44 research outputs found
A Monte Carlo Study of the 6.4 keV Emission at the Galactic Center
Strong fluorescent Fe line emission at 6.4 keV has been observed from the Sgr
B2 giant molecular cloud located in the Galactic Center region. The large
equivalent width of this line and the lack of an apparent illuminating nearby
object indicate that a time-dependent source, currently in a low-activity
state, is causing the fluorescent emission. It has been suggested that this
illuminator is the massive black hole candidate, Sgr A*, whose X-ray luminosity
has declined by an unprecedented six orders of magnitude over the past 300
years. We here report the results of our Monte Carlo simulations for producing
this line under a variety of source configurations and characteristics. These
indicate that the source may in fact be embedded within Sgr B2, although
external sources give a slightly better fit to the data. The weakened
distinction between the internal and external illuminators is due in part to
the instrument response function, which accounts for an enhanced equivalent
width of the line by folding some of the continuum radiation in with the
intrinsic line intensity. We also point out that although the spectrum may be
largely produced by K emission in cold gas, there is some evidence in
the data to suggest the presence of warm (~10^5 K) emitting material near the
cold cloud.Comment: 11 pages, 4 figure
Lepton asymmetry and the cosmic QCD transition
We study the influence of lepton asymmetry on the evolution of the early
Universe. The lepton asymmetry is poorly constrained by observations and
might be orders of magnitude larger than the baryon asymmetry , . We find that lepton asymmetries that are large compared to the
tiny baryon asymmetry, can influence the dynamics of the QCD phase transition
significantly. The cosmic trajectory in the phase diagram of strongly
interacting matter becomes a function of lepton (flavour) asymmetry. Large
lepton asymmetry could lead to a cosmic QCD phase transition of first order.Comment: 23 pages, 14 figures; matches published version, including Erratum.
Conclusions, pictures, numerics remained unchange
The Formation of Broad Line Clouds in the Accretion Shocks of Active Galactic Nuclei
Recent work on the gas dynamics in the Galactic Center has improved our
understanding of the accretion processes in galactic nuclei, particularly with
regard to properties such as the specific angular momentum distribution,
density, and temperature of the inflowing plasma. This information can be
valuable in trying to determine the origin of the Broad Line Region (BLR) in
Active Galactic Nuclei (AGNs). In this paper, we explore various scenarios for
the cloud formation based on the underlying principle that the source of plasma
is ultimately that portion of the gas trapped by the central black hole from
the interstellar medium. Based on what we know about the Galactic Center, it is
likely that in highly dynamic environments such as this, the supply of matter
is due mostly to stellar winds from the central cluster. Winds accreting onto a
central black hole are subjected to several disturbances capable of producing
shocks, including a Bondi-Hoyle flow, stellar wind-wind collisions, and
turbulence. Shocked gas is initially compressed and heated out of thermal
equilibrium with the ambient radiation field; a cooling instability sets in as
the gas is cooled via inverse-Compton and bremsstrahlung processes. If the
cooling time is less than the dynamical flow time through the shock region, the
gas may clump to form the clouds responsible for broad line emission seen in
many AGN spectra. Clouds produced by this process display the correct range of
densities and velocity fields seen in broad emission lines. Very importantly,
the cloud distribution agrees with the results of reverberation studies, in
which it is seen that the central line peak responds slower to continuum
changes than the line wings.Comment: 22 pages, 5 figure
Line profile and continuum variability in the very broad-line Seyfert galaxy Mrk 926
We present results of an intensive spectroscopic variability campaign of the
very broad-line Seyfert 1 galaxy Mrk 926. Our aim is to investigate the
broad-line region (BLR) by studying the intensity and line profile variations
of this galaxy on short timescales. High signal-to-noise ratio(S/N) spectra
were taken with the 9.2m Hobby-Eberly Telescope (HET) in identical conditions
during two observing campaigns in 2004 and 2005. After the spectral reduction
and internal calibration we achieved a relative flux accuracy of better than
1%. The rms profiles of the very broad Balmer lines have shapes that differ
from their mean line profiles, consisting of two inner (v 6000
km s) and two outer (v 6000 km s) line components
in addition to a central component (v 600 km s). These
outer and inner line segments varied with different amplitudes during our
campaign. The radius of the BLR is very small with an upper limit of 2
light-days for the H BLR size. We derived an upper limit to the central
black hole mass of . The 2-D
cross-correlation functions CCF(,) of H and H are flat
within the error limits. The response of the Balmer line segments with respect
to continuum variations is different in the outer and inner wings of H
and H. This double structure in the response curves - of two separate
inner and outer components - has also been seen in the rms line profiles. We
conclude that the outer and inner line segments originate in different regions
and/or under different physical conditions.Comment: 13 pages, 18 figures, to be published in A&