Tahir Yaqoob

ASCA observations of Seyfert 1 galaxies. 2. Relativistic iron K-alpha emission

We present evidence for widespread relativistic effects in the central regions of active galactic nuclei. In a sample of 18 Seyfert 1 galaxies observed by ASCA, 14 show an iron K? line that is resolved, with mean width ?K? = 0.43 ? 0.12 keV for a Gaussian profile (full width at half-maximum, FWHM ~ 50,000 km s-1). However, many of the line profiles are asymmetric. A strong red wing is indicative of gravitational redshifts close to a central black hole, and accretion disk models provide an excellent description of the data. The peak energy of the line is 6.4 keV, which indicates that it arises by fluorescence in near-neutral material. Our fits imply a low inclination for the disk in these Seyfert 1 galaxies, with a mean of 30?, consistent with orientation-dependent unification schemes. Differences in the line profiles from source to source imply slight variations in geometry, which cannot be accounted for solely by inclination. In most cases, we require that the line emission arises from a range of radii. Although a small contribution to the emission from a region other than the disk is not ruled out, it is not generally required and has little effect on our conclusions regarding the disk line. Our data are fit equally well with rotating (Kerr) and nonrotating (Schwarzschild) black hole models. We find a mean spectral index in the 3-10 keV range of ?3-10 = 1.91 ? 0.07 after accounting for the effects of reflection. Such observations probe the innermost regions of active galactic nuclei and arguably provide the best evidence yet obtained for the existence of supermassive black holes in the centers of active galaxies.

ASCA Observations of Seyfert 1 Galaxies. I. Data Analysis, Imaging, and Timing

We present the first in a series of papers describing the X-ray properties of a sample of 18 Seyfert 1 galaxies, using data obtained by ASCA. The imaging data reveal a number of serendipitous hard X-ray sources in some source fields, but none contribute significantly to the hard X-ray flux of the active galactic nuclei. All but one of the Seyferts show evidence for variability on timescales of minutes to hours, with the amplitude anticorrelated with the source luminosity, confirming previous results. In at least eight sources there is evidence that the variability amplitude below 2 keV is greater than that in the hard X-ray band, perhaps indicating variable components other than the power law in the soft band. Ultrarapid variability, implying significant power at frequencies greater than 10-3 Hz is detected in at least five sources but is difficult to detect in most cases, because of the sampling and signal-to-noise ratio. In Mrk 766 and MCG -6-30-15 there is also an indication that the high-frequency power spectra are variable in shape and/or intensity. There is similar evidence in NGC 4151 but on longer timescales.

Evidence for ultra-fast outflows in radio-quiet AGNs★ I. Detection and statistical incidence of Fe K-shell absorption lines

Context. Blue-shifted Fe K absorption lines have been detected in recent years between 7 and 10 keV in the X-ray spectra of several radio-quiet AGNs. The derived blue-shifted velocities of the lines can often reach mildly relativistic values, up to 0.2–0.4c. These findings are important because they suggest the presence of a previously unknown massive and highly ionized absorbing material outflowing from their nuclei, possibly connected with accretion disk winds/outflows. Aims. The scope of the present work is to statistically quantify the parameters and incidence of the blue-shifted Fe K absorption lines through a uniform analysis on a large sample of radio-quiet AGNs. This allows us to assess their global detection significance and to overcome any possible publication bias. Methods. We performed a blind search for narrow absorption features at energies greater than 6.4 keV in a sample of 42 radio-quiet AGNs observed with XMM-Newton. A simple uniform model composed by an absorbed power-law plus Gaussian emission and absorption lines provided a good fit for all the data sets. We derived the absorption lines parameters and calculated their detailed detection significance making use of the classical F-test and extensive Monte Carlo simulations. Results. We detect 36 narrow absorption lines on a total of 101 XMM-Newton EPIC pn observations. The number of absorption lines at rest-frame energies higher than 7 keV is 22. Their global probability to be generated by random fluctuations is very low, less than 3 × 10 −8 , and their detection have been independently confirmed by a spectral analysis of the MOS data, with associated random probability 7 keV and to overcome their publication bias. These lines indicate that UFOs are a rather common phenomenon observable in the central regions of these sources and they are probably the direct signature of AGN accretion disk winds/ejecta. The detailed photo-ionization modeling of these absorbers is presented in a companion paper.

An X-ray spectral model for Compton-thick toroidal reprocessors

The central engines of both type 1 and type 2 active galactic nuclei are thought to harbour a toroidal structure that absorbs and reprocesses high-energy photons from the central X-ray source. Unique features in the reprocessed spectra can provide powerful physical constraints on the geometry, column density, element abundances and orientation of the circumnuclear matter. If the reprocessor is Compton-thick, the calculation of emission-line and continuum spectra that are suitable for direct fitting to X-ray data is challenging because the reprocessed emission depends on the spectral shape of the incident continuum, which may not be directly observable. We present new Monte Carlo calculations of Greens functions for a toroidal reprocessor that provide significant improvements over currently available models. The Greens function approach enables the construction of X-ray spectral fitting models that allow arbitrary incident spectra as part of the fitting process. The calculations are fully relativistic and have been performed for column densities that cover the Compton-thin to Compton-thick regime, for incident photon energies up to 500 keV. The Greens function library can easily be extended cumulatively to provide models that are valid for higher input energies and a wider range of element abundances and opening angles of the torus. The reprocessed continuum and fluorescent line emission due to Fe Kα, Fe Kβ and Ni Kα are treated self-consistently, eliminating the need for ad hoc modelling that is currently common practice. We find that the spectral shape of the Compton-thick reflection spectrum in both the soft and hard X-ray bands in our toroidal geometry is different compared with that obtained from disc models. A key result of our study is that a Compton-thick toroidal structure that subtends the same solid angle at the X-ray source as a disc can produce a reflection spectrum that is ∼6 times weaker than that from a disc. This highlights the widespread and erroneous interpretation of the so-called ‘reflection-fraction’ as a solid angle, obtained from fitting disc-reflection models to Compton-thick sources without regard for proper consideration of geometry.

X-Ray Observations of Optically Selected, Radio-quiet Quasars. I. The ASCA Results

We present the result of 27 ASCA observations of 26 radio-quiet quasars (RQQs) from the Palomar-Green (PG) survey. The sample is not statistically complete, but it is reasonably representative of RQQs in the PG survey. For many of the sources, the ASCA data are presented here for the first time. All the RQQs were detected except for two objects, both of which contain broad absorption lines in the optical band. We find the variability characteristics of the sources to be consistent with Seyfert 1 galaxies. A power law offers an acceptable description of the time-averaged spectra in the 2-10 keV (quasar frame) band for all but one data set. The best-fitting values of the photon index vary from object to object over the range 1.5 ?2-10 3, with a mean ?2-10 2 and dispersion ?(?2-10) 0.25. The distribution of ?2-10 is therefore similar to that observed in other RQ active galactic nuclei (AGNs) and seems to be unrelated to X-ray luminosity. No single model adequately describes the full 0.6-10 keV (observed frame) continuum of all the RQQs. Approximately 50% of the sources can be adequately described by a single power law or by a power law with only very subtle deviations. All but one of the remaining data sets were found to have convex spectra (flattening as one moves to higher energies). The exception is PG 1411+442, in which a substantial column density (NH,z ~ 2 ? 1023 cm-2) obscures ~98% of the continuum. We find only five (maybe six) of 14 objects with z 0.25 to have soft excesses at energies 1 keV, but we find no universal shape for these spectral components. The spectrum of PG 1244+026 contains a rather narrow emission feature centered at an energy ~1 keV (quasar frame). The detection rate of absorption due to ionized material in these RQQs is lower than that seen in Seyfert 1 galaxies. In part, this may be due to selection effects. However, when detected, the absorbers in the RQQs exhibit a similar range of column density and ionization parameter as Seyfert 1 galaxies. We find evidence of Fe K-shell emission in at least eight RQQs. These are all low-luminosity objects, and the line parameters are consistent with those of other low-luminosity RQ AGNs. However the construction of the mean data/model ratios for various luminosity ranges reveals a trend whereby the profile and strength of the Fe K-shell emission changes as a function of luminosity.

A soft X-ray study of type I active galactic nuclei observed with Chandra high-energy transmission grating spectrometer

We present the results of a uniform analysis of the soft X-ray spectra of fifteen type I AGN observed with the high resolution X-ray gratings on board Chandra. We found that ten of the fifteen AGN exhibit signatures of an intrinsic ionized absorber. The absorbers are photoionized and outflowing, with velocities in the range ∼ 10−10 km s. The column density of the warm absorbing gas is ∼ 10cm. Nine of the ten AGN exhibiting warm absorption are best–fit by multiple ionization components and three of the ten AGN require multiple kinematic components. The warm absorbing gas in our AGN sample has a wide range of ionization parameter, spanning roughly four orders of magnitude (ξ ∼ 10 ergs cm s) in total, and often spanning three orders of magnitude in the same gas. Warm absorber components with ionization parameter ξ < 10 generate an unresolved transition array due to Fe in seven of the ten AGN exhibiting warm absorption. These low ionization state absorbers may also carry away the largest mass outflows from the AGN. The mass outflow rate depends critically on the volume filling factor of the gas, which cannot yet be directly measured. However, upper limits on the mass outflow rates for filling factors of unity can be much greater than the expected accretion rate onto the central supermassive black hole and filling factors as small as 1% can give outflow rates comparable to the accretion rate. There appears to be a gap in the outflow velocities in our sample between ∼ 300 − 500 km s, the origin of which is not clear. The outflow components with velocities below this gap tend to be associated with lower column densities than those with with velocities above the gap.

AN EXTENDED SCHEME FOR FITTING X-RAY DATA WITH ACCRETION DISK SPECTRA IN THE STRONG GRAVITY REGIME

Accreting black holes are believed to emit X-rays, which then mediate information about strong gravity in the vicinity of the emission region. We report on a set of new routines for the XSPEC package for analyzing X-ray spectra of black-hole accretion disks. The new computational tool significantly extends the capabilities of the currently available fitting procedures that include the effects of strong gravity and allows one to systematically explore the constraints on more model parameters than previously possible (e.g., black-hole angular momentum). Moreover, axial symmetry of the disk intrinsic emissivity is not assumed, although it can be imposed to speed up the computations. The new routines can be used also as a stand-alone and flexible code with the capability of handling time-resolved spectra in the regime of strong gravity. We have used the new code to analyze the mean X-ray spectrum from the long XMM-Newton 2001 campaign of the Seyfert 1 galaxy MCG -6-30-15. Consistent with previous findings, we obtained a good fit to the broad Fe K line profile for a radial line intrinsic emissivity law in the disk that is not a simple power law, and for near maximal value of black hole angular momentum. However, equally good fits can be obtained also for small values of the black hole angular momentum. The code has been developed with the aim of allowing precise modeling of relativistic effects. Although we find that current data cannot constrain the parameters of black-hole/accretion disk system well, the code allows, for a given source or situation, detailed investigations of what features of the data future studies should be focused on in order to achieve the goal of uniquely isolating the parameters of such systems.

The Cores of the Fe K Lines in Seyfert 1 Galaxies Observed by the Chandra High Energy Grating

We report on the results of 18 observations of the core, or peak, of the Fe Kα emission line at ~6.4 keV in 15 Seyfert 1 galaxies using the Chandra High Energy Grating (HEG). These data afford the highest precision measurements of the peak energy of the Fe Kα line and the highest spectral resolution measurements of the width of the core of the line to date. We were able to measure the peak energy in 17 data sets, and, excluding a very deep observation of NGC 3783, we obtained a weighted mean of 6.404 ± 0.005 keV. In all 15 sources the two-parameter, 99% confidence errors on the line peak energy do not exclude fluorescent Kα line emission from Fe I, although two sources (Mrk 509 and 3C 120) stand out as very likely being dominated by Kα emission from Fe XVII or so. We were able to measure the line core width in 14 data sets and obtained a weighted mean of 2380 ± 760 km s-1 FWHM (excluding the NGC 3783 deep exposure), a little larger than the instrument resolution (~1860 km s-1 FWHM). However, there is evidence of underlying broad-line emission in at least four sources. In fact, the width of the peak varies widely from source to source and may in general have a contribution from the outer parts of an accretion disk and more distant matter. For the disk contribution to also peak at 6.4 keV requires greater line emissivity at hundreds of gravitational radii than has been deduced from previous studies of the Fe Kα line.

The XMM-Newton Iron Line Profile of NGC 3783

We report on observations of the iron K line in the nearby Seyfert 1 galaxy, NGC 3783, obtained in a long, two- orbit (� 240 ks) XMM-Newton observation. The line profile obtained exhibits two strong narrow peaks at 6.4 and 7.0 keV, with measured line equivalent widths of 120 and 35 eV, respectively. The 6.4 keVemission is the Kline from near neutral Fe, while the 7.0 keV feature probably originates from a blend of the neutral Fe Kline and the hydrogen-like line of Fe at 6.97 keV. The relatively narrow velocity width of the Kline (P5000 km s � 1 ), its lack of response to the continuum emission on short timescales, and the detection of a neutral Compton reflection component are all consistent with a distant origin in Compton-thick matter such as the putative molecular torus. A strong absorption line from highly ionized iron (at 6.67 keV) is detected in the time-averaged iron line profile, while the depth of the feature appears to vary with time, being strongest when the continuum flux is higher. The iron absorption line probably arises from the highest ionization component of the known warm absorber in NGC 3783, with an ionization of log � � 3 and column density of NH � 5 � 10 22 cm � 2 and may originate from within 0.1 pc of the nucleus. Aweak red wing to the iron K line profile is also detected below 6.4 keV. However, when the effect of the highly ionized warm absorber on the underlying continuum is taken into account, the requirement for a relativistic iron line component from the inner disk is reduced. Subject headings: galaxies: active — galaxies: individual (NGC 3783) — galaxies: Seyfert — X-rays: galaxies

On the Dependence of the Iron K-Line Profiles with Luminosity in Active Galactic Nuclei

We present evidence for changes in the strength and profile of the iron Kα line in active galactic nuclei (AGNs), based on X-ray observations with ASCA. There is a clear decrease in the strength of the line with increasing luminosity. This relation is not due solely to radio power, as it persists when only radio-quiet AGNs are considered and therefore cannot be fully explained by relativistic beaming. In addition to the change in strength, the line profile also appears to be different in higher luminosity sources. We discuss these results in terms of a model where the accretion disk becomes ionized as a function of the accretion rate.