An upper limit to coronal X-rays from single,magnetic white dwarfs |
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Authors: | Rob Cavallo Keith A Arnaud Virginia Trimble |
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Institution: | (1) Astronomy Department, University of Maryland, College Park, 20742 MD, USA;(2) Code 666, Goddard Space Flight Center, 20771 Greenbelt, MD, USA;(3) Physics Department, University of California, 92717 Irvine, CA |
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Abstract: | Pointed ROSAT PSPC exposures of 9277 and 6992 sec, directed toward the nearby, single, cool, magnetic white dwarfs GR 290
and EG 250 yielded no counts significantly above the expected background rate. The corresponding flux limits (for an assumed
source temperature of l keV) are 1.0 and 1.7 × 10−14erg cm−2 s−1, within the 0.1–2.5 keV bandpass of the instrument (99% confidence limits). This is more than an order of magnitude below
the tentative detection level (for GR 290) and limits (for four other similar stars) obtained from archival Einstein data
in 1991. The corresponding limits on coronal electron density are comparable with those implied if cyclotron emission is not
responsible for any of the features observed in the optical spectra of magnetic white dwarfs. X-ray data currently provide
no evidence for the existence of coronae around these stars. A final long observation (25,000 sec of GD 356) is scheduled
for later this year on ROSAT, along with coordinated EUVE observations. |
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Keywords: | White dwarfs stellar coronae X-ray emission |
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