Low Energy Transmission Grating Research Articles

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.

The Continued Spectral Evolution of the Neutron Star RX J0720.4-3125

We observed the isolated neutron star RX J720.4-3125 with Chandra's Low Energy Transmission Grating Spectrometer, following the XMM-Newton discovery of long term spectral evolution of this source. The new observation shows that the spectrum of RX J720.4-3125 has continued to change in the course of 5 months. It has remained hard, similar to the last XMM-Newton observation, but the strong depression observed with XMM-Newton at long wavelengths has disappeared. Contrary to the XMM-Newton observations, the new Chandra observation shows that the flux increase at short wavelengths and the decrease at long wavelengths do not necessarily occur simultaneously.

Doppler Shifts and Broadening and the Structure of the X‐Ray Emission from Algol

In a study of Chandra High Energy Transmission Grating spectra of Algol, we clearly detect Doppler shifts caused by the orbital motion of Algol B. These data provide the first definitive proof that the X-ray emission of Algol is dominated by the secondary, in concordance with expectations that the primary B8 component should be X-ray-dark. However, the measured Doppler shifts are slightly smaller than might be expected, implying an effective orbital semimajor axis of about 10 R☉ instead of 11.5 R☉ for the Algol B center of mass. This could be caused by a small contribution of Algol A, possibly through accretion, to the observed X-ray flux, in which case such a contribution does not exceed 10%-15%. We suggest that the more likely explanation is an asymmetric corona biased toward the system center of mass by the tidal distortion of the surface of Algol B. A detailed analysis of the profiles of the strongest lines indicates the presence of excess line broadening amounting to approximately 150 km s-1 above that expected from thermal motion and surface rotation. Possible explanations for this additional broadening include turbulence, flows or explosive events, or rotational broadening from a radially extended corona. We favor the latter scenario and infer that a significant component of the corona at temperatures less than 107 K has a scale height of order the stellar radius. This interpretation is supported by the shape of the X-ray light curve and tentative detection of a shallow dip at secondary eclipse. We also examine the O VII intercombination and forbidden lines in a Low Energy Transmission Grating Spectrograph observation and find no change in their relative line fluxes as the system goes from quadrature to primary eclipse. Since these lines appear to be strongly affected by UV irradiation from Algol A through radiative excitation of the 2 3S → 2 3P transition, this supports the conjecture that the corona of Algol B at temperatures of several million kelvins must be significantly extended and/or located toward the poles to avoid being shadowed from Algol A during primary eclipse.

The Origin of Emission and Absorption Features in Ton S180ChandraObservations

We present a new interpretation of the Ton S180 spectrum obtained by the Chandra Spectrometer (Low Energy Transmission Grating). Several narrow absorption lines and a few emission disk lines have been successfully fitted to the data. We have not found any significant edges accompanying line emission. We propose an interpretation of narrow lines consistent with that of the recent paper by Krolik, in which a warm absorber is strongly inhomogeneous. Such a situation is possible in the so-called multiphase medium, where regions with different ionization states, densities, and temperatures may coexist in thermal equilibrium under constant pressure. We illustrate this scenario with theoretical spectra of radiation transferred through a stratified cloud with constant pressure (instead of constant density) computed by the TITAN code in plane-parallel approximation. Detected spectral features are faint, and their presence does not alter the broadband continuum. We model the broadband continuum of Ton S180 assuming an irradiated accretion disk with a dissipative warm skin. The set of parameters appropriate for the data cannot be determined uniquely, but models with low values of the black hole mass have too hot and radially extended a warm skin to explain the formation of soft X-ray disk lines seen in the data.

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.

An Observational Evidence of the Common Envelope Phase

AbstractChandra 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 Å, at shorter wavelengths the observed X-ray emission is entirely due to coronal emission from the K dwarf. The H-like 2p 2Р3/2, 1/2 → 1s 2S1/2 resonance lines of С 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

Absorption Components in the Nucleus of NGC 3227

X-ray spectroscopy provides a wealth of information about the properties of the surrounding reprocessing material of AGN. We present results from a 96 ks exposure of NGC 3227, taken with the Low Energy Transmission Grating Spectrometer on board the Chandra X-ray Observatory. A partial covering model fits well the resulting X-ray spectrum. We investigate the dusty nuclear environment of this galaxy, and in particular concentrate on its warm absorber.

Modeling the NeixTriplet Spectral Region of Capella with theChandraandXMM‐NewtonGratings

High-resolution X-ray spectroscopy with the diffraction gratings of Chandra and XMM-Newton offers new chances to study a large variety of stellar coronal phenomena. A popular X-ray calibration target is Capella, which has been observed with all gratings with significant exposure times. We gathered together all available data of the High Energy Transmission Grating Spectrometer (HETGS; 155 ks), Low Energy Transmission Grating Spectrometer (LETGS; 219 ks), and Reflection Grating Spectrometer (RGS; 53 ks) for comparative analysis, focusing on the Ne IX triplet at around 13.5 A, a region that is severely blended by strong iron lines. We identify 18 emission lines in this region of the High-Energy Grating (HEG) spectrum, including many from Fe XIX, and find good agreement with predictions from a theoretical model constructed using the Astrophysical Plasma Emission Code. The model uses an emission measure distribution derived from Fe XV to Fe XXIV lines. The success of the model is due in part to the inclusion of accurate wavelengths from laboratory measurements. While these 18 emission lines cannot be isolated in the LETGS or RGS spectra, their wavelengths and fluxes as measured with HEG are consistent with the lower resolution spectra. In the Capella model for HEG, the weak intercombination line of Ne IX is significantly blended by iron lines, which contribute about half the flux. After accounting for blending in the He-like diagnostic lines, we find the density to be consistent with the low-density limit (ne < 2 × 1010 cm-3); however, the electron temperature indicated by the Ne IX G-ratio is surprisingly low (~2 MK) compared to the peak of the emission measure distribution (~6 MK). Models show that the Ne IX triplet is less blended in cooler plasmas and in plasmas with an enhanced neon-to-iron abundance ratio.

AD Leonis: Flares observed by XMM-Newton and Chandra

The M-dwarf AD Leonis has been observed with the Reflection Grating Spectrometers and the European Photon Imaging Camera aboard XMM-Newton and also with the Low Energy Transmission Grating Spectrometer aboard the Chandra X-ray Observatory. In the observation taken with XMM-Newton five large flares produced by AD Leo were identified and only one in the observation taken with Chandra. A quiescent level to the lightcurves is difficult to define, since several smaller flares mutually overlap each other. However, we defined a quasi-steady state outside of obvious flares or flare decays. The spectra from the flare state and the quasi-steady state are analysed separately. From these spectra the temperature structure was derived with a multi-temperature model and with a differential emission measure model. The multi-temperature model was also used to determine the relative abundances of C, N, O, Ne, Mg, Si, S, and Fe. He-like ions, such as O VII and Ne IX, produce line triplets which are used to determine or constrain the electron temperature and electron density of the corresponding ion. During the flare state a higher emission measure at the hottest temperature is found for both XMM-Newton and Chandra observations. The derived abundances suggest the presence of an inverse First Ionization Potential effect in the corona of AD Leo.

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.

On coronal structures and their variability in active stars: The case of Capella observed with Chandra/LETGS

In this paper we present a detailed analysis of two X-ray spectra of Capella, taken eleven months apart with the Low Energy Transmission Grating Spectrometer (LETGS) of the Chandra Observatory. We have studied variability of the coronal emission over dierent time scales, both in the whole X-ray band and in narrow temperature ranges identified by lines. The comparison of the two observations shows that the whole coronal emission of Capella in March 2000 was 3% higher than in February 2001; there also appears to be a tendency, albeit a marginal one, for the hottest lines to show the largest changes between the two observations. A detailed search for short-term variability (on time scales ranging from 10 2 to 10 4 s) in the emission of individual lines shows that in all cases the emission is compatible with a constant source; the firm upper limits of 5% 10% to the source variability on short time scales suggests that the intense X-ray emission is due to stable coronal structures and not to flaring activity. We have also determined the coronal thermal structure, as described with the emission measure distribution vs. temperature and with the help of plasma density, derived from the analysis of the O vii ,N eix ,M gxi and Sixiii He-like ion triplets. The emission measure distribution, em(T ), and the element abundances, have been reconstructed with the Markov-Chain Monte Carlo method by Kashyap & Drake; the em(T ) presents a previously known sharp peak around log T = 6:8 6:9, but we have also found evidence of a small amount of plasma at T > 10 7 K. With the help of the em(T ) and the density values we have estimated the pressure and volume of the emitting plasma at dierent temperatures, and we have derived information about the structure of individual loops and about the population of loops having dierent maximum temperatures. Our results indicate that loops with higher maximum temperature have higher pressure and smaller volume than lower temperature loops.

Separating the X‐Ray Emissions of UV Ceti A and B withChandra

We present the first spatially separated X-ray observation of the active UV Ceti binary with Chandra, providing important new information on the relative X-ray activity levels of its nearly identical dM5.5e components. The two components had similar X-ray luminosities during low-level periods, and emission measure loci derived from Low Energy Transmission Grating spectra reveal similar coronal temperatures (3-6 MK). However, the B component showed a higher degree of variability, resulting in a higher average X-ray luminosity than that of UV Cet A. We discuss possible causes of this enhanced activity in UV Cet B.

High‐Resolution Grating Spectroscopy of GRB 020405 with theChandraLow Energy Transmission Grating Spectrometer

We present high-resolution X-ray spectroscopy of GRB 020405 obtained with the Low Energy Transmission Grating Spectrometer (LETGS) on board the Chandra X-Ray Observatory starting 1.68 days after the burst. The spectrum appears featureless, with no evidence for emission lines, absorption edges, or narrow radiative recombination continua. The continuum can be fitted by a power law of photon index Γ = 1.72 ± 0.21 and temporal decay index α = 1.87 ± 1.10, with a marginally significant excess column density of cold gas NH = (4.7 ± 3.7) × 1021 cm-2 at the redshift of the host galaxy. The absence of iron lines indicates that the density of surrounding material was unlikely to be very dense (n 5 × 1012 cm-3) at the time of the Chandra observation. In the case of recombination following photoionization in an optically thin medium, most ionic species would be completely stripped at lower gas densities than this. In the case of a power-law spectrum reflecting off a cold, opaque medium of low density, negligible emission features would be produced. Alternatively to these possible explanations for the lack of emission features, any X-ray line emission taking place in a dense medium in a nearby reprocessor scenario might have been overwhelmed by the bright afterglow continuum. Although the absence of discrete features does not unambiguously test for a connection between GRB 020405 and nucleosynthesis, it emphasizes the need for high-resolution X-ray spectroscopy to determine the exact emission mechanism responsible for the reported discrete lines in other gamma-ray burst afterglows.

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.

X‐Ray Spectroscopy of the Accreting Millisecond Pulsar XTE J0929−314 in Outburst

We present the high-resolution spectrum of the accretion-powered millisecond pulsar XTE J0929-314 during its 2002 outburst, measured using the Low Energy Transmission Grating Spectrometer on board the Chandra X-Ray Observatory. The 1.5-25.3 Å (0.5-8.3 keV) Chandra spectrum is well fitted by a power-law+blackbody model with photon index Γ = 1.55 ± 0.03, blackbody temperature kTbb = 0.65 ± 0.03 keV, and blackbody normalization Rbb,km/d10 kpc = 7.6 ± 0.8. No emission or absorption features are found in the high-resolution spectrum, with a 3 σ equivalent width upper limit of less than 0.007 Å at 1.5 Å and less than 0.12 Å at 24 Å. The neutral absorption edge depths are consistent with the estimated interstellar absorption along the line of sight to the source. We found no orbital modulation of the 2-10 keV X-ray flux, to a 3 σ upper limit of 1.1%, which implies an upper limit on the binary inclination angle of i ≲ 85° for a Roche lobe-filling companion. We also present the broadband spectrum measured over the course of the outburst by the Rossi X-Ray Timing Explorer (RXTE). The RXTE spectrum of XTE J0929-314 is also well fitted with a power-law+blackbody model, with average values of Γ = 1.76 ± 0.03, kTbb = 0.66 ± 0.06 keV, and Rbb,km/d10 kpc = 5.9 ± 1.3 in the 2-50 keV energy range. The blackbody flux remained constant over the course of the outburst, while the power-law flux was strongly correlated to the (decreasing) flux of the source. We find that the difference in power-law photon indices measured from Chandra and RXTE spectra can be explained by a change in the power-law photon index at low energies.

The soft X-ray spectrum from NGC 1068 observed with LETGS on Chandra

Using the combined spectral and spatial resolving power of the Low Energy Transmission Grating (LETGS) on board Chandra, we obtain separate spectra from the bright central source of NGC 1068 (Primary region), and from a fainter bright spot 4 to the NE (Secondary region). Both spectra are dominated by line emission from H- and He-like ions of C through S, and from Fe L-shell ions, but also include narrow radiative recombination continua, indicating that most of the soft X-ray emission arises in low-temperature (kT few eV) photoionized plasma. We confirm the conclusions of Kinkhabwala et al. (2002), based on XMM-Newton RGS observations, that the entire nuclear spectrum can be explained by recombination/radiative cascade following photoionization, and radiative decay following photoexcitation, with no evidence for hot, collisionally ionized plasma. In addition, this model also provides an excellent fit to the spectrum of the Secondary region, albeit with radial column densities a factor of three lower, as would be expected given its distance from the source of the ionizing continuum. The remarkable overlap and kinematical agreement of the optical and X-ray line emission, coupled with the need for a distribution of ionization parameter to explain the X-ray spectra, collectively imply the presence of a distribution of densities (over a few orders of magnitude) at each radius in the ionization cone. Relative abundances of all elements are consistent with Solar abundance, except for N, which is 2-3 times Solar. The long wavelength spectrum beyond 30 A is rich of L-shell transitions of Mg, Si, S, and Ar, and M-shell transitions of Fe. The velocity dispersion decreases with increasing ionization parameter, as deduced from these long wavelength lines and the Fe-L shell lines.