|Session 1: Plasma Modelling|
Ehud Behar (Columbia University / Technion)
Current Status of X-ray Spectroscopy and Relevant Atomic Data
With the advent of the high-resolution grating spectrometers on board the Chandra and XMM-Newton observatories, X-ray spectroscopy has become the most powerful tool for X-ray measurements in space. The increasing role of spectroscopy has naturally drawn attention to the relevant atomic physics and atomic data, as those are directly linked with our ability to draw meaningful conclusions from observed spectra. A review of the atomic data needs for analyzing cosmic X-ray spectra will be presented including the various line-excitation mechanisms. The adequacy of the existing data will be assessed. In general, a combination of state-of-the-art atomic codes and high precision laboratory measurements provide a reasonable description of the overwhelming majority of cosmic spectra. However, in a few intriguing cases, non-standard models and special laboratory measurements are still required, examples of which are the 2p - 3s lines of Fe-L ions and inner-shell absorption lines of low-Z elements.
Randall Smith (Smithsonian Astrophysical Observatory)
Spectroscopic Data for Modelling Highly Ionized Astrophysical Plasmas
Calculating the spectrum of a thermal plasma remains an ongoing challenge involving both atomic and plasma physics, as well as numeric computational issues. The size and complexity of modern spectroscopic data required for modelling high energy plasmas present a challenge for compilation and maintenance within the traditional context of plasma emission codes. Similarly the output of spectral synthesis codes require compilation of large numbers of individual emission lines and continuum bins appropriate for high resolution/broad band grating data from Chandra and XMM. We have created a code (APEC) and atomic database (APED) which combine to form the ATOMDB. The ATOMDB consists of raw atomic data and calculated collisional equilibrium output files which can be used to fit high-resolution spectral data and simultaneously identify lines or features by their explicit transition. We will present results from the latest ATOMDB release, version 1.2.0, for a selection of ionic sequences and compare with results from SPEX, Chianti, and the Raymond-Smith code. We will also discuss the future directions for the APEC/APED project.
Giulio Del Zanna (DAMTP, University of Cambridge)
CHIANTI: Applications to X-ray High Resolution Spectroscopy
The new version 4 (release 2002) of the CHIANTI atomic database and software is briefly described. New atomic calculations for ions important in the X-rays are included, together with proton rates, new relativistic continuum calculations and photoexcitation. Applications to Chandra/XMM stellar X-ray spectra are presented, together with comparisons between CHIANTI and other spectral codes such as SPEX. Some spectroscopic diagnostic methods are reviewed and discussed, in particular those that strongly depend on the assumption of ionization equilibrium (such as the elemental abundances determination).
|Session 2: Stellar Coronae|
Manuel Güdel (Paul Scherrer Institut, Villigen)
Coronae of Cool Stars
High-resolution X-ray spectroscopy of stars allows us for the first time to investigate physical mechanisms on stars hitherto accessible only on the Sun. Spectroscopic diagnostics using individual line fluxes and the continuum are now regularly applied to measure elemental abundances, dominant temperatures, electron densities, mass motions, or optical depths. I will review results from 3 years of high-resolution X-ray spectroscopy of cool stars (from XMM-Newton and Chandra) and discuss them in the context of coronal physics and stellar evolution. The focus will be on the general coronal structure and on flares and their underlying physical mechanisms. Associated data from the XMM-Newton EPIC and OM instruments will be presented as well. I also discuss a number of caveats in the determination of coronal parameters such as densities, optical depths, and abundances and the bias these caveats may introduce into the physical interpretation based on simplistic coronal models.
Marc Audard (Columbia University)
Chandra and XMM-Newton X-ray Spectroscopy of Stars at High Levels of Coronal Activity
Highly magnetically active stellar coronae usually display numerous continual flares and/or a very high temperature component. Here we present Chandra Low Energy Transmission Grating Spectrometer data of the flare M dwarf binary UV Ceti. Both components are well resolved in the zeroth order image, but their separation (1 arcsec) does not allow to obtain distinct clean spectra. Since the binary orientation is almost parallel to the dispersion axis, emission lines can however be assigned to each component. The positive and negative dispersion spectra further help for the assignment. Light curves from each component of the binary have been constructed and line fluxes measured. Using several line ratios we obtain estimates of the temperature components in each corona. XMM-Newton has observed UV Ceti for 50 ksec at different epoch than Chandra, however simultaneously with the radio VLA. We discuss these data as well. If time allows, we also present data from another highly active star, the extremely hot giant YY Mensae, observed with Chandra High-Energy Transmission Grating Spectrometer and with XMM-Newton.
Jürgen Schmitt (Hamburger Sternwarte)
Coronal Abundance Diagnostics with the Chandra LETGS
With its large band pass and high spectral resolution the Chandra LETGS is a perfect instrument to perform plasma diagnostics of stellar coronae. The Ly-Alpha and He-like triplet lines from low and high temperature ions are in the bandpass and can be spectrally resolved. We emphasise the need to construct abundance independent differential emission distributions for any reliable determination of coronal abundances. We show that rather simple DEMS are consistent with the LETGS data and lead to robust estimates of abundance. We apply the method to Chandra data of both high and low activity stars.
David Huenemoerder (MIT)
Stellar Coronal Spectroscopy with the Chandra HETGS
Grating spectra of cool stars from Chandra and XMM reveal a wealth of detail in their emission lines. Ability to resolve lines and detect the continuum allows detailed modelling of the temperature structure, abundances, and sometimes density. The abundances often show anomalies, such as strong neon and weak iron. Line strengths are often anomalous in flux ratios when compared to models or laboratory data. Temperature distributions are highly structured, but differ from star to star. We will present results from a variety of objects: the pre-main sequence star TW Hya, which seems to be accretion driven, to neon-strong, iron-weak and highly variable RS CVn stars, II Peg, TY Pyx, and AR Lac, to the rapidly rotating giant, FK Com. We will also present our techniques for line-based analysis of coronal X-ray emission.
|Session 3: Hot Stars|
Ton Raassen (SRON)
The X-ray Spectrum of Tau Sco
The XMM-MOS and -RGS X-ray spectra of the B0.2V star Tau Scorpii are simultaneously fitted to obtain self-consistent temperatures, emission measures, and elemental abundances. The nitrogen lines are relatively very strong: the N/O abundance ratio is about 3 times solar. Multi-temperature fitting yields 4 components at temperatures of 1.6, 5.2, 8.2, and >20 MK which are confirmed by DEM modeling. The X-ray luminosity (0.3-10 keV) is 3.2e31 erg/s at d=132 pc. The sensitivity of the He-like forbidden and intercombination lines to a strong UV stellar radiation field yields upper limits to the radial distances at which the He-like lines of Mg, Ne, O, and N originate. The results suggest that the soft X-rays (<8 MK) originate from shocks low in the wind produced by the common mechanism of radiation line-driven instabilities. This is consistent with the observed emission line profiles that are much narrower (<500 km/s) than the broad lines (up to 1500 km/s) observed high up in the wind of Zeta Puppis. The hot (~20-40 MK) component may be explained by a model involving dense clumps embedded in a wind which is approaching at high relative velocity (~1400-1700 km/s) and the interaction produces strong shocks.
David Cohen (Swarthmore College)
Analysis of Doppler-Broadened Emission Lines in the Chandra Spectra of OB Stars
The massive winds of hot stars have characteristic velocities of several thousand kilometers per second, which is many times greater than the velocities corresponding to the spectral resolutions of the Chandra gratings and the XMM-Newton RGS. X-rays produced in these winds generate spectrally resolved emission line profiles, with the line shapes providing kinematic information about the X-ray emitting plasma and also its spatial relation to the cold, bulk wind. We use this information, as derived from roughly half-a-dozen hot stars observed with the Chandra HETGS, to constrain several models of X-ray production in OB stars.
|Session 4: X-ray Binaries, Pulsars, Novae|
Masao Sako (Caltech)
Structure and Dynamics of Stellar Winds in High-Mass X-ray Binaries
A review of spectroscopic results obtained from Chandra HETG and XMM-Newton RGS observations of several wind-fed high-mass X-ray binaries is presented. Recent observations allow us to study the structure and dynamics of the stellar wind in more detail, but at the same time they are providing us with numerous puzzles that cannot be understood in terms of simple models. For example, in most cases, simple spherically-symmetric wind models cannot explain the observed orbital-phase variability of the line spectrum, which may be simply due to intrinsic asymmetry and/or more complicated radiative transfer effects. The observed line shifts are smaller than those expected from simple extensions of wind models of isolated OB supergiants. In addition, several novel spectroscopic discoveries have been made, including: (1) P-Cygni lines from an expanding wind, (2) Detection of multiple Si K fluorescent lines from a wide range of charge states, (3) Compton scattered Fe K lines from a cold medium. We discuss how these spectroscopic diagnostics can be used to understand some of the global properties of stellar winds in HMXBs.
Katherine McGowan (LANL)
Deep XMM-Newton Observations of the X-ray Emission from PSR 1706-44
PSR 1706-44 is a young, energetic pulsar which emits pulsed X-ray and gamma-ray emission at the 102 ms radio period. Until now the low signal-to-noise in previous observations has not allowed one to distinguish between the two leading models for generation of the high energy emission. The key to discriminating between these two mechanisms is time resolved spectroscopy. We present the first results from analysis of a 90 ks observation of PSR 1706-44 taken with XMM-Newton. These high signal-to-noise X-ray observations, combined with gamma-ray measurements, provide powerful new constraints on the radiation models.
Silvia Zane (UCL-MSSL)
RX J1856.5-3754: Bare Quark Star or Naked Neutron Star?
Recent Chandra observations have convincingly shown that the soft X-ray emission from the isolated neutron star candidate RX J1856.5-3754 is best represented by a featureless blackbody spectrum, in apparent contrast with the predictions of current neutron star atmospheric models. Moreover, the recently measured star distance (~120-140 pc) implies a radiation radius of at most ~5-6 km, too small for any neutron star equation of state. Proposed explanations include a reduced X-ray emitting region (a heated polar cap), or the presence of a bare quark/strange star. While both interpretations rely on the presumption that the star radiates a pure blackbody spectrum, no justification for this assumption has been presented yet. Here we discuss an alternative possibility. Cool neutron stars (T < 106 K) endowed with a rather high magnetic field (B > 1013 G) may suffer a phase transition in the outermost layers. As a consequence the neutron star is left bare of the gaseous atmosphere ("naked"). We computed spectra from naked neutron stars with a surface Fe composition. Depending on B, we found that the emission in the 0.1-2 keV range can be featureless and virtually indistinguishable from a blackbody. Moreover, owing to the reduced surface emissivity, the star only radiates ~30-50% of the blackbody power and this implies an apparent radius larger than the radiation radius. When applied to RX J1856.5-3754 our model accounts for the observed X-ray properties and predicts an apparent star radius of ~10-12 km, consistent with equations of state of a neutron star. The optical emission of RX J1856.5-3754 may be explained by the presence a thin gaseous shell on the top of the Fe condensate.
Jean Cottam (NASA/GSFC)
Gravitationally Redshifted Absorption Lines in the Burst Spectra of the Neutron Star in EXO 0748-676
EXO 0748-676 was observed with the Reflection Grating Spectrometer onboard the XMM-Newton Observatory for almost 400ks during the commissioning and calibration phases. During this time 28 type I X-ray bursts were observed. I will present an analysis of the average burst spectra. We see an absorption spectrum arising in highly ionized material surrounding the neutron star with evidence of a change in the ionization of this material in response to the X-ray bursts. After self-consistently accounting for absorption in the circumstellar material we find significant residual absorption lines in the spectrum. These lines are consistent with Fe XXVI and XXV n=2-3 and O VIII n=1-2 transitions, all with a redshift of z=0.35. This constitutes the first direct measurement of the grativational redshift in a neutron star.
Peter Wheatley (University of Leicester)
XMM and Chandra Grating Spectroscopy of Dwarf Novae
I will review recent grating spectroscopy of dwarf novae with the Chandra HETG, LETG, and XMM RGS. This includes the detection of radial velocity variations in the emission of WZ Sge in outburst.
Vadim Burwitz (MPI für extraterrestrische Physik, Garching)
The Chandra LETGS and ACIS-I Spectra of Nova Velorum 1999 (V382 Vel)
Results from the analysis of the highly line rich X-ray spectra of Nova Velorum 1999 (V382 Vel) obtained with the Chandra LETGS high resolution spectrograph as well as with ACIS-I CCDs will be presented. Details on the abundances, temperatures and densities from line ratios of He-like Ne, O, and N ions and shell expansion from the line widths as well as evolution of this ONeMg will be discussed at length.
|Session 5: Supernova Remnants|
Jacco Vink (Columbia University)
XMM-Newton RGS Spectra of SN1006 and RCW 86
Although dispersive spectrometers are mainly designed for spectroscopy of unresolved sources, in a number of cases they can be used to obtain high resolution spectra of extended objects. In the case of supernova remnants the Magellanic Cloud remnants are a case in point. Here, however, I would like to present new results based on XMM RGS data of the large Galactic supernova remnants SN1006 and RCW 86. Although the objects are very large, by pointing at small (< 1') "knots" (SN1006) and narrow features spectra have been obtained with a much better spectral resolution than CCD spectra. For SN1006 I will focus on the issue of the ion/electron temperature relaxation and present upper limits on the ion temperature. For RCW 86 the high resolution spectrum is used to derive constraints on the state of the plasma in relation to the morphology of this remnant (probably the result of a cavity explosion) and the unusual hard X-ray continuum, which is accompanied by Fe K emission at 6.4 keV.
|Session 6: Active Galactic Nuclei|
Ali Kinkhabwala (Columbia University)
XSPEC Model "PHOTOION": Modelling Photoionized X-ray Plasmas Wherever They May Occur
There are two main types of X-ray line-emitting plasmas: hot, collisionally-ionized plasmas and warm, photoionized plasmas. Atomic data and spectral models for hot, collisionally-ionized plasmas are highly reliable, having been extensively tested in the laboratory, in the Sun, and, very recently, in a host of other astrophysical sources. However, atomic data and spectral models of photoionized plasmas have not received the same level of attention, primarly due to the infeasibility of laboratory testing and, until only very recently, the lack of high-resolution X-ray spectra of astrophysical sources. We present a new, simple model of a photoionized plasma (XSPEC model "PHOTOION"), which uses atomic data calculated by the public atomic code FAC and verified (where possible) with other atomic codes. "PHOTOION" was first applied to the spectacular RGS spectrum of the Seyfert 2 galaxy, NGC 1068. This constituted the first detailed, quantitative test of a photoionization code. "PHOTOION" can similarly be used to fit a wide range of other X-ray sources in which warm, photoionized gas may be present, including Seyfert 1 galaxies, X-ray binaries, cataclysmic variables, and gamma ray bursts.
Tahir Yaqoob (Johns Hopkins University)
New Results from Chandra Grating Observations of AGN
We present new results from several Chandra grating observations of Seyfert 1 galaxies. Some of these observations were simultaneous with RXTE and one of them (Mkn 509) was simultaneous with HST/STIS, as well as RXTE. We discuss the detailed spectroscopy of the soft X-ray spectra, obtaining new constraints on the ionization state and kinematics of the absorber. The simultaneous UV data for Mkn 509 shows that the UV and X-ray absorbers share the same velocity space but have completely different ionization parameters and column densities. We also discuss spectroscopy of the iron K lines and show that the simultaneous RXTE data can deconvolve the narrow and broad lines, as well as simultaneously constraining the Compton-reflection continuum.
Raquel Morales (Harvard-Smithsonian CfA)
A Comparative View of the Warm Absorbers/Emitters Observed with the Chandra High Resolution Gratings
We present preliminary results from a systematic comparative study of the ionized absorbers/emitters in 5 Seyfert galaxies observed with the Chandra HETGS and LETGS: NGC 5548, NGC 3783, NGC 3227, NGC 4051 and NGC 4151. The main objective of this study is to derive the physical and dynamical properties of this important ionized component of the gaseous AGN environment, in a uniform manner using the same modelling, for a sample of well studied nearby Seyferts. For some of these sources multiple observations are available, so we also study variations of the ionization of the gas in response to changes in the ionizing X-ray continuum, and apply our non-equilibrium photoionization models, to tightly constrain the electron density of the gas, and so its distance from the central ionizing source.
Mat Page (UCL-MSSL)
X-ray Emission Line Gas in M81
The soft X-ray spectrum of the LINER galaxy M81, from a long observation with the XMM-Newton RGS contains emission lines from Fe-L, H-like and He-like N, O, and Ne. The emission lines are significantly broader than the RGS line spread function and are found adjacent to as well as coincident with the active nucleus. This implies that they originate in a region a few arcminutes (kpc) extent. The flux ratios of the OVII triplet suggest that the gas is collisionally ionized. A good fit to the whole RGS spectrum can be obtained with an absorbed power law from the active nucleus and a 3-temperature optically thin thermal plasma. I will discuss the nature and origin of the emission line gas.
|Session 7: Clusters of Galaxies|
Michael Wise (MIT)
High Resolution HETG Spectroscopy of Cooling Flow Clusters
We present results for a sample of cooling flow clusters observed with the HETG and ACIS detectors onboard Chandra. In the standard cooling flow model, one expects to see X-ray emission lines from gas over the full range of temperatures from that of the hot, ambient medium down to very low temperatures. One can show that the strength of these emission lines should be directly proportional to the total cooling rate. We present HETG observations with spectral resolutions of E/dE ~100-300 in the Fe L region where we expect line emission from the cooling gas to appear or be noticeably absent. As with recent XMM observations of these objects, we see no evidence for emission lines corresponding to gas temperatures less than about 3 keV. We discuss the implications of these observations for the standard cooling flow paradigm.
|Session 8: Instrumentation/Data Analysis|
Dan Dewey (MIT)
Extended Source Analysis for Grating Spectrometers
Dispersive spectrometers such as the XMM-Newton RGS and the Chandra HETGS are nominally designed and used to study "point sources", that is sources unresolved by the telescope system. When used to observe "extended sources" the resolving power of these spectrometers is generally degraded; however, useful high-resolution information may still be extracted using non-standard analysis techniques. Which technique to use and the amount of information extractable from an observation depend on the spatial and spectral properties of the object. Examples will be given of "spatial-spectral" techniques that are in development, in particular ones being used to extract high-resolution results from Chandra HETGS observations of the supernova remnants E0102, N132D, N103B, and Cas A.
John R. Peterson (Columbia University)
Monte Carlo Methods for the Reflection Grating Spectrometer on XMM-Newton
We discuss multivariate Monte Carlo methods for the Reflection Grating Spectrometer (RGS) on XMM-Newton. The RGS provides high resolution spectra of many moderately extended X-ray sources. The analysis of RGS data from extended sources, however, can become complicated both in the calculation of the instrument response functions as well as in the formulation of spectral models which depend on spatial position. Monte Carlo methods are the natural solution to these challenges, but techniques for their use are not well-developed. We describe a number of methods to produce a highly efficient and flexible multivariate Monte Carlo. These techniques include multi-dimensional response interpolation and multi-dimensional event comparison. We discuss how these methods have been extensively used in the XMM-Newton Reflection Grating Spectrometer in-flight calibration program and in the analysis of elliptical galaxies and clusters of galaxies.
John E. Davis (MIT)
A Pile-up Correction Technique for Dispersed Spectra
The effectiveness of the Chandra X-ray Observatory's High Energy Grating Spectrometer (HETGS) for determining the spectrum of a bright X-ray source, or a source with bright lines, can be severely limited by the effects of pile-up in the CCDs. In such cases, the standard technique for flux-correction breaks down, producing false absorption and emission features in the spectra. A new method for flux-correcting grating data affected by photon pile-up will be discussed. The usefulness of the model will be illustrated using several Chandra HETGS observations of bright sources.
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