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Spectral changes during dipping in low-mass X-ray binaries due to highly-ionized absorbers

Spectral changes during dipping in low-mass X-ray binaries due to highly-ionized absorbers

Publication date: 02 March 2006

Authors: Diaz Trigo, M., et al.

Journal: Astronomy & Astrophysics
Volume: 445
Page: 179
Year: 2006

Copyright: ESO

X-ray observations have revealed that many microquasars and low-mass X-ray binaries (LMXBs) exhibit narrow absorption features identified with resonant absorption from FeXXV and FeXXVI and other abundant ions. In many well studied systems there is evidence for blue-shifts, indicating outflowing plasmas. The changes in both the X-ray continuum and the Fe absorption features during dips from the LMXB XB 1323-619 have been modeled as resulting primarily from an increase in column density and a decrease in the ionization state of a highly-ionized absorber. We successfully fit the same ionized absorber model to the persistent and dipping emission from all the other bright dipping LMXBs observed by XMM-Newton (EXO 0748-676, XB 1254-690, X 1624-490, MXB 1659-298, 4U 1746-371 and XB 1916-053) and find similar changes in the properties of the ionized absorber as for XB 1323-619. This implies that the complex spectral changes in the X-ray continua observed from the dip sources as a class can be most simply explained primarily by changes in the highly ionized absorbers present in these systems. There is no need to invoke unusual abundances or partial covering of extended emission regions. Outside of the dips, the absorption line properties do not vary strongly with orbital phase. This implies that the ionized plasma has a cylindrical geometry with a maximum column density close to the plane of the accretion disk. Since dipping sources are simply normal LMXBs viewed from close to the orbital plane this implies that ionized plasmas are a common feature of LMXBs.

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