X-ray diagnostics of chemical composition of the accretion disk and donor star in ultra-compact X-ray binaries

Abstract

Non-solar composition of the donor star in ultra-compact X-ray binaries may have a pronounced effect on the fluorescent lines appearing in their spectra due to reprocessing of primary radiation by the accretion disk and the white dwarf surface. We show that the most dramatic and easily observable consequence of the anomalous C/O abundance, is the significant, by more than an order of magnitude, attenuation of the Ka line of iron. It is caused by screening of the presence of iron by oxygen - in the C/O dominated material the main interaction process for a E ~ 7keV photon is absorption by oxygen rather than by iron, contrary to the solar composition case. Ionization of oxygen at high mass accretion rates adds a luminosity dependence to this behavior - the iron line is significantly suppressed only at low luminosity, log(LX) less than 37-37.5, and should recover its nominal strength at higher luminosity. The increase of the EW of the Ka lines of carbon and oxygen, on the other hand, saturates at rather moderate values. Screening by He is less important, due to its low ionization threshold and because in the accretion disk it is mostly ionized. Consequently, in the case of the He-rich donor, the iron line strength remains close to its nominal value, determined by the iron abundance in the accretion disk. This opens the possibility of constraining the nature of donor stars in UCXBs by means of X-ray spectroscopy with moderate energy resolution.

Accepted by MNRAS

References in corpus (12)

• Thermonuclear (type-I) X-ray bursts observed by the Rossi X-ray Timing Explorer
• X-ray reflected spectra from accretion disk models. III. A complete grid of ionized reflection calculations
• Broad relativistic iron emission line observed in SAX J1808.4-3658
• XMM-Newton detects a relativistically broadened iron line in the spectrum of the ms X-ray pulsar SAX J1808.4-3658
• Optical spectroscopy of (candidate) ultra-compact X-ray binaries: constraints on the composition of the donor stars
• A relativistically smeared spectrum in the neutron star X-ray Binary 4U 1705-44: Looking at the inner accretion disc with X-ray spectroscopy
• Testing reflection features in 4U 1705-44 with XMM-Newton, BeppoSAX and RXTE in the hard and soft state
• Time-resolved optical photometry of the ultra-compact binary 4U0614+091
• VLT spectroscopy and non-LTE modeling of the C/O-dominated accretion disks in two ultracompact X-ray binaries
• Long-term luminosity behavior of 14 ultracompact X-ray binaries
• Detection of Highly Ionized Metal Absorption Lines in the Ultracompact X-ray Dipper 4U 1916-05
• The Fermi-GBM X-ray burst monitor: thermonuclear bursts from 4U 0614+09

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• Formation constraints indicate a black-hole accretor in 47 Tuc X9
• Luminosity dependent change of the emission diagram in the X-ray pulsar 4U 1626-67
• Disappearance of the Fe K$\alpha$ emission line in Ultra Compact X-ray Binaries 4U 1543-624 and Swift J1756.9-2508