Photoionization modeling of oxygen K absorption in the interstellar medium: the Chandra grating spectra of XTE J1817-330

E. Gatuzz (1), J. García (2), C. Mendoza (1,4), T. R. Kallman (3), M. Witthoeft (3), A. Lohfink (2), M. A. Bautista (5), P. Palmeri (6) and P. Quinet (7) ((1) Centro de Física, Instituto Venezolano de Investigaciones Científicas, PO Box 20632, Caracas 1020A,  Venezuela; (2) Department of Astronomy and Maryland Astronomy Center for Theory and Computation, University of Maryland, College Park, MD 20742, USA; (3) NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA; (4) Centro Nacional de Cálculo Científico, Universidad de Los Andes, Corporación Parque Tecnológico de Mérida 5101, Venezuela; (5) Department of Physics, Western Michigan University, Kalamazoo, MI 49008, USA; (6) Astrophysique et Spectroscopie, Université de Mons – UMONS, B-7000 Mons, Belgium; (7) IPNAS, Sart Tilman B15, Université de Liége, B-4000 Liége,

We present detailed analyses of oxygen K absorption in the interstellar medium (ISM) using four high-resolution Chandra spectra towards the X-ray low-mass binary XTE J1817-330. The 11-25 A broadband is described with a simple absorption model that takes into account the pileup effect and results in an estimate of the hydrogen column density. The oxygen K-edge region (21-25 A) is fitted with the physical warmabs model, which is based on a photoionization model grid generated with the xstar code with the most up-to-date atomic database. This approach allows a benchmark of the atomic data which involves wavelength shifts of both the K lines and photoionization cross sections in order to fit the observed spectra accurately. As a result we obtain: a column density of N(H)=1.38+/-0.01\times 10^21 cm^-2; ionization parameter of log(xi)=-2.70+/-0.023; oxygen abundance of A(O)= 0.689^{+0.015}_{-0.010}; and ionization fractions of OI/O = 0.911, OII/O = 0.077, and OIII/O = 0.012 that are in good agreement with previous studies. Since the oxygen abundance in warmabs is given relative to the solar standard of Grevesse et al. (1998), a rescaling with the revision by Asplund et al. (2009) yields A(O)=0.952^{+0.020}_{-0.013}, a value close to solar that reinforces the new standard. We identify several atomic absorption lines Kalpha, Kbeta, and Kgamma in OI and OII; and Kalpha in OIII, OVI, and OVII – last two probably residing in the neighborhood of the source rather than in the ISM. This is the first firm detection of oxygen K resonances with principal quantum numbers n>2 associated to ISM cold absorption.

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