X-Ray IGM in the Local Group

Recent observations with the dispersive X-ray spectrometers aboard Chandra and Newton Observatory have begun to probe the properties of the X-ray intergalactic medium (IGM) at small redshifts. Using large quantities (~950 ksec) of spectroscopic data acquired using the RGS aboard Newton Observatory, we investigated the intervening material toward three low redshift, high Galactic latitude AGNs with nominally featureless spectra: Mrk421, PKS2155-304 and 3C273. Each spectrum provides clear evidence for what appears to be a local (z~0), highly ionized absorbing medium betrayed by the OVII 1s-2p resonance transition feature seen at 21.6A (N[OVII] ~ 1E16 cm-2). Measurements are also made for the Lyman alpha transition of the adjacent ionization state, (OVIII; 18.97A), which potentially constrains the absorber's temperature. Finally, in a collisional equilibrium approximation, upper limits to diffuse emission intensities place upper limits on the electron density (ne < 2E-4 cm-3), lower limits on the scale length of the absorber (L > 140 kpc) and lower limits on its mass (M > 5E10 M[sun]). Limits on the absorber's scale length and its velocity distribution lead us to identify it with the Local Group.Comment: 6 pages, 1 table and 2 figs. Latex. To appear in "The IGM/Galaxy Connection: The Distribution of Baryons at z=0" to be published by Kluwer Academic Publishing. Resubmitted with 2 typos corrected, page 5 and figure 2. Thanks to Masao Sako for pointing these ou

The Gaia-LSST Synergy

We discuss the synergy of Gaia and the Large Synoptic Survey Telescope (LSST) in the context of Milky Way studies. LSST can be thought of as Gaia's deep complement because the two surveys will deliver trigonometric parallax, proper-motion, and photometric measurements with similar uncertainties at Gaia's faint end at $r=20$, and LSST will extend these measurements to a limit about five magnitudes fainter. We also point out that users of Gaia data will have developed data analysis skills required to benefit from LSST data, and provide detailed information about how international participants can join LSST.Comment: Presented at "The Milky Way Unravelled by Gaia", Barcelona, Dec 1-5, 2014; 7 pages, 1 color figur

X-Ray Spectroscopic Laboratory Experiments in Support of the X-Ray Astronomy Program

Our program is to perform a series of laboratory investigations designed to resolved significant atomic physics uncertainties that limit the interpretation of cosmic X-ray spectra. Specific goals include a quantitative characterization of Fe L-shell spectra; the development of new techniques to simulate Maxwellian plasmas using an Electron Beam Ion Trap (EBIT); and the measurement of dielectronic recombination rates for photoionized gas. New atomic calculations have also been carried out in parallel with the laboratory investigations

Soft X-Ray Absorption by Fe0+^{0+} to Fe15+^{15+} in Active Galactic Nuclei

A full set of calculations is presented for inner-shell n = 2 to 3 photoexcitation of the 16 iron charge states: Fe I through Fe XVI. The blend of the numerous absorption lines arising from these excitations (mainly 2p - 3d) forms an unresolved transition array (UTA), which has been recently identified as a prominent feature between 16 - 17 \AA in the soft X-ray spectra of active galactic nuclei (AGN). Despite the blending within charge-states, the ample separation between the individual-ion features enables precise diagnostics of the ionization range in the absorbing medium. Column density and turbulent velocity diagnostics are also possible, albeit to a lesser accuracy. An abbreviated set of atomic parameters useful for modeling the Fe 2p - 3d UTA is given. It is shown that the effects of accompanying photoexcitation to higher levels ($n \ge$ 4), as well as the associated photoionization edges, may also be relevant to AGN spectra.Comment: Accepted for publication in The Astrophysical Journa

The Chandra Iron-L X-Ray Line Spectrum of Capella

An analysis of the iron L-shell emission in the publicly available spectrum of the Capella binary system, as obtained by the High Energy Transmission Grating Spectrometer on board the Chandra X-ray Observatory, is presented. The atomic-state model, based on the HULLAC code, is shown to be especially adequate for analyzing high-resolution x-ray spectra of this sort. Almost all of the spectral lines in the 10 - 18 Angstrom wavelength range are identified. It is shown that, for the most part, these lines can be attributed to emission from L-shell iron ions in the Capella coronae. Possibilities for electron temperature diagnostics using line ratios of Fe16+ are demonstrated. It is shown that the observed iron-L spectrum can be reproduced almost entirely by assuming a single electron temperature of kTe= 600 eV. This temperature is consistent with both the measured fractional ion abundances of iron and with the temperature derived from ratios of Fe16+ lines. A volume emission measure of 1053 cm-3 is calculated for the iron L-shell emitting regions of the Capella coronae indicating a rather small volume of 1029 cm3 for the emitting plasma if an electron density of 1012 cm-3 is assumed.Comment: Accepted to Ap