15 research outputs found
Active cooling control of the CLEO detector using a hydrocarbon coolant farm
We describe a novel approach to particle-detector cooling in which a modular
farm of active coolant-control platforms provides independent and regulated
heat removal from four recently upgraded subsystems of the CLEO detector: the
ring-imaging Cherenkov detector, the drift chamber, the silicon vertex
detector, and the beryllium beam pipe. We report on several aspects of the
system: the suitability of using the aliphatic-hydrocarbon solvent PF(TM)-200IG
as a heat-transfer fluid, the sensor elements and the mechanical design of the
farm platforms, a control system that is founded upon a commercial programmable
logic controller employed in industrial process-control applications, and a
diagnostic system based on virtual instrumentation. We summarize the system's
performance and point out the potential application of the design to future
high-energy physics apparatus.Comment: 21 pages, LaTeX, 5 PostScript figures; version accepted for
publication in Nuclear Instruments and Methods in Physics Research
Can accretion disk properties distinguish gravastars from black holes?
Gravastars, hypothetic astrophysical objects, consisting of a dark energy
condensate surrounded by a strongly correlated thin shell of anisotropic
matter, have been proposed as an alternative to the standard black hole picture
of general relativity. Observationally distinguishing between astrophysical
black holes and gravastars is a major challenge for this latter theoretical
model. In the context of stationary and axially symmetrical geometries, a
possibility of distinguishing gravastars from black holes is through the
comparative study of thin accretion disks around rotating gravastars and
Kerr-type black holes, respectively. In the present paper, we consider
accretion disks around slowly rotating gravastars, with all the metric tensor
components estimated up to the second order in the angular velocity. Due to the
differences in the exterior geometry, the thermodynamic and electromagnetic
properties of the disks (energy flux, temperature distribution and equilibrium
radiation spectrum) are different for these two classes of compact objects,
consequently giving clear observational signatures. In addition to this, it is
also shown that the conversion efficiency of the accreting mass into radiation
is always smaller than the conversion efficiency for black holes, i.e.,
gravastars provide a less efficient mechanism for converting mass to radiation
than black holes. Thus, these observational signatures provide the possibility
of clearly distinguishing rotating gravastars from Kerr-type black holes.Comment: 12 pages, 12 figures. V2: 14 pages, significant discussion and
references added, to appear in Class.Quant.Gra
X-ray bursts in neutron star and black hole binaries from Unconventional Stellar Aspect experiment and Rossi X-ray Timing Explorer data: Detections and upper limits
Narayan & Heyl have developed a theoretical framework to convert suitable upper limits on type I X-ray bursts from accreting black hole candidates (BHCs) into evidence for an event horizon. However, no appropriate observational limit exists in the literature. In this paper we survey 2101.2 ks of data from the Unconventional Stellar Aspect (USA) X-ray timing experiment and 5142 ks of data from the Rossi X-Ray Timing Explorer (RXTE) experiment to obtain a formal constraint of this type. We find that 1122 ks of neutron star data yield a population-averaged mean burst rate of (1.7 ± 0.4) × 10 -5 bursts s -1, while 6081 ks of BHC data yield a 95% confidence level upper limit of 4.9 × 10 -7 bursts s -1. This is the first published limit of this type for BHCs. Applying the theoretical framework of Narayan & Heyl, we calculate regions of unstable luminosity, where the neutron stars are expected to burst and the BHCs would be expected to burst if they had a surface. In this unstable luminosity region, 464 ks of neutron star data yield an averaged meah burst rate of (4.1 ± 0.8) × 10 -5 bursts s -1, and 1512 ks of BHC data yield a 95% confidence level upper limit of 2.0 × 10 -6 bursts s -1 and a strong limit that BHCs do not burst with a rate similar to the rate of neutron stars in these unstable regions. This gives further evidence that BHCs do not have surfaces unless there is some new physics occurring on their surface.link_to_subscribed_fulltex
The ACT vision mission study simulation effort
The Advanced Compton Telescope (ACT) has been selected by NASA for a one-year “vision mission” study. The study’s main goal is to determine feasible instrument configurations to achieve ACT’s sensitivity requirements, and to give recommendations for technology development. Space-based instruments operating in the energy range of nuclear lines are subject to complex backgrounds generated by cosmic rays, earth albedo radiations, trapped particles, and diffuse gamma rays; typically measurements are significantly background-dominated. Therefore accurate, detailed simulations of the background induced in different ACT configurations, and exploration of event selection and reconstruction techniques for reducing these backgrounds, are crucial to determining the capabilities of a given instrument configuration. The ACT simulation team has assembled a complete suite of tools that allows the generation of particle backgrounds for a given orbit, their propagation through any instrument and spacecraft geometry – including delayed photon emission from instrument activation – as well as the selection and reconstruction of Compton events in the given detectors. We describe here the scope of the ACT simulation effort and the suite of tools used