42 research outputs found
The X-ray Ridge Surrounding Sgr A* at the Galactic Center
We present the first detailed simulation of the interaction between the
supernova explosion that produced Sgr A East and the wind-swept inner ~ 2-pc
region at the Galactic center. The passage of the supernova ejecta through this
medium produces an X-ray ridge ~ 9'' to 15'' to the NE of the supermassive
black hole Sagittarius A* (Sgr A*). We show that the morphology and X-ray
intensity of this feature match very well with recently obtained Chandra
images, and we infer a supernova remnant age of less than 2,000 years. This
young age--a factor 3--4 lower than previous estimates--arises from our
inclusion of stellar wind effects in the initial (pre-explosion) conditions in
the medium. The supernova does not clear out the central ~ 0.2-pc region around
Sgr~A* and does not significantly alter the accretion rate onto the central
black hole upon passage through the Galactic center.Comment: 10 pages, 3 figures, submitted to ApJ
The Environments around Long-Duration Gamma-Ray Burst Progenitors
Gamma-ray burst (GRB) afterglow observations have allowed us to significantly
constrain the engines producing these energetic explosions. Te redshift and
position information provided by these afterglows have already allowed us to
limit the progenitors of GRBs to only a few models. The afterglows may also
provide another observation that can place further constraints on the GRB
progenitor: measurements telling us about the environments surrounding GRBs.
Current analyses of GRB afterglows suggest that roughly half of long-duration
gamma-ray bursts occur in surroundings with density profiles that are uniform.
We study the constraints placed by this observation on both the classic
``collapsar'' massive star progenitor and its relative, the ``helium-merger''
progenitor. We study several aspects of wind mass-loss and find that our
modifications to the standard Wolf-Rayet mass-loss paradigm are not sufficient
to produce constant density profiles. Although this does not rule out the
standard ``collapsar'' progenitor, it does suggest a deficiency with this
model. We then focus on the He-merger models and find that such progenitors can
fit this particular constraint well. We show how detailed observations can not
only determine the correct progenitor for GRBs, but also allow us to study
binary evolution physics.Comment: 44 pages including 11 figure
Probing the Density in the Galactic Center Region: Wind-Blown Bubbles and High-Energy Proton Constraints
Recent observations of the Galactic center in high-energy gamma-rays (above
0.1TeV) have opened up new ways to study this region, from understanding the
emission source of these high-energy photons to constraining the environment in
which they are formed. We present a revised theoretical density model of the
inner 5pc surrounding Sgr A* based on the fact that the underlying structure of
this region is dominated by the winds from the Wolf-Rayet stars orbiting Sgr
A*. An ideal probe and application of this density structure is this high
energy gamma-ray emission. We assume a proton-scattering model for the
production of these gamma-rays and then determine first whether such a model is
consistent with the observations and second whether we can use these
observations to further constrain the density distribution in the Galactic
center.Comment: 36 pages including 17 figures, submitted to ApJ, comments welcom
Spin-Induced Disk Precession in Sagittarius A*
In Sgr A* at the Galactic center, by far the closest and easiest supermassive
black hole we can study, the observational evidence is increasingly pointing to
the presence of a compact, hot, magnetized disk feeding the accretor. In such
low-Mach-number plasmas, forces arising, e.g., from pressure gradients in the
plasma, can altogether negate the warping of disks around Kerr black holes
caused by the Bardeen-Petterson effect and can lead to coherent precession of
the entire disk. In this Letter, we present for the first time highly detailed
3D SPH simulations of the accretion disk evolution in Sgr A*, guided by
observational constraints on its physical characteristics, and conclude that
indeed the Bardeen-Petterson effect is probably absent in this source. Given
what we now understand regarding the emission geometry in this object, we
suggest that a ~ 50-500-day modulation in Sgr A*'s spectrum, arising from the
disk precession, could be an important observational signature; perhaps the ~
106-day period seen earlier in its radio flux, if confirmed, could be due to
this process. On the other hand, if future observations do not confirm this
long modulation in Sgr A*'s spectrum, this would be an indication that either
the disk size or orientation is very different from current estimates, or that
the black hole is not spinning at all (unlikely), or that our current
understanding of how it produces its radiative output is incorrect.Comment: 14 pages, 3 figures; small changes to sections 2.1 and 3; accepted
for publication in Ap
Nucleosynthetic Yields from "Collapsars"
The "collapsar" engine for gamma-ray bursts invokes as its energy source the
failure of a normal supernova and the formation of a black hole. Here we
present the results of the first three-dimensional simulation of the collapse
of a massive star down to a black hole, including the subsequent accretion and
explosion. The explosion differs significantly from the axisymmetric scenario
obtained in two-dimensional simulations; this has important consequences for
the nucleosynthetic yields. We compare the nucleosynthetic yields to those of
hypernovae. Calculating yields from three-dimensional explosions requires new
strategies in post-process nucleosynthesis; we discuss NuGrid's plan for
three-dimensional yields.Comment: To appear in the Conference Proceedings for the "10th Symposium on
Nuclei in the Cosmos (NIC X)", July 27 - August 1 2008, Mackinack Island,
Michigan, US
Difficulties in Probing Nuclear Physics: A Study of Ti and Ni
The nucleosynthetic yield from a supernova explosion depends upon a variety
of effects: progenitor evolution, explosion process, details of the nuclear
network, and nuclear rates. Especially in studies of integrated stellar yields,
simplifications reduce these uncertainties. But nature is much more complex,
and to actually study nuclear rates, we will have to understand the full,
complex set of processes involved in nucleosynthesis. Here we discuss a few of
these complexities and detail how the NuGrid collaboration will address them.Comment: To appear in the Conference Proceedings for the "10th Symposium on
Nuclei in the Cosmos (NIC X)", July 27 - August 1 2008, Mackinack Island,
Michigan, US