Recent Publications by School Staff

Staff at the School of Physical Sciences endeavour to engage in high level research in their respective fields in addition to teaching duties. Over the summer Lamrpos Nikolopoulos and Masha Chernyakova published interesting papers in the fields of theoretical atomic physics and gamma ray astronomy, respectively.

Lampros' paper was published in Physical Review Letters (PRL 111, 093001 (2013) and is entitled: Time-Dependent Theory of Angular Correlations in Sequential Double Ionization. The paper deals with the ionisation of noble gases under intense and ultrashort wavelength radiation. The abstract is below:

In this work, we emphasize the importance of the bound-state dynamics to the two-electron ejection in
double ionization processes. The conclusions of the present study are pertinent to all excitation or decay
processes that proceed via well-defined intermediate states. A general strong-field time-dependent density
matrix theory is established and applied to the case of neon, allowing us to analyze the role of the ionizing
field in the interpretation of reported angular patterns [M. Kurka et al., J. Phys. B42, 141002 (2009); A. S.
Kheifets, J. Phys. B 42, 134016 (2009)] and in the dynamic ionic alignment. The present analysis reveals
that short-pulse coherent excitation of the neon ionic doublet 2
P1=2;3=2 leads to quantum beats in the twoelectron angular correlation patterns.

Masha's paper was published in the Montly Notes of the Royal Astronomical Society (Mon. Not. R. Astron. Soc. 000, 1–10 (2013)) and the research was carried out in collaboration with the Bogolyubov Institute for Theoretical Physics in the Ukraine and the Centrum Astronomiczne im. M. Kopernika in Poland. The paper is entitled: High-energy gamma-ray emission from Cyg X-1 measured by Fermi and its theoretical implications. The paper addresses measurements from the Fermi Gamma-Ray Telescope as detailed in the abstract:

We have obtained measurements and upper limits on the emission of Cyg X-1 in the photon
energy range of 0.03–300 GeV based on observations by Fermi. We present the results separately for 
the hard and soft spectral states, as well for all of the analysed data. 
In the hard state, we detect a weak steady emission in the 0.1–10 GeV range with a power-law photon
index of Γ ≃ 2.6 ± 0.2 at a 4σ statistical significance. This measurement, even if considered
to be an upper limit, strongly constrains Compton emission of the steady radio jet, present in
that state. The number of relativistic electrons in the jet has to be low enough for the spectral
components due to Compton upscattering of the stellar blackbody and synchrotron radiation
to be within the observed fluxes. If optically-thin synchrotron emission of the jet is to account
for the MeV tail, as implied by the recently-claimed strong polarization in that energy range,
the magnetic field in the jet has to be much above equipartition. The GeV-range measurements
also strongly constrain models of hot accretion flows, most likely present in the hard state, in
which γ-rays are produced from decay of neutral pions produced in collisions of energetic
ions in an inner part of the flow. In the soft state, the obtained upper limits constrain electron
acceleration in a non-thermal corona, most likely present around a blackbody accretion disc.
The coronal emission above 30 MeV has to be rather weak, which is most readily explained
by absorption of γ-rays in pair-producing photon-photon collisions. Then, the size of the bulk
of the corona is less than a few tens of the gravitational radii.