Showing papers by "Fabrizio Nicastro published in 2006"

CIAO: Chandra's data analysis system

Abstract: The CIAO (Chandra Interactive Analysis of Observations) software package was first released in 1999 following the launch of the Chandra X-ray Observatory and is used by astronomers across the world to analyze Chandra data as well as data from other telescopes. From the earliest design discussions, CIAO was planned as a general-purpose scientific data analysis system optimized for X-ray astronomy, and consists mainly of command line tools (allowing easy pipelining and scripting) with a parameter-based interface layered on a flexible data manipulation I/O library. The same code is used for the standard Chandra archive pipeline, allowing users to recalibrate their data in a consistent way. We will discuss the lessons learned from the first six years of the software's evolution. Our initial approach to documentation evolved to concentrate on recipe-based "threads" which have proved very successful. A multi-dimensional abstract approach to data analysis has allowed new capabilities to be added while retaining existing interfaces. A key requirement for our community was interoperability with other data analysis systems, leading us to adopt standard file formats and an architecture which was as robust as possible to the input of foreign data files, as well as re-using a number of external libraries. We support users who are comfortable with coding themselves via a flexible user scripting paradigm, while the availability of tightly constrained pipeline programs are of benefit to less computationally-advanced users. As with other analysis systems, we have found that infrastructure maintenance and re-engineering is a necessary and significant ongoing effort and needs to be planned in to any long-lived astronomy software.

The unprecedented optical outburst of the quasar 3C 454.3 - The WEBT campaign of 2004–2005

Abstract: Context. The radio quasar 3C 454.3 underwent an exceptional optical outburst lasting more than 1 year and culminating in spring 2005. The maximum brightness detected was R = 12.0, which represents the most luminous quasar state thus far observed (M B ∼ 31.4). Aims. In order to follow the emission behaviour of the source in detail, a large multiwavelength campaign was organized by the Whole Earth Blazar Telescope (WEBT). Methods. Continuous optical, near-IR and radio monitoring was performed in several bands. ToO pointings by the Chandra and INTEGRAL satellites provided additional information at high energies in May 2005. Results. The historical radio and optical light curves show different behaviours. Until about 2001.0 only moderate variability was present in the optical regime, while prominent and long-lasting radio outbursts were visible at the various radio frequencies, with higher-frequency variations preceding the lower-frequency ones. After that date, the optical activity increased and the radio flux is less variable. This suggests that the optical and radio emissions come from two separate and misaligned jet regions, with the inner optical one acquiring a smaller viewing angle during the 2004-2005 outburst. Moreover, the colour-index behaviour (generally redder-when-brighter) during the outburst suggests the presence of a luminous accretion disc. A huge mm outburst followed the optical one, peaking in June-July 2005. The high-frequency (37-43 GHz) radio flux started to increase in early 2005 and reached a maximum at the end of our observing period (end of September 2005). VLBA observations at 43 GHz during the summer confirm the brightening of the radio core and show an increasing polarization. An exceptionally bright X-ray state was detected in May 2005, corresponding to the rising mm flux and suggesting an inverse-Compton nature of the hard X-ray spectrum. Conclusions. A further multifrequency monitoring effort is needed to follow the next phases of this unprecedented event.

Rapid N_H Changes in NGC 4151

Abstract: We have analyzed the two longest (elapsed time > 3 days) BeppoSAX observations of the X-ray brightest Seyfert galaxy, NGC 4151, to search for spectral variability on time-scales from a few tens of ksec to years. We found in both cases highly significant spectral variability below ~ 6 keV down to the shortest time-scales investigated. These variations can be naturally explained in terms of variations in the low energy cut-off due to obscuring matter along the line of sight. If the cut-off is modeled by two neutral absorption components, one fully covering the source and the second covering only a fraction of the source, the shortest time-scale of variability of a few days constrains the location of the obscuring matter to within 3.4 X 10^4 Schwarzschild radii from the central X-ray source. This is consistent with the distance of the Broad Emission Line Region, as inferred from reverberation mapping, and difficult to reconcile with the parsec scale dusty molecular torus of Krolik & Begelman (1988). We have also explored a more complex absorption structure, namely the presence of an ionized absorber. Although the behaviour of the ionization parameter is nicely consistent with the expectations, the results are not completely satisfactory from the statistical point of view. The overall absorption during the 2001 December observation is lower than in all other historical observations with similar 2-10 keV flux. This suggests that absorption variability plays a crucial role in the observed flux variability of this source.

Chandra detection of local warm-hot gas toward markarian 279

Abstract: We report the Chandra detection of O VII Kα absorption at z = 0 in the direction of the z = 0.03 Seyfert 1 galaxy Mrk 279. The high-velocity cloud Complex C lies along this line of sight, with H I 21 cm emission and O VI λ1032 absorption both observed at velocities of roughly-150 km s-1 relative to the local standard of rest. We present an improved method for placing limits on the Doppler parameter and column density of a medium when only one unresolved line can be measured; this method is applied to the O VII absorption seen here, indicating that the O VII Doppler parameter is inconsistent with that of any low-velocity (Galactic thick disk) or high-velocity O VI [(O VI)HV] component. Direct association of the O VII with the (O VI)HV is further ruled out by the high temperatures required to produce the observed (O VI)HV/O VII ratio and the significant velocity difference between the O VII and (O VI)HV lines. If the O VII absorption is associated with a very broad, undetected O VI component, then the absorption must be broadened by primarily nonthermal processes. The large velocity dispersion and possible slight redshift of the O VII absorption (as well as limits on the absorber's temperature and density) may be indicative of a local intergalactic medium origin, although absorption from a hot, low-density Galactic corona cannot be ruled out.

The weak absorbing outflow in AGN Mrk 279: evidence of super-solar metal abundances

Abstract: We present analysis and photoionization modeling of the Chandra high resolution spectrum of Mrk 279. There is clear evidence of an absorbing outflow which is best fit by a two component model, one with a low ionization parameter and one with a higher ionization parameter. The column density of the X-ray warm absorber, about log N_H=20, is the smallest known of all AGNs in which X-ray absorbing outflows are observed. We find that the X-ray and UV/FUV absorbers are part of the same overall outflow. There is some evidence of super-solar carbon, nitrogen, oxygen and iron in the Chandra spectrum of Mrk 279. While this is not a robust result in itself, Chandra data in combination with the UV data and the pressure equilibrium between two phases of the outflow, support the scenario of super-solar abundances. This the first case where super-solar abundances are reported in the nucleus of a normal Seyfert galaxy. The data suggest that the outflow originates from a compact region around the nuclear black hole and that it carries insignificant amount of mass and energy.

XMM-Newton View of the z > 0 Warm-Hot Intergalactic Medium toward Markarian 421

Abstract: The recent detection with Chandra of two warm-hot intergalactic medium (WHIM) filaments toward Mrk 421 by Nicastro et al. provides a measurement of the bulk of the "missing baryons" in the nearby universe. Since Mrk 421 is a bright X-ray source, it is also frequently observed by the XMM-Newton Reflection Grating Spectrometer (RGS) for calibration purposes. Using all available archived XMM observations of this source with small pointing offsets (<15''), we construct the highest quality XMM grating spectrum of Mrk 421 to date with a net exposure time (excluding periods of high background flux) of 437 ks and ~15,000 counts per resolution element at 21.6 A, more than twice that of the Chandra spectrum. Despite the long-exposure time, neither of the two intervening absorption systems is seen, although the upper limits derived are consistent with the Chandra equivalent width measurements. This appears to result from (1) the larger number of narrow instrumental features caused by bad detector columns, (2) the degraded resolution of XMM/RGS as compared to the Chandra/LETG, and (3) fixed-pattern noise at λ 29 A. The nondetection of the WHIM absorbers by XMM is thus fully consistent with the Chandra measurement.

The properties of the circumnuclear regions in the Circinus galaxy

Abstract: We analysed the spectra from the XMM-Newton and the three longest Chandra observations of the Circinus Galaxy, the X-ray brightest, Compton-thick Seyfert 2 galaxy, to derive the physical and morphological parameters of the circumnuclear matter. We compare the line and continuum parameters with extensive calculations performed with the well-known photoionisation code CLOUDY. The line spectrum cannot be explained by a single zone, as three different reflecting regions are required. The first has low ionisation, accounting for the prominent iron Kα line, and is likely to be identified with the "torus" envisaged in unification models for Seyfert galaxies; a lower limit of 0.05 pc to the distance of this matter from the nucleus has been estimated, using new constraints on the ionisation state of iron (≤FeX). The second zone is moderately ionised and optically thin (T 0.2), and it accounts for the oxygen to silicon lines. Finally, a third, highly ionised and optically thin reflector is required for the detection of the He-like iron line.

The XMM-Newton/INTEGRAL monitoring campaign of IGR J16318-4848

Abstract: IGR J16318-4848 is the prototype and one of the more extreme examples of the new class of highly obscured Galactic X-ray sources discovered by INTEGRAL. A monitoring campaign on this source has been carried out by XMM-Newton and INTEGRAL, consisting in three simultaneous observations performed in February, March and August 2004. The long-term variability of the Compton-thick absorption and emission line complexes will be used to probe the properties of the circumstellar matter. A detailed timing and spectral analysis of the three observations is performed, along with the reanalysis of the XMM-Newton observation performed in February 2003. The results are compared with predictions from numerical radiative transfer simulations to derive the parameters of the circumstellar matter. Despite the large flux dynamic range observed (almost a factor 3 between observations performed a few months apart), the source remained bright (suggesting it is a persistent source) and Compton-thick (NH >1.2x10^24 cm-2). Large Equivalent Width (EW) emission lines from Fe Kalpha, Fe Kbeta and Ni Kalpha were present in all spectra. The addition of a Fe Kalpha Compton Shoulder improves the fits, especially in the 2004 observations. Sporadic occurrences of rapid X-ray flux risings were observed in three of the four observations. The Fe Kalpha light curve followed the continuum almost instantaneously, suggesting that the emission lines are produced by illumination of small-scale optically-thick matter around the high-energy continuum source. Using the iron line EW and Compton Shoulder as diagnostic of the geometry of the matter, we suggest that the obscuring matter is in a flattened configuration seen almost edge-on.

ESTREMO/WFXRT: Extreme phySics in the TRansient and Evolving COsmos

Abstract: We present a mission designed to address two main themes of the ESA Cosmic Vision Programme: the Evolution of the Universe and its Violent phenomena. ESTREMO/WFXRT is based on innovative instrumental and observational approaches, out of the mainstream of observatories of progressively increasing area, i.e.: Observing with fast reaction transient sources, like GRB, at their brightest levels, thus allowing high resolution spectroscopy. Observing and surveying through a X-ray telescope with a wide field of view and with high sensitivity extended sources, like cluster and Warm Hot Intragalactic Medium (WHIM). ESTREMO/WFXRT will rely on two cosmological probes: GRB and large scale X-ray structures. This will allow measurements of the dark energy, of the missing baryon mass in the local universe, thought to be mostly residing in outskirts of clusters and in hot filaments (WHIM) accreting onto dark matter structures, the detection of first objects in the dark Universe, the history of metal formation. The key asset of ESTREMO/WFXRT with regard to the study of Violent Universe is the capability to observe the most extreme objects of the Universe during their bursting phases. The large flux achieved in this phase allows unprecedented measurements with high resolution spectroscopy. The mission is based on a wide field X-ray/hard X-ray monitor, covering >1/4 of the sky, to localize transients; fast (min) autonomous follow-up with X-ray telescope (2000 cm2) equipped with high resolution spectroscopy transition edge (TES) microcalorimeters (2eV resolution below 2 keV) and with a wide field (1°) for imaging with 10" resolution (CCD) extended faint structures and for cluster surveys. A low background is achieved by a 600 km equatorial orbit. The performances of the mission on GRB and their use as cosmological beacons are presented and discussed.

Chandra and Far Ultraviolet Spectroscopic Explorer Observations of z~0 Warm-Hot Gas Toward PKS 2155-304

Abstract: The X-ray bright z=0.116 quasar PKS 2155-304 is frequently observed as a Chandra calibration source, with a total of 483 ksec of Low Energy Transmission Grating (LETG) exposure time accumulated through May 2006. Highly-ionized metal absorption lines, including numerous lines at z=0 and a putative OVIII K-alpha line at z=0.055, have been reported in past Chandra studies of this source. Using all available Chandra LETG spectra and analysis techniques developed for such z=0 X-ray absorption along other sightlines, we revisit these previous detections. We detect 4 absorption lines at >3\sigma significance (OVII K-alpha/beta, OVIII K-alpha, and NeIX K-alpha), with OVII K-alpha being a 7.3\sigma detection. The 1\sigma ranges of z=0 OVII column density and Doppler parameter are consistent with those derived for Mrk 421 and within 2\sigma of the Mrk 279 absorption. Temperatures and densities inferred from the relative OVII and other ionic column densities are found to be consistent with either the local warm-hot intergalactic medium or a Galactic corona. Unlike the local X-ray absorbers seen in other sightlines, a link with the low- or high-velocity far-ultraviolet OVI absorption lines cannot be ruled out. The z=0.055 OVIII absorption reported by Fang et al. is seen with 3.5\sigma confidence in the ACIS/LETG spectrum, but no other absorption lines are found at the same redshift.