2,506 research outputs found
A search for millimetric emission from Gamma Ray Bursts
We have used the 2- year Differential Microwave Radiometer data from the
COsmic Background Explorer (COBE) satellite to systematically search for
millimetric (31 - 90 GHz) emission from the Gamma Ray Bursts (GRBs) in the
Burst And Transient Source Experiment (BATSE) GRB 3B catalog. The large
beamsize of the COBE instrument (7 degs FWHM) allows for an efficient search of
the large GRB positional error boxes, although it also means that fluxes from
(point source) GRB objects will be somewhat diluted. A likelihood analysis has
been used to look for a change in the level of millimetric emission from the
locations of 81 GRB events during the first two years (1990 & 1991) of the COBE
mission. The likelihood analysis determined that we did not find any
significant millimetric signal before or after the occurance of the GRB. We
find 95% confidence level upper limits of 175, 192 and 645 Jy or, in terms of
fluxes, of 9.6, 16.3 and 54.8 10^{-13} erg/cm^2/s, respectively at 31, 53 and
90 GHz. We also look separately at different classes of GRBs, including a study
of the top ten (in peak flux) GRBs, the "short burst" and "long burst" subsets,
finding similar upper limits. While these limits may be somewhat higher than
one would like, we estimate that using this technique with future planned
missions could push these limits down to \sim 1 mJy.Comment: 21 pages, 5 figures, to be published in The Astrophysical Journa
Design parameters for the Very Large Hadron Collider
The goal of the Very Large Hadron Collider (VLHC) is to extend the energy frontier beyond LHC. The proposed design center-of-mass energy for the VLHC pp collider is 100 TeV, with a luminosity of le34 cm/sup -2/ sec/sup -1/. At present accelerator designs and calculations are being carried out for two different magnet technologies, one using superferric magnets at 2 Tesla (T), the other using high-field design with B>10 T. This paper summarizes the accelerator parameters for these two designs. We discuss the design parameters that have the largest effects on the performance of the accelerator and therefore need careful optimization. (11 refs)
COBE Observations of the Microwave Counterparts of Gamma Ray Bursts
We have used the data from the COBE satellite to search for delayed microwave
emission (31 - 90 GHz) from Gamma Ray Bursts (GRBs). The large beam
of COBE is well matched to the large positional uncertainties in the GRB
locations, although it also means that fluxes from (point source) GRB objects
will be diluted. In view of this we are doing a statistical search of the GRBs
which occurred during the currently released COBE DMR data (years 1990 and
1991), which overlap GRBs recorded by GRO. Here we concentrate on
just the top 10 GRBs (in peak counts/second). We obtain the limits on the
emission by comparing the COBE fluxes before and after the GRB at the GRB
location. Since it is thought that the microwave emission should lag the GRB
event, we have searched the GRB position for emission in the few months
following the GRB occurrence.Comment: 5 pages, LaTE
A Large-Diameter Hollow-Shaft Cryogenic Motor Based on a Superconducting Magnetic Bearing for Millimeter-Wave Polarimetry
In this paper we present the design and measured performance of a novel
cryogenic motor based on a superconducting magnetic bearing (SMB). The motor is
tailored for use in millimeter-wave half-wave plate (HWP) polarimeters, where a
HWP is rapidly rotated in front of a polarization analyzer or
polarization-sensitive detector. This polarimetry technique is commonly used in
cosmic microwave background (CMB) polarization studies. The SMB we use is
composed of fourteen yttrium barium copper oxide (YBCO) disks and a contiguous
neodymium iron boron (NdFeB) ring magnet. The motor is a hollow-shaft motor
because the HWP is ultimately installed in the rotor. The motor presented here
has a 100 mm diameter rotor aperture. However, the design can be scaled up to
rotor aperture diameters of approximately 500 mm. Our motor system is composed
of four primary subsystems: (i) the rotor assembly, which includes the NdFeB
ring magnet, (ii) the stator assembly, which includes the YBCO disks, (iii) an
incremental encoder, and (iv) the drive electronics. While the YBCO is cooling
through its superconducting transition, the rotor is held above the stator by a
novel hold and release mechanism (HRM). The encoder subsystem consists of a
custom-built encoder disk read out by two fiber optic readout sensors. For the
demonstration described in this paper, we ran the motor at 50 K and tested
rotation frequencies up to approximately 10 Hz. The feedback system was able to
stabilize the the rotation speed to approximately 0.4%, and the measured rotor
orientation angle uncertainty is less than 0.15 deg. Lower temperature
operation will require additional development activities, which we will
discuss
The MAP Satellite Feed Horns
We present the design, manufacturing methods, and characterization of 20
microwave feed horns currently in use on the Microwave Anisotropy Probe (MAP)
satellite. The nature of the cosmic microwave background (CMB) anisotropy
requires a detailed understanding of the properties of every optical component
of a microwave telescope. In particular, the properties of the feeds must be
known so that the forward gain and sidelobe response of the telescope can be
modeled and so that potential systematic effects may be computed. MAP requires
low emissivity, azimuthally symmetric, low-sidelobe feeds in five microwave
bands (K, Ka, Q, V, and W) that fit within a constrained geometry. The beam
pattern of each feed is modeled and compared with measurements; the agreement
is generally excellent to the -60 dB level (80 degrees from the beam peak).
This agreement verifies the beam-predicting software and the manufacturing
process. The feeds also affect the properties and modeling of the microwave
receivers. To this end, we show that the reflection from the feeds is less than
-25 dB over most of each band and that their emissivity is acceptable. The
feeds meet their multiple requirements.Comment: 9 pages with 7 figures, of which 2 are in low-resolution versions;
paper is available with higher quality figures at
http://map.gsfc.nasa.gov/m_mm/tp_links.htm
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