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W. C. Priedhorsky (LANL)
All-sky monitors (ASMs) track changes in the highly variable X-ray sky, and trigger time-critical observations. Previous ASMs have given us only a glimpse of the rich variability of the X-ray sky on timescales less than a day, and have no better sensitivity than pointed instruments circa 1970. An order-of-magnitude improvement, to a few times 10-12 erg cm-2 s-1 (1 day, 5 sigma) would allow the monitoring of all classes of x-ray sources, including cataclysmic variables, late-type stars, and pre-main sequence stars, and provide the first census of time variability in active galactic nuclei. This improvement can be had with wide-field ("Lobster-eye") x-ray optics. There is also room for improvement in the time domain. On-board processing and real-time data links can open our eyes to fast phenomena in the x-ray sky. Immediate transient locations would enable rapid multi-wavelength observations of counterparts.
X-ray flashes (XRFs) are a particular target of improved monitoring. These intense outbursts, with timescales from seconds to hours, have been seen by numerous instruments. Although only a few hundred have been seen, the all-sky rate is tens of thousands per year. We have assembled archival data to produce a global fluence-frequency relationship (log N-log S), and find a power law with slope -1.0 +0.2/0.3. Based on archival measurements of gamma-ray burst (GRB) numbers and spectra, we find that the fraction of X-ray flashes that are counterparts of conventional GRBs is a function of fluence. Although the exact fraction remains uncertain, most XRFs are not GRB counterparts. The fraction of XRFs from non-GRB sources, such as magnetic stars, is greatest for the faintest flashes. Our understanding of XRFs remains poor, much like the murky understanding of GRBs before BATSE, and would greatly benefit from a large, homogenous data set from a next-generation all-sky monitor.
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Bulletin of the American Astronomical Society, 34
© 2002. The American Astronomical Soceity.