331 research outputs found
Small-Angle Scattering of X-Rays from Extragalactic Sources by Dust in Intervening Galaxies
Gamma-ray bursts are now known to be a cosmological population of objects,
which are often accompanied by X-ray and optical afterglows. The total energy
emitted in the afterglow can be similar to the energy radiated in the gamma-ray
burst itself. If a galaxy containing a large column density of dust is near the
line of sight to a gamma-ray burst, small-angle scattering of the X-rays due to
diffraction by the dust grains will give rise to an X-ray echo of the
afterglow. A measurement of the angular size of the echo at a certain time
after the afterglow is observed yields a combination of the angular diameter
distances to the scattering galaxy and the gamma-ray burst that can be used to
constrain cosmological models in the same way as a time delay in a
gravitational lens. The scattering galaxy will generally cause gravitational
lensing as well, and this should modify the shape of the X-ray echo from a
circular ring.
The main difficulty in detecting this phenomenon is the very low flux
expected for the echo. The flux can be increased when the gamma-ray burst is
highly magnified by gravitational lensing, or when the deflecting galaxy is at
low redshift. X-ray echos of continuous (but variable) sources, such as
quasars, may also be detectable with high-resolution instruments and would
allow similar measurements.Comment: To be published in Ap
An XMM-Newton observation of Ton S180: Constraints on the continuum emission in ultrasoft Seyfert galaxies
We present an XMM-Newton observation of the bright, narrow-line, ultrasoft
Seyfert 1 galaxy Ton S180. The 0.3-10 keV X-ray spectrum is steep and curved,
showing a steep slope above 2.5 keV (Gamma ~ 2.3) and a smooth, featureless
excess of emission at lower energies. The spectrum can be adequately
parameterised using a simple double power-law model. The source is strongly
variable over the course of the observation but shows only weak spectral
variability, with the fractional variability amplitude remaining approximately
constant over more than a decade in energy. The curved continuum shape and weak
spectral variability are discussed in terms of various physical models for the
soft X-ray excess emission, including reflection off the surface of an ionised
accretion disc, inverse-Compton scattering of soft disc photons by thermal
electrons, and Comptonisation by electrons with a hybrid thermal/non-thermal
distribution. We emphasise the possibility that the strong soft excess may be
produced by dissipation of accretion energy in the hot, upper atmosphere of the
putative accretion disc.Comment: 9 pages, accepted for publication in MNRA
Spectroscopic Confirmation of a Radio-Selected Galaxy Overdensity at z=1.11
We report the discovery of a galaxy overdensity at z=1.11 associated with the
z=1.110 high-redshift radio galaxy MG0442+0202. The group, CL0442+0202, was
found in a near-infrared survey of z>1 radio galaxies undertaken to identify
spatially-coincident regions with a high density of objects red in I-K' color,
typical of z>1 elliptical galaxies. Spectroscopic observations from the Keck
telescope reveal five galaxies within 35" of MG0442+0202 at 1.10<z<1.11. These
member galaxies have broad-band colors and optical spectra consistent with
passively-evolving elliptical galaxies formed at high redshift. A 45ks Chandra
X-Ray Observatory observation detects the radio galaxy and four point sources
within 15" of the radio galaxy, corresponding to a surface density two orders
of magnitude higher than average for X-ray sources at these flux levels,
S(0.5-2keV) > 5e-16 erg/cm2/s. One of these point sources is identified with a
radio-quiet, typeII quasar at z=1.863, akin to sources recently reported in
deep Chandra surveys. The limit on an extended hot intracluster medium in the
Chandra data is S(1-6keV) < 1.9e-15 erg/cm2/s (3-sigma, 30" radius aperture).
Though the X-ray observations do not confirm the existence of a massive, bound
cluster at z>1, the success of the optical/near-infrared targeting of
early-type systems near the radio galaxy validates searches using radio
galaxies as beacons for high-redshift large-scale structure. We interpret
CL0442+0202 to be a massive cluster in the process of formation.Comment: 23 pages, 7 figure
Is MS1054-03 an exceptional cluster? A new investigation of ROSAT/HRI X-ray data
We reanalyzed the ROSAT/HRI observation of MS1054-03, optimizing the channel
HRI selection and including a new exposure of 68 ksec. From a wavelet analysis
of the HRI image we identify the main cluster component and find evidence for
substructure in the west, which might either be a group of galaxies falling
onto the cluster or a foreground source. Our 1-D and 2-D analysis of the data
show that the cluster can be fitted well by a classical betamodel centered only
20arcsec away from the central cD galaxy. The core radius and beta values
derived from the spherical model(beta = 0.96_-0.22^+0.48) and the elliptical
model (beta = 0.73+/-0.18) are consistent. We derived the gas mass and total
mass of the cluster from the betamodel fit and the previously published ASCA
temperature (12.3^{+3.1}_{-2.2} keV). The gas mass fraction at the virial
radius is fgas = (14[-3,+2.5]+/-3)% for Omega_0=1, where the errors in brackets
come from the uncertainty on the temperature and the remaining errors from the
HRI imaging data. The gas mass fraction computed for the best fit ASCA
temperature is significantly lower than found for nearby hot clusters,
fgas=20.1pm 1.6%. This local value can be matched if the actual virial
temperature of MS1054-032 were close to the lower ASCA limit (~10keV) with an
even lower value of 8 keV giving the best agreement. Such a bias between the
virial and measured temperature could be due to the presence of shock waves in
the intracluster medium stemming from recent mergers. Another possibility, that
reconciles a high temperature with the local gas mass fraction, is the
existence of a non zero cosmological constant.Comment: 12 pages, 5 figures, accepted for publication in Ap
The Heisenberg model on the 1/5-depleted square lattice and the CaV4O9 compound
We investigate the ground state structure of the Heisenberg model on the
1/5-depleted square lattice for arbitrary values of the first- and
second-neighbor exchange couplings. By using a mean-field Schwinger-boson
approach we present a unified description of the rich ground-state diagram,
which include the plaquette and dimer resonant-valence-bond phases, an
incommensurate phase and other magnetic orders with complex magnetic unit
cells. We also discuss some implications of ours results for the experimental
realization of this model in the CaV4O9 compound.Comment: 4 pages, Latex, 7 figures included as eps file
Constraints on the high-density nuclear equation of state from the phenomenology of compact stars and heavy-ion collisions
A new scheme for testing nuclear matter equations of state (EsoS) at high
densities using constraints from neutron star phenomenology and a flow data
analysis of heavy-ion collisions is suggested. An acceptable EoS shall not
allow the direct Urca process to occur in neutron stars with masses below
, and also shall not contradict flow and kaon production data of
heavy-ion collisions. Compact star constraints include the mass measurements of
2.1 +/- 0.2 M_sun (1 sigma level) for PSR J0751+1807, of 2.0 +/- 0.1 M_sun from
the innermost stable circular orbit for 4U 1636-536, the baryon mass -
gravitational mass relationships from Pulsar B in J0737-3039 and the
mass-radius relationships from quasiperiodic brightness oscillations in 4U
0614+09 and from the thermal emission of RX J1856-3754. This scheme is applied
to a set of relativistic EsoS constrained otherwise from nuclear matter
saturation properties with the result that no EoS can satisfy all constraints
simultaneously, but those with density-dependent masses and coupling constants
appear most promising.Comment: 15 pages, 8 figures, 5 table
Spin wave analysis to the spatially-anisotropic Heisenberg antiferromagnet on triangular lattice
We study the phase diagram at T=0 of the antiferromagnetic Heisenberg model
on the triangular lattice with spatially-anisotropic interactions. For values
of the anisotropy very close to J_alpha/J_beta=0.50, conventional spin wave
theory predicts that quantum fluctuations melt the classical structures, for
S=1/2. For the regime J_beta<J_alpha, it is shown that the incommensurate
spiral phases survive until J_beta/J_alpha=0.27, leaving a wide region where
the ground state is disordered. The existence of such nonmagnetic states
suggests the possibility of spin liquid behavior for intermediate values of the
anisotropy.Comment: Revised version, 4 pages, Latex (twocolumn), 4 figures as eps files.
To appear in PR
Effect of magnetic field on the strange star
We study the effect of a magnetic field on the strage quark matter and apply
to strange star. We found that the strange star becomes more compact in
presence of strong magnetic field.Comment: 10 pages (LaTex) and 3 postscript figures available on reques
Emerging technologies in physics education
Three emerging technologies in physics education are evaluated from the
interdisciplinary perspective of cognitive science and physics education
research. The technologies - Physlet Physics, the Andes Intelligent Tutoring
System (ITS), and Microcomputer-Based Laboratory (MBL) Tools - are assessed
particularly in terms of their potential at promoting conceptual change,
developing expert-like problem-solving skills, and achieving the goals of the
traditional physics laboratory. Pedagogical methods to maximize the potential
of each educational technology are suggested.Comment: Accepted for publication in the Journal of Science Education and
Technology; 20 page
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