Chandra Low Energy Transmission Grating Research Articles

Two Shell-burning White Dwarfs in Symbiotics of the Small Magellanic Cloud

SMC3 and Lin358 are super-soft X-ray sources (SSS) in symbiotics of the Small Magellanic Cloud. They have been observed as SSS for over 30 yr. We present new observations done in 2014, 2021 and in 2023 with XMM-Newton, NICER, and Chandra. Lin358 is modeled with an effective temperature around 200,000 K, and a maximum flux value around 8.25 × 10−13 erg cm−2 s−1 in 2021. An observation close to periastron revealed hardening of the source. SMC3 is known to undergo an X-ray obscuration approximately every 4.5 yr; we measured the peak X-ray flux of 4.12 × 10−13 in 2014 with the Chandra Low Energy Transmission Grating. Models indicate an effective temperature of about 500,000 K, consistent with previous findings, and increasing luminosity excursion between maxima and minima. We suggest that with such high effective temperature, indicative of steady shell burning, the possibility these systems are on the path to type Ia supernova is worth considering.

Evidence for a moderate spin from X-ray reflection of the high-mass supermassive black hole in the cluster-hosted quasar H1821+643

ABSTRACT We present an analysis of deep Chandra Low-Energy and High-Energy Transmission Grating archival observations of the extraordinarily luminous radio-quiet quasar H1821+643, hosted by a rich and massive cool-core cluster at redshift z = 0.3. These data sets provide high-resolution spectra of the AGN at two epochs, free from contamination by the intracluster medium and from the effects of photon pile-up, providing a sensitive probe of the iron-K band. At both epochs, the spectrum is well described by a power-law continuum plus X-ray reflection from both the inner accretion disc and cold, slowly moving distant matter. Adopting this framework, we proceed to examine the properties of the inner disc and the black hole spin. Using Markov chain Monte Carlo (MCMC) methods, we combine constraints from the two epochs assuming that the black hole spin, inner disc inclination, and inner disc iron abundance are invariant. The black hole spin is found to be modest, with a 90 per cent credible range of ${a}^{*}=0.62^{+0.22}_{-0.37}$; and, with a mass MBH in the range log (MBH/M⊙) ∼ 9.2–10.5, this is the most massive black hole candidate for which a well-defined spin constraint has yet been obtained. The modest spin of this black hole supports previous suggestions that the most massive black holes may grow via incoherent or chaotic accretion and/or SMBH-SMBH mergers.

Pointing Chandra toward the Extreme Ultraviolet Fluxes of Very Low Mass Stars

Abstract The X-ray and EUV emission of stars plays a key role in the loss and evolution of the atmospheres of their planets. The coronae of dwarf stars later than M6 appear to behave differently from those of earlier spectral types and are more X-ray dim and radio bright. Too faint to have been observed by the Extreme Ultraviolet Explorer, their EUV behavior is currently highly uncertain. We have devised a method to use the Chandra X-ray Observatory High Resolution Camera to provide a measure of EUV emission in the 50–170 Å range and applied it to the M6.5 dwarf LHS 248 in a pilot 10 ks exposure. Analysis with model spectra using simple, idealized coronal emission measure distributions inspired by the Chandra Low Energy Transmission Grating spectra of the M5.5 dwarf Proxima Cen and results from the literature finds the greatest consistency with a very shallow emission measure distribution slope, DEM ∝ T 3/2 or shallower, in the range log T = 5.5–6.5, although this could be an artifact of systematic errors. Instead, cooler, more solar-like differential emission measures (DEMs) with a wide range of slopes were able to match the observations. Within the limitations of systematic errors, model spectra constrained by this method can provide accurate (within a factor of 2–4) synthesis and extrapolation of EUV spectra for wavelengths <400–500 Å. At longer wavelengths, models are uncertain by an order of magnitude or more and depend on the details of the emission measure distribution at temperatures log T < 5.5. The method is sensitive to the possible incompleteness of plasma radiative loss models in the 30–170 Å range for which reexamination would be warranted.

Diffuse low-ionization gas in the galactic halo casts doubts on z ≃ 0.03 WHIM detections

Abstract In this Letter, we demonstrate that the two claims of z ≃ 0.03 O vii K α absorption lines from Warm Hot Intergalactic Medium (WHIM) along the lines of sight to the blazars H 2356-309 (Buote et al.; Fang et al.) and Mkn 501 (Ren, Fang & Buote) are likely misidentifications of the z = 0 O ii K β line produced by a diffuse Low-Ionization Metal Medium in the Galaxy's interstellar and circum-galactic mediums. We perform detailed modelling of all the available high signal-to-noise Chandra Low Energy Transmission Grating (LETG) and XMM–Newton Reflection Grating Spectrometer (RGS) spectra of H 2356-309 and Mkn 501 and demonstrate that the z ≃ 0.03 WHIM absorption along these two sightlines is statistically not required. Our results, however, do not rule out a small contribution from the z ≃ 0.03 O vii K α absorber along the line of sight to H 2356-309. In our model the temperature of the putative z = 0.031 WHIM filament is T = 3 × 105 K and the O vii column density is ${\rm N}_{\rm O\,\small {VII}} \lesssim 4\times 10^{15}$ cm−2, twenty times smaller than the O viicolumn density previously reported, and now more consistent with the expectations from cosmological hydrodynamical simulations.

Chandragrating spectroscopy of three hot white dwarfs

High-resolution soft X-ray spectroscopic observations of single hot white dwarfs are scarce. With the Chandra Low-Energy Transmission Grating, we have observed two white dwarfs, one is of spectral type DA (LB 1919) and the other is a non-DA of spectral type PG1159 (PG 1520+525). The spectra of both stars are analyzed, together with an archival Chandra spectrum of another DA white dwarf (GD 246). The soft X-ray spectra of the two DA white dwarfs are investigated in order to study the effect of gravitational settling and radiative levitation of metals in their photospheres. LB 1919 is of interest because it has a significantly lower metallicity than DAs with otherwise similar atmospheric parameters. GD 246 is the only white dwarf known that shows identifiable individual iron lines in the soft X-ray range. For the PG1159 star, a precise effective temperature determination is performed in order to confine the position of the blue edge of the GW Vir instability region in the HRD. (abridged)

TESTING EUV/X-RAY ATOMIC DATA FOR THESOLAR DYNAMICS OBSERVATORY

The Atmospheric Imaging Assembly (AIA) and the Exteme-ultraviolet Variability Experiment (EVE) onboard the Solar Dynamics Observatory include spectral windows in the X-ray/EUV band. Accuracy and completeness of the atomic data in this wavelength range is essential for interpretation of the spectrum and irradiance of the solar corona, and of SDO observations made with the AIA and EVE instruments. Here we test the X-ray/EUV data in the CHIANTI database to assess their completeness and accuracy in the SDO bands, with particular focus on the 94A and 131A AIA passbands. Given the paucity of solar observations adequate for this purpose, we use high-resolution X-ray spectra of the low-activity solar-like corona of Procyon obtained with the Chandra Low Energy Transmission Grating Spectrometer (LETGS). We find that while spectral models overall can reproduce quite well the observed spectra in the soft X-ray range ll < 50A, and at the EUV wavelengths ll >130A, they significantly underestimate the observed flux in the 50-130A wavelength range. The model underestimates the observed flux by a variable factor ranging from \approx 1.5, at short wavelengths below \sim50A, up to \approx5-7 in the \sim 70-125A range. In the AIA bands covered by LETGS, i.e. 94A and 131A, we find that the observed flux can be underestimated by large factors (\sim 3 and \sim 1.9 respectively, for the case of Procyon presented here). We discuss the consequences for analysis of AIA data and possible empirical corrections to the AIA responses to model more realistically the coronal emission in these passbands.

ChandraLETGS Spectroscopy of the Quasar MR 2251−178 and Its Warm Absorber

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 NH ≈ 2 × 1021 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 unresolved transition arrays [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 extra cold material in the line of sight, and set an upper limit of NH ≈ 1.2 × 1020 cm−2. The X-ray spectrum does not appear to show evidence for dusty material, although an upper limit in the neutral carbon and oxygen column densities can only be set to cm−2 and cm−2, respectively.

High-Resolution X-Ray Spectroscopy of RX J0806.3+1527 with Chandra

RX J0806.3+1527 is a candidate double-degenerate binary with possibly the shortest known orbital period. The source shows an ≈100% X-ray intensity modulation at the putative orbital period of 321.5 s, but there still is no direct detection of orbital motion at this period. Here we report the results of high-resolution X-ray spectroscopic observations of the source, obtained with the Chandra Low Energy Transmission Grating (LETG) spectrometer. We obtained a total of ≈80 ks of exposure in two pointings separated by about 5 months. We detect the source in both the zeroth-order image and the dispersed spectrum. Detection of the 321.5 s pulsations in the zeroth-order image allows us to extract dispersed spectra as a function of the pulse phase of the X-ray modulation. The bulk of the source flux appears in the 20-60 A band. We find evidence for emission features in the 25-50 A range. The strongest feature (detected at better than 3 σ confidence) is an ≈0.6 A-wide (FWHM) emission line centered at 27 A. Weaker features at 31.6 and 46.4 A are also suggested, but they require confirmation with deeper exposures. The continuum is adequately described by an absorbed blackbody model with a temperature of 65 ± 5 eV and column depth of (2 ± 0.5) × 1020 cm−2. Interestingly, the spectrum does not show strong emission lines of typically abundant elements such as carbon, nitrogen, oxygen, and neon. We briefly discuss possible identifications for the emission features.

Confirming the Detection of an Intergalactic X‐Ray Absorber toward PKS 2155−304

We present new observations of PKS 2155-304 with the Chandra Low Energy Transmission Grating (LETG), using the Advanced CCD Imaging Spectrometer (ACIS). We confirm the detection of an absorption line plausibly identified as O VIII Ly? from a warm-hot intergalactic medium associated with a small group of galaxies along the line of sight, as originally reported by Fang and coworkers. Combining the previous observations by Fang and coworkers with five new long observations on the same target, we increase the total exposure time by a factor of 3 and the total counts per resolution element by a factor of 5. The measured line equivalent width is smaller than that observed by Fang and coworkers, but still consistent at 90% confidence. We also analyze the XMM-Newton observations on the same target, as well as observations using the combination of the Chandra LETG and the High Resolution Camera (HRC). These observations have been used to challenge our reported detection. While no line is seen in either the XMM-Newton or the Chandra LETG+HRC data, we find that our result is consistent with the upper limits from both data sets. We attribute the nondetection to (1) the higher quality of the Chandra LETG+ACIS spectrum and (2) the rather extended wings of the line-spread functions of both the XMM-Newton Reflection Grating Spectrometer (RGS) and the Chandra LETG+HRC. We discuss the implication of our observation on the temperature and density of the absorber. We also confirm the detection of z ~ 0 O VII absorption, and, comparing with previous Chandra analysis, we obtain much tighter constraints on the line properties.

ChandraandFar Ultraviolet Spectroscopic ExplorerObservations ofz∼ 0 Warm–Hot Gas toward PKS 2155−304

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.

An Assessment of the Fe xviii and Fe xix Line Ratios from the Chandra Grating Observations of Capella

Observations of Fe XVIII and Fe XIX X-ray, extreme-UV, and far-UV line emission, formed at the peak of Capella's (α Aurigae's) emission measure distribution and ubiquitous in spectra of many cool stars and galaxies, provide a unique opportunity to test the robustness of Fe XVIII and Fe XIX spectral models. The Astrophysical Plasma Emission Code (APEC) is used to identify over 35 lines from these two ions alone, and to compare model predictions with spectra obtained with the Chandra Low Energy Transmission Grating and High Energy Transmission Grating Spectrometers, the Far Ultraviolet Spectroscopic Explorer (FUSE), and the Extreme Ultraviolet Explorer. Some flux discrepancies larger than a factor of 2 are found between observations of Fe XVIII and Fe XIX lines and predictions by APEC and other models in common use. In particular, the X-ray resonance lines for both ions are stronger than predicted by all models relative to the EUV resonance lines. The multiwavelength observations demonstrate the importance of including dielectronic recombination and proton-impact excitation, and of using accurate wavelengths in spectral codes. These ions provide important diagnostic tools for 107 K plasmas currently observed with Chandra, XMM-Newton, and FUSE.

ChandraLow Energy Transmission Grating Spectrum of SS Cygni in Outburst

We have fitted the Chandra Low Energy Transmission Grating spectrum of SS Cygni in outburst with a single temperature blackbody suffering the photoelectric opacity of a neutral column density and the scattering opacity of an outflowing wind. We find that this simple model is capable of reproducing the essential features of the observed spectrum with the blackbody temperature T_bl = 250+/-50 kK, hydrogen column density N_H = 5.0^{+2.9}_{-1.5} x 10^{19} cm^{-2}, fractional emitting area f = 5.6^{+60}_{-4.5} x 10^{-3}, boundary layer luminosity L_bl = 5^{+18}_{-3} x 10^{33} erg/s, wind velocity v ~ 2500 km/s, wind mass-loss rate Mdot_w ~ 1.1 x 10^{16} g/s, and arbitrary values of the wind ionization fractions of 20 ions of O, Ne, Mg, Si, S, and Fe. Given that in outburst the accretion disk luminosity L_disk ~ 1 x 10^{35} erg/s, L_bl/L_disk ~ 0.05^{+0.18}_{-0.03}, which can be explained if the white dwarf (or an equatorial belt thereon) is rotating with an angular velocity Omega_wd ~ 0.7^{+0.1}_{-0.2} Hz, hence V_rot sin i ~ 2300 km/s.

A Simple, Unifying Model of the Extreme Ultraviolet Spectra of Dwarf Novae in Outburst

AbstractA simple model of the boundary layer and accretion disk wind of dwarf novae in outburst is briefly described and applied to the Chandra Low Energy Transmission Grating spectra of WZ Sge and SS Cyg in outburst.

Chandra LETGS and XMM-Newton observations of NGC 4593

In this paper, we analyze spectra of the Seyfert 1 galaxy NGC 4593 obtained with the Chandra Low Energy Transmission Grating Spectrometer (LETGS), the Reflection Grating Spectrometer (RGS) and the European Photon Imaging Camera's (EPIC) onboard of XMM-Newton. The two observations were separated by ~7 months. In the LETGS spectrum we detect a highly ionized warm absorber corresponding to an ionization state of 400x10^{-9} W m, visible as a depression at 10-18 \AA. This depression is formed by multiple weak Fe and Ne lines. A much smaller column density was found for the lowly ionized warm absorber, corresponding to xi = 3x10^{-9} W m. However, an intermediate ionization warm absorber is not detected. For the RGS data the ionization state is hard to constrain. The EPIC results show a narrow Fe Kalpha line.

From the Heart of the Ghoul: C and N Abundances in the Corona of Algol B

Chandra Low Energy Transmission Grating Spectrograph observations of Algol have been used to determine the abundances of C and N in the secondary star for the first time. In order to minimize errors arising from an uncertain coronal differential emission measure as a function of temperature, the analysis was performed relative to similar observations of an adopted "standard" star, HR 1099. It is demonstrated that HR 1099 and Algol are coronal twins in many respects and that their X-ray spectra are very similar in nearly all details, except for the observed strengths of C and N lines. The H-like 2p2P3/2,1/2 → 1s2S1/2 transitions of C and N in the coronae of Algol and HR 1099 demonstrate that the surface abundances of Algol B have been strongly modified by CN processing, as shown earlier by Schmitt & Ness. It is found that N is enhanced in Algol B by a factor of 3 compared with HR 1099. No C lines are detected in the Algol spectrum, indicating a C depletion relative to HR 1099 by a factor of 10 or more. These C and N abundances indicate that Algol B must have lost at least half of its initial mass and are consistent with predictions of evolutionary models that include nonconservative mass transfer and angular momentum loss through magnetic activity. Based on H-like and He-like transitions in O and Ne, it is estimated that Algol is slightly metal poor, by 0.2 dex in terms of the coronal abundances of light elements relative to HR 1099, while the Fe XVII 2p53d1P1 → 2p6 1S0 transition indicates a very similar Fe abundance. In reviewing coronal abundance results for active stars in the literature and drawing on an earlier Chandra study of the coronal abundances of HR 1099, it is concluded that Fe is very likely depleted in the coronae of both Algol and HR 1099 by 0.5 dex relative to their photospheric compositions, but that Ne is enhanced by a similar magnitude. Light elements such as C, N, and O are likely depleted in both stars by of order 0.3 dex. The similarities in these large-abundance anomalies in HR 1099 and Algol are notable. Despite such compositional fractionation in these coronae, the relative C and N abundances in HR 1099, determined by comparing observed line strengths with theoretical C/N line ratios, are consistent with recent solar values, indicating that differential fractionation between these elements is not significant and that little or no dredge-up of material subjected to CN processing has occurred on the subgiant component.

X-Ray Evidence of the Common Envelope Phase of V471 Tauri

Chandra Low Energy Transmission Grating Spectrograph observations of the pre-cataclysmic binary V471 Tau have been used to estimate the C/N abundance ratio of the K dwarf component for the first time. While the white dwarf component dominates the spectrum longward of 50 AA, at shorter wavelengths the observed X-ray emission is entirely due to coronal emission from the K dwarf. The H-like resonance lines of C and N yield an estimate of their logarithmic abundance ratio relative to the Sun of [C/N]=-0.38+/-0.15 - half of the currently accepted solar value. We interpret this result as the first clear observational evidence for the presumed common envelope phase of this system, during which the surface of the K dwarf was contaminated by CN-cycle processed material dredged up into the red giant envelope. We use the measured C/N ratio to deduce that 0.015-0.04 Msun was accreted by the K dwarf while engulfed, and show that this is consistent with a recent tentative detection of 13C in the K dwarf photosphere, and with the measured Li abundance in the scenario where the red giant companion was Li-rich during the common envelope phase.

A Chandra Low Energy Transmission Grating Spectrometer Observation of V4743 Sagittarii: A Supersoft X-Ray Source and a Violently Variable Light Curve

V4743 Sagittarii (Nova Sgr 2002 No. 3) was discovered on 2002 September 20. We obtained a 5 ks ACIS-S spectrum in 2002 November and found that the nova was faint in X-rays. We then obtained a 25 ks Chandra Low Energy Transmission Grating Spectrometer (LETGS) observation on 2003 March 19. By this time, it had evolved into the supersoft X-ray phase exhibiting a continuous spectrum with deep absorption features. The light curve from the observation showed large-amplitude oscillations with a period of 1325 s (22 minutes) followed by a decline in the total count rate after ~13 ks of observations. The count rate dropped from ~40 counts s-1 to practically zero within ~6 ks and stayed low for the rest of the observation (~6 ks). The spectral hardness ratio changed from maxima to minima in correlation with the oscillations and then became significantly softer during the decay. Strong H-like and He-like lines of oxygen, nitrogen, and carbon were found in absorption during the bright phase, indicating temperatures between 1 and 2 MK, but they were shifted in wavelength corresponding to a Doppler velocity of -2400 km s-1. The spectrum obtained after the decline in count rate showed emission lines of C VI, N VI, and N VII, suggesting that we were seeing expanding gas ejected during the outburst, probably originating from CNO-cycled material. An XMM-Newton Target of Opportunity observation, obtained on 2002 April 4 and a later LETGS observation from 2003 July 18 also showed oscillations, but with smaller amplitudes.

The Remarkably Featureless High-Resolution X-Ray Spectrum of Markarian 478

An observation of Mrk 478 using the Chandra Low Energy Transmission Grating Spectrometer is presented. The source exhibited 30%–40% flux variations on timescales of ∼104 s together with a slow decline in the spectral softness over the full 80 ks observation. The 0.15–3.0 keV spectrum is well fitted by a single power law with photon index Γ = 2.91 ± 0.03. Combined with high-energy data from BeppoSAX, the spectrum from 0.15 to 10 keV is well fitted as the sum of two power laws with Γ1 = 3.03 ± 0.04, which dominates below 2 keV, and Γ2 = 1.4 ± 0.2, which dominates above 2 keV (quoting 90% confidence uncertainties). No significant emission or absorption features are detected in the high-resolution spectrum, supporting our previous findings using the Extreme Ultraviolet Explorer but contradicting the earlier claims of emission lines by Hwang & Bowyer. There is no evidence of a warm absorber as found in the high-resolution spectra of many Seyfert 1 galaxies, including others classified as narrow-line Seyfert 1's such as Mrk 478. We suggest that the X-ray continuum may result from Comptonization of disk thermal emission in a hot corona through a range of optical depths.

High-resolution X-ray spectroscopy of late-type stars with CHANDRA

We have analyzed high-resolution (Δλ ≅ 0.06 Å) X-ray spectra in the region 6–180 Å of the coronae of the cool stars Capella, Procyon, and α Centauri. These stars were observed with the the CHANDRA Low Energy Transmission Grating Spectrometer (LETGS) between Sep. and Dec. 1999. Temperatures are derived from line ratios of helium-like lines and long-wavelength iron lines. Electron densities are obtained for the relatively cooler (few MK) and more tenuous (⪅ 10 11 cm −3) plasma components from the forbidden to intercombination line ratios in the helium-like triplets of O, N, and C and for the hotter (⪆ 5 MK) and denser (⪆ 10 12 cm −3) components (such as occur in Capella) from the helium-like triplets of Mg and Si and the ratios of Fe XIX-Fe XXII 2ℓ-2ℓ′ lines above 90 Å. The implications of these results for the coronal structure are discussed.

Using the High‐Resolution X‐Ray Spectrum of PSR B0656+14 to Constrain the Chemical Composition of the Neutron Star Atmosphere

Observations of PSR B0656+14 using the Chandra Low Energy Transmission Grating Spectrometer are presented. The zeroth order events are pulsed at an amplitude of 10 +/- 2% and the image may be slightly extended. The extended emission is modelled as a Gaussian with a FWHM of about 0.75, for a linear size (at a distance of 760 pc) of 8.5e15 cm. In the absence of systematic errors in the detector point spread function, the extended emission comprises <~ 50% of the observed flux in the 0.2-2.0 keV band, for a luminosity of <~ 3e32 erg/s. The spectrum is well modelled by a dominant blackbody with T = 8.0e5 +/- 3e4 K and a size of 22.5 +/- 2.1 km in addition to a harder component that is modelled as a hotter and much smaller blackbody. No significant absorption features are found in the spectrum that might be expected from ionization edges of H or He or bound-bound transitions of Fe in magnetized atmospheres. Such features are expected to be deep but could vary in position or strength with rotation phase. There are no strong absorption features in the pulse-phased spectra, however, so we conclude that the atmosphere is not dominated by Fe or other heavy elements that would be partially ionized at a temperature of 1e6 K.