Unveiling the Secrets of Gamma Ray Bursts

Gamma Ray Bursts (GRBs) are unpredictable and brief flashes of gamma rays that occur about once a day in random locations in the sky. Since gamma rays do not penetrate the Earth's atmosphere, they are detected by satellites, which automatically trigger ground-based telescopes for follow-up observations at longer wavelengths. In this introduction to Gamma Ray Bursts we review how building a multi-wavelength picture of these events has revealed that they are the most energetic explosions since the Big Bang and are connected with stellar deaths in other galaxies. However, in spite of exceptional observational and theoretical progress in the last 15 years, recent observations raise many questions which challenge our understanding of these elusive phenomena. Gamma Ray Bursts therefore remain one of the hottest topics in modern astrophysics.Comment: 20 pages, 11 figures, review article, draft version, final version will appear in Contemporary Physic

The mass fallback rate of the debris in relativistic stellar tidal disruption events

Highly energetic stellar tidal disruption events (TDEs) provide a way to study black hole characteristics and their environment. We simulate TDEs with the PHANTOM code in a general relativistic and Newtonian description of a supermassive black hole's gravity. Stars, which are placed on parabolic orbits with different parameters $\beta$, are constructed with the stellar evolution code MESA and therefore have realistic stellar density profiles. We study the mass fallback rate of the debris $\dot{M}$ and its dependence on the $\beta$, stellar mass and age as well as the black hole's spin and the choice of the gravity's description. We calculate peak value $\dot{M}_\mathrm{peak}$, time to the peak $t_\mathrm{peak}$, duration of the super-Eddington phase $t_\mathrm{Edd}$, time $t_{>0.5\dot{M}_\mathrm{peak}}$ during which $\dot{M}>0.5\dot{M}_\mathrm{peak}$, early rise-time $\tau_\mathrm{rise}$ and late-time slope $n_\infty$. We recover the trends of $\dot{M}_\mathrm{peak}$, $t_\mathrm{peak}$, $\tau_\mathrm{rise}$ and $n_\infty$ with $\beta$, stellar mass and age, which were obtained in previous studies. We find that $t_\mathrm{Edd}$, at a fixed $\beta$, scales primarily with the stellar mass, while $t_{>0.5\dot{M}_\mathrm{peak}}$ scales with the compactness of stars. The effect of SMBH's rotation depends on the orientation of its rotational axis relative to the direction of the stellar motion on the initial orbit. Encounters on prograde orbits result in narrower $\dot{M}$ curves with higher $\dot{M}_\mathrm{peak}$, while the opposite occurs for retrograde orbits. We find that disruptions, at the same pericenter distance, are stronger in a relativistic tidal field than in a Newtonian. Therefore, relativistic $\dot{M}$ curves have higher $\dot{M}_\mathrm{peak}$, and shorter $t_\mathrm{peak}$ and $t_\mathrm{Edd}$.Comment: 18 pages, 11 figures, accepted for publication in Ap

An Autonomous Reference Frame for Relativistic GNSS

Current GNSS systems rely on global reference frames which are fixed to the Earth (via the ground stations) so their precision and stability in time are limited by our knowledge of the Earth dynamics. These drawbacks could be avoided by giving to the constellation of satellites the possibility of constituting by itself a primary and autonomous positioning system, without any a priori realization of a terrestrial reference frame. Our work shows that it is possible to construct such a system, an Autonomous Basis of Coordinates, via emission coordinates. Here we present the idea of the Autonomous Basis of Coordinates and its implementation in the perturbed space-time of Earth, where the motion of satellites, light propagation, and gravitational perturbations are treated in the formalism of general relativity.Comment: 5 pages, 3 figures, in proceedings of the 4th International Colloquium: Scientific and Fundamental Aspects of the Galileo Programme, 4-6 December 2013, Prague, Czech Republic; removed unnecessary indices from eqs. 3,6,7 and corrected minus signs in eqs. 6 and

Spin-induced offset stream self-crossing shocks in tidal disruption events

Tidal disruption events occur when a star is disrupted by a supermassive black hole, resulting in an elongated stream of gas that partly falls back to pericenter. Due to apsidal precession, the returning stream may collide with itself, leading to a self-crossing shock that launches an outflow. If the black hole spins, this collision may additionally be affected by Lense-Thirring precession that can cause an offset between the two stream components. We study the impact of this effect on the outflow properties by carrying out local simulations of collisions between offset streams. As the offset increases, we find that the geometry of the outflow becomes less spherical and more collimated along the directions of the incoming streams, with less gas getting unbound by the interaction. However, even the most grazing collisions we consider significantly affect the trajectories of the colliding gas, likely promoting subsequent strong interactions near the black hole and rapid disc formation. We analytically compute the dependence of the offset to stream width ratio, finding that even slowly spinning black holes can cause both strong and grazing collisions. We propose that the deviation from outflow sphericity may enhance the self-crossing shock luminosity due to a reduction of adiabatic losses, and cause significant variations of the efficiency at which X-ray radiation from the disc is reprocessed to the optical band depending on the viewing angle. These potentially observable features hold the promise of constraining the black hole spin from tidal disruption events.Comment: 14 pages, 11 figures, submitted to MNRAS. Movies from the simulations are available at https://github.com/tajjankovic/Spin-induced-offset-stream-self-crossing-shocks-in-TDEs/tree/main/Movies. Comments welcome!

The host-galaxy response to the afterglow of GRB 100901A

For Gamma-Ray Burst 100901A, we have obtained Gemini-North and Very Large Telescope optical afterglow spectra at four epochs: one hour, one day, three days and one week after the burst, thanks to the afterglow remaining unusually bright at late times. Apart from a wealth of metal resonance lines, we also detect lines arising from fine-structure levels of the ground state of Fe II, and from metastable levels of Fe II and Ni II at the host redshift (z = 1.4084). These lines are found to vary significantly in time. The combination of the data and modelling results shows that we detect the fall of the Ni II 4 F9/2 metastable level population, which to date has not been observed. Assuming that the population of the excited states is due to the UV-radiation of the afterglow, we estimate an absorber distance of a few hundred pc. This appears to be a typical value when compared to similar studies. We detect two intervening absorbers (z = 1.3147, 1.3179). Despite the wide temporal range of the data, we do not see significant variation in the absorption lines of these two intervening systems.Comment: 17 pages, 9 figures. Accepted by Monthly Notices of the Royal Astronomical Society on Jan 11th 201

The Liverpool Telescope: Rapid follow-up observation of Targets of opportunity with a 2 m robotic telescope

The Liverpool Telescope, situated at Roque de los Muchachos Observatory, La Palma, Canaries, is the first 2-m, fully instrumented robotic telescope. It recently began observations. Among Liverpool Telescope's primary scientific goals is to monitor variable objects on all timescales from seconds to years. An additional benefit of its robotic operation is rapid reaction to unpredictable phenomena and their systematic follow up, simultaneous or coordinated with other facilities. The Target of Opportunity Programme of the Liverpool Telescope includes the prompt search for and observation of GRB and XRF counterparts. A special over-ride mode implemented for GRB/XRF follow-up enables observations commencing less than a minute after the alert, including optical and near infrared imaging and spectroscopy. In particular, the moderate aperture and rapid automated response make the Liverpool Telescope excellently suited to help solving the mystery of optically dark GRBs and for the investigation of currently unstudied short bursts and XRFs.Comment: 4 pages, 1 figure. To appear in the Proceedings of The Restless High-Energy Universe, 5-8 May 2003, Amsterdam, E.P.J. van den Heuvel, J.J.M. in 't Zand, and R.A.M.J. Wijers (eds.