6,710 research outputs found
The Discovery of Quasisoft and Supersoft Sources in External Galaxies
We apply a uniform procedure to select very soft sources from point sources
observed by Chandra in 4 galaxies. This sample includes one elliptical galaxy
(NGC 4967), 2 face-on spirals (M101 and M83), and an interacting galaxy (M51).
We have found very soft X-ray sources (VSSs) in every galaxy. Some of these fit
the criteria for canonical supersoft sources (SSSs), while others are somewhat
harder. These latter have characteristic values of kT < 300 eV; we refer to
them as quasisoft sources (QSSs). We found a combined total of 149 VSSs in the
4 galaxies we considered; 77 were SSSs and 72 were QSSs. (See the paper for the
original long abstract)Comment: 20 pages, 6 figures. Accepted for publication in Ap
Review of the "Bottom-Up" scenario
Thermalization of a longitudinally expanding color glass condensate with
Bjorken boost invariant geometry is investigated within parton cascade BAMPS.
Our main focus lies on the detailed comparison of thermalization, observed in
BAMPS with that suggested in the Bottom-Up scenario. We demonstrate that the
tremendous production of soft gluons via , which is shown in the
Bottom-Up picture as the dominant process during the early preequilibration,
will not occur in heavy ion collisions at RHIC and LHC energies, because the
back reaction hinders the absolute particle multiplication.
Moreover, contrary to the Bottom-Up scenario, soft and hard gluons thermalize
at the same time. The time scale of thermal equilibration in BAMPS calculations
is of order \as^{-2} (\ln \as)^{-2} Q_s^{-1}. After this time the gluon
system exhibits nearly hydrodynamic behavior. The shear viscosity to entropy
density ratio has a weak dependence on and lies close to the lower bound
of the AdS/CFT conjecture.Comment: Quark Matter 2008 Proceeding
Thermalization through Hagedorn states - the importance of multiparticle collisions
Quick chemical equilibration times of hadrons within a hadron gas are
explained dynamically using Hagedorn states, which drive particles into
equilibrium close to the critical temperature. Within this scheme master
equations are employed for the chemical equilibration of various hadronic
particles like (strange) baryon and antibaryons. A comparison of the Hagedorn
model to recent lattice results is made and it is found that for both Tc =176
MeV and Tc=196 MeV, the hadrons can reach chemical equilibrium almost
immediately, well before the chemical freeze-out temperatures found in thermal
fits for a hadron gas without Hagedorn states.Comment: 8 pages, 3 figures, talk presented at the International Conference on
Strangeness in Quark Matter, Buzios, Rio de Janeiro, Brazil, Sept. 27 - Oct.
2, 200
On the GBM event seen 0.4 sec after GW 150914
In view of the recent report by Connaughton we analyse continuous TTE data of
Fermi-GBM around the time of the gravitational wave event GW 150914. We find
that after proper accounting for low count statistics, the GBM transient event
at 0.4 s after GW 150914 is likely not due to an astrophysical source, but
consistent with a background fluctuation, removing the tension between the
INTEGRAL/ACS non-detection and GBM. Additionally, reanalysis of other short
GRBs shows that without proper statistical modeling the fluence of faint events
is over-predicted, as verified for some joint GBM-ACS detections of short GRBs.
We detail the statistical procedure to correct these biases. As a result, faint
short GRBs, verified by ACS detections, with significances in the broad-band
light curve even smaller than that of the GBM-GW150914 event are recovered as
proper non-zero source, while the GBM-GW150914 event is consistent with zero
fluence.Comment: 13 pages, 12 figures, ApJL (acc.); subm. 2016 March 10, Apr 21 (1st
rev.), May 13 (2nd rev), Jun 1 (3rd rev), and editorial changes by Jun 2 (4th
rev), Jun 8 (5th rev): Our manuscript refers exclusively to
arXiv:1602.03920.v3 since we had no prior access to arXiv:1602.03920.v4/5
(2016 May 31). Note that JG and HFY are not co-authors on
arXiv:1602.03920.v4/
General U(N) gauge transformations in the realm of covariant Hamiltonian field theory
A consistent, local coordinate formulation of covariant Hamiltonian field
theory is presented. While the covariant canonical field equations are
equivalent to the Euler-Lagrange field equations, the covariant canonical
transformation theory offers more general means for defining mappings that
preserve the action functional - and hence the form of the field equations -
than the usual Lagrangian description. Similar to the well-known canonical
transformation theory of point dynamics, the canonical transformation rules for
fields are derived from generating functions. As an interesting example, we
work out the generating function of type F_2 of a general local U(N) gauge
transformation and thus derive the most general form of a Hamiltonian density
that is form-invariant under local U(N) gauge transformations.Comment: 36 pages, Symposium on Exciting Physics: Quarks and gluons/atomic
nuclei/biological systems/networks, Makutsi Safari Farm, South Africa, 13-20
November 2011; Exciting Interdisciplinary Physics, Walter Greiner, Ed., FIAS
Interdisciplinary Science Series, Springer International Publishing
Switzerland, 201
Distillation of Strangelets for low initial mu/T
We calculate the evolution of quark-gluon-plasma droplets during the
hadronization in a thermodynamical model. It is speculated that cooling as well
as strangeness enrichment allow for the formation of strangelets even at very
high initial entropy per baryon and low initial
baryon numbers of . It is shown that the
droplet with vanishing initial chemical potential of strange quarks and a very
moderate chemical potential of up/down quarks immediately charges up with
strangeness. Baryon densities of and strange chemical
potentials of ~MeV are reached if strangelets are stable. The
importance of net--baryon and net--strangeness fluctuations for the possible
strangelet formation at RHIC and LHC is emphasized
Entropy Production in Collisions of Relativistic Heavy Ions -- a signal for Quark-Gluon Plasma phase transition?
Entropy production in the compression stage of heavy ion collisions is
discussed within three distinct macroscopic models (i.e. generalized RHTA,
geometrical overlap model and three-fluid hydrodynamics). We find that within
these models \sim 80% or more of the experimentally observed final-state
entropy is created in the early stage. It is thus likely followed by a nearly
isentropic expansion. We employ an equation of state with a first-order phase
transition. For low net baryon density, the entropy density exhibits a jump at
the phase boundary. However, the excitation function of the specific entropy
per net baryon, S/A, does not reflect this jump. This is due to the fact that
for final states (of the compression) in the mixed phase, the baryon density
\rho_B increases with \sqrt{s}, but not the temperature T. Calculations within
the three-fluid model show that a large fraction of the entropy is produced by
nuclear shockwaves in the projectile and target. With increasing beam energy,
this fraction of S/A decreases. At \sqrt{s}=20 AGeV it is on the order of the
entropy of the newly produced particles around midrapidity. Hadron ratios are
calculated for the entropy values produced initially at beam energies from 2 to
200 AGeV.Comment: 17 pages, 8 figures, uses epsfig.sty; Submitted to Nucl.Phys.
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