arXiv subjects

W. Brinkmann

Publications and source records attributed to W. Brinkmann.

At least 19 recordsLinked to original sources

arXiv2012

Revisiting algorithms for generating surrogate time series

The method of surrogates is one of the key concepts of nonlinear data analysis. Here, we demonstrate that commonly used algorithms for generating surrogates often fail to generate truly linear time series. Rather, they create surrogate realizations with Fourier phase correlations leading to non-detections of nonlinearities. We argue that reliable surrogates can only be generated, if one tests separately for static and dynamic nonlinearities.

physics.data-an↗

arXiv2009

XMM-Newton long-look observation of the narrow line Seyfert 1 galaxy PKS0558-504; I: Spectral analysis

We present results from the spectral analysis of a long XMM-Newton observation of the radio-loud NLS1 galaxy PKS0558-504. The source is highly variable, on all sampled time scales. We did not observe any absorption features in either the soft or hard X-ray band. We found weak evidence for the presence of an iron line at ~6.8 keV, which is indicative of emission from highly ionized iron. The 2-10 keV band spectrum is well fitted by a simple power law model, whose slope steepens with increasing flux, similar to what is observed in other Seyferts as well. The soft excess is variable both in flux and shape, and it can be well described by a low-temperature Comptonisation model, whose slope flattens with increasing flux. The soft excess flux variations are moderately correlated with the hard band variations, and we found weak evidence that they are leading them by ~20 ksec. Our results rule out a jet origin for the bulk of the X-ray emission in this object. The observed hard band spectral variations suggest intrinsic continuum slope variations, caused by changes in the "heating/cooling" ratio of the hot corona. The low-temperature Comptonising medium, responsible for the soft excess emission, could be a hot layer in the inner disc of the source, which appears due to the fact that the source is accreting at a super-Eddington rate. The soft excess flux and spectral variations could be caused by random variations of the accretion rate.

astro-ph.CO↗

arXiv2008

The X-ray variability and the near-IR to X-ray spectral energy distribution of four low luminosity Seyfert 1 galaxies

We present the results from a study of the X-ray variability and the near-IR to X-ray spectral energy distribution of four low-luminosity, Seyfert 1 galaxies. We compared their variability amplitude and broad band spectrum with those of more luminous AGN in order to investigate whether accretion in low-luminosity AGN operates as in their luminous counterparts. We used archival XMM-Newton and, in two cases, ASCA data to estimate their X-ray variability amplitude and determine their X-ray spectral shape and luminosity. We also used archival HST data to measure their optical nuclear luminosity, and near-IR measurements from the literature, in order to construct their near-IR to X-ray spectra. The X-ray variability amplitude of the four Seyferts is what one would expect, given their black hole masses. Their near-IR to X-ray spectrum has the same shape as the spectrum of quasars which are 10^2-10^5 times more luminous. The objects in our sample are optically classified as Seyfert 1-1.5. This implies that they host a relatively unobscured AGN-like nucleus. They are also of low luminosity and accrete at a low rate. They are therefore good candidates to detect radiation from an inefficient accretion process. However, our results suggest that they are similar to AGN which are 10^2-10^5 times more luminous. The combination of a "radiative efficient accretion disc plus an X-ray producing hot corona" may persist at low accretion rates as well.

astro-ph↗

arXiv2008

The soft X-ray properties of AGN from the CJF sample; A correlation analysis between soft X-ray and VLBI properties

Context: We present the soft X-ray properties obtained in the ROSAT All-Sky survey and from pointed PSPC observations for the AGN in the complete flux-density limited Caltech-Jodrell Bank flat spectrum sample (hereafter CJF). CJF is a VLBI survey (VLBA observations at 5 GHz) of 293 AGN with detailed information on jet component motion. Aims: We investigate and discuss the soft X-ray properties of this AGN sample and examine the correlations between X-ray and VLBI properties, test beaming scenarios, and search for the discriminating properties between the sub-samples detected and not detected by ROSAT. Methods: Comparing the observed and the predicted X-ray fluxes by assuming an Inverse Compton (IC) origin for the observed X-rays, we compute the beaming or Doppler factor and contrast the Doppler factors with other beaming indicators derived from the VLBI observations, such as the value of the expansion velocity, and the observed and intrinsic brightness temperature. In addition, we investigate the large-scale radio structure of the AGN and the difference between the pc- and kpc-scale structure (misalignment) with regard to the X-ray observations.Results: We find a nearly linear relation between X-ray and radio luminosities, and a similar but less stringent behaviour for the relation between optical and X-ray luminosities. The quasars detected by ROSAT have a different $β_{\rm app}$-redshift relationship compared to the non-detected ones. ROSAT-detected sources tend to reveal extended large-scale radio structures more often. Conclusions: We conclude that beaming alone cannot explain the observed dichotomy of ROSAT detection or non-detection and assume that the large-scale jet structure plays a decisive role.

astro-ph↗

arXiv2007

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 ksec 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 AGN observed so far, which have only one detected break, and resemble Galactic binary systems in a 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, localised, 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.

astro-ph↗

arXiv2007

Spectral variability analysis of an XMM-Newton observation of Ark 564

We present a spectral variability analysis of the X-ray emission of the Narrow Line Seyfert 1 galaxy Ark 564 using the data from a ~100 ks XMM-Newton observation. Taking advantage of the high sensitivity of this long observation and the simple spectral shape of Ark 564, we determine accurately the spectral variability patterns in the source. We use standard cross-correlation methods to investigate the correlations between the soft and hard energy band light curves. We also generated 200 energy spectra from data stretches of 500 s duration each and fitted each one of them with a power law plus a bremsstrahlung component (for the soft excess) and we investigated the correlations between the various best fit model parameter values. The ``power law plus bremsstrahlung'' model describes the spectrum well at all times. The iron line and the absorption features, which are found in the time-averaged spectrum of the source are too weak to effect the results of the time resolved spectral fits. We find that the power law and the soft excess flux are variable, on all measured time scales. The power law slope is also variable, and leads the flux variations of both the power law and the bremsstrahlung components. Our results can be explained in the framework of time-dependent Comptonization models. They are consistent with a picture where instabilities propagate through an extended X-ray source, affecting first the soft and then the hard photons producing regions. The soft excess could correspond to ionized disc reflection emission, in which case it responds fast to the primary continuum variations. The time scales are such that light travel times might additionally influence the observed variability structure.

astro-ph↗

arXiv2006

Long-Term X-Ray spectral variability of the radio-loud NLS1 galaxy PKS 0558-504

We present X-ray observations of the Narrow-Line Seyfert 1 galaxy PKS 0558-504 obtained with RXTE during a 1-year monitoring campaign. This source, which is one of the very few radio-loud NLS1 galaxies, shows strong X-ray flux variability on time scales of weeks-months accompanied by spectral variability. The main goal of this study is to investigate the spectral variability with model-independent methods and time-resolved spectroscopy in order to shed light on the origin of the X-rays. The main results can be summarized as follows: 1) The flux typically changes by a factor of 1.5-2 on time scales of 10-30 days, with few extreme events where the flux increases by a factor of ~4 in 3 days. 2) We do not observe any large amplitude, flux related spectral variations. During the flux variations, the spectrum varies mainly in normalization and not in shape. We do observe some small amplitude spectral variations, which do not correlate with flux, although there is a hint of spectral hardening as the source brightens. 3) There is no evidence for reprocessing features such as the Fe Kalpha line or a Compton hump. We argue that PKS 0558-504 is a peculiar object that appears to be different from most of the radio-quiet and radio-loud AGN. If a jet is responsible for the bulk of the X-rays, it must operate in an unusual way. If instead a corona is responsible for the X-rays, the system might be a large-scale analog of the Galactic black holes in the transient intermediate state.

astro-ph↗

arXiv2006

XMM-Newton observations of the eastern jet of SS433

The radio supernova remnant W50 hosts at its center the peculiar galactic X-ray binary SS 433. It shows a central spherical structure with two `ears' which are supposed to be formed by the interaction of the precessing jets of SS 433 with the supernova shell. In two pointings in September/October 2004 for 30 ks each the eastern jet of SS 433 was observed with XMM-Newton to study the outermost parts of the `ear' and the X-ray bright emission region about 35 arcmin from SS 433. The spectra consist of two components: a non-thermal power law with photon index Γ~ 2.17+/-0.02 and a thermal component at a typical temperature of kT ~ 0.3 keV. The X-ray emission seems to fill the whole interior region of the radio remnant W50. The jet terminates in the eastern `ear' in a ring-like terminal shock which indicates a flow with a kind of hollow-cone morphology. The spatial coincidence of X-ray and radio emission suggests physical conditions similar to those found at the outer shocks of ordinary supernova remnants. The bright emission region closer to SS 433 radiates non-thermally in a spatially well confined geometry at higher X-ray energies. At soft X-rays the shape of the region gets blurred, centered on the hard lenticular emission. The shape of this region and the bend in the jet propagation direction might be caused by the interaction of a re-collimated jet with the outer, non homogeneous interstellar matter distribution. The physical conditions leading to the re-collimation of the jet and the peculiar emission morphology are far from being understood and require deeper observations as well as a detailed modeling of the interaction of a jet with its surroundings.

astro-ph↗

arXiv2006

XMM-Newton observation of the NLS1 Galaxy Ark 564: I. Spectral analysis of the time-average spectrum

We use the data from a recent, 100 ksec XMM-Newton observation of the Narrow Line Seyfert 1 galaxy Ark 564 to obtain its time average, X-ray spectrum. The 3-11 keV spectrum is well fitted by a power-law of slope 2.43. We detect a weak (equivalent width ~80 eV) emission line at ~6.7 keV, which implies emission from ionized iron. There is no compelling evidence for significant broadening of the line.We also detect a possible Doppler shifted absorption line at 8.1 keV. At energies lower than 2 keV, the spectrum is dominated by a smooth soft excess component which can be well fitted either by a two black body components (kT~0.15 and 0.07 keV) or by a black body plus a relativistically blurred photoionized disc reflection model. We detect a broad, shallow flux deficit in the 0.65-0.85 keV band, reminiscent of the iron unresolved transition array (UTA) features. We do not detect neither a strong absorption edge around 0.7 keV nor an emission line around 1 keV. The soft excess emission is consistent with being reflected emission from a geometrically flat disc, with solar abundances, illuminated by an isotropic source. The weakness of the iron line emission can be explained by relativistic blurring. The UTA feature implies the presence of warm material with a column density of 2-5*10^{20} cm^{-2}. If the absorption line at 8.1 keV corresponds to FeXXVI Kalpha, it suggests the presence of a highly ionized absorbing material with N_H > 10^{23} cm^{-2}, outflowing at a high velocity of ~0.17c.

astro-ph↗

arXiv2006

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 ~ 10^{-6} to 10^{-3} Hz. This is the widest frequency range for which these spectral-timing properties have been calculated accurately for any AGN. The 0.7-2 and 2--0 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} - 5x10^{-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.

astro-ph↗

arXiv2006

The spectral energy distribution of PKS 2004-447: a compact steep-spectrum source and possible radio-loud narrow-line Seyfert 1 galaxy

(abridged) The spectral energy distribution (SED) of the compact steep spectrum (CSS) source and possible radio-loud narrow-line Seyfert 1 galaxy (NLS1), PKS2004-447, is presented. Five out of six well studied RL NLS1 share this dual classification (optically defined as a NLS1 with radio definition of a CSS or giga-hertz peaked spectrum (GPS) source). The SED is created from simultaneous observations at radio (ATCA), optical/NIR (Siding Spring) and UV/X-ray (XMM-Newton) wavelengths. The X-ray data show evidence of short-term variability, a possible soft excess, and negligible absorption. Together with the rest of the SED, the X-ray emission is excessive in comparison to synchrotron plus synchrotron self-Compton (SSC) models. The SED can be described with a two component model consisting of extended synchrotron/SSC emission with Comptonisation in the X-rays, though SSC models with a very high electron-to-magnetic energy density ratio cannot be excluded either. The peak emission in the SED appears to be in the near infrared, which can be attributed to thermal emission from a dusty torus. Analysis of a non-contemporaneous, low resolution optical spectrum suggests that the narrow-line region (NLR) is much more reddened than the X-ray emitting region suggesting that the gas-to-dust ratio in PKS2004-447 may be very different then in our own Galaxy. Long-term radio monitoring of PKS2004-447 shows a rather constant light curve over nearly a six month period with the exception of one outburst when the 6.65GHz flux increased by ~35% over 19 days. In comparison to general samples of GPS sources, which appear to be X-ray weak, NLS1-CSS/GPS sources possess stronger X-ray emission relative to radio, and lower intrinsic absorption than GPS sources of similar X-ray luminosity.

astro-ph↗

arXiv2005

XMM-Newton observations of bright ROSAT selected active galactic nuclei with low intrinsic absorption

We present a sample of twenty-one ROSAT bright active galactic nuclei (AGN), representing a range of spectral classes, and selected for follow-up snap-shot observations with XMM-Newton. The typical exposure was between 5-10 ks. The objects were primarily selected on the bases of X-ray brightness and not on hardness ratio; thus the sample cannot be strictly defined as a `soft' sample. One of the main outcomes from the XMM-Newton observations was that all of the AGN, including eleven type 1.8-2 objects, required low levels of intrinsic absorption (nh < 10^21 cm^-2). The low absorption in type 2 systems is a challenge to account for in the standard orientation-based unification model, and we discuss possible physical and geometrical models which could elucidate the problem. Moreover, there does not appear to be any relation between the strength and shape of the soft excess, and the spectral classification of the AGN in this sample. We further identify a number of AGN which deserve deeper observations or further analysis: for example, the LINERs NGC 5005 and NGC 7331, where optically thin thermal and extended emission is detected, and the narrow-line Seyfert 1 II Zw 177, which shows a broad emission feature at 5.8keV.

astro-ph↗

arXiv2005

XMM-Newton timing mode observations of Mrk 421

We present the results of a detailed temporal analysis of the bright BL Lac object Mrk 421 using the three available long timing mode observations by the EPIC PN camera. This detector mode is characterized by its long life time and is largely free of photon pile-up problems. The source was found in different intensity and variability states differing by up to more than a factor of three in count rate. A time resolved cross correlation analysis between the soft and hard energy bands revealed that the characteristics of the correlated emission, with lags of both signs, change on time scales of a few thousand seconds. Individual spectra, resolved on time scales of a few hundread seconds, can be quite well fitted by a broken power law. We find significant spectral variations on time scales as short as 500-1000 sec. Both the hard and the soft band spectral indices show a non-linear correlation with the source flux. A comparison of the observed light curves with numerical results from relativistic hydrodynamic computer simulations of the currently favored shock-in-jet models indicates that any determination of the jet's physical parameters from `simple' emission models must be regarded with caution: at any time we are seeing the emission from several emission regions distinct in space and time, which are connected by the complex hydrodynamic evolution of the non-uniform jet.

astro-ph↗

arXiv2005

Chandra Observation of the Interaction between the Hot Plasma Nebula RCW89 and the Pulsar Jet of PSR B1509-58

We present a Chandra observation of the H II region RCW89. The nebula lies on 10' north from the central pulsar PSR B1509-58, and it has been suggested that the nebula is irradiated by the pulsar jet. We performed a spectral analysis of the seven brightest emitting regions aligned in a ``horse-shoe'' shape, and found that the temperature of the knots increases along the ``horse-shoe'' in the clockwise direction, while, in contrast, the ionization parameter decreases. This strongly supports a picture of energy transfer via the precessing pulsar jet. We examined the energy budget assuming that RCW89 is powered by the pulsar jet, and confirmed that the pulsar rotational energy loss is sufficient to drive the nebula. The rate of energy injection into RCW89 by the jet was estimated from the synchrotron radiation flux. We obtained a heating time-scale of 1400 yr, which is consistent with the pulsar characteristic age of 1700 yr. To explain the temperature gradient, we discuss the cooling process for plasma clouds in RCW89. We argue that the plasma clouds can be cooled down by the adiabatic expansion within 70 yr, and form the temperature gradient reflecting the sequential heating by the precessing pulsar jet. We also determined the velocities of the individual plasma clouds by spectral fitting. The plasma clouds in RCW89 are moving away at 240 ~ 860 km/s, which constrains the inclination angle of the pulsar spin axis i > 50 degree and the expanding velocity of the shell as v > 1100 km/s.

astro-ph↗

arXiv2004

X-ray to UV Variability Correlation in MCG-6-30-15

We used a ~300 ks long XMM-Newton observation of the Seyfert 1 galaxy MCG-6-30-15 to study the correlation between the 0.2-10 keV X-ray and the 3000-4000 A bands. We found a significant correlation peak at a time lag of 160 ks where the UV flux variations preceded the variations in the X-ray band. We interpret this result as evidence in favour of Comptonisation models where the observed X-rays are produced through Compton up-scattering of thermal UV seed photons from an accretion disc, as this process naturally predicts the UV variations to precede similar flux variations in the X-rays. The length of the time lag favours models where the observed UV and the seed-photon-emitting regions are connected by perturbations of the accretion flow traveling inwards through the disc, affecting first the main U-band-emitting radii and then the innermost region where the bulk of the seed photons is expected to be produced. Finally, the absence of significant features in the correlation function with X-ray flux variations preceding those in the UV indicates that the observed U-band photons are not mainly produced through reprocessing of hard X-rays in this source.

astro-ph↗

arXiv2004

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 ~1-1.5keV, whilst two objects (PG1114+445 and IZw1) show a deficit of soft X-ray flux due to the presence of a strong warm absorber. At least half of the objects appear to harbor a warm absorber, as found previously in Seyfert 1 galaxies. We find significant detections of FeKalpha 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 6.7-7keV, 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 and the optical Hbeta width, in both the soft and hard X-ray bands, whereby the steepest X-ray spectra are found in objects with narrow Hbeta 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.

astro-ph↗

arXiv2004

Synthetic X-ray light curves of BL Lacs from relativistic hydrodynamic simulations

We present the results of relativistic hydrodynamic simulations of the collision of two dense shells in a uniform external medium, as envisaged in the internal shock model for BL Lac jets. The non-thermal radiation produced by highly energetic electrons injected at the relativistic shocks is computed following their temporal and spatial evolution. The acceleration of electrons at the relativistic shocks is parametrized using two different models and the corresponding X-ray light curves are computed. We find that the interaction time scale of the two shells is influenced by an interaction with the external medium. For the chosen parameter sets, the efficiency of the collision in converting dissipated kinetic energy into the observed X-ray radiation is of the order of one percent.

astro-ph↗

arXiv2003

X-ray variability of the Narrow Line Seyfert 1 Galaxy PKS 0558-504

We present results from several XMM-Newton observations of the radio loud Narrow-line Seyfert 1 galaxy (NLS1) PKS 0558-504. We find evidence for strong and persistent X-ray variability, both on short and long time-scales. On short time scales of ~2 hours the source varies smoothly by 15-20%; long term variations by a factor of >2 could not be resolved in the relatively short exposures: we find the source mostly in a `low' state, in 2 out of the 11 observations in a `high state'. X-ray flares seem to be recurrent with a time scale of ~24 ksec which, if interpreted as the Keplerian time scale in the disc, would place the emission region just outside the last stable orbit. The X-ray spectrum of PKS 0558-504 can be well fitted by two Comptonization components, one at moderate temperatures of kT ~ 4.5 keV and optical depths of \~ 2, the other at high temperatures kT > 50 keV and low optical depths < 1.0. These parameters are, however, subject to large errors due to the inherent degeneracy of the Comptonization models. Flux variations of the source are caused by changes of the colder component only, the hot component with parameters very similar to those of BLS1 galaxies, stays constant. All results fit nicely the picture that NLS1 galaxies are lower mass objects, accreting close to the Eddington rate emitting X-rays from a very active magnetically powered accretion disc corona.

astro-ph↗