arXiv subjects

W. Brinkmann

Publications and source records attributed to W. Brinkmann.

At least 37 records · Page 2Linked to original sources

arXiv2003

XMM-Newton observations of four high-z quasars

We present the results of XMM-Newton observations of four high-z quasars, two radio-loud and two radio-quiet. One of the radio-loud objects, PKS 2126-158, clearly shows absorption in excess of the galactic value as claimed before from ASCA and ROSAT observations. For PKS 2149-306 the evidence for excess absorption is only marginal in contrast to previous results. The location of the absorber in PKS 2126-158 is compatible with the redshift of the source. Both, a warm and a cold absorber are allowed by the X-ray data. Both quasars have very flat photon spectra (Γ\la 1.5) and the high photon statistics reveal small deviations from a simple power law form. For the two radio-quiet objects, Q 0000-263 and Q 1442+2931, we determine, for the first time, reliable spectral parameters. Both quasars have steeper power laws (Γ\sim 2) and show absorption consistent with the galactic value, similar to radio-quiet quasars at low redshifts. In the case of Q 0000-263 the presence of the damped Ly$α$ system (N(HI)\sim 2.6\times 10^{21} cm^{-2} at z = 3.39) cannot be examined further due to the limited photon statistics in all instruments.

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arXiv2002

XMM-Newton observation of PG 0844 + 349

In a ~ 20 ksec XMM-Newton observation the X-ray transient radio-quiet quasar PG 0844+349 was found in a historically high state compared to previous X-ray observations. The quasar showed a featureless spectrum with a strong soft excess over the extrapolation of a hard power law. Comptonization models or a broken power law with Gamma_{soft} ~ 2.75, Gamma_{hard} ~ 2.25 and a break energy of E_{break} ~ 1.35 keV represent acceptable descriptions of the spectral continuum. In the Comptonization models the temperature of the Comptonizing gas is considerably lower than generally found in (broad line) Seyfert galaxies whereas the optical depth is much higher. As a similar behavior has been seen in NLSy1 galaxies, it might be an indicator of the different physical conditions in these two classes of AGN. During the XMM-Newton observation the flux of PG 0844+349 varied achromatically in a smooth, nearly linear fashion, by ~ 25 % on time scales of a few thousand seconds, which puts some constraints on current models of Comptonizing accretion disk coronae.

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arXiv2002

On the Nature of X-ray Variability in Ark 564

We use data from a recent long ASCA observation of the Narrow Line Seyfert 1 Ark 564 to investigate in detail its timing properties. We show that a thorough analysis of the time series, employing techniques not generally applied to AGN light curves, can provide useful information to characterize the engines of these powerful sources.We searched for signs of non-stationarity in the data, but did not find strong evidences for it. We find that the process causing the variability is very likely nonlinear, suggesting that variability models based on many active regions, as the shot noise model, may not be applicable to Ark 564. The complex light curve can be viewed, for a limited range of time scales, as a fractal object with non-trivial fractal dimension and statistical self-similarity. Finally, using a nonlinear statistic based on the scaling index as a tool to discriminate time series, we demonstrate that the high and low count rate states, which are indistinguishable on the basis of their autocorrelation, structure and probability density functions, are intrinsically different, with the high state characterized by higher complexity.

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arXiv2001

Detection of a high frequency break in the X-ray power spectrum of Ark 564

We present a power spectrum analysis of the long ASCA observation of Ark 564 in June/July 2001. The observed power spectrum covers a frequency range of ~ 3.5 decades. We detect a high frequency break at ~ 0.002 Hz. The power spectrum has an rms of ~30% and a slope of ~ -1 and ~ -2 below and above the break frequency. When combined with the results from a long RXTE observation (Pounds et al. 2001), the observed power spectra of Ark 564 and Cyg X-1 (in the low/hard state) are almost identical, showing a similar shape and rms amplitude. However, the ratio of the high frequency breaks is very small (~ 10e{3-4}), implying that these characteristic frequencies are not indicative of the black hole mass. This result supports the idea of a small black hole mass/high accretion rate in Ark 564.

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arXiv2001

BeppoSAX observations of the Narrow-Line Seyfert 1 galaxy RX J1702.5+3247

We report optical, radio and X-ray observations of the Narrow-Line Seyfert 1 galaxy RX J1702.5+3247. The soft (0.1-2 keV) X-ray flux, measured by BeppoSAX, is characterized by strong variability on short time scales (< 500 sec). The most extreme amplitude variations require a radiative efficiency exceeding the maximum for a Kerr black hole, implying the presence of relativistic effects. A comparison with archival ROSAT data reveals long term temporal and spectral variability. The 0.1-10 keV spectrum is equally well fitted either by an ionized reflection disk model, or by a broken power law plus a Gaussian line, consistent with a hydrogen-like iron line at 6.97 keV from a highly ionized accretion disk.

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arXiv2001

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

We present the results of a two-year long optical monitoring program of the narrow-line Seyfert 1 galaxy Akn 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 & ASCA) simultaneously with the ground-based observations. Rapid and large-amplitude variations seen in the X-ray band, on a daily and hourly time-scale, were not detected at optical and UV wavelengths, which in turn exhibited much lower variability either on short (one day) or long (several months) time-scales. 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 dataset 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.

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arXiv2000

Optical and X-ray monitoring of the NLS1 galaxy Ark 564

The Narrow-line Seyfert galaxy Ark 564 was monitored between June 04, 2000 and June 24, 2000 from Skinakas Observatory. The B,V,R and I band light curves show night to night variations of 0.1 mag. Comparison with X-ray data from a simultaneous ASCA observation shows that the X-ray variations are more pronounced than in the optical band. Cross correlation analysis yields a time lag of ~ 0.8 days in the sense that the X-rays are leading the optical but with low statistical significance. We discuss possible implications of our results to current physical models of AGN.

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arXiv2000

XMM - Newton observations of Markarian 421

The BL Lac object Mrk 421 was observed on May 25, 2000 during the XMM - Newton CAL/PV phase. The high throughput of the X-ray telescopes and the spectral capabilities of the instruments allow an uninterrupted temporal and spectral study of the source with unprecedented time resolution. Mrk 421 was found at a relatively high state with a 2-6 keV flux of (1.3 - 1.9)E-10 erg cm**-2 s **-1. The observed intensity variations by more than a factor of three at highest X-ray energies are accompanied by complex spectral variations with only a small time lag (tau = 265 {+116} {-102} seconds) between the hard and soft photons. The (0.2-10) keV spectrum can be well fitted by a broken power law and no absorption structures are found in the source spectrum at the high spectral resolution of the transmission gratings.

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arXiv2000

XMM-Newton monitoring of X-ray variability in the quasar PKS 0558-504

We present the temporal analysis of X-ray observations of the radio-loud Narrow-Line Seyfert 1 galaxy (NLS1) PKS 0558-504 obtained during the XMM-Newton Calibration and Performance Verification (Cal/PV) phase. The long term light curve is characterized by persistent variability with a clear tendency for the X-ray continuum to harden when the count rate increases. Another strong correlation on long time scales has been found between the variability in the hard band and the total flux. On shorter time scales the most relevant result is the presence of smooth modulations, with characteristic time of ~ 2 hours observed in each individual observation. The short term spectral variability turns out to be rather complex but can be described by a well defined pattern in the hardness ratio-count rate plane.

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arXiv2000

XMM-Newton observations of the BL Lac MS 0737+7441

We report on the XMM-Newton observations of the BL Lac object MS 0737.9+7441 during the performance verification phase. A simple power-law fit provides an adequate description of the integrated spectrum in the 0.2-10 keV energy band. The photon index is slightly steeper in the EPIC pn data with 2.38+-0.01 compared to the EPIC MOS data (2.28+-0.01). The difference is most probably due to the present uncertainties in the calibration of the EPIC MOS and EPIC pn data sets. We report evidence for intrinsic absorption in the distant BL Lac above the Galactic column N_H,Gal=3.2*10^20 cm^-2 which is N_H,fit(z=0.315)= (2.70+-0.20)*10^20cm^-2 in the EPIC pn data and N_H,fit(z=0.315)= (3.25+-0.25)*10^20cm^-2 in the EPIC MOS data assuming neutral gas and solar abundances. The flux variations are found to be of the order of 10 %.

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arXiv2000

XMM-Newton Detection of a Comptonized Accretion Disc in the Quasar PKS0558-504

We present XMM-Newton observations of the bright quasar PKS0558-504. The 0.2-10 keV spectrum is dominated by a large, variable soft X-ray excess. The fastest flux variations imply accretion onto a Kerr black hole. The XMM-Newton data suggest the presence of a `big blue bump' in PKS0558-504 extending from the optical band to ~3 keV. The soft X-ray spectrum shows no evidence for significant absorption or emission-line features. The most likely explanation for the hot big blue bump is Comptonization by the multi-temperature corona of a thermal accretion disc running at a high accretion rate.

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arXiv2000

The X-Ray Absorber in Broad Absorption Line Quasars

Recent observations of Broad Absorption Line (BAL) quasars demonstrated that the soft X-ray emission of these objects is extremely weak and convincing evidence for very strong absorption by a high column density (~ 10^23.5 cm^-2) was obtained for PG 1411+442, even though it is one of the few BAL QSOs strongly detected in soft X-rays. This paper examines the ionization status and geometry of the X-ray absorber by combining the properties of the UV lines with the X-ray continuum absorption. We show that the gas has to have large column densities in ions of major UV absorption lines, such as CIV, NV, OVI and Ne VIII, in order to have sufficient opacity around 0.2 to 0.35 keV. The UV absorption lines have to be saturated if the X-ray absorber intersects the line of sight to the UV continuum emission region. A uniformly covering UV and X-ray absorption model can be constructed for PG 1411+442 but in some other soft X-ray detected BAL QSOs, such as PG 1001+054, the observed line optical depth is much lower than expected from the X-ray absorbing material. We propose a scheme in which a substantial fraction of the line of sight to the continuum source may be covered by either an optically thick flow or clouds in a narrow velocity range, but in which the total covering factor of either the whole flow or all clouds is close to unity.

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arXiv2000

X-ray observation of the super-luminal quasar 4C+73.18 (1928+738) by ASCA

The results of an X-ray observation of the nearby core-dominated, super- luminal quasar 4C+73.18 (1928+738) with ASCA are reported. Variations of the 1-10keV flux on time-scales of the order of 10 hours were found while the 0.5-1keV flux remained almost unchanged. The quasar shows a curved spectrum in the 0.8-10keV band, which can be modeled as a power law plus a soft component. Both, the spectrum and the uncorrelated flux variations indicate the presence of a soft X-ray excess, which might be the high-energy extension of the intense UV bump emission found in this quasar. The hard X-ray emission is dominated by the flat power law (photon index 1.1-1.5). The results are consistent with models in which the hard X-rays are emitted from the relativistic jet. An iron line emission as reported in a previous Ginga observation (Lawson & Turner 1997) was not detected. We suggest that the line detected by Ginga might be not associated with the quasar. There are indications for a tentative weak emission line-like feature around 1.25keV (1.63keV in the quasar frame) in both, the SIS and GIS spectra. The energy is surprisingly close to that of an unexplained line found in the flat-spectrum quasar PKS0637-752 by Yaqoob et al. (1998).

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arXiv2000

ROSAT HRI monitoring of X-ray variability in the NLS1 galaxy PKS 0558-504

We present the results of ROSAT High Resolution Imager (HRI) observations and the survey data of the radio-loud Narrow-Line Seyfert 1 galaxy (NLS1) PKS 0558--504. We find strong and persistent X-ray variability on both short and medium time-scales. The most extreme amplitude variations require a radiative efficiency exceeding the theoretical maximum for a Schwarzschild black hole, suggesting the presence of a rotating black hole or the influence of relativistic beaming effects. The spatial analysis rules out the possibility that the high luminosity and the strong variability are related to a nearby source.

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arXiv1999

ROSAT HRI observations of radio-loud AGN

We present the results of three ROSAT HRI observations of AGN expected to reside in clusters of galaxies. Although the exposures were truncated by the premature end of the ROSAT mission, valuable information could be achieved which greatly improved upon the previous PSPC results. For RGB 1745+398 we could separate the cluster emission from that of the BL Lac and could confirm the cluster parameters obtained from optical follow-up observations. In MRC 0625-536 the flux from the central point source contributes less than 3% to the total X-ray flux and the eastern component of the dumbbell galaxy seems to be the X-ray emitter. RXJ1234.6+2350 appears to be extended in X-rays. The X-ray flux is centered on a quasar, but optical spectroscopy indicates that the nearby radio galaxies reside in a previously unknown cluster at redshift z=0.134.

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arXiv1999

The putative pulsar-wind nebulae of the three Musketeers PSR B1055-52, B0656+14 and Geminga revisited

We report on the analysis of archival ASCA, ROSAT and BeppoSAX data of PSR B1055-52, PSR B0656+14 and Geminga, performed in order to investigate the reality of the putative ~10-20 arcmin wide X-ray pulsar-wind nebulae recently proposed to exist around these pulsars. In all these cases, the ROSAT and/or BeppoSAX data show that the diffuse and clumpy X-ray nebulae found by ASCA can be resolved in the contribution of unidentified point sources, unlikely related to the pulsars.

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arXiv1999

Spatial and spectral X-ray properties of the powerful radio galaxy Hercules A: environment and jet/ICM interaction

We investigate the X-ray properties of the powerful radio galaxy Hercules A (3C 348) using ROSAT HRI, PSPC and ASCA observations. The ASCA data are well fit by a thermal plasma model with a temperature of about 4.3 keV and abundances 0.4 solar. The HRI observation clearly reveals extended and elongated X-ray emission. For radii greater than 10 arcsec, the surface brightness profile perpendicular to the elongation is well fit by an isothermal beta--model with beta = 0.63 and a core radius of about 120 kpc. We derive a total mass of the putative cluster of 8.4e+13 M_sun and a gas mass fraction of about 18 per cent. The inner part of the surface brightness profile clealry reveals the presence of a point source, which contributes about 8 per cent to the total flux. The 0.1-2.4 keV luminosity of the point-like and the extended emission is 3.4e+43 and 4.3e+44 erg/s, respectively. After subtracting the cluster X-ray emission from the HRI image, residual structures are visible, which partly coincide with the radio jet and lobes. This indicates an interaction of the radio jet with the intracluster medium.

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arXiv1999

Recent ASCA and SAX observations of intermediate BL Lac objects

We present 6 ASCA and SAX observations of 4 intermediate BL Lac objects (1034+5727, 1055+5644, 1424+2401, 1741+1936). Their X-ray spectral properties and spectral energy distributions are compared to typical X-ray and radio selected BL Lacs. 1055+5644 varied by a factor of ten in flux without significant spectral changes.

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