X-ray diagnostics of ultra-compact X-ray binaries

Koliopanos, Filippos (2015): X-ray diagnostics of ultra-compact X-ray binaries. Dissertation, LMU München: Fakultät für Physik

Abstract

Non-solar composition of the donor star in ultra-compact X-ray binaries (UCXBs) may have a pronounced effect on the fluorescent lines appearing in their spectra due to reprocessing of primary radiation by the accretion disc 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 Kα 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 an E ~ 7 keV 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 behaviour - the iron line is significantly suppressed only at low luminosity, log (LX) < 37-37.5, and should recover its nominal strength at higher luminosity. The increase of the equivalent width of the Kα 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 disc 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 disc. This opens the possibility of constraining the nature of donor stars in UCXBs by means of X-ray spectroscopy with moderate energy resolution.

Abstract

We search for the Fe Kα line in spectra of ultra-compact X-ray binaries (UCXBs). For this purpose we have analysed XMM-Newton observations of five confirmed UCXBs. We find that the object 2S 0918-549 - whose optical spectrum bears tentative signatures of a C/O accretion disc - is devoid of any emission features in the 6-7 keV range, with an upper limit of less than 10 eV for the equivalent width (EW) of the Fe line. 4U 1916-05 - whose optical spectrum is consistent with reflection from a He-rich accretion disc - exhibits a bright broad iron emission line. This behaviour is in agreement with the theoretical predictions presented in Koliopanos et al. Namely, we expect strong suppression of the Fe Kα emission line in spectra originating in moderately bright (log LX less than ≈37.5) UCXBs with C/O- or O/Ne/Mg-rich donors. On the other hand the EW of the Fe line in spectra from UCXBs with He-rich donors is expected to retain its nominal value of ~100 eV. Our analysis also reveals a strong Fe Kα line in the spectrum of 4U 0614+091. This detection points towards a He-rich donor and seems to be at odds with the source's classification as C/O rich. Nevertheless, a He-rich donor would explain the bursting activity reported for this system. Lastly, based on our theoretical predictions, we attribute the lack of a strong iron emission line - in the two remaining UCXB sources in our sample (XTE J1807-294 and 4U 0513-40) - as an indication of a C/O or O/Ne/Mg white dwarf donor. From the upper limits of the Fe Kα line EW in 4U 0513-40, 2S 0918-549 and XTE J1807-294 we obtain a lower limit on the oxygen-to-iron ratio, O/Fe > 10[O/Fe]sol

Abstract

We detect variability of the Fe Kalpha emission line in the spectrum of X-ray pulsar 4U 1626-67, correlated with changes in its luminosity and in the shape of its pulse profile. Analysis of archival Chandra and RXTE observations revealed the presence of an intrinsically narrow Fe Kalpha emission line in the spectrum obtained during the source's current high luminosity period. However, the line was not present during an XMM- Newton observation seven years earlier, when the source was ~three times fainter. The small intrinsic width of the line, sigma ~ 36 eV, as measured by the high energy grating of Chandra, suggests reflection off the outer accretion disk, at R ~ 1800Rs,assuming a Keplerian disk. This value is consistent with the truncation radius of the disk by the magnetic field of the neutron star, ~ 3 x 10^12 Gauss, known from cyclotron line measurements. Timing analysis of the XMM-Newton and RXTE data revealed a major change in the pulse profile of the source from a distinct double peaked shape during the high luminosity state when the line was present, to a much more complex multi-peak structure during the low luminosity state. We argue that the appearance of the line and the change in the shape of the pulse profile are correlated and are the result of a major change in the emission diagram of the accretion column, from a pencil-beam pattern at low luminosity, to a fan-beam pattern at high luminosity.

• Dokumententyp: Dissertationen (Dissertation, LMU München)
• Keywords: High energy astrophysics, accretion disks, emission lines, X-ray binaries
• Themengebiete: 500 Naturwissenschaften und Mathematik ; 500 Naturwissenschaften und Mathematik > 530 Physik
• Fakultäten: Fakultät für Physik
• Sprache der Hochschulschrift: Englisch
• Datum der mündlichen Prüfung: 13. März 2015
• 1. Bericht­erstatter:in: Sunyaev, Rashid