324 research outputs found
Exploring the Physics of Type Ia Supernovae Through the X-ray Spectra of their Remnants
We present the results of an ongoing project to use the X-ray observations of
Type Ia Supernova Remnants to constrain the physical processes involved in Type
Ia Supernova explosions. We use the Tycho Supernova Remnant (SN 1572) as a
benchmark case, comparing its observed spectrum with models for the X-ray
emission from the shocked ejecta generated from different kinds of Type Ia
explosions. Both the integrated spectrum of Tycho and the spatial distribution
of the Fe and Si emission in the remnant are well reproduced by delayed
detonation models with stratified ejecta. All the other Type Ia explosion
models fail, including well-mixed deflagrations calculated in three dimensions.Comment: 5 pages, 3 figures, to appear in the proceedings of the "Stellar end
products" workshop, 13-15 April 2005, Granada, Spain, ed. M.A. Perez-Torres,
Vol. 77 (Jan 2006) of MmSA
Supernova Ejecta in the Youngest Galactic Supernova Remnant G1.9+0.3
G1.9+0.3 is the youngest known Galactic supernova remnant (SNR), with an
estimated supernova (SN) explosion date of about 1900, and most likely located
near the Galactic Center. Only the outermost ejecta layers with free-expansion
velocities larger than about 18,000 km/s have been shocked so far in this
dynamically young, likely Type Ia SNR. A long (980 ks) Chandra observation in
2011 allowed spatially-resolved spectroscopy of heavy-element ejecta. We
denoised Chandra data with the spatio-spectral method of Krishnamurthy et al.,
and used a wavelet-based technique to spatially localize thermal emission
produced by intermediate-mass elements (IMEs: Si and S) and iron. The spatial
distribution of both IMEs and Fe is extremely asymmetric, with the strongest
ejecta emission in the northern rim. Fe Kalpha emission is particularly
prominent there, and fits with thermal models indicate strongly oversolar Fe
abundances. In a localized, outlying region in the northern rim, IMEs are less
abundant than Fe, indicating that undiluted Fe-group elements (including 56Ni)
with velocities larger than 18,000 km/s were ejected by this SN. But in the
inner west rim, we find Si- and S-rich ejecta without any traces of Fe, so
high-velocity products of O-burning were also ejected. G1.9+0.3 appears similar
to energetic Type Ia SNe such as SN 2010jn where iron-group elements at such
high free-expansion velocities have been recently detected. The pronounced
asymmetry in the ejecta distribution and abundance inhomogeneities are best
explained by a strongly asymmetric SN explosion, similar to those produced in
some recent 3D delayed-detonation Type Ia models.Comment: 6 pages, 3 figures, submitted to ApJ Letter
Nonuniform Expansion of the Youngest Galactic Supernova Remnant G1.9+0.3
We report measurements of X-ray expansion of the youngest Galactic supernova
remnant, G1.9+0.3, using Chandra observations in 2007, 2009, and 2011. The
measured rates strongly deviate from uniform expansion, decreasing radially by
about 60% along the X-ray bright SE-NW axis from 0.84% +/- 0.06% per yr to
0.52% +/- 0.03% per yr. This corresponds to undecelerated ages of 120-190 yr,
confirming the young age of G1.9+0.3, and implying a significant deceleration
of the blast wave. The synchrotron-dominated X-ray emission brightens at a rate
of 1.9% +/- 0.4% per yr. We identify bright outer and inner rims with the blast
wave and reverse shock, respectively. Sharp density gradients in either ejecta
or ambient medium are required to produce the sudden deceleration of the
reverse shock or the blast wave implied by the large spread in expansion ages.
The blast wave could have been decelerated recently by an encounter with a
modest density discontinuity in the ambient medium, such as found at a wind
termination shock, requiring strong mass loss in the progenitor. Alternatively,
the reverse shock might have encountered an order-of-magnitude density
discontinuity within the ejecta, such as found in pulsating delayed-detonation
Type Ia models. We demonstrate that the blast wave is much more decelerated
than the reverse shock in these models for remnants at ages similar to
G1.9+0.3. Similar effects may also be produced by dense shells possibly
associated with high-velocity features in Type Ia spectra. Accounting for the
asymmetry of G1.9+0.3 will require more realistic 3D Type Ia models.Comment: 6 pages, 4 figures, accepted for publication in ApJ Letters, minor
revision
Non-adiabatic spin torque investigated using thermally activated magnetic domain wall dynamics
Using transmission electron microscopy, we investigate the thermally
activated motion of domain walls (DWs) between two positions in permalloy
(Ni80Fe20) nanowires at room temperature. We show that this purely thermal
motion is well described by an Arrhenius law, allowing for a description of the
DW as a quasi-particle in a 1D potential landscape. By injecting small
currents, the potential is modified, allowing for the determination of the
non-adiabatic spin torque: the non-adiabatic coefficient is 0.010 +/- 0.004 for
a transverse DW and 0.073 +/- 0.026 for a vortex DW. The larger value is
attributed to the higher magnetization gradients present
XMM-Newton observation of Kepler's supernova remnant
We present the first results coming from the observation of Kepler's
supernova remnant obtained with the EPIC instruments on board the XMM-Newton
satellite. We focus on the images and radial profiles of the emission lines (Si
K, Fe L, Fe K) and of the high energy continuum. Chiefly, the Fe L and Si K
emission-line images are generally consistent with each other and the radial
profiles show that the Si K emission extends to a larger radius than the Fe L
emission (distinctly in the southern part of the remnant). Therefore, in
contrast to Cas A, no inversion of the Si- and Fe-rich ejecta layers is
observed in Kepler. Moreover, the Fe K emission peaks at a smaller radius than
the Fe L emission, which implies that the temperature increases inwards in the
ejecta. The 4-6 keV high energy continuum map shows the same distribution as
the asymmetric emission-line images except in the southeast where there is a
strong additional emission. A two color image of the 4-6 keV and 8-10 keV high
energy continuum illustrates that the hardness variations of the continuum are
weak all along the remnant except in a few knots. The asymmetry in the Fe K
emission-line is not associated with any asymmetry in the Fe K equivalent width
map. The Si K maps lead to the same conclusions. Hence, abundance variations do
not cause the north-south brightness asymmetry. The strong emission in the
north may be due to overdensities in the circumstellar medium. In the
southeastern region of the remnant, the lines have a very low equivalent width
and the X-ray emission is largely nonthermal.Comment: 15 pages, 15 figures, accepted for publication in A&
The Angular Expansion and Distance of the Planetary Nebula BD+30 3639
The WFPC2 camera aboard the HST was used to obtain images of the planetary
nebula BD+30 3639 at two epochs separated by 5.663 years. The expansion of the
nebula in the H-alpha and [N II] bands has been measured using several methods.
Detailed expansion maps for both emission lines were constructed from nearly
200 almost independent features. There is good agreement between the
(independent) H-alpha and [N II] proper motions. The central velocity split is
measured from the STIS echelle spectra of the C II] 2326A multiplet to be
+/-36.3 km/s at a position angle of 99 degrees. The angular displacement along
this slit position was measured to be 4.25 mas/yr at 2".47 from the center. We
constructed a tilted ellipsoidal shell model by fitting the radio brightness
variation of the 5 and 15 GHz VLA observations, and making use of the
ground-based echelle spectra from Bryce & Mellema (1999), to estimate distance.
Our model has an axial ratio of 1.56, is inclined to the line of sight by 9.7
degrees, and exhibits an expansion in the plane of the sky which is 2/3 that in
the radial direction, leading to a distance of 1.2 kpc. Not all the kinematic
data fits this simple model, so the distance must still be regarded as
uncertain. Based on the recent model atmosphere of Crowther et al. (2002), a
distance of 1.2 kpc implies a stellar luminosity of 4250 L_sun. The kinematic
age of the nebula varies somewhat from region to region; a good average value
is 800 years, while the expansion along the position of the echelle slit gives
about 600 years.Comment: 21 pages, 9 figures, accepted by AJ, May 200
Langevin Simulation of Thermally Activated Magnetization Reversal in Nanoscale Pillars
Numerical solutions of the Landau-Lifshitz-Gilbert micromagnetic model
incorporating thermal fluctuations and dipole-dipole interactions (calculated
by the Fast Multipole Method) are presented for systems composed of nanoscale
iron pillars of dimension 9 nm x 9 nm x 150 nm. Hysteresis loops generated
under sinusoidally varying fields are obtained, while the coercive field is
estimated to be 1979 14 Oe using linear field sweeps at T=0 K. Thermal
effects are essential to the relaxation of magnetization trapped in a
metastable orientation, such as happens after a rapid reversal of an external
magnetic field less than the coercive value. The distribution of switching
times is compared to a simple analytic theory that describes reversal with
nucleation at the ends of the nanomagnets. Results are also presented for
arrays of nanomagnets oriented perpendicular to a flat substrate. Even at a
separation of 300 nm, where the field from neighboring pillars is only 1
Oe, the interactions have a significant effect on the switching of the magnets.Comment: 19 pages RevTeX, including 12 figures, clarified discussion of
numerical technique
Status and Plans for the Array Control and Data Acquisition System of the Cherenkov Telescope Array
The Cherenkov Telescope Array (CTA) is the next-generation atmospheric
Cherenkov gamma-ray observatory. CTA will consist of two installations, one in
the northern, and the other in the southern hemisphere, containing tens of
telescopes of different sizes. The CTA performance requirements and the
inherent complexity associated with the operation, control and monitoring of
such a large distributed multi-telescope array leads to new challenges in the
field of the gamma-ray astronomy. The ACTL (array control and data acquisition)
system will consist of the hardware and software that is necessary to control
and monitor the CTA arrays, as well as to time-stamp, read-out, filter and
store -at aggregated rates of few GB/s- the scientific data. The ACTL system
must be flexible enough to permit the simultaneous automatic operation of
multiple sub-arrays of telescopes with a minimum personnel effort on site. One
of the challenges of the system is to provide a reliable integration of the
control of a large and heterogeneous set of devices. Moreover, the system is
required to be ready to adapt the observation schedule, on timescales of a few
tens of seconds, to account for changing environmental conditions or to
prioritize incoming scientific alerts from time-critical transient phenomena
such as gamma ray bursts. This contribution provides a summary of the main
design choices and plans for building the ACTL system.Comment: In Proceedings of the 34th International Cosmic Ray Conference
(ICRC2015), The Hague, The Netherlands. All CTA contributions at
arXiv:1508.0589
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