1,105 research outputs found
The Integral Burst Alert System (IBAS)
We describe the INTEGRAL Burst Alert System (IBAS): the automatic software
for the rapid distribution of the coordinates of the Gamma-Ray Bursts detected
by INTEGRAL. IBAS is implemented as a ground based system, working on the
near-real time telemetry stream. During the first six months of operations, six
GRB have been detected in the field of view of the INTEGRAL instruments and
localized by IBAS. Positions with an accuracy of a few arcminutes are currently
distributed by IBAS to the community for follow-up observations within a few
tens of seconds of the event.Comment: 7 pages, latex, 5 figures, Accepted for publication on A&A Special
Issue on First Science with INTEGRA
Thermal X-Ray Emission from Shocked Ejecta in Type Ia Supernova Remnants II: Parameters Affecting the Spectrum
The supernova remnants left behind by Type Ia supernovae provide an excellent
opportunity for the study of these enigmatic objects. In a previous work, we
showed that it is possible to use the X-ray spectra of young Type Ia supernova
remnants to explore the physics of Type Ia supernovae and identify the relevant
mechanism underlying these explosions. Our simulation technique is based on
hydrodynamic and nonequilibrium ionization calculations of the interaction of a
grid of Type Ia explosion models with the surrounding ambient medium, coupled
to an X-ray spectral code. In this work we explore the influence of two key
parameters on the shape of the X-ray spectrum of the ejecta: the density of the
ambient medium around the supernova progenitor and the efficiency of
collisionless electron heating at the reverse shock. We also discuss the
performance of recent 3D simulations of Type Ia SN explosions in the context of
the X-ray spectra of young SNRs. We find a better agreement with the
observations for Type Ia supernova models with stratified ejecta than for 3D
deflagration models with well mixed ejecta. We conclude that our grid of Type
Ia supernova remnant models can improve our understanding of these objects and
their relationship to the supernovae that originated them.Comment: Accepted for publication in Ap
Real time localization of Gamma Ray Bursts with INTEGRAL
The INTEGRAL satellite has been successfully launched in October 2002 and has
recently started its operational phase. The INTEGRAL Burst Alert System (IBAS)
will distribute in real time the coordinates of the GRBs detected with
INTEGRAL. After a brief introduction on the INTEGRAL instruments, we describe
the main IBAS characteristics and report on the initial results. During the
initial performance and verification phase of the INTEGRAL mission, which
lasted about two months, two GRBs have been localized with accuracy of about
2-4 arcmin. These observations have allowed us to validate the IBAS software,
which is now expected to provide quick (few seconds delay) and precise (few
arcmin) localization for about 10-15 GRBs per year.Comment: 6 pages, latex, 3 figures, submitted to Adv. Sp. Res., Proceedings of
the 34th COSPAR Scientific Assembly, Houston, 10-19 October 200
Rate constant for the reaction NH2 + NO from 216 to 480 K
The absolute rate constant was measured by the technique of flash photolysis-laser induced fluorescence (FP-LIF). NH2 radicals were produced by the flash photolysis of ammonia and the fluorescent NH2 photons were measured by multiscaling techniques. At each temperature, the results were independent of variations in total pressure, and flash intensity. The results are compared with previous determinations using the techniques of mass spectrometry, absorption spectroscopy, laser absorption spectroscopy, and laser induced fluorescence. The implications of the results are discussed with regard to combustion, post combustion, and atmospheric chemistry. The results are also discussed theoretically
Beam Alignment Techniques Based on the Current Multiplication Effect in Photoconductors Summary Technical Progress Report
Beam alignment techniques based on current multiplication effect in photoconductors for application to spacecraft communications syste
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
Do recent supernovae Ia observations tend to rule out all the cosmologies?
Dark energy and the accelerated expansion of the universe have been the
direct predictions of the distant supernovae Ia observations which are also
supported, indirectly, by the observations of the CMB anisotropies,
gravitational lensing and the studies of galaxy clusters. Today these results
are accommodated in what has become the `concordance cosmology': a universe
with flat spatial sections t=constant with about 70% of its energy in the form
of Einstein's cosmological constant \Lambda.
However, we find that as more and more supernovae Ia are observed, more
accurately and towards higher redshift, the probability that the data are well
explained by the cosmological models decreases alarmingly, finally ruling out
the concordance model at more than 95% confidence level. This raises doubts
against the `standard candle'-hypothesis of the supernovae Ia and their use to
constrain the cosmological models. We need a better understanding of the entire
SN Ia phenomenon in order to have cosmological consequences from them.Comment: Replaced with published versio
Observations of X-rays and Thermal Dust Emission from the Supernova Remnant Kes 75
We present Spitzer Space Telescope and Chandra X-ray Observatory observations
of the composite Galactic supernova remnant Kes 75 (G29.7-0.3). We use the
detected flux at 24 microns and hot gas parameters from fitting spectra from
new, deep X-ray observations to constrain models of dust emission, obtaining a
dust-to-gas mass ratio M_dust/M_gas ~0.001. We find that a two-component
thermal model, nominally representing shocked swept-up interstellar or
circumstellar material and reverse-shocked ejecta, adequately fits the X-ray
spectrum, albeit with somewhat high implied densities for both components. We
surmise that this model implies a Wolf-Rayet progenitor for the remnant. We
also present infrared flux upper limits for the central pulsar wind nebula.Comment: 7 pages, 2 tables, 4 figures, uses emulateapj. Accepted for
publication in Ap
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