arXiv subjects

Aneta Siemiginowska

Publications and source records attributed to Aneta Siemiginowska.

106 records · Page 6Linked to original sources

arXiv2003

The X-ray absorber of PKS2126-158

BeppoSAX observed the z=3.27 quasar PKS2126-158 on 1999 May 24-28 when its 2-10keV and 0.1-2.5keV fluxes were 1.1E-11 and 4.4E-12 cgs respectively, a factor of 2 higher than in all previous ROSAT and ASCA observations and 40% higher than in two more recent Chandra and XMM-Newton observations. The shortest detected rest frame variability timescale is of a few months, comparable to the causal timescale associated to an emission region of ~10 Schwarzschild radii around a few 1E10 solar masses black hole. The source is detected with a signal to noise ratio S/N>=3 up to ~50 keV, 215 keV rest frame. The BeppoSAX observations confirm the presence of low energy absorption along the line of sight, independent on the continuum model adopted, at high confidence level. Despite the limited spectral resolution of the BeppoSAX LECS and MECS it is possible to put constraints on different absorption and continuum models, but not to unambiguously determine the redshift of the absorber. If the absorber is not significantly ionized the BeppoSAX data do prefer an absorber at z<=2.7. Strong and complex metal line systems along the line of sight to PKS2126-158 have been found at z=0.6631 and at 2.64<z<2.82. They could well be associated to the X-ray absorption. Conversely, an ionized (``warm'') absorber at the quasar redshift provides a good fit only if the iron abundance is smaller than ~0.3 solar, while that of the other elements is fixed to the solar value.

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arXiv2003

X-ray emission from GPS/CSS sources

The high spatial resolution of the Chandra X-ray Observatory allows us to study the environment of GPS/CSS sources to within an arcsec of the strong compact core. We present the discovery of X-ray jets in two GPS quasars, PKS1127-145 and B2 0738+393, indicating that X-ray emission associated with the relativistic plasma is present at large distances from the GPS nucleus. We also discuss first results from Chandra observations of our GPS/CSS sample. We find that 6 out of 10 sources show intrinsic absorption at a level which may be sufficient to confine the GPS source.

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arXiv2003

Chandra Discovery of an X-ray jet and Extended X-ray Structure in z=0.63 quasar, B2 0738+313

We have made a 30 ksec Chandra observation of the redshift z=0.63 GPS quasar B2 0738+313. We detected X-ray emission from the core and have discovered a 200 kpc (projected on the sky) X-ray jet. The X-ray jet is narrow and curves, following the extended radio structure to the south of the quasar, and ending with a hot spot at the southernmost part of the radio lobe. The jet has a knot at ~13 arcsec away from the core. The knot emission is consistent with the X-rays being created by the inverse Compton scattering of the cosmic microwave background (CMB) photons and requires jet bulk Lorentz factors of a few (Gamma_{bulk} ~ 5-7). We discuss the emission mechanisms that may be responsible for the jet emission. We present new VLA data of the core and jet, and discuss the relation between the extended radio and X-ray emission. Extended emission observed in several GPS sources has been interpreted as a signature of the source past activity, while the GPS source is young and newly expanded. We argue that B2~0738+313 may be an example of a new class of radio sources similar to the FRII radio galaxies in their high jet bulk velocities, but with the powerful GPS-like nucleus. B2 0738+313 also has two damped Lyman-alpha systems along the line of sight, at z_{abs} = 0.0912 and 0.2212. We discuss the possible connection between the X-ray absorption (7.2+/-0.9 e20 cm(-2)) detected in the ACIS spectrum and these two intervening absorbers. We also investigate an extended structure within the central 10 arcsec of the core in the relation to structure seen in the optical.

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arXiv2002

Radiation pressure instability driven variability in the accreting black holes

The time dependent evolution of the accretion disk around black hole is computed. The classical description of the $α$-viscosity is adopted so the evolution is driven by the instability operating in the innermost radiation-pressure dominated part of the accretion disk. We assume that the optically thick disk always extends down to the marginally stable orbit so it is never evacuated completely. We include the effect of the advection, coronal dissipation and vertical outflow. We show that the presence of the corona and/or the outflow reduce the amplitude of the outburst. If only about half of the energy is dissipated in the disk (with the other half dissipated in the corona and carried away by the outflow) the outburst amplitude and duration are consistent with observations of the microquasar GRS 1915+105. Viscous evolution explains in a natural way the lack of direct transitions from the state C to the state B in color-color diagram of this source. Further reduction of the fraction of energy dissipated in the optically thick disk switches off the outbursts which may explain why they are not seen in all high accretion rate sources being in the Very High State.

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arXiv2002

Chandra Survey of Radio-quiet, High-redshift Quasars

We observed 17 optically-selected, radio-quiet high-redshift quasars with the Chandra Observatory ACIS, and detected 16 of them. The quasars have redshift between 3.70 and 6.28 and include the highest redshift quasars known. When compared to low-redshift quasars observed with ROSAT, these high redshift quasars are significantly more X-ray quiet. We also find that the X-ray spectral index of the high redshift objects is flatter than the average at lower redshift. These trends confirm the predictions of models where the accretion flow is described by a cold, optically-thick accretion disk surrounded by a hot, optically thin corona, provided the viscosity parameter alpha >= 0.02. The high redshift quasars have supermassive black holes with masses ~10^{10} M_{sun}, and are accreting material at ~0.1 the Eddington limit. We detect 10 X-ray photons from the z=6.28 quasar SDS 1030+0524, which may have a Gunn-Peterson trough and be near the redshift of reionization of the intergalactic medium. The X-ray data place an upper limit on the optical depth of the intergalactic medium tau(IGM) < 10^6, compared to the lower limit from the spectrum of Lyalpha and Lybeta, which implies tau(IGM) > 20.

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arXiv2002

Chandra Observations of the QSO Pair Q2345+007: Binary Quasar or Massive Dark Lens?

The components of the wide (7.3") separation quasar pair Q2345+007A,B (z=2.15) have the most strikingly similar optical spectra seen to date (Steidel & Sargent 1991) yet no detected lensing mass, making this system the best candidate known for a massive (1e14 Msun) dark matter lens system. Here we present results from a 65ksec Chandra observation designed to investigate whether it is a binary quasar or a gravitational lens. We find no X-ray evidence for a lensing cluster to a (0.5-2keV) flux limit of 2e-15 cgs, which is consistent with lensing only for a reduced baryon fraction. Using the Chandra X-ray observations of the quasars themselves, together with new and published optical measurements, we use the observed emission properties of the quasars for further tests between the lens and binary hypotheses. Assuming similar line-of-sight absorption to the images, we find that their X-ray continuum slopes are inconsistent (Gamma_A=2.30 and Gamma_B=0.83) as are their X-ray to optical flux ratios. The probability that B suffers absorption sufficient to account for these spectral differences is negligible. We present new optical evidence that the flux ratio of the pair is variable, so the time-delay in a lens scenario could cause some of the discrepancies. However, adequately large variations in overall spectral energy distribution are rare in individual QSOs. All new evidence here weighs strongly toward the binary interpretation. Q2345+007 thus may represent the highest redshift example known of interaction-triggered but as-yet unmerged luminous AGN.

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arXiv2002

Statistics: Handle with Care, Detecting Multiple Model Components with the Likelihood Ratio Test

The likelihood ratio test (LRT) and the related $F$ test, do not (even asymptotically) adhere to their nominal $χ^2$ and $F$ distributions in many statistical tests common in astrophysics, thereby casting many marginal line or source detections and non-detections into doubt. Although there are many legitimate uses of these statistics, in some important cases it can be impossible to compute the correct false positive rate. For example, it has become common practice to use the LRT or the $F$ test for detecting a line in a spectral model or a source above background despite the lack of certain required regularity conditions. In these and other settings that involve testing a hypothesis that is on the boundary of the parameter space, {\it contrary to common practice, the nominal $χ^2$ distribution for the LRT or the $F$ distribution for the $F$ test should not be used}. In this paper, we characterize an important class of problems where the LRT and the $F$ test fail and illustrate this non-standard behavior. We briefly sketch several possible acceptable alternatives, focusing on Bayesian posterior predictive probability-values. We present this method in some detail, as it is a simple, robust, and intuitive approach. This alternative method is illustrated using the gamma-ray burst of May 8, 1997 (GRB 970508) to investigate the presence of an Fe K emission line during the initial phase of the observation.

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arXiv2002

Chandra Discovery of a 300 kpc X-ray Jet in the GPS Quasar PKS1127-145

We have discovered an X-ray jet with Chandra imaging of the z=1.187 radio-loud quasar PKS1127-145. In this paper we present the Chandra X-ray data, follow-up VLA observations, and optical imaging using the HST WFPC2. The X-ray jet contains 273+/-5 net counts in 27ksec and extends ~30 arcsec, from the quasar core, corresponding to a minimum projected linear size of ~330/h_50 kpc. The evaluation of the X-ray emission processes is complicated by the observed offsets between X-ray and radio brightness peaks. We discuss the problems posed by these observations to jet models. In addition, PKS1127-145 is a Giga-Hertz Peaked Spectrum radio source, a member of the class of radio sources suspected to be young or ``frustrated'' versions of FRI radio galaxies. However the discovery of an X-ray and radio jet extending well outside the host galaxy of PKS1127-145 suggests that activity in this and other GPS sources may be long-lived and complex.

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arXiv2001

Sherpa: a Mission-Independent Data Analysis Application

The ever-increasing quality and complexity of astronomical data underscores the need for new and powerful data analysis applications. This need has led to the development of Sherpa, a modeling and fitting program in the CIAO software package that enables the analysis of multi-dimensional, multi-wavelength data. In this paper, we present an overview of Sherpa's features, which include: support for a wide variety of input and output data formats, including the new Model Descriptor List (MDL) format; a model language which permits the construction of arbitrarily complex model expressions, including ones representing instrument characteristics; a wide variety of fit statistics and methods of optimization, model comparison, and parameter estimation; multi-dimensional visualization, provided by ChIPS; and new interactive analysis capabilities provided by embedding the S-Lang interpreted scripting language. We conclude by showing example Sherpa analysis sessions.

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arXiv2001

150 keV Emission from PKS2149-306 with BeppoSAX

A BeppoSAX observation of the z=2.34 quasar PKS2149-306 produced a strong signal in the high energy PDS instrument up to a maximum observed energy of nearly 50 keV, 150 keV in the quasar frame. The Beppo-SAX spectrum spans almost 3 decades (0.3-150 keV, quasar frame) and shows an extremely hard (alpha=0.4+/-0.05) X-ray spectrum above 3 keV (comparable to the X-ray background slope), and either a softer (alpha=1.0(+0.6, -0.3)) low energy component, or an ionized absorber at zero redshift. No evidence is seen of an Fe-K emission line (EW<167 eV at 6.5 keV quasar frame) or a Compton hump (R<0.3). A bremsstrahlung fit gives kT(rest)=46(+32, -16) keV, similar to the X-ray background value, and a high energy cut-off power law requires E(cut)>120 keV (quasar frame). The SED of PKS 2149-306 shows two peaks at ~ 10(12+/-0.5}Hz and \~10(21+/-1.0)Hz (~ 0.3 mm and ~ 4 MeV), strongly resembling a low energy cutoff BL~Lac object (LBL). The ratio of the two peaks shows an extreme Compton dominance (C_D=1.4+/- 0.4), as in flat spectrum radio quasars (FSRQs). The presence of an additional `optical/UV big bump' component may provide photons that cool the jet, suppressing the radio emission.

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arXiv2001

Chandra Detection of X-ray Absorption Associated with a Damped Lyman Alpha System

We have observed three quasars, PKS 1127-145, Q 1331+171 and Q0054+144, with the ACIS-S aboard the Chandra X-ray Observatory, in order to measure soft X-ray absorption associated with intervening 21-cm and damped Ly$α$ absorbers. For PKS 1127-145, we detect absorption which, if associated with an intervening z_{abs}=0.312 absorber, implies a metallicity of 23% solar. If the absorption is not at z_{abs}=0.312, then the metallicity is still constrained to be less than 23% solar. The advantage of the X-ray measurement is that the derived metallicity is insensitive to ionization, inclusion of an atom in a molecule, or depletion onto grains. The X-ray absorption is mostly due to oxygen, and is consistent with the oxygen abundance of 30% solar derived from optical nebular emission lines in a foreground galaxy at the redshift of the absorber. For Q1331+171 and Q 0054+144, only upper limits were obtained, although the exposure times were intentionally short, since for these two objects we were interested primarily in measuring flux levels to plan for future observations. The imaging results are presented in a companion paper.

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arXiv2000

Radiation pressure instability as a variability mechanism in the microquasar GRS 1915+105

Physical mechanism responsible for high viscosity in accretion disks is still under debate. Parameterization of the viscous stress as $αP$ proved to be a successful representation of this mechanism in the outer parts of the disk, explaining the dwarf novae and X-ray novae outbursts as due to ionization instability. We show that this parameterization can be also adopted in the innermost part of the disk where the adoption of the $α$-viscosity law implies the presence of the instability in the radiation pressure dominated region. We study the time evolution of such disks. We show that the time-dependent behavior of GRS 1915+105 can be well reproduced if $α$-viscosity disk model is calculated accurately (with proper numerical coefficients in vertically averaged equations and with advection included), and if the model is supplemented with (i) moderate corona dissipating 50% of energy (ii) jet carrying luminosity-dependent fraction of energy. These necessary modifications in the form of the presence of a corona and a jet are well justified observationally. The model predicts outbursts at luminosity larger than 0.16$\dot M_{Edd}$, as required, correct outburst timescales and amplitudes, including the effect of increasing outburst timescale with mean luminosity. This result strongly suggests that the $α$-viscosity law is a good description of the actual mechanism responsible for angular momentum transfer also in the innermost, radiation pressure dominated part of the disk around a black hole.

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arXiv2000

Analysis of Energy Spectra with Low Photon Counts via Bayesian Posterior Simulation

Over the past 10 years Bayesian methods have rapidly grown more popular as several computationally intensive statistical algorithms have become feasible with increased computer power. In this paper, we begin with a general description of the Bayesian paradigm for statistical inference and the various state-of-the-art model fitting techniques that we employ (e.g., Gibbs sampler and Metropolis- Hastings). These algorithms are very flexible and can be used to fit models that account for the highly hierarchical structure inherent in the collection of high-quality spectra and thus can keep pace with the accelerating progress of new space telescope designs. The methods we develop, which will soon be available in the CIAO software package, explicitly model photon arrivals as a Poisson process and, thus, have no difficulty with high resolution low count X-ray and gamma-ray data. We expect these methods to be useful not only for the recently launched Chandra X-ray observatory and XMM but also new generation telescopes such as Constellation X, GLAST, etc. In the context of two examples (Quasar S5 0014+813 and Hybrid-Chromosphere Supergiant Star alpha TrA) we illustrate a new highly structured model and how Bayesian posterior sampling can be used to compute estimates, error bars, and credible intervals for the various model parameters.

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arXiv2000

Variability Model for the Microquasar GRS 1915+105

We analyse the radiation pressure instability of the standard, optically thisc disc and propose it as an explanation of the obseved variability of the superluminal microquasar GRS 1915+105. The quantitave agreement with the observed outburst amplitudes is obtained under the assumption that substantial part of the energy is carried away by the jet.

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arXiv1998

Q1208+1011: Search for the lensing galaxy

We present a high-resolution spectrum of the high redshift, lensed quasar Q1208+1101, obtained with the echellette spectrograph on the Multiple Mirror Telescope. We examine the new and published spectra and provide an updated list of high-confidence metal-line absorption systems at z=1.1349, 2.8626, 2.9118, 2.9136, 2.9149. Combining this with a simple model of the gravitational lens system allows us to constrain the possible lens redshifts. The high-redshift (z > 2.5) and low-redshift (z < 0.4) candidates can be ruled out with high confidence. The current spectra effectively probe about 40% of the redshift range in which the lens is expected. In that range, there is only one known metal-line absorption system, an MgII absorber at z=1.1349. We consider the possibility that this system is the lensing galaxy and discuss the implied parameters of the galaxy.

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arXiv1997

Deriving the Quasar Luminosity Function from Accretion Disk Instabilities

We have derived the quasar luminosity function assuming that the quasar activity is driven by a thermal-viscous unstable accretion disk around a supermassive black hole. The instabilities produce large amplitude, long-term variability of a single source. We take a light curve of a single source and calculate the luminosity function, from the function of time it spends at each luminosity. Convolving this with an assumed mass distribution we fit well the observed optical luminosity function of quasars at four redshifts. As a result we obtain the evolution of the mass distribution between redshifts 2.5 and 0.5. The maximum of the active black hole mass function moves towards lower mass by a factor ~10 at the low redshift. The number of high mass sources declines rapidly, and so low mass sources become dominant at lower redshift. The main conclusions are following: 1) The quasar long-term variability due to the disk thermal-viscous instabilities provides a natural explanation for the observed quasar luminosity function. 2) The peak of the mass function evolves towards lower black hole masses at lower redshifts by a factor ~10. 3) High mass sources die subsequently when redshift gets smaller. 4) The number of high mass sources declines rapidly, and so low mass sources become dominant at lower redshift. 5) The periodic outbursts of activity appear as long as the matter is supplied to the accretion disk. 6) Since the time-averaged accretion rate is low, the remnant sources (or sources in the low activity phase) do not grow to very massive black holes. 7) A continuous fuel supply at a relatively low accretion rate (~0.01 - 0.1 \dot M_{Edd}$) for each single source is required over the lifetime of the entire quasar population.

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