717 research outputs found
X-ray observations of the hot phase in Sgr~A*
We analyze 134 ks Chandra ACIS-I observations of the Galactic Centre (GC)
performed in July 2011. The X-ray image with the field of view
contains the hot plasma surrounding the Sgr~A*. The obtained surface brightness
map allow us to fit Bondi hot accretion flow to the innermost hot plasma around
the GC. We have fitted spectra from region up to from Sgr~A* using a
thermal bremsstrahlung model and four Gaussian profiles responsible for
K emission lines of Fe, S, Ar, and Ca. The X-ray surface brightness
profile up to from Sgr~A* found in our data image, was successfully fitted
with the dynamical model of Bondi spherical accretion. By modelling the surface
brightness profile, we derived the temperature and number density profiles in
the vicinity of the black hole. The best fitted model of spherical Bondi
accretion shows that this type of flow works only up to and implies outer
plasma density and temperature to be:
cm and keV respectively. We show
that the Bondi flow can reproduce observed surface brightness profile up to
from Sgr~A* in the Galactic Center. This result strongly suggests the
position of stagnation radius in the complicated dynamics around GC. The
Faraday rotation computed from our model towards the pulsar PSR J1745-2900 near
the GC agrees with the observed one, recently reported.Comment: 10 pages, 7 figures, accepted for publication in A&
Tau longitudinal polarization in B -> D tau nu and its role in the search for charged Higgs boson
We study the longitudinal polarization of the tau lepton in B -> D tau nu
decay. After discussing possible sensitivities of tau decay modes to the tau
polarization, we examine the effect of charged Higgs boson on the tau
polarization in B -> D tau nu. We find a relation between the decay rate and
the tau polarization, and clarify the role of the tau polarization measurement
in the search for the charged Higgs boson.Comment: 17 pages, 3 figures. Tau -> l nu nu is included in Sec. II.
Accordingly the title is changed. References are adde
X-ray Variability of AGN and the Flare Model
Short-term variability of X-ray continuum spectra has been reported for
several Active Galactic Nuclei. Significant X-ray flux variations are observed
within time scales down to 10^3-10^5 seconds. We discuss short variability time
scales in the frame of the X-ray flare model, which assumes the release of a
large hard X-ray flux above a small portion of the accretion disk. The
resulting observed X-ray spectrum is composed of the primary radiation and of a
reprocessed Compton reflection component that we model with numerical radiative
transfer simulations. The incident hard X-rays of the flare will heat up the
atmosphere of the accretion disk and hence induce thermal expansion.
Eventually, the flare source will be surrounded by an optically thick medium,
which should modify the observed spectra.Comment: 4 pages, 3 figures, accepted proceedings for a talk at the conference
"AGN variability from the X-rays to the radio", June 2004, Crimean
Observator
Iron lines from transient and persisting hot spots on AGN accretion disks
[abridged] We model the X-ray reprocessing from a strong co-rotating flare
above an accretion disk in active galactic nuclei. We explore the horizontal
structure and evolution of the underlying hot spot. To obtain the spectral
evolution seen by a distant observer, we apply a general relativity ray-tracing
technique. We concentrate on the energy band around the iron K-line, where the
relativistic effects are most pronounced. Persistent flares lasting for a
significant fraction of the orbital time scale and short, transient flares are
considered. In our time-resolved analysis, the spectra recorded by a distant
observer depend on the position of the flare/spot with respect to the central
black hole. If the flare duration significantly exceeds the light travel time
across the spot, then the spot horizontal stratification is unimportant. On the
other hand, if the flare duration is comparable to the light travel time across
the spot radius, the lightcurves exhibit a typical asymmetry in their time
profiles. The sequence of dynamical spectra proceeds from more strongly to less
strongly ionized re-emission. At all locations within the spot the spectral
intensity increases towards edge-on emission angles, revealing the limb
brightening effect. Future X-ray observatories with significantly larger
effective collecting areas will enable to spectroscopically map out the
azimuthal irradiation structure of the accretion disk and to localize
persistent flares. If the hot spot is not located too close to the marginally
stable orbit of the black hole, it will be possible to probe the reflecting
medium via the sub-structure of the iron K-line. Indications for transient
flares will only be obtained from analyzing the observed lightcurves on the
gravitational time scale of the accreting supermassive black hole.Comment: 15 pages, 8 figures, accepted by Astronomy & Astrophysic
Testing wind as an explanation for the spin problem in the continuum-fitting method
The continuum-fitting method is one of the two most advanced methods of
determining the black hole spin in accreting X-ray binary systems. There are,
however, still some unresolved issues with the underlying disk models. One of
them manifests as an apparent decrease in spin for increasing source
luminosity. Here, we perform a few simple tests to establish whether outflows
from the disk close to the inner radius can address this problem. We employ
four different parametric models to describe the wind and compare these to the
apparent decrease in spin with luminosity measured in the sources LMC~X-3 and
GRS~1915+105. Wind models in which parameters do not explicitly depend on the
accretion rate cannot reproduce the spin measurements. Models with mass
accretion rate dependent outflows, however, have spectra that emulate the
observed ones. The assumption of a wind thus effectively removes the artifact
of spin decrease. This solution is not unique; the same conclusion can be
obtained with a truncated inner disk model. To distinguish among valid models,
high resolution X-ray data and a realistic description of the Comptonization in
the wind will be needed.Comment: 14 pages, 11 figures, accepted by Ap
Constraints on New Scalars from the LHC 125 GeV Higgs Signal
We study the implications the recent results from the LHC Higgs searches have
on scalar new physics. We study the impact on both the Higgs production and
decay from scalars with and without colour, and in cases where decoupling do
and do not happen. We investigate possible constraints on scalar parameters
from the production rate in the diphoton channel, and also the two vector boson
channels. Measurements from both channels can help disentangle new physics due
to colour from that due to charge, and thus reveal the nature of the new scalar
states.Comment: 23 pages, 8 figures; references addded, typos corrected. Published in
PR
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