Chandra LETG Research Articles

A novel approach to detect line emission under high background in high-resolution X-ray spectra

ABSTRACT We develop a novel statistical approach to identify emission features or set upper limits in high-resolution spectra in the presence of high background. The method relies on detecting differences from the background using smooth tests and using classical likelihood ratio tests to characterize known shapes like emission lines. We perform signal detection or place upper limits on line fluxes while accounting for the problem of multiple comparisons. We illustrate the method by applying it to a Chandra LETGS + HRC-S observation of symbiotic star RT Cru, successfully detecting previously known features like the Fe line emission in the 6–7 keV range and the Iridium-edge due to the mirror coating on Chandra. We search for thermal emission lines from Ne x, Fe xvii, O viii, and O vii, but do not detect them, and place upper limits on their intensities consistent with a ≈1 keV plasma. We serendipitously detect a line at 16.93 Å (0.732 keV) that we attribute to photoionization or a reflection component.

Probing the Hot Circumgalactic Medium with Broad O vi and X-Rays

Abstract Most of the baryonic mass in the circumgalactic medium (CGM) of a spiral galaxy is believed to be warm-hot, with temperature around 106 K. The narrow O vi absorption lines probe a somewhat cooler component at , but broad O vi absorbers have the potential to probe the hotter CGM. Here we present 376 ks Chandra LETG observations of a carefully selected galaxy in which the presence of broad O vi together with the non-detection of Lyα was indicative of hot gas. The strongest line expected to be present at ≈106 K is O vii λ21.602. There is a hint of an absorption line at the redshifted wavelength, but the line is not detected with better than 2σ significance. A physical model, taking into account strengths of several other lines, provides better constraints. Our best-fit absorber model has and . These parameters are consistent with the hot plasma model based on UV observations; other O vi models of cooler gas phases are ruled out at better than 99% confidence. Thus we have suggestive, but not conclusive evidence for the broad O vi absorber probing the hot gas from the shallow observations of this pilot program. About 800 ks of XMM-Newton observations will detect the expected absorption lines of O vii and O viii unequivocally. Future missions like XRISM, Arcus, and Athena will revolutionize the CGM science.

Detection of the Missing Baryons toward the Sightline of H1821+643

Abstract Based on constraints from Big Bang nucleosynthesis and the cosmic microwave background, the baryon content of the high-redshift Universe can be precisely determined. However, at low redshift, about one-third of the baryons remain unaccounted for, which poses the long-standing missing baryon problem. The missing baryons are believed to reside in large-scale filaments in the form of warm-hot intergalactic medium (WHIM). In this work, we employ a novel stacking approach to explore the hot phases of the WHIM. Specifically, we utilize the 470 ks Chandra LETG data of the luminous quasar, H 1821+643, along with previous measurements of UV absorption line systems and spectroscopic redshift measurements of galaxies toward the quasar’s sightline. We repeatedly blueshift and stack the X-ray spectrum of the quasar corresponding to the redshifts of the 17 absorption line systems. Thus, we obtain a stacked spectrum with 8.0 Ms total exposure, which allows us to probe X-ray absorption lines with unparalleled sensitivity. Based on the stacked data, we detect an O vii absorption line that exhibits a Gaussian line profile and is statistically significant at the 3.3σ level. Since the redshifts of the UV absorption line systems were known a priori, this is the first definitive detection of an X-ray absorption line originating from the WHIM. The equivalent width of the O vii line is (4.1 ± 1.3) mÅ, which corresponds to an O vii column density of . We constrain the absorbing gas to have a density of for a single WHIM filament. We derive for the cosmological mass density of O vii, assuming that all 17 systems contribute equally.

A possibleChandraandHubble Space Telescopedetection of extragalactic WHIM towards PG 1116+215

(Abridged) We have analyzed Chandra LETG and XMM-Newton RGS spectra towards the z=0.177 quasar PG 1116+215, a sightline that is rendered particularly interesting by the HST detection of several OVI and HI broad Lyman-alpha absorption lines that may be associated with the warm-hot intergalactic medium. We performed a search for resonance K-alpha absorption lines from OVII and OVIII at the redshifts of the detected far-ultraviolet lines. We detected an absorption line in the Chandra spectra at 5.2 sigma confidence level at wavelengths corresponding to OVIII K-alpha at z=0.0911+-0.0004+-0.0005 (statistical followed by systematic error). This redshift is within 3 sigma of that of a HI broad Lyman-alpha of b=130 km/s at z=0.09279+-0.00005. We have also analyzed the available XMM-Newton RGS data towards PG 1116+215. Unfortunately, the XMM-Newton data are not suitable to investigate this line because of instrumental features at the wavelengths of interest. At the same redshift, the Chandra and XMM-Newton spectra have OVII K-alpha absorption line features of significance 1.5 sigma and 1.8 sigma, respectively. We also analyzed the available SDSS spectroscopic galaxy survey data towards PG 1116+215 in the redshift range of interest. We found evidence for a galaxy filament that intersects the PG 1116+215 sightline and additional galaxy structures that may host WHIM. The combination of HST, Chandra, XMM-Newton and SDSS data indicates that we have likely detected a multi-temperature WHIM at z=0.091-0.093 towards PG 1116+215.

Model Atmosphere Spectrum Fit to the Soft X-Ray Outburst Spectrum of SS Cyg

The X-ray spectrum of SS Cyg in outburst has a very soft component that can be interpreted as the fast-rotating optically thick boundary layer on the white dwarf surface. This component was carefully investigated by Mauche (2004) using the Chandra LETG spectrum of this object in outburst. The spectrum shows broad ( ≈5 °A) spectral features that have been interpreted as a large number of absorption lines on a blackbody continuum with a temperature of ≈250 kK. Because the spectrum resembles the photospheric spectra of super-soft X-ray sources, we tried to fit it with high gravity hot LTE stellar model atmospheres with solar chemical composition, specially computed for this purpose. We obtained a reasonably good fit to the 60–125 °A spectrum with the following parameters: T<sub>eff</sub> = 190 kK, log g = 6.2, and NH = 8 · 10<sup>19</sup> cm<sup>−2</sup>, although at shorter wavelengths the observed spectrum has a much higher flux. The reasons for this are discussed. The hypothesis of a fast rotating boundary layer is supported by the derived low surface gravity.

A new soft X-ray spectral model for polars with an application to AM Herculis

We present a simple heuristic model for the time-averaged soft X-ray temperature distribution in the accretion spot on the white dwarf in polars. The model is based on the analysis of the Chandra LETG spectrum of the prototype polar AM Her and involves an exponential distribution of the emitting area vs. blackbody temperature a(T) = a0 exp(-T/T0). With one free parameter besides the normalization, it is mathematically as simple as the single blackbody, but is physically more plausible and fits the soft X-ray and far-ultraviolet spectral fluxes much better. The model yields more reliable values of the wavelength-integrated flux of the soft X-ray component and the implied accretion rate than reported previously.

CHANDRAHIGH-RESOLUTION SPECTROSCOPY OF THE CIRCUMNUCLEAR MATTER IN THE BROAD-LINE RADIO GALAXY 3C 445

We present evidence for X-ray line emitting and absorbing gas in the nucleus of the Broad-Line Radio Galaxy (BLRG), 3C 445. A 200ks Chandra LETG observation of 3C 445 reveals the presence of several highly ionized emission lines in the soft X-ray spectrum, primarily from the He and H-like ions of O, Ne, Mg and Si. Radiative recombination emission is detected from O VII and O VIII, indicating that the emitting gas is photoionized. The He-like emission appears to be resolved into forbidden and intercombination line components, which implies a high density of >10^{10} cm^{-3}, while the Oxygen lines are velocity broadened with a mean width of ~2600 km s^{-1} (FWHM). The density and widths of the ionized lines indicate an origin of the gas on sub-parsec scales in the Broad Line Region (BLR).The X-ray continuum of 3C 445 is heavily obscured either by a partial coverer or by a photoionized absorber of column density N_{H}=2x10^{23} cm^{-2} and ionization parameter log(xi)=1.4 erg cm s^{-1}. However the view of the X-ray line emission is unobscured, which requires the absorber to be located at radii well within any parsec scale molecular torus. Instead, we suggest that the X-ray absorber in 3C 445 may be associated with an outflowing, but clumpy, accretion disk wind with an observed outflow velocity of ~10000 km/s.

X-raying the AU Microscopii debris disk

AU Mic is a young, nearby X-ray active M-dwarf with an edge-on debris disk. Debris disk are the successors of the gaseous disks usually surrounding pre-main sequence stars which form after the first few Myrs of their host stars' lifetime, when - presumably - also the planet formation takes place. Since X-ray transmission spectroscopy is sensitive to the chemical composition of the absorber, features in the stellar spectrum of AU Mic caused by its debris disk can in principle be detected. The upper limits we derive from our high resolution Chandra LETGS X-ray spectroscopy are on the same order as those from UV absorption measurements, consistent with the idea that AU Mic's debris disk possesses an inner hole with only a very low density of sub-micron sized grains or gas.

THE TWO-PHASE, TWO-VELOCITY IONIZED ABSORBER IN THE SEYFERT 1 GALAXY NGC 5548

We present an analysis of X-ray high quality grating spectra of the Seyfert 1 galaxy NGC 5548 using archival Chandra HETGS and LETGS observations for a total exposure time of 800ks. The continuum emission is well represented by a powerlaw plus a black-body component. We find that the well known X-ray warm absorber in this source consists of two different outflow velocity systems. Recognizing the presence of these kinematically distinct components allows each system to be fitted independently, each with two absorption components with different ionization levels. The high velocity system consists of a component with temperature of 2.7X10^6K and another component with temperature of 5.8X10^5K. The low-velocity system required also two absorbing components, one with temperature of 5.8X10^5K; the other with lower temperature (3.5X10^4K). Once these components are considered, the data do not require any further absorbers. In particular, a model consisting of a continuous radial range of ionization structures is not required. The two absorbing components in each velocity system are in pressure equilibrium with each other. This suggests that each velocity system consists of a multi-phase medium. This is the first time that different outflow velocity systems have been modelled independently in the X-ray band for this source. The kinematic components and column densities found from the X-rays are in agreement with the main kinematic components found in the UV absorber. This supports the idea that the UV and X-ray absorbing gas is part of the same phenomenon. NGC 5548 can now be seen to fit in a pattern established for other warm absorbers: 2 or 3 discrete phases in pressure equilibrium. There are no remaining cases of a well studied warm absorber in which a model consisting of a multi-phase medium is not viable.

R-matrix electron-impact excitation data for B-like Si and its application in cool stars

We present the results of an intermediate coupling frame-transformation R-matrix calculation for the electron-impact excitation of Si 9+ . The target and close-coupling expansions are both taken to be the 125 fine-structure levels (58 LS terms) belonging to the configurations 2s x 2p y (x + y = 3) and 2s α 2p β 3l (α+ β = 2, l = s, p, and d). Due to the additional resonances included in our calculation, we find significant differences at low temperatures with the widely used n = 2 → 2 excitation rates, also obtained with the R-matrix method, as well as with the n = 2 → 3 excitation rates calculated by using the distorted wave (DW) approximation. We present a list of prominent transition lines and comparisons with SERTS and Hinode/EIS EUV spectra of the solar corona, SUMER observations for the quiet sun, as well as Chandra LETG and Rocket soft X-ray spectra of the Procyon corona and solar flares, respectively. Line emissivities of some transitions are enhanced up to 40% when compared with those obtained from using the previous atomic data at the same electron density (1.6 × 10 9 cm −3 ) and temperature (1.3 × 10 6 K). The comparison with Chandra LETG observation of Procyon reveals that the 3s−2p line flux was significantly underestimated (by a factor of 4−5) in previous analyses. Some EUV and soft X-ray emission line ratios are ne-sensitive and T e-insensitive. Estimated electron densities from them shift downwards due to the new resonant-enhanced excitation data used in the present modelling.

X-RAY ABSORPTION BY WHIM IN THE SCULPTOR WALL

We present XMM RGS and Chandra LETG observations of the blazar, H 2356-309, located behind the Sculptor Wall, a large-scale galaxy structure expected to harbor high-density Warm-Hot Intergalactic Medium (WHIM). Our simultaneous analysis of the RGS and LETG spectra yields a 3-sigma detection of the crucial redshifted O vii K-alpha line with a column density (>~ 10^{16} cm^{-2}) consistent with similar large-scale structures produced in cosmological simulations. This represents the first detection of non-local WHIM from X-ray absorption studies where XMM and Chandra data are analyzed simultaneously and the absorber redshift is already known, thus providing robust evidence for the expected repository of the "missing baryons".

HIGH-RESOLUTION X-RAY SPECTROSCOPY OF SNR 1987A: CHANDRA LETG AND HETG OBSERVATIONS IN 2007

We present an extended analysis of the deep Chandra LETG and HETG observations of the supernova remnant 1987A (SNR 1987A) carried out in 2007. The global fits to the grating spectra show that the temperature of the X-ray emitting plasma in the slower shocks in this system has remained stable for the last three years, while that in the faster shocks has decreased. This temperature evolution is confirmed by the first light curves of strong X-ray emission lines and their ratios. On the other hand, bulk gas velocities inferred from the X-ray line profiles are too low to account for the post-shock plasma temperatures inferred from spectral fits. This suggests that the X-ray emission comes from gas that has been shocked twice, first by the blast wave and again by shocks reflected from the inner ring of SNR 1987A. A new model that takes these considerations into account gives support to this physical picture.

The Fainting of α Centauri A, Resolved

Beginning in 2003, XMM-Newton snapshot monitoring of α Centauri (HD 128620, 128621: G2 V, K1 V) documented a steady fading of the primary's X-ray corona, which had all but disappeared by early 2005. The steep decline in LX was at odds with the previous two decades of high-energy measurements, which showed only modest variability of the Sun-like star. A Chandra LETGS spectrum in 2007 June, however, fully resolved the source of the curious X-ray darkening: a depletion of plasma above ~2 MK had substantially depressed the line spectrum where the XMM-Newton response peaks (λ 30 A), even though the overall coronal luminosity, dominated by longer wavelength emissions, had declined only slightly. This is reminiscent of the Sun's magnetic activity cycle, where the 2-3 MK active regions of sunspot maximum give way to the spatially pervasive, but cycle-independent, 1 MK quiet corona at minimum. This emphasizes that any discussion of cyclic coronal variability in low-activity stars will depend crucially on the energy coverage of the measurements.

Chandra HETG Spectra of SN 1987A at 20 Years

We have undertaken deep, high-resolution observations of SN 1987A at ≈20 years after its explosion with the Chandra HETG and LETG spectrometers. Here we present the HETG X-ray spectra of SN 1987A having unprecedented spectral resolution and signal-to-noise ratios in the 6–20 bandpass, which includes the H-like u A and He-like lines of Si, Mg, and Ne, as well as O viii lines and bright Fe xvii lines. In joint analysis with LETG data, we find that there has been a significant decrease from 2004 to 2007 in the average temperature of the highest temperature component of the shocked-plasma emission. Model fitting of the profiles of individual HETG lines yields bulk kinematic velocities of the higher-Z ions, Mg and Si, that are significantly lower than those inferred from the LETG 2004 observations. Subject headings: ISM: individual (SN 1987A) — radiation mechanisms: thermal — supernova remnants — techniques: spectroscopic — X-rays: ISM

RGS X-Ray Spectroscopy of the Crab Nebula

The Crab nebula and pulsar have been widely used as a calibration source for X-ray instruments. The in-flight effective area calibration of the Reflection Grating Spectrometers (RGS) of XMM-Newton depends upon the availability of reliable calibration sources. We analyse RGS observations of the Crab using different instrument configurations and spatial offsets, and make use of previous determinations of the continuum spectrum of the nebula plus pulsar. Due to the high spectral resolution of the RGS, we resolve the main absorption edges. We get an excellent fit to the Crab spectrum using this fixed continuum and the absorption spectrum determined by RGS. We get accurate column densities for the neutral atoms of H, N, O, Ne, Mg and Fe as well as a clear detection of Fe II and firm upper limits for other ions (O II, Mg II). We find solar-like abundances for N, O and Mg, and Fe (adding Fe I and Fe II), while Ne is overabundant. Finally, we show how we can determine the absolute flux of Crab with high accuracy by combining RGS and Chandra LETGS spectra of different sources.

The OviiX‐Ray Forest toward Markarian 421: Consistency betweenXMM‐NewtonandChandra

Recently the first detections of highly ionised gas associated with two Warm-Hot Intergalactic Medium (WHIM) filaments have been reported. The evidence is based on X-ray absorption lines due to O VII and other ions observed by Chandra towards the bright blazar Mrk 421. We investigate the robustness of this detection by a re-analysis of the original Chandra LETGS spectra, the analysis of a large set of XMM-Newton RGS spectra of Mrk 421, and additional Chandra observations. We address the reliability of individual spectral features belonging to the absorption components, and assess the significance of the detection of these components. We also use Monte Carlo simulations of spectra. We confirm the apparent strength of several features in the Chandra spectra, but demonstrate that they are statistically not significant. This decreased significance is due to the number of redshift trials that are made and that are not taken into account in the original discovery paper. Therefore these features must be attributed to statistical fluctuations. This is confirmed by the RGS spectra, which have a higher signal to noise ratio than the Chandra spectra, but do not show features at the same wavelengths. Finally, we show that the possible association with a Ly-alpha absorption system also lacks sufficient statistical evidence. We conclude that there is insufficient observational proof for the existence of the two proposed WHIM filaments towards Mrk 421, the brightest X-ray blazar on the sky. Therefore, the highly ionised component of the WHIM still remains to be discovered.

Capella Corona Revisited: A Combined View fromXMM‐NewtonRGS andChandraHETGS and LETGS

We present a combined analysis of the X-ray emission of the Capella corona obtained with XMM-Newton RGS and Chandra HETGS and LETGS. An improved atomic line database and a new differential emission measure (DEM) deconvolution method are developed for this purpose. Our new atomic database is based on the Astrophysical Plasma Emission Database and incorporates improved calculations of ionization equilibrium and line emissivities for L-shell ions of abundant elements using the Flexible Atomic Code. The new DEM deconvolution method uses a Markov Chain Monte Carlo (MCMC) technique that differs from existing MCMC or χ2-fitting-based methods. We analyze the advantages and disadvantages of each individual instrument in determining the DEM and elemental abundances. We conclude that results from either RGS or HETGS data alone are not robust enough due to their failure to constrain the DEM in some temperature region or the lack of significant continuum emission in the wavelength band of the spectrometers, and that the combination of HETGS and RGS produces more stringent constraints on the DEM and abundance determinations. Using the LETGS data, we show that the recently discovered inconsistencies between the EUV and X-ray lines of Fe XVIII and XIX also exist in more highly charged iron ions, up to Fe XXIII, and that enhanced interstellar absorption due to partially ionized plasma along the Capella line of sight may explain some, but not all, of these discrepancies.

ChandraLETG Observations of Supernova Remnant 1987A

We discuss the results from deep Chandra LETG observations of the supernova remnant 1987A (SNR 1987A). We find that a distribution of shocks, spanning the same range of velocities (from � 300 to 1700 km s � 1 ) as deduced in the first part of our analysis, can account for the entire X-ray spectrum of this object. The postshock temperature distribution isbimodal, peaking atkT � 0:5and� 3 keV. Abundances inferred from the X-ray spectrum have values similartothosefortheinnercircumstellarring,exceptthattheabundancesofnitrogenandoxygenareapproximately a factor of 2 lower than those inferred from the optical/UV spectrum. The velocity of the X-ray-emitting plasma has decreased since 1999, apparently because the blast wave has entered the main body of the inner circumstellar ring. Subject headingg supernova remnants — supernovae: individual (SNR 1987A) — X-rays: ISM

Evidence for Gravitational Infall of Matter onto the Supermassive Black Hole in the Quasar PG 1211+143?

We report the detection of redshifted iron Kα absorption lines in the Chandra LETG spectrum of the narrow-line quasar PG 1211+143. The absorption lines are observed at 4.22 and 4.93 keV in the quasar spectrum, corresponding to 4.56 and 5.33 keV in the rest frame of PG 1211+143. From Monte Carlo simulations, the chance probability of both lines being false detections is low at 1.36 × 10-4. Highly redshifted ionized iron Kα (Fe XXV or Fe XXVI) is the most plausible identification for the lines at their observed energies. If identified with H-like iron Kα at 6.97 keV, then the relativistic velocity shifts required are 0.40c and 0.26c. The extreme velocities can be explained by pure gravitational redshift if the matter exists in a stable orbit within 6 gravitational radii of the black hole. This would require a Kerr metric for the black hole. Alternatively, the absorption may be the result of matter infalling directly onto the black hole, with a maximum observed velocity of 0.38c at 6Rg in the Schwarzschild metric. This matter may originate in a failed outflow or jet, which does not escape the gravitational potential of the black hole.

ChandraObservations of Shock Kinematics in Supernova Remnant 1987A

We report the first results from deep X-ray observations of the SNR 1987A with the Chandra LETG. Temperatures inferred from line ratios range from 0.1 - 2 keV and increase with ionization potential. Expansion velocities inferred from X-ray line profiles range from 300 - 1700 km/s, much less than the velocities inferred from the radial expansion of the radio and X-ray images. We can account for these observations with a scenario in which the X-rays are emitted by shocks produced where the supernova blast wave strikes dense protrusions of the inner circumstellar ring, which are also responsible for the optical hot spots.