Jose M Ramirez | Instituto Venezolano de Investigaciones Cientificas (original) (raw)

Papers by Jose M Ramirez

We have reanalyzed the 900 ks Chandra X-ray spectrum of NGC 3783, finding evidence for asymmetry ... more We have reanalyzed the 900 ks Chandra X-ray spectrum of NGC 3783, finding evidence for asymmetry of the spectral absorption lines. The lines are fit with a parametric expression that results from an analytical treatment of radiatively driven winds (Lamers et al. 1987). The line asymmetry distribution derived from the spectrum is consistent with a non-spherical outflow with a finite

We report on the performance of the statistical, X-ray absorption lines identification procedure ... more We report on the performance of the statistical, X-ray absorption lines identification procedure xline-id. As illustration, it is used to estimate the time averaged gas density n H (r) of a representative AGN's warm absorber (T ≈ 10 5 K) X-ray simulated spectrum. The method relies on three key ingredients: (1) a well established emission continuum level; (2) a robust grid of photoionization models spanning several orders of magnitude in gas density (n H), plasma column density (N H), and in ionization states; (3) theoretical curves of growth for a large set of atomic lines. By comparing theoretical and observed equivalent widths of a large set of lines, spanning highly ionized charge states from O, Ne, Mg, Si, S, Ar, and the Fe L-shell and K-shell, we are able to infer the location of the X-ray warm absorber.

Eight X-ray observations of V4743 Sgr (2002), observed with Chan-dra and XMM-Newton, are presente... more Eight X-ray observations of V4743 Sgr (2002), observed with Chan-dra and XMM-Newton, are presented, covering three phases: Early optically thin hard emission (day 50.2), photospheric emission from the ejecta (days 180.4, 196.1, 301.9, 371, 526), and faint post-outburst emission (days 742 and 1286). The flux level at Earth during the first and last phase is of order 10 −12 erg cm −2 s −1 over the energy range 0.3-2.5 keV. These values are higher than an upper limit obtained in September 1990 with ROSAT. The nova thus continued fading in the soft band (0.1-2.4 keV). The nova turned off some time between days 301.9 and 371, and the X-ray flux subsequently decreased from day 301.9 to 526, following an exponential decline time scale of (96 ± 3) days. We use the absorption lines present in the SSS spectrum for diagnostic purposes, and characterize the physics and the dynamics of the expanding atmosphere during the explosion of the nova. The information extracted from this first stage is then used as input for computing full photoion-ization models of the ejecta in V4743 Sgr. The SSS spectrum is modeled with a simple black-body and multiplicative Gaussian lines, which provides us of a general kinematical picture of the system, before it decays to its faint phase (Ness et al. 2003). In the grating spectra taken between days 180.4 and 370, we can resolve the line profiles of absorption lines arising from H-like and He-like C, N, and O, including transitions involving higher principal quantum numbers. Except for a few interstellar lines, all lines are significantly blue-shifted, yielding velocities between 1000 and 6000 km s −1 which implies an ongoing mass loss. It is shown that significant expansion and mass loss occur during this phase of the explosion, at a rate ˙ M ≈ (3 − 5) × 10 −4 (L/L 38) M /yr. Our measurements show that the efficiency of the amount of energy used for the motion of the ejecta, defined as the ratio between the kinetic luminosity L kin and the radiated luminosity L rad , is of the order of one.

We present a technique to obtain detailed resonance structures from Rmatrix calculations of atomi... more We present a technique to obtain detailed resonance structures from Rmatrix calculations of atomic cross sections for both collisional and radiative processes. The resolving resonances (RR) method relies on the QB method of Quigley-Berrington (Quigley L, Berrington K A and Pelan J 1998 Comput. Phys. Commun. 114 225) to find the position and width of resonances directly from the reactance matrix. Then one determines the symmetry parameters of these features and generates an energy mesh whereby fully resolved cross sections are calculated with minimum computational cost. The RR method is illustrated with the calculation of the photoionization cross sections and the unified recombination rate coefficients of Fe XXIV, O VI, and Fe XVII. The RR method reduces numerical errors arising from unresolved Rmatrix cross sections in the computation of synthetic bound-free opacities, thermally averaged collision strengths and recombination rate coefficients.

We present an analysis of a 100 ks X-ray spectrum of the Narrow-Line Seyfert 1 Galaxy Ark 564, wh... more We present an analysis of a 100 ks X-ray spectrum of the Narrow-Line Seyfert 1 Galaxy Ark 564, which we have taken with the Low Energy Transmission Grating Spectrometer (LETGS) on board the X-ray telescope Chandra. We fit several spectral continuum models to the time-averaged X-ray spectrum of this galaxy, and study the possible origin of the steep soft X-ray spectrum. Also, we characterize the average intrinsic absorption of the system, based on photoionization modeling. Using χ 2 −statistics, we compare several of these spectral models, and arrive at possible solutions for the description of (1) the intrinsic emission continuum, and (2) the intrinsic absorption of the system.

We present an analysis of the 100 ks X-ray spectrum of the narrow-line Seyfert 1 Galaxy Ark 564, ... more We present an analysis of the 100 ks X-ray spectrum of the narrow-line Seyfert 1 Galaxy Ark 564, taken with the Low Energy Transmission Grating Spectrometer (LETGS) on board Chandra. Using χ 2 statistics, several continuum models of the time-averaged spectrum of this object are compared, obtaining a semi-empirical solution for the description of the intrinsic emission continuum and a physical solution for the intrinsic absorption of the system. We find that the 0.1−10 keV spectrum can be well described by a power law plus two thermal components that account for the soft step. We are also able to detect and measure several narrow, unresolved absorption lines arising from highly ionized species of C, N, O, and Fe. The material seems to have a velocity consistent with the systemic velocity of the galaxy. This piece of evidence, in addition to the very low observed column density of N H ∼ 10 20 cm −2 , is in good agreement with the scenario of a transverse biconical outflow with a gas density of n 3 × 10 12 cm −3 at distances beyond the broad-line region r 10 lt-days, but a dust torus origin cannot be ruled out.

Aims. We describe here a new full 2D parameterization of the PSFs of the three XMM-Newton EPIC te... more Aims. We describe here a new full 2D parameterization of the PSFs of the three XMM-Newton EPIC telescopes as a function of instrument, energy, off-axis angle and azimuthal angle, covering the whole field-of-view (FoV) of the three EPIC detectors. It models the general PSF envelopes, the primary and secondary spokes, their radial dependencies, and the large-scale azimuthal variations.

We investigate the X-ray absorption structure of oxygen in the interstellar medium by analyzing X... more We investigate the X-ray absorption structure of oxygen in the interstellar medium by analyzing XMM-Newton observations of the low-mass X-ray binary Sco X-1. Simple models based on the O i atomic photoabsorption cross section from different sources are used to fit the data and evaluate the impact of the atomic data on the interpretation of the observations. We show that relatively small differences in the atomic calculations can yield spurious results, and that the most complete and accurate set of atomic cross sections successfully reproduce the observed data in the 21.0-24.5 Å wavelength region of the spectrum. Our fits indicate that the absorption is mainly due to neutral gas with an ionization parameter of ξ = 10 −4 erg cm s −1 and an oxygen column density of N O ≈ (8-10) × 10 17 cm −2 . The models are able to reproduce both the K edge and the Kα absorption line from O i which are the two main features in this region. We find no conclusive evidence for absorption by anything other than atomic oxygen.

Construimos un modelo de fotoionización dependiente de la velocidad para el absorbedor tibio de N... more Construimos un modelo de fotoionización dependiente de la velocidad para el absorbedor tibio de NGC 3783. Adoptando formas funcionales de la velocidad del flujo y su densidad del número de partículas con el radio, apropiadas para un viento acelerado por radiación, calculamos el nivel de ionización, la temperatura, el corrimiento Doppler y las profundidadesópticas de las líneas como función de la distancia. El modelo reproduce la relación observada entre la ionización del gas y el corrimiento del centroide de las líneas de absorción en NGC 3783. Este espectro requiere de la presencia de dos flujos: uno altamente ionizado responsable de las alas azules de las líneas de alto grado de ionización y al mismo tiempo las alas rojas de las líneas de menor grado de ionización; y uno de baja ionización que produce las alas azules de las líneas de oxígeno.

An exploration of the relationship between bolometric luminosity and outflow velocity, for two cl... more An exploration of the relationship between bolometric luminosity and outflow velocity, for two classes of X-ray outflows in a large sample of active galactic nuclei has been performed. We find that line radiation pressure could be one physical mechanism that might accelerate the gas we observe in warm absorber, v ∼ 100 − 1000 km s −1 , and on comparable but less stringent grounds the ultra-fast outflows (UFOs), v ∼ 0.03 − 0.3c. If comparable with the escape velocity of the system; the first is naturally located at distances of the dusty torus, ≈ 1 pc, and the second at sub-parsec scales, ≈ 0.01 pc, in accordance with large set of observational evidence existing in the literature. The presentation of this relationship might give us key clues for our understanding of the different physical mechanisms acting in the center of galaxies, the feedback process and its impact on the evolution of the host galaxy.

We present an analysis of our Chandra Low Energy Transmission Grating Spectrometer (LETGS) observ... more We present an analysis of our Chandra Low Energy Transmission Grating Spectrometer (LETGS) observation of the quasar MR 2251−178. The warm absorber of MR 2251−178 is well described by a hydrogen column density, N H ≈ 2 × 10 21 cm −2 , and an ionization parameter log(ξ) ≈ 0.6. We find in the spectrum weak evidence for narrow absorption lines from Carbon and Nitrogen which indicate that the ionized material is in outflow. We note changes (in time) of the absorption structure in the band (0.6 − 1) keV (around the UTAs plus the O vii and O viii K-edges) at different periods of the observation. We measure a (0.1 − 2) keV flux of 2.58 ×10 −11 ergs cm −2 s −1 . This flux implies that the nuclear source of MR 2251−178 is in a relatively low state. No significant variability is seen in the light curve. We do not find evidence for an extra cold material in the line of sight, and set an upper limit of N H ≈ 1.2 × 10 20 cm −2 . The X-ray spectrum does not appear to show evidence for dusty material, though an upper limit in the neutral carbon and oxygen column densities can only be set to N C I ≈ 2 × 10 19 cm −2 and N O I ≈ 9 × 10 19 cm −2 , respectively.

Context. We have re-analyzed the X-ray spectra of the gravitational lensed high-redshift BAL QSO ... more Context. We have re-analyzed the X-ray spectra of the gravitational lensed high-redshift BAL QSO APM 08279+5255, observed with the XMM-Newton and Chandra observatories. Previous studies ) detected unusual, highly-ionized iron absorption features, but differed in their interpretation of these features, regarding the kinematical and ionization structure. Aims. We seek one physical model that can be successfully applied to both observations. Methods. For the first time we have performed detailed photoionization modeling on the X-ray spectrum of APM 08279+5255. Results. The absorbing gas in APM 08279+5255 can be represented by a two-absorbers model with column densities N H (1) ≈ 7×10 22 cm −2 , N H (2) ≈ 6 × 10 22 cm −2 , and ionization parameters log ξ(1) ≈ 1.5 and log ξ(2) ≈ 3, with one of them (the high-ionization component) outflowing at v ≈ 0.18(±0.01)c, carrying large amount of gas out of the system. We find that the Chandra spectrum of APM 08279+5255 requires the same Fe/O ratio overabundance (previously) indicated by the XMM-Newton observation, showing that both absorber components underwent similar chemical evolution and/or have similar origin.