High Energy Transmission Grating Observation Research Articles

Gravitational Redshift Detection from the Magnetic White Dwarf Harbored in RX J1712.6–2414

Gravitational redshift is a fundamental parameter that allows us to determine the mass-to-radius ratio of compact stellar objects, such as black holes, neutron stars, and white dwarfs (WDs). In the X-ray spectra of the close binary system, RX J1712.6−2414, obtained from the Chandra High Energy Transmission Grating observation, we detected significant redshifts for characteristic X-rays emitted from hydrogen-like magnesium, silicon (ΔE/E rest ∼ 7 × 10−4), and sulfur (ΔE/E rest ∼ 15 × 10−4) ions, which are over the instrumental absolute energy accuracy (ΔE/E rest ∼ 3.3 × 10−4). Considering some possible factors, such as Doppler shifts associated with the plasma flow, systemic velocity, and optical depth, we concluded that the major contributor to the observed redshift is the gravitational redshift of the WD harbored in the binary system, which is the first gravitational redshift detection from a magnetic WD. Moreover, the gravitational redshift provides us with a new method of WD mass measurement by invoking the plasma-flow theory with strong magnetic fields in close binaries. Regardless of large uncertainty, our new method estimated the WD mass to be M WD > 0.9 M ⊙.

High-cadence Dispersed Spectral Analysis of Supernova Remnant 1987A

Abstract We present an analysis of the dispersed spectral data from 11 epochs (2011 March–2018 September) of supernova remnant (SNR) 1987A observations performed with Chandra. These observations were performed with the High Energy Transmission Grating (HETG) as part of our ongoing Chandra monitoring campaign of SNR 1987A, whose first-order dispersed spectrum provides a significantly greater energy resolution than the previously published zeroth-order spectrum. Our data sets with moderate exposure times of ∼50–70 ks per epoch cover the time period between deep Chandra HETG observations (with individual exposures >∼200 ks) taken in 2011 March and 2018 March. These data have a much higher cadence than the widely spaced deep high-resolution spectra, at the expense of total exposure time. While statistical uncertainties are large due to low photon count statistics in the observed first-order spectra, we find that spectral model parameters are generally in line with the shock wave propagating into the medium beyond the dense inner ring, as suggested by Frank et al. (2016). As the reverse shock begins ionizing the heavier elements of the supernova ejecta interior to the equatorial ring, spectral fit parameters are expected to change as the chemical makeup and physical properties of the shocked gas evolve. Based on our broadband spectral model fits, we find that abundance values appear to be constant in this time period. While our results are somewhat limited due to photon statistics, we demonstrate the utility of the dispersed HETG spectral analysis that can be performed with our regular Chandra monitoring observations of SNR 1987A.

WAVELENGTH MEASUREMENTS OF K TRANSITIONS OF OXYGEN, NEON, AND MAGNESIUM WITH X-RAY ABSORPTION LINES

Accurate atomic transition data are important in many astronomical research areas especially in line spectroscopy study. Whereas transition data of He-like and H-like ions (i.e., ions at high-charge states) are accurately calculated, that of K transitions of neutral or low-ionized metal elements are still very uncertain. Spectroscopy of absorption lines produced in the interstellar medium (ISM) has been proven to be an effective way to measure the central wavelengths of these atomic transitions. In this work we analyze 36 Chandra High Energy Transmission Grating observations and search for and measure the ISM absorption lines along sight lines of 11 low-mass X-ray binaries. We correct the Galactic rotation velocity to the rest frame for every observation and then use two different methods to merge all the corrected spectra to a co-added spectrum. However the co-added spectra obtained by these methods exhibit biases, either to the observations of high counts or high signal-to-noise ratios of the lines. We make Bayesian analysis to several significantly detected lines to obtain the systematic uncertainty and the bias correction of other lines. Compared to previous studies (e.g., Yao et al. 2009), our results improve the accuracy of wavelengths by a factor from two to five and significantly reduce the systematic uncertainties and biases. Several weak transitions (e.g., 1s--2p of Mg IV and Mg V; 1s--3p of Mg III and Mg V) are also detected for the first time, albeit with low significance; future observations with improved accuracy are required to confirm these detections.

EVOLUTION AND HYDRODYNAMICS OF THE VERY BROAD X-RAY LINE EMISSION IN SN 1987A

Observations of SN 1987A by the Chandra High Energy Transmission Grating (HETG) in 1999 and the XMM-Newton Reflection Grating Spectrometer (RGS) in 2003 show very broad (v-b) lines with a full-width at half-maximum (FWHM) of order 10^4 kms; at these times the blast wave was primarily interacting with the HII region around the progenitor. Since then, the X-ray emission has been increasingly dominated by narrower components as the blast wave encounters dense equatorial ring (ER) material. Even so, continuing v-b emission is seen in the grating spectra suggesting that interaction with HII region material is on-going. Based on the deep HETG 2007 and 2011 data sets, and confirmed by RGS and other HETG observations, the v-b component has a width of 9300 +/-2000 kms FWHM and contributes of order 20% of the current 0.5--2 keV flux. Guided by this result, SN 1987A's X-ray spectra are modeled as the weighted sum of the non-equilibrium-ionization (NEI) emission from two simple 1D hydrodynamic simulations, this "2x1D" model reproduces the observed radii, light curves, and spectra with a minimum of free parameters. The interaction with the HII region (rho_init \sim 130 amu/cc, +/- 15 degrees opening angle) produces the very-broad emission lines and most of the 3-10 keV flux. Our ER hydrodynamics, admittedly a crude approximation to the multi-D reality, gives ER densities of order 10^4 amu/cc, requires dense clumps (x5.5 density enhancement in \sim 30% of the volume), and it predicts that the 0.5-2 keV flux will drop at a rate of \sim 17% per year once no new dense ER material is being shocked.

PHOTOIONIZED FEATURES IN THE X-RAY SPECTRUM OF EX HYDRAE

We present the first results from a long (496 ks) Chandra High Energy Transmission Grating observation of the intermediate polar EX Hydrae. In addition to the narrow emission lines from the cooling post-shock gas, for the first time we have detected a broad component in some of the X-ray emission lines, namely O VIII 18.97, Mg XII 8.42, Si XIV 6.18, and Fe XVII 16.78. The broad and narrow components have widths of ~ 1600 km s^-1 and ~ 150 km s^-1, respectively. We propose a scenario where the broad component is formed in the pre-shock accretion flow, photoionized by radiation from the post-shock flow. Because the photoionized region has to be close to the radiation source in order to produce strong photoionized emission lines from ions like O VIII, Fe XVII, Mg XII, and Si XIV, our photoionization model constrains the height of the standing shock above the white dwarf surface. Thus, the X-ray spectrum from EX Hya manifests features of both magnetic and non-magnetic cataclysmic variables.

A broad-line region origin for the iron Kα line in NGC 7213

Abstract The X-ray spectrum of NGC 7213 is known to present no evidence for Compton reflection, a unique result among bright Seyfert 1s. The observed neutral iron Kα line, therefore, cannot be associated with a Compton-thick material, like the disc or the torus, but is due to Compton-thin gas, with the broad-line region (BLR) as the most likely candidate. To check this hypothesis, a long Chandra High-Energy Transmission Grating observation, together with a quasi-simultaneous optical spectroscopic observation at the ESO NTT EMMI were performed. We found that the iron line is resolved with a full width at half-maximum (FWHM) = 2400+1100−600 km s−1, in perfect agreement with the value measured for the broad component of the Hα, 2640+110−90 km s−1. Therefore, NGC 7213 is the only Seyfert 1 galaxy whose iron Kα line is unambiguously produced in the BLR. We also confirmed the presence of two ionized iron lines and studied them in greater detail than before. The resonant line is the dominant component in the Fe xxv triplet, therefore suggesting an origin in collisionally ionized gas. If this is the case, the blueshift of around 1000 km s−1 of the two ionized iron lines could be the first measure of the velocity of a starburst wind from its X-ray emission.

X‐Atlas: An Online Archive ofChandra's Stellar High‐Energy Transmission Grating Observations

The high-resolution X-ray spectroscopy made possible by the 1999 deployment of the Chandra X-ray Observatory has revolutionized our understanding of stellar X-ray emission. Many puzzles remain, however, particularly regarding the mechanisms of X-ray emission from OB stars. Although numerous individual stars have been observed in high resolution, realizing the full scientific potential of these observations will necessitate studying the high-resolution Chandra data set as a whole. To facilitate the rapid comparison and characterization of stellar spectra, we have compiled a uniformly processed database of all stars observed with the Chandra High-Energy Transmission Grating (HETG). This database, known as X-Atlas, is accessible through a Web interface with searching, data retrieval, and interactive plotting capabilities. For each target, X-Atlas also features predictions of the low-resolution ACIS spectra convolved from the HETG data for comparison with stellar sources in archival ACIS images. Preliminary analyses of the hardness ratios, quantiles, and spectral fits derived from the predicted ACIS spectra reveal systematic differences between the high- and low-mass stars in the atlas and offer evidence for at least two distinct classes of high-mass stars. A high degree of X-ray variability is also seen in both high- and low-mass stars, including Capella, long thought to exhibit minimal variability. X-Atlas contains over 130 observations of approximately 25 high-mass stars and 40 low-mass stars and will be updated as additional stellar HETG observations become public. The atlas has recently expanded to nonstellar point sources, and Low-Energy Transmission Grating (LETG) observations are currently being added as well.

Chandra X-ray observations of the young stellar cluster NGC 6193 in the Ara OB1 association

A 90-ks Chandra High Energy Transmission Grating observation of the young stellar cluster NGC 6193 in the southern Ara OB1 association detected 43 X-ray sources in a 2 × 2 arcmin2 core region centred on the massive O stars HD 150135 (O6.5 V) and HD 150136 (O3 + O6V). The cluster is dominated by exceptionally bright X-ray emission from the two O stars, which are separated by only 10 arcsec. The X-ray luminosity of HD 150136 is log LX= 33.39 (erg s−1), making it one of the most luminous O-star X-ray sources known. All of the fainter X-ray sources in the core region have near-infrared (near-IR) counterparts, but existing JHK photometry provides little evidence for near-IR excesses. These core sources have typical mean photon energies 〈E〉 ≈ 2 keV and about one-third are variable. It is likely that some are young low-mass stars in the cluster, but cluster membership remains to be determined. Grating spectra show that the X-ray properties of HD 150135 and HD 150136 are similar, but not identical. Both have moderately broadened unshifted emission lines and their emission is dominated by cool plasma at kT ≈ 0.3 keV, pointing to a wind-shock origin. However, the emission of HD 150136 is slightly hotter and four times more luminous than its optical twin HD 150135. We discuss the possibility that a radiative colliding wind shock contributes to the prodigious X-ray output of the short-period (2.66 d) spectroscopic binary HD 150136. A surprising result is that the X-ray emission of HD 150136 is slowly variable on a time-scale of <1 d. The origin of the variability is not yet known but the observed behaviour suggests that it is an occultation effect.

X-Ray Doppler Imaging of 44[CLC]i[/CLC] Bootis with [ITAL]Chandra[/ITAL

Chandra High-Energy Transmission Grating observations of the bright eclipsing contact binary 44i Bootis show X-ray line profiles that are Doppler-shifted by orbital motions. The X-ray emission spectrum contains a multitude of lines superposed on a weak continuum, with strong lines of O VIII, Ne X, Fe XVII, and Mg XII. The profiles of these lines from the total observed spectrum show Doppler-broadened widths of ~550 km s-1. Line centroids vary with orbital phase, indicating velocity changes of greater than 180 km s-1. The first-order light curve shows significant variability but no clear evidence for either primary or secondary eclipses. Flares are observed for all spectral ranges; additionally, the light curve constructed near the peak of the emission measure distribution [Te = × 106 K] shows quiescent variability as well as flares. The phase dependences of line profiles and light curves together imply that at least half of the emission is localized at high latitude. A simple model with two regions on the primary star at relatively high latitude reproduces the observed line profile shifts and quiescent light curve. These first clear X-ray Doppler shifts of stellar coronal material illustrate the power of Chandra.

ITAL]Chandra[/ITAL] Detection of Doppler-shifted X-Ray Line Profiles from the Wind of ζ Puppis (O4 [CLC]f[/CLC

We report on a 67 ks High-Energy Transmission Grating observation of the optically brightest early O star z Puppis (O4 f). Many resolved X-ray lines are seen in the spectra over a wavelength range of 5–25 A u . Chandra has sufficient spectral resolution to study the velocity structure of isolated X-ray line profiles and to distinguish the individual forbidden, intercombination, and resonance (fir) emission lines in several He-like ions, even where the individual components are strongly Doppler-broadened. In contrast to X-ray line profiles in other hot stars, z Pup shows blueshifted and skewed line profiles, providing the clearest and most direct evidence that the Xray sources are embedded in the stellar wind. The broader the line, the greater the blueward centroid shift tends to be. The N vii line at 24.78 A u is a special case, showing a flat-topped profile. This indicates that it is formed in regions beyond most of the wind attenuation. The sensitivity of the He-like ion fir lines to a strong UV radiation field is used to derive the radial distances at which lines of S xv ,S ixiii ,M gxi ,N eix, and O vii originate. The formation radii correspond well with a continuum optical depth of unity at the wavelength of each line complex, indicating that the X-ray line emission is distributed throughout the stellar wind. However, the S xv emission lines form deeper in the wind than expected from standard wind-shock models. Subject headings: line: profiles — stars: early-type — stars: individual ( z Puppis) — stars: mass loss — stars: winds, outflows — X-rays: stars

The [ITAL]Chandra[/ITAL] High-Energy Transmission Grating Observation of an X-Ray Ionization Cone in Markarian 3

We present a preliminary analysis of the first high-resolution X-ray spectrum of a Seyfert 2 galaxy, Mkn 3, obtained with the High Energy Transmission Grating Spectrometer onboard the Chandra X-ray Observatory. The high-energy spectrum (lambda < 4 Ang) is dominated by reflection of the AGN continuum radiation in a cold optically thick medium and contains bright K-alpha fluorescent lines from iron and silicon, as well as weak, blended lines from sulfur and magnesium. The soft X-ray emission (4 < lambda < 23 Ang) is spatially extended along the [O III] ionization cone and shows discrete signatures of emission following recombination and photoexcitation produced in a warm photoionized region. The measured iron L line fluxes indicate that emission from collisionally ionized plasma is almost completely negligible, and does not contribute significantly to the total energy budget of the X-ray emission. We find that significant fractions of the H- and He-like resonance lines, as well as the observed iron L lines are produced through re-emission from the warm absorbing medium observed in Seyfert 1 galaxies. Its X-ray spectral properties are qualitatively consistent with those of a typical Seyfert 1 galaxy viewed at a different orientation, and provide further convincing evidence for the existence of an obscured Seyfert 1 nucleus in Mkn 3.