Publications by Year: 2002

2002
Kazanas D, Georganopoulos M, Mastichiadis A. The ``Supercritical Pile'' Model for Gamma-Ray Bursts: Getting the νFν Peak at 1 MeV. [Internet]. 2002;578:L15 - L18. WebsiteAbstract
We propose that the internal energy of the gamma-ray burst (GRB) blast waves, thought to be stored in the form of relativistic protons comoving with the blast wave, is converted explosively (i.e., on light crossing timescales) into relativistic electrons of the same Lorentz factor, which are responsible for the production of observed prompt γ-ray emission of the burst. This conversion is the result of the combined effects of the reflection of photons produced within the flow by upstream located matter, their reinterception by the blast wave, and their eventual conversion into e+e- pairs in interactions with the relativistic protons of the blast wave (via the pγ-->pe+e- reaction). This entire procedure is contingent on two conditions on the relativistic protons: a kinematic one imposed by the threshold of the pγ-->pe+e- reaction and a dynamic one related to the column density of the postshock matter to the same process. This latter condition is in essence identical to that of the criticality of a nuclear pile, hence the terminology. It is argued that the properties of relativistic blast waves operating under these conditions are consistent with GRB phenomenology, including the recently found correlation between quiescence periods and subsequent flare fluence. Furthermore, it is shown that, when operating near threshold, the resulting GRB spectrum produces its peak luminosity at an energy (in the lab frame) E~=mec2, thereby providing an answer to this outstanding question of GRBs.
Akylas A, Georgantopoulos I, Griffiths RG, Papadakis IE, Mastichiadis A, Warwick RS, Nandra K, Smith DA. Monitoring RXTE observations of Markarian 348: the origin of the column density variations. [Internet]. 2002;332:L23 - L27. WebsiteAbstract
We analyse 37 RXTE observations of the type 2 Seyfert galaxy Mrk 348 obtained over a period of 14 months. We confirm the spectral variability previously reported by Smith, Georgantopoulos & Warwick, in the sense that the column density decreases by a factor of ~3 as the count rate increases. Column density variations could possibly originate either from the random drift of clouds within the absorption screen, or from photoionization processes. Our modelling of the observed variations implies that the first scenario is more likely. These clouds should lie within a distance of >2 light-years from the source, having a diameter of a few light-days and a density of >107 cm-3 , hence probably residing outside the broad-line region.
Mastichiadis A, Kirk JG. Models of Variability in Blazar Jets. [Internet]. 2002;19:138 - 142. WebsiteAbstract
During the last decade multiwavelength observations of blazars have revealed many interesting patterns in their emission across the EM spectrum. In the present article we will review the time-dependent one-zone models and the models which advocate an acceleration and a radiation zone, and we will make some comparisons between them, especially in light of recent observations of the so-called TeV blazars.
Mastichiadis A. Radiative Processes in Relativistic Outflows. In: Vol. 589. ; 2002. pp. 1. WebsiteAbstract
1. Introduction 2. Superlumminal Motion 3. Doppler Boosting 4. Photon-Photon Pair Binding 5. Synchrotron Radiation 6. Inverse Compton Scattering 6.1. Thomson Scattering 6.2. Klein-Nishina Limit 7. Synchro Self-Compton Radiation 8. Continuity Equation 9. Coda References