218 research outputs found
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 , are constructed with the stellar evolution
code MESA and therefore have realistic stellar density profiles. We study the
mass fallback rate of the debris and its dependence on the ,
stellar mass and age as well as the black hole's spin and the choice of the
gravity's description. We calculate peak value , time to
the peak , duration of the super-Eddington phase
, time during which
, early rise-time and
late-time slope . We recover the trends of ,
, and with , stellar
mass and age, which were obtained in previous studies. We find that
, at a fixed , scales primarily with the stellar mass,
while 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 curves with higher
, 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
curves have higher , and shorter and
.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!
A flash of polarized optical light points to an aspherical ‘cow’
The astronomical transient AT2018cow is the closest example of the new class of luminous, fast blue optical transients (FBOTs). Liverpool telescope RINGO3 observations of AT 2018cow are reported here, which constitute the earliest polarimetric observations of an FBOT. At 5.7 days post-explosion, the optical emission of AT2018cow exhibited a chromatic polarization spike that reached ∼ 7% at red wavelengths. This is the highest intrinsic polarization recorded for a non-relativistic explosive transient and is observed in multiple bands and at multiple epochs over the first night of observations, before rapidly declining. The apparent wavelength dependence of the polarization may arise through depolarization or dilution of the polarized flux, due to conditions in AT 2018cow at early times. A second 'bump' in the polarization is observed at blue wavelengths at ∼ 12 days. Such a high polarization requires an extremely aspherical geometry that is only apparent for a brief period (<1 d), such as shock breakout through an optically thick disk. For a disk-like configuration, the ratio of the thickness to radial extent must be ∼ 10%.</p
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
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