Abstracts of Visitor Talks
| Date | Speaker | Title | Time | Abstract | Venue |
|---|---|---|---|---|---|
| 22/03/2019 | Guglielmo Mastroserio | X-ray Reverberation Mass Measurement of Cygnus X-1 | 15:30 | Both galactic and supermassive black holes display characteristic features in their energy spectra, including an Fe K$\alpha$ line and a Compton hump, that result from reprocessing of hard X-ray photons by the accretion disk. This reflected emission provides a probe of the innermost region of the accretion disk through general relativistic distortions to the line profile. However, these spectral distortions are insensitive to black hole mass, since they depend on disk geometry in units of gravitational radii. Measuring the reverberation lag resulting from the difference in path length between direct and reflected emission gives a measure of absolute photon path length differences. Therefore the length of the gravitational radius can be calibrated by a combined spectral-timing analysis, providing a means to measure black hole mass. I will present the application of our new reverberation model to RXTE data from the black hole X-ray binary Cygnus X-1. We jointly fit the time-averaged X-ray spectrum and the real and imaginary parts of the cross-spectrum as a function of energy for a range of Fourier frequencies, in order to constrain the mass of the black hole. I will show how introducing a radial ionisation profile in the disk changes our results and I will compare our reverberation mass measurement with the dynamical mass measurement of Cygnus X-1. | Sackler |
| 16/11/2018 | Ioanna Psaradaki | The accretion disk atmosphere of X-ray binaries: The case of EXO 0748-676 | 15:30 | Low mass X-ray binaries exhibit ionized emission from an extended disk atmosphere that surrounds the accretion disk. However, its nature and geometry is still unclear. I will present a spectral analysis of the extended atmosphere of EXO 0748-676 using high-resolution spectra from archival XMM-Newton observations. We model the RGS spectrum that is obtained during the eclipses. This enables us to model the emission lines that come only from the extended atmosphere of the source, and study its physical structure and properties. The RGS spectrum reveals a series of emission lines consistent with transitions of O VIII, O VII, Ne IX and N VII. We perform both Gaussian line fitting and photoionization modelling. Our results suggest that there are two photoionization gas components, out of pressure equilibrium with respect to each other. One with ionization parameter of 2.5 and a large opening angle, and one with 1.3. The second component is possibly covering a smaller fraction of the source. From the density diagnostics of the O VII triplet using photoionization modelling, we detect a rather high density plasma of > 10^13 cm^-3 for the lower ionization component. This latter component also displays an inflow velocity. We propose a scenario where the high ionization component constitutes an extended upper atmosphere of the accretion disk. The lower ionization component may instead be a clumpy gas created from the impact of the accretion stream with the disk | Hoyle |
| 30/11/2018 | Filippos Koliopanos | - | 15:30 | In light of recent discoveries of pulsating ultra luminous X-ray sources (ULXs) and recently introduced models placing neutron stars as the engines of ULXs, we revisit the spectra of eighteen well-known ULXs, in search of indications that favor or reject this hypothesis. We find that the notable (>6keV) spectral curvature observed in most ULXs, is commensurate with the Wien tail of a hot (T>1keV) multicolor black-body component and confirm that a double thermal model (comprised of a "cool" and "hot" thermal component) with the addition of a faint non-thermal tail describes all ULX spectra in our list. More importantly, we offer a new physical interpretation for the dual thermal spectrum, where it is the result of accretion onto high-B NSs rather than black holes. We estimate the magnetic-field strength and demonstrate that it correlates strongly with the source luminosity and the temperature of the hot component. We also discuss the application of our model on the most recent pulsating ULX "NGC 300 ULX1", casting doubts on the claimed presence of a cyclotron scattering feature in its spectrum. Our findings offer an additional and compelling argument in favor of NSs as prime candidates for powering ULXs, as has been recently suggested by (e.g.) King, Lasota 2016; King et al. 2017. In my talk I will briefly review the observational history of ULXs and present the most widely accepted theoretical considerations for their origin. I will then present our novel physical interpretation along with its major implications. | Hoyle |