20,876 research outputs found
A Chandra Search for Coronal X Rays from the Cool White Dwarf GD 356
We report observations with the Chandra X-ray Observatory of the single,
cool, magnetic white dwarf GD 356. For consistent comparison with other X-ray
observations of single white dwarfs, we also re-analyzed archival ROSAT data
for GD 356 (GJ 1205), G 99-47 (GR 290 = V1201 Ori), GD 90, G 195-19 (EG250 = GJ
339.1), and WD 2316+123 and archival Chandra data for LHS 1038 (GJ 1004) and GD
358 (V777 Her). Our Chandra observation detected no X rays from GD 356, setting
the most restrictive upper limit to the X-ray luminosity from any cool white
dwarf -- L_{X} < 6.0 x 10^{25} ergs/s, at 99.7% confidence, for a 1-keV
thermal-bremsstrahlung spectrum. The corresponding limit to the electron
density is n_{0} < 4.4 x 10^{11} cm^{-3}. Our re-analysis of the archival data
confirmed the non-detections reported by the original investigators. We discuss
the implications of our and prior observations on models for coronal emission
from white dwarfs. For magnetic white dwarfs, we emphasize the more stringent
constraints imposed by cyclotron radiation. In addition, we describe (in an
appendix) a statistical methodology for detecting a source and for constraining
the strength of a source, which applies even when the number of source or
background events is small.Comment: 27 pages, 4 figures, submitted to the Astrophysical Journa
Search for Acoustic Signals from Ultra-High Energy Neutrinos in 1500 km^3 of Sea Water
An underwater acoustic sensor array spanning ~1500 km^3 is used to search for
cosmic-ray neutrinos of ultra-high energies (UHE, E > 10^18 eV). Approximately
328 million triggers accumulated over an integrated 130 days of data taking are
analysed. The sensitivity of the experiment is determined from a Monte Carlo
simulation of the array using recorded noise conditions and expected waveforms.
Two events are found to have properties compatible with showers in the energy
range 10^24 to 5x10^24 eV and 10^22 to 5x10^22 eV. Since the understanding of
impulsive backgrounds is limited, a flux upper limit is set providing the most
sensitive limit on UHE neutrinos using the acoustic technique.Comment: Submitted to PRD. 8 pages, 12 figure
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