Photoionization modeling of oxygen K absorption in the interstellar medium:the Chandra grating spectra of XTE J1817-330. (arXiv:1303.2396v1 [astro-ph.HE])

Photoionization modeling of oxygen K absorption in the interstellar medium:the Chandra grating spectra of XTE J1817-330. (arXiv:1303.2396v1 [astro-ph.HE]):
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.