arXiv subjects

M. Elvis

Publications and source records attributed to M. Elvis.

At least 181 records · Page 10Linked to original sources

arXiv2005

XMM-Newton RGS and Chandra LETGS observations of the WHIM in 1ES 1028+511

We report preliminary results on the detection of the Warm-Hot Intergalactic Medium (WHIM) along the line of sight toward the blazar 1ES 1028+511 (z = 0.361). 1ES 1028+511 was observed for 150 ks with the low energy transmission grating in combination with the high resolution camera onboard Chandra. An additional 300 ks observation was obtained using the reflection grating spectrometers (RGS) onboard XMM-Newton. We report the detection of three absorption lines which can be attributed to the WHIM, and compare the results with theoretical predictions.

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arXiv2005

Highly ionized iron absorption lines from outflowing gas in the X-ray spectrum of NGC 1365

We present the discovery of four absorption lines in the X-ray spectrum of the Seyfert Galaxy NGC 1365, at energies between 6.7 and 8.3 keV. The lines are detected with high statistical confidence (from >20sigma for the strongest to \~4sigma for the weakest) in two XMM-Newton observations 60 ksec long. We also detect the same lines, with lower signal-to-noise (but still >2sigma for each line) in two previous shorter (~10 ksec) XMM observations. The spectral analysis identifies these features as FeXXV and FeXXVI Kalpha and Kbeta lines, outflowing with velocities varying between ~1000 to ~5000 km/s among the observations. These are the highest quality detections of such lines so far. The high equivalent widths (EW(Kalpha)~100 eV) and the Kalpha/Kbeta ratios imply that the lines are due to absorption of the AGN continuum by a highly ionized gas with column density N_H~5x10^23 cm^{-2}

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arXiv2005

The SDSS-XMM Quasar Sample: First Results

We have searched in the XMM-Newton public archive for quasars in the Sloan Digital Sky Survey (SDSS) First Data Release (DR1), and found 55 lying in the field of a XMM-Newton observation with exposure times >20 ksec (as of August, 2004). The 35 quasars which yielded good X-ray spectra span redshifts from 0.5 to 2.5. The large collecting area of XMM-Newton allows us to investigate the dependence of the X-ray spectra of quasars on luminosity, redshift and optical colors. We find: (1) no evolution of X-ray slope Gamma with either redshift or luminosity; (2) no correlation between Gamma or absorbing column density and the optical to X-ray ratio, alpha_{OX}; (4) no relation between alpha_{OX} and optical colors. The two latter results suggest that obscuration is not the dominant cause of the spread in X-ray slope or optical color. We find four unusual quasars, 10% of the sample: three are absorbed (N_H>10^{22} cm^{-2}), of which one has high luminosity (1.5x10$^{44} erg/s); the fourth has Gamma=0.6+-0.2, far flatter than the typical value of 1.8-2.0, and a strong emission line (EW=1.2+-$0.4 keV) which, if Fe-K, implies a redshift of ~1.4.

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arXiv2005

Extreme X-ray spectral variability in the Seyfert 2 Galaxy NGC 1365

We present multiple Chandra and XMM-Newton observations of the type 1.8 Seyfert Galaxy NGC 1365, which shows the most dramatic X-ray spectral changes observed so far in an AGN: the source switched from reflection dominated to transmission dominated and back in just 6 weeks. During this time the soft thermal component, arising from a ~1 kpc region around the center, remained constant. The reflection component is constant at all timescales, and its high flux relative to the primary component implies the presence of thick gas covering a large fraction of the solid angle. The presence of this gas, and the fast variability time scale, suggest that the Compton-thick to Compton thin change is due to variation in the line-of-sight absorber, rather than to extreme intrinsic emission variability. We discuss a structure of the circumnuclear absorber/reflector which can explain the observed X-ray spectral and temporal properties.

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arXiv2004

A Mass Measurement for the Missing Baryons in the Warm-Hot Intergalactic Medium via the X-ray Forest

Recent Cosmological measurements indicate that baryons comprise about four percent of the total mass-energy density of the Universe, which is in accord with the predictions arising from studies of the production of the lightest elements. It also is in agreement with the actual number of baryons detected at early times (redshifts>2). However, close to our own epoch (z<2), the number of baryons actually detected add up to just over half (~55 percent) of the number seen at z>2, meaning that about 45 percent are 'missing'. Here we report a determination of the mass-density of a previously undetected population of baryons, in the warm-hot phase of the intergalactic medium. We show that this mass-density is consistent, within the uncertainties, with the mass-density of the missing baryons.

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arXiv2004

Opacity Variations in the Ionized Absorption in NGC 3783: A Compact Absorber

We show that the Fe (VII-XII) M-shell unresolved transition array (UTA) in the NGC 3783 900 ks Chandra HETGS observation clearly changes in opacity in a timescale of 31 days responding to a factor of 2 change in the ionizing continuum. The opacity variation is observed at a level >10 sigma. There is also evidence for variability in the O VI K edge (at 3 sigma level). The observed changes are consistent with the gas producing these absorption features (the low ionization component) being close to photoionization equilibrium. The gas responsible for the Fe (XVII-XXII) L-shell absorption (the high ionization component), does not seem to be responding as expected in photoionization equilibrium. The observed change in opacity for the UTA implies a density >1E4 cm-3, and so locates the gas within 6 pc of the X-ray source. The scenario in which the gas is composed of a continuous radial range of ionization structures is ruled out, as in such scenario, no opacity variations are expected. Rather, the structure of the absorber is likely composed by heavily clumped gas.

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arXiv2004

Chandra discovery of activity in the quiescent nuclear black hole of NGC821

We report the results of the Chandra ACIS-S observations of the elliptical galaxy NGC 821, which harbors a supermassive nuclear black hole (of 3.5x10^7 solar masses), but does not show sign of AGN activity. A small, 8.5" long (~1 kpc at the galaxy's distance of 23 Mpc), S-shaped, jet-like feature centered on the nucleus is detected in the 38 ksec ACIS-S integrated exposure of this region. The luminosity of this feature is L_X ~ 2.6 x 10^39 erg/s (0.3-10 keV), and its spectrum is hard (described by a power-law of Gamma = 1.8^{+0.7}_{-0.6}; or by thermal emission with kT > 2 keV). We discuss two possibilities for the origin of this feature: (1) a low-luminosity X-ray jet, or (2) a hot shocked gas. In either case, it is a clear indication of nuclear activity, detectable only in the X-ray band. Steady spherical accretion of the mass losses from the central stellar cusp within the accretion radius, when coupled to a high radiative efficiency, already provides a power source exceeding the observed radiative losses from the nuclear region.

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arXiv2004

The Ionized Nuclear Environment in NGC 985 as seen by Chandra and BeppoSAX

(Abridged) We investigate the ionized environment of the Seyfert 1 galaxy NGC 985 with a new Chandra-HETGS observation and an archival BeppoSAX observation. Two absorption components are clearly required to fit absorption features observed in the Chandra spectrum. The components have a difference of 29 in ionization parameter and 3 in column density. The presence of the low ionization component is evidenced by an Fe M-shell unresolved transition array (UTA) produced by charge states VII to XIII. The high ionization phase is required by the presence of broad absorption features arising from several blends of Fe L-shell transitions (Fe XVII-XXII). A third highly ionized component might also be present, but the data does not allow to constrain its properties. An X-ray luminosity variation by a factor 2.3 is observed between the BeppoSAX and Chandra observations (separated by almost 3 years). Variability in the opacity of the absorbers is detected in response to the continuum variation, but while the colder component is consistent with a simple picture of photoionization equilibrium, the ionization state of the hotter component seems to increase while the continuum flux drops. The most striking result in our analysis is that during both the Chandra and the BeppoSAX observations, the two absorbing components appear to have the same pressure. Thus, we suggest that the absorption arises from a multi-phase wind. Such scenario can explain the change in opacity of both absorption components during the observations, but requires that a third hotter component is pressure-confining the two phases. Hence, our analysis points to a 3-phase medium similar to the wind found in NGC 3783, and further suggests that such a wind might be a common characteristic in AGN.

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arXiv2004

The Faint X-ray Source Population Near the 3C 295 cluster

We present a statistical analysis of the Chandra observation of the source field around the 3C 295 galaxy cluster (z=0.46). Three different methods of analysis, namely a chip by chip logN-logS, a two dimentional Kolmogorov-Smirnov (KS) test, and the angular correlation function (ACF) show a strong overdensity of sources in the North-East of the field, that may indicate a filament of the large scale structure of the Universe toward 3C 295.

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arXiv2004

Chandra's Discovery of Activity in the Quiescent Nuclear Black Hole of NGC 821

We report the results of the {\it Chandra} ACIS-S observations of the elliptical galaxy NGC 821, which harbors a supermassive nuclear black hole (of $3.5 \times 10^7 M_{\odot}$), but does not show sign of AGN activity. A small, 8.5$^{\prime\prime}$ long ($\sim 1$ kpc at the galaxy's distance of 23 Mpc), S-shaped, jet-like feature centered on the nucleus is detected in the 38 ksec ACIS-S integrated exposure of this region. The luminosity of this feature is $L_X \sim 2.6 \times 10^{39} \rm ergs s^{-1}$ (0.3-10 keV), and its spectrum is hard (described by a power-law of $Γ= 1.8^{+0.7}_{-0.6}$; or by thermal emission with $kT >2$ keV). We discuss two possibilities for the origin of this feature: (1) a low-luminosity X-ray jet, or (2) a hot shocked gas. In either case, it is a clear indication of nuclear activity, detectable only in the X-ray band. Steady spherical accretion of the mass losses from the central stellar cusp within the accretion radius, when coupled to a high radiative efficiency, already provides a power source exceeding the observed radiative losses from the nuclear region.

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arXiv2004

A panchromatic view of AGN

We review the continuum emission of Active Galactic Nuclei (AGN) over the entire electromagnetic spectrum. After a brief historical introduction, we describe the main spectral properties of unobscured AGN, discussing the selection biases which prevent us from having a complete view of the AGN population in the universe, and trying to build an updated spectral energy distribution of optically selected quasars. In the second part of the review, we describe the spectral properties of obscured AGN. Finally, we discuss the main observational methods in the different wavelength bands for disentangling AGN and stellar emission, and the ability of these methods to find new (mainly obscured) AGN, a significant fraction of which are probably still missing in current surveys.

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arXiv2003

Rapid N_H Changes in NGC 4151

We have analyzed two long BeppoSAX observations of the bright Seyfert galaxy NGC 4151, searching for short timescale (10-200 ksec) X-ray spectral variability. The light curve of a softness ratio, chosen as most sensitive to pinpoint changes of the column density of the absorbing gas along the line of sight, shows significant variations. We try to model these variations by performing a detailed, time resolved, spectral analysis. We find significant, large (factors of 1.5-6) variations of the absorber column densities on time scales of 40-200 ksec. These values are 10-100 times shorter than those found by Risaliti et al. 2002 in a sample of Seyfert 2 galaxies, and provide strong constraints on the geometry of the obscuring medium.

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arXiv2003

X-Ray Sources Overdensity Around the 3C 295 Galaxy Cluster

We present a statistical analysis of the Chandra observation of the source field around the 3C 295 galaxy cluster ($z=0.46$). The logN-logS of this field is in good agreement with that computed for the Chandra Deep Field South in this work and in previous ones. Nevertheless, the logN-logS computed separately for the four ACIS-I chips reveals that there is a significant excess of sources to the North-North East and a void to the South of the central cluster. Such an asymmetric distribution is confirmed by the two-dimensional Kolmogorov-Smirnov test, which excludes ($P \sim 3%$) a uniform distribution. In addition, a strong spatial correlation emerges from the study of the angular correlation function of the field: the angular correlation function is above that expected for X-ray sources on a few arcmin scales. In synthesis, the present analysis may indicate a filament of the large scale structure of the Universe toward 3C 295. This kind of studies may open-up a new way to map (with high efficiency) high-density peaks of large scale structures at high-z.

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arXiv2003

X-Ray Sources Overdensity Around 3C 295

We present a statistical analysis of the Chandra observation of the source field around the 3C 295 galaxy cluster ($z=0.46$). Three different methods of analysis, namely a chip by chip logN-logS, a two dimentional Kolmogorov-Smirnov (KS) test, and the angular correlation function (ACF) show a strong overdensity of sources in the North-East of the field, that may indicate a filament of the large scale structure of the Universe toward 3C 295.

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arXiv2003

Long term X-ray spectral variability of the nucleus of M81

We have analysed the soft X-ray emission from the nuclear source of the nearby spiral galaxy M81, using the available data collected with ROSAT, ASCA, BeppoSAX and Chandra. The source flux is highly variable, showing (sometimes dramatic: a factor of 4 in 20 days) variability at different timescales, from 2 days to 4 years, and in particular a steady increase of the flux by a factor of >~ 2 over 4 years, broken by rapid flares. After accounting for the extended component resolved by Chandra, the nuclear soft X-ray spectrum (from ROSAT/PSPC, BeppoSAX/LECS and Chandra data) cannot be fitted well with a single absorbed power-law model. Acceptable fits are obtained adding an extra component, either a multi-color black body (MCBB) or an absorption feature. In the MCBB case the inner accretion disk would be far smaller than the Schwartzchild radius for the 3-60X 10^6 solar masses nucleus requiring a strictly edge-on inclination of the disk, even if the nucleus is a rotating Kerr black hole. The temperature is 0.27 keV, larger than expected from the accretion disk of a Schwartzchild black hole, but consistent with that expected from a Kerr black hole. In the power-law + absorption feature model we have either high velocity (0.3 c) infalling C_v clouds or neutral C_i absorption at rest. In both cases the C:O overabundance is a factor of 10.

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arXiv2003

MM-Newton Observations of two high redshift quasars: RX J1028-0844 and BR 0351-1034

We report the results of XMM-Newton observations of two high redshift quasars, one radio-loud, RX J1028.6-0844 (z=4.276) and one radio-quiet, BR 0351-1034 (z=4.351). We find that the evidence for strong excess absorption towards RX J1028-0844 is marginal at best, contrary to previous claims. The superior sensitivity and broader, softer energy range of XMM-Newton (0.2-10 keV) allows better determination of spectral parameters than much deeper ASCA observations (0.8-7 keV). Our XMM-Newton observations call into question several other ASCA results of strong absorption towards high redshift radio-loud quasars. RX J1028.6-0844 occupies the same parameter space in broad band spectral properties as the low redshift BL Lac objects, showing no obvious evolution with redshift. The radio-quiet quasar BR 0351-1024 became fainter between ROSAT and XMM observations by a factor of at least 5, but with the present data we cannot determine whether there is an associated spectral change. These observations do not support previous claims of weaker X-ray emission from high redshift radio-quiet quasars. The soft X-ray spectral slope required to reconstruct the ROSAT PSPC hardness ratio of BR 0351-1034 is about alpha-X=3.5, the steepest X-ray slope ever observed in a high redshift quasar, and similar to that of low redshift Narrow Line Seyfert 1 galaxies.

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arXiv2003

Toward a Self Consistent Model of the Ionized Absorber in NGC 3783

We present a detailed model for the ionized absorbing gas evident in the 900 ksec Chandra HETGS spectrum of NGC 3783. The analysis was carried out with PHASE a new tool designed to model X-ray and UV absorption features in ionized plasmas. The 0.5-10 keV intrinsic continuum of the source is well represented by a single power law (Gamma=1.53) and a soft black-body component (kT=10 keV). The spectrum contains over 100 features, which are well fit by PHASE with just six free parameters. The model consists of a simple two phase absorber with difference of 35 in the ionization parameter and difference of 4 in the column density of the phases. The two absorption components turned out to be in pressure equilibrium, and are consistent with a single outflow (750 kms-1) an a single turbulent velocity (300 km s-1), and with solar elemental abundances. The main features of the low ionization phase are an Fe M-shell unresolved transition array (UTA) and the OVII lines. The OVII features, usualy identified with the OVIII and a warm absorber, are instead produced in a cooler medium also producing OVI lines. The UTA sets tight constraints on the ionization degree of the absorbers, making the model more reliable. The high ionization phase is required by the OVII and the Fe L-shell lines, and there is evidence for an even more ionized component in the spectrum. A continuous range of ionization parameters is disfavored by the fits, particularly to the UTA. The low ionizaton phase can be decomposed into three subcomponents based on the outflow velocity, FWHM, and H column densities found for three out of the four UV absorbers detected in NGC 3783. However, the ionization parametes are systematically smaller in our model than derived from UV data, indicating a lower degree of ionization.

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arXiv2003

Chandra Observation of 3C 212: a New Look at the X-ray and UV Absorbers

The red quasar 3C 212 (z=1.049) is one of the most distant and most luminous AGN which has shown evidence of an X-ray warm absorber. In order to further investigate this unusual quasar, we used Chandra/ACIS-S to observe 3C 212 for 19.5 ksec, resulting in a net detection of ~4000 counts. The Chandra data confirm the presence of an excess absorbing column N_H ~ 4 x 10^21 cm^-2 at the quasar redshift, but we find no compelling evidence for a warm absorber. Using both the Chandra and archival ROSAT PSPC data, we obtain very good fits for both a partially covered neutral absorber and a low-ionization (U = 0.03) photo-ionized absorber. In the ultraviolet, 3C 212 shows a strong associated MgII absorber. Based on a moderate resolution (80 km/s) MMT spectrum we show that the absorber is highly saturated and has a covering fraction less than 60%, implying that the absorber is truly intrinsic to the quasar. Photo-ionization modeling of the MgII absorber yields a constraint on the ionization parameter of U < 0.03, inconsistent with a warm UV/X-ray absorber. In addition to our spectral analysis, we find evidence in the ACIS image data for weak extended emission surrounding the quasar as well as emission corresponding to the radio lobes at a distance of 5 arcsec from 3C 212. The statistical significance of these features is low, but we briefly explore the implications if the detections are valid.

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