3,657 research outputs found
Strong lensing by fermionic dark matter in galaxies
It has been shown that a self-gravitating system of massive keV fermions in
thermodynamic equilibrium correctly describes the dark matter (DM) distribution
in galactic halos and predicts a denser quantum core towards the center of the
configuration. Such a quantum core, for a fermion mass in the range of keV
keV, can be an alternative interpretation of the
central compact object in Sgr A*. We present in this work the gravitational
lensing properties of this novel DM model in Milky Way-like spiral galaxies. We
describe the lensing effects of the pure DM component both on halo scales,
where we compare them to the effects of the Navarro-Frenk-White and the
Non-Singular Isothermal Sphere DM models, and near the galaxy center, where we
compare them with the effects of a Schwarzschild BH. For the particle mass
leading to the most compact DM core, keV, we draw the
following conclusions. At distances pc from the center of the
lens the effect of the central object on the lensing properties is negligible.
However, we show that measurements of the deflection angle produced by the DM
distribution in the outer region at a few kpc, together with rotation curve
data, could help to discriminate between different DM models. We show that at
distances pc strong lensing effects, such as multiple images and
Einstein rings, may occur. Large differences in the deflection angle produced
by a DM central core and a central BH appear at distances
pc; in this regime the weak-field formalism is no longer applicable and the
exact general-relativistic formula has to be used. We find that quantum DM
cores do not show a photon sphere what implies that they do not cast a shadow.
Similar conclusions apply to the other DM distributions for other fermion
masses in the above specified range and for other galaxy types.Comment: 10 pages, 8 figures. v2: Version published in PR
Induced gravitational collapse at extreme cosmological distances: the case of GRB 090423
CONTEXT: The induced gravitational collapse (IGC) scenario has been
introduced in order to explain the most energetic gamma ray bursts (GRBs),
Eiso=10^{52}-10^{54}erg, associated with type Ib/c supernovae (SNe). It has led
to the concept of binary-driven hypernovae (BdHNe) originating in a tight
binary system composed by a FeCO core on the verge of a SN explosion and a
companion neutron star (NS). Their evolution is characterized by a rapid
sequence of events: [...]. AIMS: We investigate whether GRB 090423, one of the
farthest observed GRB at z=8.2, is a member of the BdHN family. METHODS: We
compare and contrast the spectra, the luminosity evolution and the
detectability in the observations by Swift of GRB 090423 with the corresponding
ones of the best known BdHN case, GRB 090618. RESULTS: Identification of
constant slope power-law behavior in the late X-ray emission of GRB 090423 and
its overlapping with the corresponding one in GRB 090618, measured in a common
rest frame, represents the main result of this article. This result represents
a very significant step on the way to using the scaling law properties, proven
in Episode 3 of this BdHN family, as a cosmological standard candle.
CONCLUSIONS: Having identified GRB 090423 as a member of the BdHN family, we
can conclude that SN events, leading to NS formation, can already occur already
at z=8.2, namely at 650 Myr after the Big Bang. It is then possible that these
BdHNe originate stem from 40-60 M_{\odot} binaries. They are probing the
Population II stars after the completion and possible disappearance of
Population III stars.Comment: 9 pages, 9 figures, to appear on A&
On the thermal and double episode emissions in GRB 970828
Following the recent theoretical interpretation of GRB 090618 and GRB 101023,
we here interpret GRB 970828 in terms of a double episode emission: the first
episode, observed in the first 40 s of the emission, is interpreted as the
proto-black-hole emission; the second episode, observed after t+50 s, as a
canonical gamma ray burst. The transition between the two episodes marks the
black hole formation. The characteristics of the real GRB, in the second
episode, are an energy of erg, a
baryon load of and a bulk Lorentz factor at transparency
of . The clear analogy with GRB 090618 would require also in
GRB 970828 the presence of a possible supernova. We also infer that the GRB
exploded in an environment with a large average particle density part/cm and dense clouds characterized by typical dimensions
of cm and . Such an environment
is in line with the observed large column density absorption, which might have
darkened both the supernova emission and the GRB optical afterglow.Comment: 7 pages, 10 figures, submitted to Ap
A common behavior in the late X-ray afterglow of energetic GRB-SN systems
The possibility to divide GRBs in different subclasses allow to understand
better the physics underlying their emission mechanisms and progenitors. The
induced gravitational collapse scenario proposes a binary progenitor to explain
the time-sequence in GRBs-SNe. We show the existence of a common behavior of
the late decay of the X-ray afterglow emission of this subclass of GRBs,
pointing to a common physical mechanism of their late emission, consistent with
the IGC picture.Comment: 3 pages, to appear in the proceedings of the Gamma-Ray Burst
Symposium 2012 - IAA-CSIC - Marbella, editors: Castro-Tirado, A. J.,
Gorosabel, J. and Park, I.
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