94 research outputs found
Evolution of Massive Haloes in non-Gaussian Scenarios
We have performed high-resolution cosmological N-body simulations of a
concordance LCDM model to study the evolution of virialized, dark matter haloes
in the presence of primordial non-Gaussianity. Following a standard procedure,
departures from Gaussianity are modeled through a quadratic Gaussian term in
the primordial gravitational potential, characterized by a dimensionless
non-linearity strength parameter f_NL. We find that the halo mass function and
its redshift evolution closely follow the analytic predictions of Matarrese et
al.(2000). The existence of precise analytic predictions makes the observation
of rare, massive objects at large redshift an even more attractive test to
detect primordial non-Gaussian features in the large scale structure of the
universe.Comment: 7 pages,3 figures, submitted to MNRA
GRB011211: An alternative interpretation of the optical and X-ray spectra in terms of blueshifts
The redshifts of the gamma ray burst (GRB) GRB 011211 has been determined as
2.14 from several absorption lines seen in the spectrum of its optical
afterglow. The spectrum of its X-ray afterglow exhibited several emission
lines,and their identification led to a mean redshift 1.862. A supernova model
has been proposed based on the redshift of the GRB as 2.141. It is shown here
that the redshift interpretation cannot explain the observed spectra, as some
serious inconsistencies exist in the process of redshift determinations in
spectra of both optical and X-ray afterglows. In view of that, an alternative
interpretation of the spectra is presented in terms of blueshifts. Ejection
mechanism is proposed as a possible scenario to explain the blueshifted
spectrum.Comment: 26 pages, one table; in Canadian Journal of Physics, June 200
Charged Particles from Nuclear Reactions Induced by Medium-Energy Protons
This work was supported by National Science Foundation Grant PHY 76-84033 and Indiana Universit
Effects of color superconductivity on the structure and formation of compact stars
We show that if color superconducting quark matter forms in hybrid or quark
stars it is possible to satisfy most of recent observational boundaries on
masses and radii of compact stellar objects. An energy of the order of
erg is released in the conversion from a (metastable) hadronic star
into a (stable) hybrid or quark star in presence of a color superconducting
phase. If the conversion occurs immediately after the deleptonization of the
proto-neutron star, the released energy can help Supernovae to explode. If the
conversion is delayed the energy released can power a Gamma Ray Burst. A delay
between the Supernova and the subsequent Gamma Ray Burst is possible, in
agreement with the delay proposed in recent analysis of astrophysical data.Comment: 4 pages, 2 figures. To be published in Phys.Rev.
The Magnificent Seven: Magnetic fields and surface temperature distributions
Presently seven nearby radio-quiet isolated neutron stars discovered in ROSAT
data and characterized by thermal X-ray spectra are known. They exhibit very
similar properties and despite intensive searches their number remained
constant since 2001 which led to their name ``The Magnificent Seven''. Five of
the stars exhibit pulsations in their X-ray flux with periods in the range of
3.4 s to 11.4 s. XMM-Newton observations revealed broad absorption lines in the
X-ray spectra which are interpreted as cyclotron resonance absorption lines by
protons or heavy ions and / or atomic transitions shifted to X-ray energies by
strong magnetic fields of the order of 10^13 G. New XMM-Newton observations
indicate more complex X-ray spectra with multiple absorption lines. Pulse-phase
spectroscopy of the best studied pulsars RX J0720.4-3125 and RBS 1223 reveals
variations in derived emission temperature and absorption line depth with pulse
phase. Moreover, RX J0720.4-3125 shows long-term spectral changes which are
interpreted as due to free precession of the neutron star. Modeling of the
pulse profiles of RX J0720.4-3125 and RBS 1223 provides information about the
surface temperature distribution of the neutron stars indicating hot polar caps
which have different temperatures, different sizes and are probably not located
in antipodal positions.Comment: 10 pages, 8 figures, to appear in Astrophysics and Space Science, in
the proceedings of "Isolated Neutron Stars: from the Interior to the
Surface", edited by D. Page, R. Turolla and S. Zan
Corrections to flat-space particle dynamics arising from space granularity
The construction of effective Hamiltonians describing corrections to flat
space particle dynamics arising from the granularity of space at very short
distances is discussed in the framework of an heuristic approach to the
semiclassical limit of loop quantum gravity. After some general motivation of
the subject, a brief non-specialist introduction to the basic tools employed in
the loop approach is presented. The heuristical semiclassical limit is
subsequently defined and the application to the case of photons and spin 1/2
fermions is described. The resulting modified Maxwell and Dirac Hamiltonians,
leading in particular to Planck scale corrections in the energy-momentum
relations, are presented. Alternative interpretations of the results and their
limitations, together with other approaches are briefly discussed along the
text. Three topics related to the above methods are reviewed: (1) The
determination of bounds to the Lorentz violating parameters in the fermionic
sector, obtained from clock comparison experiments.(2) The calculation of
radiative corrections in preferred frames associated to space granularity in
the framework of a Yukawa model for the interactions and (3) The calculation of
synchrotron radiation in the framework of the Myers-Pospelov effective theories
describing Lorentz invariance violations, as well as a generalized approach to
radiation in Planck scale modified electrodynamics. The above exploratory
results show that quantum gravity phenomenology provides observational guidance
in the construction of quantum gravity theories and opens up the possibility of
probing Planck scale physics.Comment: 49 pages, 6 figures and 4 tables. Extended version of the talk given
at the 339-th WE-Heraeus-Seminar: Special Relativity, will it survive the
next 100 years?, Potsdam, february 200
X-Ray Polarization of Solar Flares Measured with Rhessi
The degree of linear polarization in solar flares has not yet been precisely
determined despite multiple attempts to measure it with different missions. The
high energy range in particular has very rarely been explored, due to its
greater instrumental difficulties. We approached the subject using the Reuven
Ramaty High Energy Spectroscopic Imager (RHESSI) satellite to study 6 X-class
and 1 M-class flares in the energy range between 100 keV and 350 keV. Using
RHESSI as a polarimeter requires the application of strict cuts to the event
list in order to extract those photons that are Compton scattered between two
detectors. Our measurements show polarization values between 2% and 54%, with
errors ranging from 10% to 26% in 1 sigma level. In view of the large
uncertainties in both the magnitude and direction of the polarization vector,
the results can only reject source models with extreme properties.Comment: 26 pages, 11 figures, accepted for publication by Solar Physic
Studying Millisecond Pulsars in X-rays
Millisecond pulsars represent an evolutionarily distinct group among rotation-powered pulsars. Outside the radio band, the soft X-ray range (--10 keV) is most suitable for studying radiative mechanisms operating in these fascinating objects. X-ray observations revealed diverse properties of emission from millisecond pulsars. For the most of them, the bulk of radiation is of a thermal origin, emitted from small spots (polar caps) on the neutron star surface heated by relativistic particles produced in pulsar acceleration zones. On the other hand, a few other very fast rotating pulsars exhibit almost pure nonthermal emission generated, most probably, in pulsar magnetospheres. There are also examples of nonthermal emission detected from X-ray nebulae powered by millisecond pulsars, as well as from pulsar winds shocked in binary systems with millisecond pulsars as companions. These and other most important results obtained from X-ray observations of millisecond pulsars are reviewed in this paper, as well as results from the search for millisecond pulsations in X-ray flux of the radio-quite neutron star RX J1856.5-3754
Theory of magnetically powered jets
The magnetic theory for the production of jets by accreting objects is
reviewed with emphasis on outstanding problem areas. An effort is made to show
the connections behind the occasionally diverging nomenclature in the
literature, to contrast the different points of view about basic mechanisms,
and to highlight concepts for interpreting the results of numerical
simulations. The role of dissipation of magnetic energy in accelerating the
flow is discussed, and its importance for explaining high Lorentz factors. The
collimation of jets to the observed narrow angles is discussed, including a
critical discussion of the role of `hoop stress'. The transition between disk
and outflow is one of the least understood parts of the magnetic theory; its
role in setting the mass flux in the wind, in possible modulations of the mass
flux, and the uncertainties in treating it realistically are discussed. Current
views on most of these problems are still strongly influenced by the
restriction to 2 dimensions (axisymmetry) in previous analytical and numerical
work; 3-D effects likely to be important are suggested. An interesting problem
area is the nature and origin of the strong, preferably highly ordered magnetic
fields known to work best for jet production. The observational evidence for
such fields and their behavior in numerical simulations is discussed. I argue
that the presence or absence of such fields may well be the `second parameter'
governing not only the presence of jets but also the X-ray spectra and timing
behavior of X-ray binaries.Comment: 29 pages. Publication delays offered the opportunity for further
corrections, an expansion of sect 4.2, and one more Fig. To appear in
Belloni, T. (ed.): The Jet Paradigm - From Microquasars to Quasars, Lect.
Notes Phys. 794 (2009
- …