Chandra High-Energy Transmission Grating Spectrometer Research Articles

An Ejecta Kinematics Study of Kepler’s Supernova Remnant with High-resolution Chandra HETG Spectroscopy

Abstract We report our measurements of the bulk radial velocity from a sample of small, metal-rich ejecta knots in Kepler’s supernova remnant (SNR). We measure the Doppler shift of the He-like Si Kα line-center energy in the spectra of these knots based on our Chandra High-Energy Transmission Grating Spectrometer observation to estimate their radial velocities. We estimate high radial velocities of up to ∼8000 km s−1 for some of these ejecta knots. We also measure proper motions for our sample based on the archival Chandra Advanced CCD Imaging Spectrometer data taken in 2000, 2006, and 2014. Our measured radial velocities and proper motions indicate that some of these ejecta knots are almost freely expanding after ∼400 yr since the explosion. The fastest moving knots show proper motions of up to ∼0.″2 per year. Assuming that these high-velocity ejecta knots are traveling ahead of the forward shock of the SNR, we estimate the distance to Kepler’s SNR d ∼ 4.4–7.5 kpc. We find that the ejecta knots in our sample have an average space velocity of v s ∼ 4600 km s−1 (at a distance of 6 kpc). We note that 8 of the 15 ejecta knots from our sample show a statistically significant (at the 90% confidence level) redshifted spectrum, compared to only two with a blueshifted spectrum. This may suggest an asymmetry in the ejecta distribution in Kepler’s SNR along the line of sight; however, a larger sample size is required to confirm this result.

AstroSat and Chandra View of the High Soft State of 4U 1630–47 (4U 1630–472): Evidence of the Disk Wind and a Rapidly Spinning Black Hole

Abstract We present the X-ray spectral and timing analysis of the transient black hole X-ray binary 4U 1630–47, observed with the AstroSat, Chandra, and MAXI space missions during its soft X-ray outburst in 2016. The outburst, from the rising phase until the peak, is detected neither in hard X-rays (15–50 keV) by the Swift/BAT nor in radio. Such nondetection, along with the source behavior in the hardness–intensity and color–color diagrams obtained using MAXI data, confirms that both Chandra and AstroSat observations were performed during the HS spectral state. The High Energy Grating (HEG) spectrum from the Chandra High-Energy Transmission Grating Spectrometer shows two strong, moderately blueshifted absorption lines at keV and keV, which are produced by Fe xxv and Fe xxvi in a low-velocity ionized disk wind. The corresponding outflow velocity is determined to be 366 ± 56 km s−1. Separate spectral fits of Chandra/HEG, AstroSat/SXT+LAXPC, and Chandra/HEG+AstroSat/SXT+LAXPC data show that the broadband continuum can be well described with a relativistic disk blackbody model, with a disk flux fraction of ∼0.97. Based on the best-fit continuum spectral modeling of Chandra, AstroSat, and Chandra+AstroSat joint spectra and using the Markov chain Monte Carlo simulations, we constrain the spectral hardening factor at and the dimensionless black hole spin parameter at 0.92 ± 0.04 within the 99.7% confidence interval. Our conclusion of a rapidly spinning black hole in 4U 1630–47 using the continuum spectrum method is in agreement with a previous finding applying the reflection spectral fitting method.

Physical parameter estimation with MCMC from observations of Vela X-1

We present a parameter estimate for continua, and He-like triplets of the high resolution X-ray spectra with a Bayesian inference and a Markov Chain Monte Carlo (MCMC) tool. The method is applied for Vela X-1 with three different orbital phases ($\unicode[STIX]{x1D719}$), Eclipse, $\unicode[STIX]{x1D719}=0.25$, and $\unicode[STIX]{x1D719}=0.5$, which are adopted from the Chandra High-Energy Transmission Grating Spectrometer (HETGS). A parameterized two-component power-law model [Sako et al., Astrophys. J. 525, 921 (1999)] and a multi-Gaussian model are applied to model these continua and He-like triplets, respectively. A uniform distribution over each parameter is used as the prior belief. Posterior probability distribution functions of parameters and the covariances among them are explored by using the MCMC method. The main advantages are (i) all model-based parameters are set to be free instead of artificially fixing some of the parameters during the data-model fitting; (ii) the contributions from satellite lines are considered; (iii) backgrounds are treated as a correction to the observation errors; and (iv) the confidence interval of each parameter is given. The fitted results show that the column density of scatter component ($N_{\text{H}}^{\text{scat}}$) varies from phase to phase, which imply a non-spherical structure of the stellar wind in Vela X-1. Moreover, the wind velocities derived from main lines of each set of He-like triplets show better self-consistency than those in previous publications, which could provide a reliable approach for the diagnostics of photoionized plasma in astrophysical objects and the laboratory.

Excess Galactic Molecular Absorption Toward the Radio Galaxy 3C 111

Abstract We show the combined spectral analysis of Chandra high-energy transmission grating and XMM-Newton reflection-grating spectrometer observations of the broad-line radio galaxy 3C 111. This source is known to show excess neutral absorption with respect to the one estimated from 21 cm radio surveys of atomic H i in the Galaxy. However, previous works were not able to constrain the origin of such an absorber as local to our Milky Way or intrinsic to the source (z = 0.0485). The high signal-to-noise grating spectra allow us to constrain the excess absorption as being due to intervening gas in the Milky Way, and we estimate a time-averaged total column density of cm−2, a factor of two higher than the tabulated H i value. We recommend using the total average Galactic column density estimated here when studying 3C 111. The origin of the extra Galactic absorption of cm−2 is likely due to molecular gas associated with the Taurus molecular cloud complex toward 3C 111, which is our nearest star-forming region. We also detect a weak (EW = 16 ± 10 eV) and narrow (FWMH < 5500 km s−1, consistent with optical Hα) Fe Kα emission line at E = 6.4 keV, likely from the torus in the central regions of 3C 111, and we place an upper limit on the column density of a possible intrinsic warm absorber of N H < 2.5 × 1020 cm−2. These complexities make 3C 111 a very promising object for studying both the intrinsic properties of this active radio galaxy and the Galactic interstellar medium, if used as a background source.

A SUPER-EDDINGTON, COMPTON-THICK WIND IN GRO J1655–40?

ABSTRACT During its 2005 outburst, GRO J1655–40 was observed at high spectral resolution with the Chandra High-Energy Transmission Grating Spectrometer, revealing a spectrum rich with blueshifted absorption lines indicative of an accretion disk wind—apparently too hot, too dense, and too close to the black hole to be driven by radiation pressure or thermal pressure (Miller et al.). However, this exotic wind represents just one piece of the puzzle in this outburst, as its presence coincides with an extremely soft and curved X-ray continuum spectrum, remarkable X-ray variability (Uttley & Klein-Wolt), and a bright, unexpected optical/infrared blackbody component that varies on the orbital period. Focusing on the X-ray continuum and the optical/infrared/UV spectral energy distribution, we argue that the unusual features of this “hypersoft state” are natural consequences of a super-Eddington Compton-thick wind from the disk: the optical/infrared blackbody represents the cool photosphere of a dense, extended outflow, while the X-ray emission is explained as Compton scattering by the relatively cool, optically thick wind. This wind obscures the intrinsic luminosity of the inner disk, which we suggest may have been at or above the Eddington limit.

A TEST OF THE NATURE OF THE FE K LINE IN THE NEUTRON STAR LOW-MASS X-RAY BINARY SERPENS X-1

ABSTRACT Broad Fe K emission lines have been widely observed in the X-ray spectra of black hole systems as well as in neutron star systems. The intrinsically narrow Fe K fluorescent line is generally believed to be part of the reflection spectrum originating in an illuminated accretion disk which is broadened by strong relativistic effects. However, the nature of the lines in neutron star low-mass X-ray binaries (LMXBs) has been a matter of debate. We therefore obtained the longest, high-resolution X-ray spectrum of a neutron star LMXB to date with a 300 ks Chandra High Energy Transmission Grating Spectrometer (HETGS) observation of Serpens X-1. The observation was taken under the “continuous clocking” mode, and thus was free of photon pile-up effects. We carry out a systematic analysis and find that the blurred reflection model fits the Fe line of Serpens X-1 significantly better than a broad Gaussian component does, implying that the relativistic reflection scenario is much preferred. Chandra HETGS also provides a highest spectral resolution view of the Fe K region and we find no strong evidence for additional narrow lines.

LONG-TERM X-RAY STABILITY AND ULTRAVIOLET VARIABILITY OF THE IONIZED ABSORPTION IN NGC 3783

We present the results of recent Chandra High-Energy Transmission Grating Spectrometer and Hubble Space Telescope Cosmic Origins Spectrograph observations of the nearby Seyfert 1 galaxy NGC 3783 which shows a strong, non-varying X-ray warm absorber and physically related and kinematically varying UV absorption. We compare our new observations to high-resolution, high signal-to-noise archival data from 2001, allowing a unique investigation into the long-term variations of the absorption over a 12 yr period. We find no statistically significant changes in the physical properties of the X-ray absorber, but there is a significant drop of ~40% in the UV and X-ray flux, and a significant flattening of the underlying X-ray power-law slope. Large kinematic changes are seen in the UV absorbers, possibly due to radial deceleration of the material. Similar behavior is not observed in the X-ray data, likely due to its lower velocity resolution, which shows an outflow velocity of v ~ -655 km/s in both epochs. The narrow iron K-alpha emission line at 6.4 keV shows no variation between epochs, and its measured width places the material producing the line at a radial distance of ~0.03 pc from the central black hole.

A LINK BETWEEN X-RAY EMISSION LINES AND RADIO JETS IN 4U 1630-47?

Recently, Diaz Trigo et al. reported an XMM-Newton detection of relativistically Doppler-shifted emission lines associated with steep-spectrum radio emission in the stellar-mass black hole candidate 4U 1630-47 during its 2012 outburst. They interpreted these lines as indicative of a baryonic jet launched by the accretion disk. Here we present a search for the same lines earlier in the same outburst using high-resolution X-ray spectra from the Chandra High-Energy Transmission Grating Spectrometer. While our observations (eight months prior to the XMM-Newton campaign) also coincide with detections of steep spectrum radio emission by the Australia Telescope Compact Array, we find no evidence for any relativistic X-ray emission lines. Indeed, despite $\sim5\times$ brighter radio emission, our Chandra spectra allow us to place an upper limit on the flux in the blueshifted Fe XXVI line that is $\gtrsim20\times$ weaker than the line observed by Diaz Trigo et al. We explore several scenarios that could explain our differing results, including variations in the geometry of the jet or a mass-loading process or jet baryon content that evolves with the accretion state of the black hole. We also consider the possibility that the radio emission arises in an interaction between a jet and the nearby ISM, in which case the X-ray emission lines might be unrelated to the radio emission.

THE ACCRETION DISK CORONA AND DISK ATMOSPHERE OF 4U 1624–490 AS VIEWED BY THECHANDRA-HIGH ENERGY TRANSMISSION GRATING SPECTROMETER

We present a detailed spectral study (photoionization modeling and variability) of the 'Big Dipper' 4U 1624-490 based on a Chandra-High Energy Transmission Grating Spectrometer (HETGS) observation over the {approx}76 ks binary orbit of 4U 1624-490 . While the continuum spectrum can be modeled using a blackbody plus power law, a slightly better fit is obtained using a single {gamma} = 2.25 power-law partially (71%) covered by a local absorber of column density N{sub H,Local}=8.1{sub -0.6}{sup +0.7}x10{sup 22} cm{sup -2}. The data show a possible quasi-sinusoidal modulation with period 43{sup +13} {sub -9} ks that might be due to changes in local obscuration. Photoionization modeling with the XSTAR code and variability studies of the observed strong Fe XXV and Fe XXVI absorption lines point to a two-temperature plasma for their origin: a highly ionized component of ionization parameter {xi}{sub hot} {approx} 10{sup 4.3} erg cm s{sup -1} (T {approx} 3.0 x 10{sup 6} K) associated with an extended accretion disk corona of radius R {approx} 3 x 10{sup 10} cm, and a less-ionized more variable component of {xi} {approx} 10{sup 3.4} erg cm s{sup -1} (T {approx} 1.0 x 10{sup 6} K) and {xi}{approx}10{sup 3.1} erg cm s{sup -1} (T {approx}more » 0.9 x 10{sup 6} K) coincident with the accretion disk rim. We use this, with the observed Fe XXV and Fe XXVI absorption line variations (in wavelength, strength, and width) to construct a viewing geometry that is mapped to changes in plasma conditions over the 4U 1624-490 orbital period.« less

SPECTROSCOPIC SIGNATURES OF THE SUPERORBITAL PERIOD IN THE NEUTRON STAR BINARY LMC X-4

We present the first high-resolution X-ray study of emission line variability with superorbital phase in the neutron star binary LMC X-4. Our analysis provides new evidence from X-ray spectroscopy confirming accretion disk precession as the origin of the superorbital period. The spectra, obtained with the Chandra High-Energy Transmission Grating Spectrometer (HETGS) and the XMM-Newton Reflection Grating Spectrometer (RGS), contain a number of emission features, including lines from hydrogen-like and helium-like species of N, O, Ne, and Fe, a narrow O VII RRC, and fluorescent emission from cold Fe. We use the narrow RRC and the He-alpha triplets to constrain the temperature and density of the (photoionized) gas. By comparing spectra from different superorbital phases, we attempt to isolate the contributions to line emission from the accretion disk and the stellar wind. There is also evidence for highly ionized iron redshifted and blueshifted by ~25,000 km/s. We argue that this emission originates in the inner accretion disk, and show that the emission line properties in LMC X-4 are natural consequences of accretion disk precession.

The Nature of the Hard X‐Ray–Emitting Symbiotic Star RT Cru

We describe Chandra High-Energy Transmission Grating Spectrometer observations of RT Cru, the first of a new sub-class of symbiotic stars that appear to contain white dwarfs (WDs) capable of producing hard X-ray emission out to greater than 50 keV. The production of such hard X-ray emission from the objects in this sub-class (which also includes CD -57 3057, T CrB, and CH Cyg) challenges our understanding of accreting WDs. We find that the 0.3 -- 8.0 keV X-ray spectrum of RT Cru emanates from an isobaric cooling flow, as in the optically thin accretion-disk boundary layers of some dwarf novae. The parameters of the spectral fit confirm that the compact accretor is a WD, and they are consistent with the WD being massive. We detect rapid, stochastic variability from the X-ray emission below 4 keV. The combination of flickering variability and a cooling-flow spectrum indicates that RT Cru is likely powered by accretion through a disk. Whereas the cataclysmic variable stars with the hardest X-ray emission are typically magnetic accretors with X-ray flux modulated at the WD spin period, we find that the X-ray emission from RT Cru is not pulsed. RT Cru therefore shows no evidence for magnetically channeled accretion, consistent with our interpretation that the Chandra spectrum arises from an accretion-disk boundary layer.

Evidence for Jet Collimation in SS 433 with the Chandra HETGS

High-resolution X-ray spectra of SS 433 obtained at an anti-eclipse, during an eclipse, and after a binary egress with the Chandra High Energy Transmission Grating Spectrometer (HETGS) were studied. Many Doppler-shifted X-ray emission lines from highly ionized elements were detected. All of the Doppler- shifted emission lines are found to be broader than the instrumented resolution. Neither the measured line widths nor their dependence on the atomic number can be explained by thermal broadening alone. Alternative explanations of the observed line widths are discussed, including in particular a progressive jet collimation along its axis.

Chandra and RXTE spectroscopy of the accreting msec pulsar IGR J00291+5934

We report on an observation of the recently discovered accreting millisecond X-ray pulsar IGR J00291+5934 performed with the RXTE-Proportional Counter Array (PCA) and Chandra-High Energy Transmission Grating Spectrometer (HETGS). The RXTE data are from a twoweek follow-up of the source, while the Chandra observation took place around the end of the follow-up, about 12 days after the discovery of the source, when the source flux had decreased already by a factor of ten. The analysis of the Chandra data allowed us to extract the most precise X-ray position of IGR J00291+5934, RA = 00h 29m 03.08s, and Dec =+59◦ 34 19.2 (0.6 error), compatible with the optical and radio ones. We find that the spectra of IGR J00291+5934 can be described by a combination of a thermal component and a power-law. Along the outburst detected by PCA, the power-law photon index showed no particular trend, while the thermal component (∼1 keV, interpreted as a hot spot on the neutron star surface) became weaker until non-detection. In the simultaneous observation of the weak Chandra /RXTE spectrum, there was no longer any indication of the ∼1 keV thermal component, while we detected a colder thermal component (∼0.4 keV) that we interpret as the emission from the cold disc. A hint of a 6.4 keV iron line was detected, together with an excess around 6.8 keV and absorption feature around 7.1 keV. The last two features have never been detected in the spectra of accretion-driven millisecond pulsars before and, if confirmed, would suggest the presence of an expanding hot corona with high outflow velocities.

Identification of an Extended Accretion Disk Corona in the Hercules X‐1 Low State: Moderate Optical Depth, Precise Density Determination, and Verification of CNO Abundances

We identify an accretion disk atmosphere and corona from the high resolution X-ray spectrum of Hercules X-1, and we determine its detailed physical properties. More than two dozen recombination emission lines (from Fe XXVI at 1.78 A to N VI at 29.08 A) and Fe K-alpha, K-beta fluorescence lines were detected in a 50 ks observation with the Chandra High-Energy Transmission Grating Spectrometer (HETGS). They allow us to measure the density, temperature, spatial distribution, elemental composition, and kinematics of the plasma. We exclude HZ Her as the source of the recombination emission. We compare accretion disk model atmospheres with the observed spectrum in order to constrain the stratification of density and ionization, disk atmosphere area, elemental composition, and energetics. The atmospheric spectrum observed during the low state is photoionized by the main-on X-ray continuum, indicating that the disk is observed edge-on during the low state. We infer the mean number of scatterings N of Ly-alpha and Ly-beta line photons from H-like ions. We derive N < 69 for O VIII Ly_alpha_1, which rules out the presence of a mechanism modeled by Sako (2003) to enhance N VII emission via a line overlap with O VIII. The line optical depth diagnostics are consistent with a flattened atmosphere. Our spectral analysis, the disk atmosphere model, and the presence of intense N VII and N VI lines (plus N V in the UV), confirm the over-abundance of nitrogen relative to other metals, which was shown to be indicative of CNO cycle processing in a massive progenitor.

Stellar Coronal Spectroscopy with the Chandra HETGS

Spectroscopy with the Chandra High Energy Transmission Grating Spectrometer (HETGS) provides details on X-ray emission and activity from young and cool stars through resolution of emission lines from a variety of ions. We are beginning to see trends in activity regarding abundances, emission measures, and variability. Here we contrast spectra of TV Crt, a weak-lined T Tauri star (WTT), with TW Hya, a Classical T Tauri star (CTT). TV Crt has a spectrum more like magnetic activity driven coronae, relative to the TW Hya spectrum, which we have interpreted as due to accretion-produced X-rays. We have also observed the long period system, IM Pegasi to search for rotational modulation, and to compare activity in a long period active binary to shorter period systems and to the pre-main sequence stars. We detected no rotational modulation, but did see long-duration flares.

Evidence for Jet Collimation in SS 433 with the Chandra HETGS

Abstract High-resolution X-ray spectra of SS 433 obtained after a binary egress with the Chandra High Energy Transmission Grating Spectrometer (HETGS) were studied. Many Doppler-shifted X-ray emission lines from highly ionized elements were detected. The initial temperature of the jets is estimated to be 20 keV. The lines are found to generally be broader than the instrumental resolution. The widths of the Fe xxv$\mathrm{K}\alpha$ and Si xiii$\mathrm{K}\alpha$ lines correspond to velocity dispersions of $2100^{+600}_{-340} \,\mathrm{km} \,\mathrm{s}^{-1}$ and $840^{+180}_{-150} \,\mathrm{km} \,\mathrm{s}^{-1}$ respectively, in terms of Gaussian sigma. Neither the measured line widths nor their dependence on the atomic number can be explained by thermal broadening alone. Alternative explanations of the observed line widths are discussed, including in particular a progressive jet collimation along its axis.

ChandraObservations of Two High‐Redshift Quasars

We report the first high-resolution X-ray spectra of two high-redshift quasars, S5 0836+710 and PKS 2149-306, obtained with the Chandra High-Energy Transmission Grating Spectrometer (HETGS). The primary goal of this observation is to use the high spectral resolving power of the HETGS to detect X-ray absorption produced by a hot intergalactic medium (IGM). The continuum of both quasars can be fitted by absorbed power laws. The power-law photon index (Γ) of S5 0836+710 is consistent with that found in previous observations with ROSAT and ASCA, while the power law of PKS 2149-306 is harder than found in a previous observation by ASCA. Excess continuum absorption above the Galactic value is found in S5 0836+710, as evidenced in ASCA and ROSAT observations. No significant emission or absorption feature is detected in either source at the ±3 σ level. Based on the detection limits we constrain the properties of possible emitters and absorbers. The upper limit of the equivalent width of Fe-K emission lines could be as low as ~10 eV. Absorbers with a column density higher than 8 × 1016 cm-2 for O VIII or 5 × 1016 cm-2 for Si XIV would have been detected. We propose a method to constrain the cosmological parameters (namely, Ω0 and Ωb) via the X-ray forest theory, but current data do not give significant constraints. However, we estimate that observing about seven bright quasars should give at least one O VIII and more O VII absorption lines at 95% possibility. We also find that combined with the constraints from the distortion of the cosmic microwave background spectrum, the X-ray Gunn-Peterson test can marginally constrain a uniform, enriched IGM.

Chandra,EUVE,HST, and VLA Multiwavelength Campaign on HR 1099: Instrumental Capabilities, Data Reduction, and Initial Results

In mid-September of 1999, a multiwavelength campaign was carried out on the coronally active RS Canum Venaticorum binary HR 1099 (K1 IV + G5 V; P = 2.84 days), during commissioning of the High Energy Transmission Grating Spectrometer (HETGS) of the Chandra X-ray Observatory (CXO). The coordinated program involved the Extreme Ultraviolet Explorer (EUVE), the Hubble Space Telescope Imaging Spectrograph (STIS), and the Very Large Array (VLA). The objective was to study the system in quiescence, across as much of the electromagnetic spectrum as practical, and to catch any flares that might occur. The EUVE 80-180 A light curve of HR 1099, covering the period September 13-22, showed only a single impulsive outburst, which occurred at the very end of the 9 day pointing. The 3-25 A bremsstrahlung continuum displayed an overall decay during the 1.5 day Chandra observation on September 14-16, with a few superimposed mild enhancements. The VLA 3.5 cm and 20 cm radio light curves, obtained during a 10 hr period on September 15 immediately before the HST pointing, similarly revealed normal quiescent gyrosynchrotron emission and an absence of impulsive events. In contrast, the 7 hr STIS time series later on September 15 contained two distinct flares. The first was accompanied by intensification of the preexisting broad wings of the medium excitation species (e.g., Si IV λ1393 and C IV λ1548), while the second involved primarily the narrow cores of the lines, and even cooler temperatures. The Fe XXI λ1354 forbidden line showed little response to either flare, consistent with the contemporaneous soft X-ray and EUV light curves. The lack of coronal counterparts to the ultraviolet flares is unusual and suggests that they belong to a separate class of outbursts, sharing some similarities with the transition zone explosive events seen on the Sun. The density sensitive O IV λ1400 multiplet was not affected by either flare. The density sensitive Si III λ1300 multiplet showed little response to the first flare, but a dramatic brightening in the second, likely due to the lower peak temperature of that event. The O IV line ratios were near their low-density limits and suggest ne ~ 1010 cm-3 for the duration of the HST observations. The Si III ratios during the second flare rise indicate ne ~ 7 × 1010 cm-3. The far-UV diagnostics jointly imply electron pressures of neT ~ 2 × 1015 K cm-3, if formed close to their respective ionization equilibrium temperatures. The helium-like triplets of O VII, Ne IX, and Mg XI in the HETGS spectra have forbidden-to-intercombination line ratios consistent with average coronal electron densities of 1011 cm-3 at T ~ 0.3-1 × 107 K over the duration of the Chandra pointing. Fe XXI λ102/λ128 and λ142/λ128 ratios from EUVE suggest ne 1012-1013 cm-3, near 107 K. Thus, the coronal electron pressures could be as much as several orders of magnitude larger than those of the lower atmosphere. We constructed time-resolved spectra from the HETGS event lists and measured the centroids of the three brightest X-ray lines of HR 1099 in 60 minute bins. In high-S/N Ne X λ12.1, we believe that we can see the changing radial velocity of the K1 IV star over the half an orbital cycle recorded by Chandra. The amplitude of the velocity change is only ~100 km s-1 over the 1.5 day interval, a small fraction of the 300-500 km s-1 resolution of the HETGS at 12 A. Ne X λ12.1 also displayed a transient blueshift of ~60 km s-1 immediately prior to the first STIS flare, during which the far-UV lines briefly exhibited blueshifts of similar magnitude. The apparent displacement of Ne X appears independently in the -1 and +1 orders of the medium energy band (MEG), but there was no confirmation in the redundant high energy band (HEG), nor in other X-ray lines. Fe XXI λ1354 showed a blueshift about 30 minutes later at the beginning of the second HST orbit, but only at the 20 km s-1 level. Analogous HETGS time series of the brightest lines of comparison star Capella (α Aurigae; G8 III + G1 III) exhibited steady behavior, consistent with measurement uncertainties, without the transient Doppler bursts seen in HR 1099. Although the reality of the Ne X Lyα blueshift is in doubt, there is no question that the Chandra HETGS velocity scales are stable and free from large systematic errors.

X-Ray Imaging of the Seyfert 2 Galaxy Circinus with [ITAL]Chandra[/ITAL

We present results from the zero-order imaging of a Chandra High-Energy Transmission Grating Spectrometer observation of the nearby Seyfert 2 galaxy Circinus. Twelve X-ray sources were detected in the Advanced CCD Imaging Spectrometer-S (ACIS-S) image of the galaxy, embedded in diffuse X-ray emission. The latter shows a prominent (~18'') soft in the northwest direction, coincident with the [O III] ionization cone. The radial profiles of the brightest X-ray source at various energies are consistent with an unresolved (FWHM ~ 08) component, which we identify as the active nucleus, plus two extended components with FWHM ~ 23 and 18'', respectively. In a radius of 3'', the nucleus contributes roughly the same flux as the extended components at the softest energies (2 keV). However, at harder energies (>2 keV), the contribution of the nucleus is dominant. The zero-order ACIS spectrum of the nucleus exhibits emission lines at both soft and hard X-rays, including a prominent Fe Kα line at 6.4 keV, showing that most of the X-ray lines previously detected with ASCA originate in a compact region (15 pc). Based on its X-ray spectrum, we argue that the 23 extended component is scattered nuclear radiation from nearby ionized gas. The large-scale extended component includes the emission from the northwest plume and possibly from the outer starburst ring.

ITAL]Chandra[/ITAL] Observations of the X-Ray Narrow-Line Region in NGC 4151

We present the first high-resolution X-ray spectrum of the Seyfert 1.5 galaxy NGC 4151. Observations with the Chandra High-Energy Transmission Grating Spectrometer reveal a spectrum dominated by narrow emission lines from a spatially resolved (1.6 kpc), highly ionized nebula. The X-ray narrow-line region is composite, consisting of both photoionized and collisionally ionized components. The X-ray emission lines have similar velocities, widths, and spatial extent to the optical emission lines, showing that they arise in the same region. The clouds in the narrow-line region must contain a large range of ionization states in order to explain both the optical and X-ray photoionized emission. Chandra data give the first direct evidence of X-ray line emission from a hot plasma (T ~ 107 K) that may provide pressure confinement for the cooler (T = 3 × 104 K) photoionized clouds.