10,914 research outputs found
GRB Afterglows from Anisotropic Jets
Some progenitor models of gamma-ray bursts (GRBs) (e.g., collapsars) may
produce anisotropic jets in which the energy per unit solid angle is a
power-law function of the angle (). We calculate light
curves and spectra for GRB afterglows when such jets expand either in the
interstellar medium or in the wind medium. In particular, we take into account
two kinds of wind: one () possibly from a typical red
supergiant star and another () possibly from a Wolf-Rayet
star. We find that in each type of medium, one break appears in the late-time
afterglow light curve for small but becomes weaker and smoother as
increases. When , the break seems to disappear but the afterglow decays
rapidly. Thus, one expects that the emission from expanding, highly anisotropic
jets provides a plausible explanation for some rapidly fading afteglows whose
light curves have no break. We also present good fits to the optical afterglow
light curve of GRB 991208. Finally, we argue that this burst might arise from a
highly anisotropic jet expanding in the wind () from a red
supergiant to interpret the observed radio-to-optical-band afterglow data
(spectrum and light curve).Comment: 12 pages + 10 figures, accepted by Ap
Digging supplementary buried channels: investigating the notch architecture within the CCD pixels on ESA's Gaia satellite
The European Space Agency (ESA) Gaia satellite has 106 CCD image sensors
which will suffer from increased charge transfer inefficiency (CTI) as a result
of radiation damage. To aid the mitigation at low signal levels, the CCD design
includes Supplementary Buried Channels (SBCs, otherwise known as `notches')
within each CCD column. We present the largest published sample of Gaia CCD SBC
Full Well Capacity (FWC) laboratory measurements and simulations based on 13
devices. We find that Gaia CCDs manufactured post-2004 have SBCs with FWCs in
the upper half of each CCD that are systematically smaller by two orders of
magnitude (<50 electrons) compared to those manufactured pre-2004 (thousands of
electrons). Gaia's faint star (13 < G < 20 mag) astrometric performance
predictions by Prod'homme et al. and Holl et al. use pre-2004 SBC FWCs as
inputs to their simulations. However, all the CCDs already integrated onto the
satellite for the 2013 launch are post-2004. SBC FWC measurements are not
available for one of our five post-2004 CCDs but the fact it meets Gaia's image
location requirements suggests it has SBC FWCs similar to pre-2004. It is too
late to measure the SBC FWCs onboard the satellite and it is not possible to
theoretically predict them. Gaia's faint star astrometric performance
predictions depend on knowledge of the onboard SBC FWCs but as these are
currently unavailable, it is not known how representative of the whole focal
plane the current predictions are. Therefore, we suggest Gaia's initial
in-orbit calibrations should include measurement of the onboard SBC FWCs. We
present a potential method to do this. Faint star astrometric performance
predictions based on onboard SBC FWCs at the start of the mission would allow
satellite operating conditions or CTI software mitigation to be further
optimised to improve the scientific return of Gaia.Comment: Accepted for publication in MNRAS, 16 pages, 19 figure
Effect of continuous gamma-ray exposure on performance of learned tasks and effect of subsequent fractionated exposures on blood-forming tissue
Sixteen monkeys trained to perform continuous and discrete-avoidance and fixed-ratio tasks with visual and auditory cues were performance-tested before, during, and after 10-day gamma-ray exposures totaling 0, 500, 750, and 1000 rads. Approximately 14 months after the performance-test exposures, surviving animals were exposed to 100-rad gamma-ray fractions at 56-day intervals to observe injury and recovery patterns of blood-forming tissues. The fixed-ratio, food-reward task performance showed a transient decline in all dose groups within 24 hours of the start of gamma-ray exposure, followed by recovery to normal food-consumption levels within 48 to 72 hours. Avoidance tasks were performed successfully by all groups during the 10-day exposure, but reaction times of the two higher dose-rate groups in which animals received 3 and 4 rads per hour or total doses of 750 and 1000 rads, respectively, were somewhat slower
Proof Theory and Ordered Groups
Ordering theorems, characterizing when partial orders of a group extend to
total orders, are used to generate hypersequent calculi for varieties of
lattice-ordered groups (l-groups). These calculi are then used to provide new
proofs of theorems arising in the theory of ordered groups. More precisely: an
analytic calculus for abelian l-groups is generated using an ordering theorem
for abelian groups; a calculus is generated for l-groups and new decidability
proofs are obtained for the equational theory of this variety and extending
finite subsets of free groups to right orders; and a calculus for representable
l-groups is generated and a new proof is obtained that free groups are
orderable
HST/STIS Imaging of the Host Galaxy of GRB980425/SN1998bw
We present HST/STIS observations of ESO 184-G82, the host galaxy of the
gamma-ray burst GRB 980425 associated with the peculiar Type Ic supernova
SN1998bw. ESO 184-G82 is found to be an actively star forming SBc sub-luminous
galaxy. We detect an object consistent with being a point source within the
astrometric uncertainty of 0.018 arcseconds of the position of the supernova.
The object is located inside a star-forming region and is at least one
magnitude brighter than expected for the supernova based on a simple
radioactive decay model. This implies either a significant flattening of the
light curve or a contribution from an underlying star cluster.Comment: 12 pages, 2 figures, AASTeX v5.02 accepted for publication in ApJ
Letter
Millimeter wave satellite concepts, volume 1
The identification of technologies necessary for development of millimeter spectrum communication satellites was examined from a system point of view. Development of methodology based on the technical requirements of potential services that might be assigned to millimeter wave bands for identifying the viable and appropriate technologies for future NASA millimeter research and development programs, and testing of this methodology with selected user applications and services were the goals of the program. The entire communications network, both ground and space subsystems was studied. Cost, weight, and performance models for the subsystems, conceptual design for point-to-point and broadcast communications satellites, and analytic relationships between subsystem parameters and an overall link performance are discussed along with baseline conceptual systems, sensitivity studies, model adjustment analyses, identification of critical technologies and their risks, and brief research and development program scenarios for the technologies judged to be moderate or extensive risks. Identification of technologies for millimeter satellite communication systems, and assessment of the relative risks of these technologies, was accomplished through subsystem modeling and link optimization for both point-to-point and broadcast applications
Multi-wavelength observations of 1RXH J173523.7-354013: revealing an unusual bursting neutron star
On 2008 May 14, the Burst Alert Telescope aboard the Swift mission triggered
on a type-I X-ray burst from the previously unclassified ROSAT object 1RXH
J173523.7-354013, establishing the source as a neutron star X-ray binary. We
report on X-ray, optical and near-infrared observations of this system. The
X-ray burst had a duration of ~2 h and belongs to the class of rare,
intermediately long type-I X-ray bursts. From the bolometric peak flux of
~3.5E-8 erg/cm^2/s, we infer a source distance of D<9.5 kpc. Photometry of the
field reveals an optical counterpart that declined from R=15.9 during the X-ray
burst to R=18.9 thereafter. Analysis of post-burst Swift/XRT observations, as
well as archival XMM-Newton and ROSAT data suggests that the system is
persistent at a 0.5-10 keV luminosity of ~2E35 (D/9.5 kpc)^2 erg/s. Optical and
infrared photometry together with the detection of a narrow Halpha emission
line (FWHM=292+/-9 km/s, EW=-9.0+/-0.4 Angstrom) in the optical spectrum
confirms that 1RXH J173523.7-354013 is a neutron star low-mass X-ray binary.
The Halpha emission demonstrates that the donor star is hydrogen-rich, which
effectively rules out that this system is an ultra-compact X-ray binary.Comment: Accepted for publication in MNRAS, 13 pages, 6 figures, 5 table
Quasiparticle spectrum and dynamical stability of an atomic Bose-Einstein condensate coupled to a degenerate Fermi gas
The quasiparticle excitations and dynamical stability of an atomic
Bose-Einstein condensate coupled to a quantum degenerate Fermi gas of atoms at
zero temperature is studied. The Fermi gas is assumed to be either in the
normal state or to have undergone a phase transition to a superfluid state by
forming Cooper pairs. The quasiparticle excitations of the Bose-Einstein
condensate exhibit a dynamical instability due to a resonant exchange of energy
and momentum with quasiparticle excitations of the Fermi gas. The stability
regime for the bosons depends on whether the Fermi gas is in the normal state
or in the superfluid state. We show that the energy gap in the quasiparticle
spectrum for the superfluid state stabilizes the low energy energy excitations
of the condensate. In the stable regime, we calculate the boson quasiparticle
spectrum, which is modified by the fluctuations in the density of the Fermi
gas.Comment: 12 pages, 3 figure
Resonance superfluidity in a quantum degenerate Fermi gas
We consider the superfluid phase transition that arises when a Feshbach
resonance pairing occurs in a dilute Fermi gas. We apply our theory to consider
a specific resonance in potassium-40, and find that for achievable experimental
conditions, the transition to a superfluid phase is possible at the high
critical temperature of about 0.5 T_F. Observation of superfluidity in this
regime would provide the opportunity to experimentally study the crossover from
the superfluid phase of weakly-coupled fermions to the Bose-Einstein
condensation of strongly-bound composite bosons.Comment: 4 pages, 3 figure
The Host Galaxy and Optical Light Curve of the Gamma-Ray Burst GRB 980703
We present deep HST/STIS and ground-based photometry of the host galaxy of
the gamma-ray burst GRB 980703 taken 17, 551, 710, and 716 days after the
burst. We find that the host is a blue, slightly over-luminous galaxy with
V_gal = 23.00 +/- 0.10, (V-R)_gal = 0.43 +/- 0.13, and a centre that is
approximately 0.2 mag bluer than the outer regions of the galaxy. The galaxy
has a star-formation rate of 8-13 M_sun/yr, assuming no extinction in the host.
We find that the galaxy is best fit by a Sersic R^(1/n) profile with n ~= 1.0
and a half-light radius of 0.13 arcsec (= 0.72/h_100 proper kpc). This
corresponds to an exponential disk with a scale radius of 0.22 arcsec (=
1.21/h_100 proper kpc). Subtracting a fit with elliptical isophotes leaves
large residuals, which suggests that the host galaxy has a somewhat irregular
morphology, but we are unable to connect the location of GRB 980703 with any
special features in the host. The host galaxy appears to be a typical example
of a compact star forming galaxy similar to those found in the Hubble Deep
Field North. The R-band light curve of the optical afterglow associated with
this gamma-ray burst is consistent with a single power-law decay having a slope
of alpha = -1.37 +/- 0.14. Due to the bright underlying host galaxy the late
time properties of the light-curve are very poorly constrained. The decay of
the optical light curve is consistent with a contribution from an underlying
Type Ic supernova like SN1998bw, or a dust echo, but such contributions cannot
be securely established.Comment: 9 pages, 5 figures, LaTeX using A&A Document Class v4.05, to appear
in A&
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