantly varied (ΔNH ~ 8 × 1020 cm-2) in the case of S5 0836+71, on a timescale of approximately 0.8 yr in the quasar frame. For the remaining five sources for which ROSAT spectra were available, the two instruments gave consistent results, and the data were combined yielding unprecedented spectral coverage (typically about 0.4-40 keV in the quasar frame) for high-z quasars. This allows us to put severe limits on several different descriptions of the continuum (e.g., broken power law, bremsstrahlung, reflection component). No Fe Kα emission line is detected in any of the ASCA spectra. An absorption edge consistent with Fe Kα at the quasar redshift is marginally detected in S5 0014+81. Possible origins for the observed low energy absorption are discussed. In particular, contributions from the molecular clouds and dust present in our Galaxy (usually disregarded) are carefully considered. In light of the new results for S5 0836+71 and S5 0014+81, absorption intrinsic to the quasars is considered and discussed. The average slope obtained from the eight ASCA spectra in the observed ~0.5-10 keV energy band is Γ0.5-10 keV 1.61 ± 0.04, with a dispersion σ0.5-10 keV 0.10 ± 0.03. The average photon index in the observed 2-10 keV band, where the effect of absorption is negligible, is Γ2-10 keV 1.53 ± 0.05, with a dispersion σ2-10 keV 0.12. Furthermore, the implications of the present results on the calculations of the contribution of quasars to the cosmic X-ray and γ-ray backgrounds are briefly discussed.

Discovery of multiple Lorentzian components in the X-ray timing properties of the Narrow Line Seyfert 1 Ark 564

We present a power spectral analysis of a 100-ks XMM-Newton observation of the Narrow Line Seyfert 1 galaxy Ark 564. When combined with earlier RXTE and ASCA observations, these data produce a power spectrum covering seven decades of frequency which is well described by a power law with two very clear breaks. This shape is unlike the power spectra of almost all other active galactic nuclei (AGN) observed so far, which have only one detected break, and resemble Galactic binary systems in the soft state. The power spectrum can also be well described by the sum of two Lorentzian-shaped components, the one at higher frequencies having a hard spectrum, similar to those seen in Galactic binary systems. Previously we have demonstrated that the lag of the hard-band variations relative to the soft band in Ark 564 is dependent on variability time-scale, as seen in Galactic binary sources. Here we show that the time-scale dependence of the lags can be described well using the same two-Lorentzian model which describes the power spectrum, assuming that each Lorentzian component has a distinct time lag. Thus all X-ray timing evidence points strongly to two discrete, localized, regions as the origin of most of the variability. Similar behaviour is seen in Galactic X-ray binary systems in most states other than the soft state, i.e. in the low-hard and intermediate/very high states. Given the very high accretion rate of Ark 564 the closest analogy is with the very high (intermediate) state rather than the low-hard state. We therefore strengthen the comparison between AGN and Galactic binary sources beyond previous studies by extending it to the previously poorly studied very high accretion rate regime.

Radio-loud active galaxies in the northern ROSAT All-Sky Survey - I. Radio identifications

Abstract. We present 5 GHz high resolution VLA observations of 2,127 radio– and X-ray–emitting sources found in both the Green Bank (GB) 5 GHz radio catalog and the ROSAT All-Sky Survey (RASS). We report core flux densities and positions accurate to ±0.5′′. Combined with the GB measurements of the total radio emission, we derive the core-to-lobe ratio of objects in our sample and discuss their core-dominance relative to samples of radio galaxies and BL Lacertae objects. Our results show the RASS/Green Bank (RGB) sample is approximately an order of magnitude more core-dominated than the radio galaxy sample, but is more than an order of magnitude less core-dominated than highly beamed BL Lacertae objects. Using simple beaming models, this indicates the typical object in the RGB catalog exhibits moderately beamed radio emission and is oriented at an angle to the lineof-sight θRGB ∼ 25◦ − 35◦. The case of the origin of the X-ray emission is not as clear; the data are consistent with either an anisotropic unbeamed or moderately beamed X-ray component. Tables 2 and 3 which present the RGB catalog are available in their entirety only from the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5), via the WWW at http://cdsweb.ustrasbg.fr/Abstract.html, or at ftp://ftp.astro.psu.edu /pub/edf.We present 5 GHz high resolution VLA observations of 2,127 radio– and X-ray–emitting sources found in both the Green Bank (GB) 5 GHz radio catalog and the ROSAT All-Sky Survey (RASS). We report core flux densities and positions accurate to ± 0.5. Combined with the GB measurements of the total radio emission, we derive the core-to-lobe ratio of objects in our sample and discuss their core-dominance relative to samples of radio galaxies and BL Lacertae objects. Our results show the RASS/Green Bank (RGB) sample is approximately an order of magnitude more core-dominated than the radio galaxy sample, but is more than an order of magnitude less core-dominated than highly beamed BL Lacertae objects. Using simple beaming models, this indicates the typical object in the RGB catalog exhibits moderately beamed radio emission and is oriented at an angle to the line-of-sight . The case of the origin of the X-ray emission is not as clear; the data are consistent with either an anisotropic unbeamed or moderately beamed X-ray component. Tables 2 and 3 which present the RGB catalog are available in their entirety only from the CDS via anonymous ftp to cdsarc. u-strasbg.fr (130.79.128.5), via the WWW at http://cdsweb. u-strasbg.fr/ Abstract.html, or at ftp://ftp. astro. psu. edu /pub /edf.

R-band imaging of the host galaxies of RGB BL Lacertae objects

We present optical R-band images of 100 BL Lacertae objects in the ROSAT-Green Bank (RGB) sample obtained at the Nordic Optical Telescope (NOT). For 62 objects we could not find any previously published images in the literature, of these 35 are new BL Lacs discovered during the RGB BL Lac survey. We were able to resolve the host galaxy in 62% of the objects. We analyze the host galaxies using a two-dimensional fitting procedure and tabulate the best-fit core magnitude and host galaxy parameters (magnitude, effective radius, ellipticity, position angle and shape parameter β) for each object. With two exceptions, all objects are better fit by a model representing an elliptical galaxy (β = 0.25) than by a disk galaxy model (β = 1.0). In the two exceptions the disk host fit is only marginally better than the elliptical host fit, however, and we do not find a single clear example of a disk-dominated host galaxy. The host galaxies have an average brightness MR = −23.9 ± 0.8 and average effective radius reff = 13.2 ± 0.8 kpc. Their bulk properties are indistinguishable from normal elliptical and radio galaxies. The median β is 0.18, well below the value 0.25 that is normally used to describe ellipticals. However, the difference may be caused by as election effect due to the dependence of β on host galaxy luminosity.

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.

Multiwavelength Monitoring of the Narrow-Line Seyfert 1 Galaxy Arakelian 564. III. Optical Observations and the Optical-UV-X-Ray Connection

We present the results of a 2 yr long optical monitoring program of the narrow-line Seyfert 1 galaxy Ark 564. The majority of this monitoring project was also covered by X-ray observations (RXTE), and for a period of ~50 days, we observed the galaxy in UV (HST) and X-rays (RXTE and ASCA) simultaneously with the ground-based observations. Rapid and large-amplitude variations seen in the X-ray band, on a daily and hourly timescale, were not detected at optical and UV wavelengths, which in turn exhibited much lower variability either on short (1 day) or long (several months) timescales. The only significant optical variations can be described as two 2-4 day events with ~10% flux variations. We detect no significant optical line variations and thus cannot infer a reverberation size for the broad-line region. Similarly, the large X-ray variations seem to vanish when the light curve is smoothed over a period of 30 days. The UV continuum follows the X-rays with a lag of ~0.4 days, and the optical band lags the UV band by ~2 days. No significant correlation was found between the entire X-ray data set and the optical band. Focusing on a 20 day interval around the strongest optical event we detect a significant X-ray-optical correlation with similar events seen in the UV and X-rays. Our data are consistent with reprocessing models on the grounds of the energy emitted in this single event. However, several large X-ray flares produced no corresponding optical emission.

Spectral-timing evidence for a very high state in the narrow-line Seyfert 1 Ark 564

We use a 100 ks long XMM-Newton observation of the narrow-line Seyfert 1 galaxy Ark 564 and combine it with the month-long monitoring of the same source produced by ASCA, to calculate the phase lags and coherence between different energy bands, over frequencies from ∼10 -6 to 10 -3 Hz. This is the widest frequency range for which these spectral-timing properties have been calculated accurately for any active galactic nuclei (AGN). The 0.7-2 and 2-10 keV ASCA light curves, and the XMM-Newton light curves in corresponding energy bands, are highly coherent (∼0.9) over most of the frequency range studied. We observe time lags between the energy bands, increasing both with time-scale and with energy separation of the bands. The time lag spectrum shows a broad peak in the 10 -5 to 5 x 10 -4 Hz frequency range, where the time lags follow a power-law slope ∼- 0.7. Above ∼5 x 10 -4 Hz the lags drop below this relation significantly. This change in slope resembles the shape of the lag spectra of black hole X-ray binaries (BHXRB) in the very high or intermediate state. The lags increase linearly with the logarithm of the separation of the energy bands, which poses one more similarity between this AGN and BHXRBs.

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.