A. Kinkhabwala

XMM-Newton reflection grating spectrometer observations of discrete soft-x-ray emission features from NGC 1068

We present the first high-resolution, soft X-ray spectrum of the prototypical Seyfert 2 galaxy, NGC 1068. This spectrum was obtained with the XMM-Newton Reflection Grating Spectrometer (RGS). Emission lines from H-like and He-like low-Z ions (from C to Si) and Fe L-shell ions dominate the spectrum. Strong, nar- row radiative recombination continua (RRCs) for several ions are also present, implying that most of the observed soft X-ray emission arises in low-temperature plasma (kTea few eV). This plasma is photoion- ized by the inferred nuclear continuum (obscured along our line of sight), as expected in the unified model of active galactic nuclei (AGNs). We find excess emission (compared to pure recombination) in all resonance lines (1s!np) up to the photoelectric edge, demonstrating the importance of photoexcitation as well. We introduce a simple model of a cone of plasma irradiated by the nuclear continuum; the line emission we observe along our line of sight perpendicular to the cone is produced through recombination/radiative cas- cade following photoionization and radiative decay following photoexcitation. A remarkably good fit is obtained to the H-like and He-like ionic line series, with inferred radial ionic column densities consistent with recent observations of warm absorbers in Seyfert 1 galaxies. Previous Chandra imaging revealed a large (extending out to � 500 pc) ionization cone containing most of the X-ray flux, implying that the warm absorber in NGC 1068 is a large-scale outflow. To explain the ionic column densities, a broad, flat distribu- tion in the logarithm of the ionization parameter (� ¼ LX=ner 2 ) is necessary, spanning log � ¼ 0-3. This sug- gests either radially stratified ionization zones, the existence of a broad density distribution (spanning a few orders of magnitude) at each radius, or some combination of both. Subject headings: galaxies: individual (NGC 1068) — galaxies: Seyfert — line: formation — X-rays: galaxies

Two Types of X-Ray Spectra in Cataclysmic Variables

In a 2003 paper, Mukai et al. presented the Chandra HETG spectra of 7 cataclysmic variables (CVs) then available in the public archive, and classified them into “cooling flow” and “photoionized” types. In this presentation, I will revisit this classification. It is clear that multi-temperature plasma exists in the post-shock region of accreting white dwarfs; I will discuss why the cooling flow model works well, and what its limitations are when applied to CVs. It is also clear that even the “photoionized” CVs are also powered by the same multi-temperature plasma. I will discuss how the appearance of a much harder continuum of these “photoionized” CVs is created by the complex intrinsic absorber. I will show that there is an additional soft emission in these systems that the multi-temperature plasma models cannot explain, and discuss what evidence there is for the photoionization origin of this component. Figure 1 The Cooling Flow CVs Figure 2 The Photoionized CVs

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 discrete 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 (RRC), indicating that most of the observed soft X-ray emission arises in low-temperature (kT_e ~ few eV) photoionized plasma. We confirm the conclusions of Kinkhabwala et al. ([CITE]), based on XMM-Newton Reflection Grating Spectrometer (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 the presence of hot, collisionally ionized plasma. In addition, we show that this same model also provides an excellent fit to the spectrum of the Secondary region, albeit with radial column densities roughly 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. Finally, 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, which has been deduced from the measured line intensities of particularly these long wavelength lines in conjunction with the Fe-L shell lines.

Can a Dusty Warm Absorber Model Reproduce the Soft X-Ray Spectra of MCG –6-30-15 and Markarian 766?

XMM-Newton RGS spectra of MCG -6-30-15 and Mrk 766 exhibit complex discrete structure, which was interpreted in a paper by Branduardi-Raymont and coworkers as evidence for the existence of relativistically broadened Lyα emission from carbon, nitrogen, and oxygen, produced in the innermost regions of an accretion disk around a Kerr black hole. This suggestion was subsequently criticized in a paper by Lee and coworkers, who argued that for MCG -6-30-15, the Chandra HETG spectrum, which is partially overlapping the RGS in spectral coverage, is adequately fitted by a dusty warm absorber model, with no relativistic line emission. We present a reanalysis of the original RGS data sets in terms of the model by Lee and coworkers. Specifically, we show that (1) the explicit model given by Lee and coworkers differs markedly from the RGS data, especially at longer wavelengths, beyond the region sampled by the HETG; (2) generalizations of the Lee and coworkers model, with all parameters left free, do provide qualitatively better fits to the RGS data, but are still incompatible with the detailed spectral structure; (3) the ionized oxygen absorption-line equivalent widths are well measured with the RGS for both sources, and place very tight constraints on both the column densities and turbulent velocity widths of O VII and O VIII. The derived column densities are well below those posited by Lee and coworkers and are insufficient to play any role in explaining the observed edge-like feature near 17.5 A; (4) the lack of a significant neutral oxygen edge near 23 A places very strong limits on any possible contribution of absorption to the observed structure by dust embedded in a warm medium; and (5) the original relativistic line model with warm absorption proposed by Branduardi-Raymont and coworkers provides a superior fit to the RGS data, both in the overall shape of the spectrum and in the discrete absorption lines. We also discuss a possible theoretical interpretation for the putative relativistic Lyα line emission in terms of the photoionized surface layers of the inner regions of an accretion disk. While there are still a number of outstanding theoretical questions about the viability of such a model, it is interesting to note that simple estimates of key parameters are roughly compatible with those derived from the observed spectra.

Galactic halo models and particle dark-matter detection

Rates for detection of weakly interacting massive-particle (WIMP) dark matter are usually carried out assuming the Milky Way halo is an isothermal sphere. However, it is possible that our halo is not precisely spherical; it may have some bulk rotation; and the radial profile may differ from that of an isothermal sphere. In this paper, we calculate detection rates in observationally consistent alternative halo models that produce the same halo contributions to the local and asymptotic rotation speeds to investigate the effects of the theoretical uncertainty of the WIMP spatial and velocity distribution. We use self-consistent models to take into account the effects of various mass distributions on the local velocity distribution. The local halo density may be increased up to a factor of 2 by flattening or by an alternative radial profile (which may also decrease the density slightly). However, changes in the WIMP velocity distribution in these models produce only negligible changes in the WIMP detection rate. Reasonable bulk rotations lead to only an O(10%) effect on event rates. We also show how the nuclear recoil spectrum in a direct-detection experiment could provide information on the shape and rotation of the halo.

New Constraint on Open Cold-Dark-Matter Models

We calculate the large-angle cross correlation between the cosmic-microwave-background temperature and the x-ray-background intensity expected in an open universe with cold dark matter (CDM) and a nearly scale-invariant spectrum of adiabatic density perturbations. Results are presented as a function of the nonrelativistic-matter density Ω0 and the x-ray bias bx for both an open universe and a flat cosmological-constant universe. Recent experimental upper limits to the amplitude of this cross correlation provide a new constraint to the Ω0-bx parameter space that open-CDM models (and the open-inflation models that produce them) must satisfy.

X-ray spectroscopy of astrophysical plasmas

We provide a qualitative review of key X–ray spectral diagnostics of astrophysical plasmas. We begin with a brief discussion of the two major types of equilibria, collisional ionization and photoionization, and then consider the behaviour of hydrogen–like, helium–like, iron L–shell and iron K–shell transitions for these separate cases. Where possible, we discuss explicit examples using high–resolution spectra acquired by the grating instruments on the Chandra and XMM–Newton observatories.

Performance and results of the reflection grating spectrometers onboard XMM-Newton

XMM-Newton was launched in December 1999 and science operations started in March 2000. Following two years of very successful operations, a report on the instrument performance and a selection of exciting new results are presented. Behind two of the three telescopes of XMM-Newton Reflection Grating Spectrometers (RGS) are placed. Each spectrometer consists of an array of reflection gratings and a set of back illuminated CCDs. They cover the wavelength band between 6 and 38 Angstromwith a resolution varying between 100 and 600 (E/DE) and a maximum effective area of 140 cm2 for the two spectrometers combined. The selected wavelength band covers the K-shell transitions of C, N, O, Ne, Mg and Si as well as the L- and M-shell transitions of Fe. After a short introduction to the instrument design, the in-orbit performance is given. This includes the line spread function, the wavelength scale and the effective area including their stability during the more than 2 years of operations. Following this a number of key scientific results are briefly addressed, illustrating the power of the RGS instrument in combination with the other instruments on-board of XMM-Newton as well as the wealth of information which is obtained as the RGS instruments operate continuously.