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4U 1820-303 is entering a low/hard X-ray state

ATEL # 2533; E. Kuulkers (ESA/ESAC), D. Altamirano (UvA) and C. B. Markwardt (U. Maryland & NASA/GSFC)
on 5 Apr 2010; 16:55 UT
Distributed as an Instant Email Notice (Request for Observations)
Password Certification: Erik Kuulkers (

Subjects: X-ray, Request for Observations, Binaries, Globular Clusters, Neutron Stars, Variables

All sky measurements taken by the RXTE/ASM, MAXI/GSC and Swift/BAT, as well as pointed observations by the RXTE/PCA as part of the Galactic bulge scans, show that the ultra-compact low-mass X-ray binary 4U 1820-303 is entering a low/hard state again. The last low/hard state was seen in April-June 2009 and lasted for about 2 months (ATels #2071, #2167).

In soft X-rays (<12 keV) the source is declining steadily in flux over the last days to weeks; it reached about 100-125 mCrab on UT 2010 April 4, approaching the lowest values seen over the last decades. At such low fluxes prolonged low/hard states may emerge. Indeed, indication of a hardening of the source comes from the Swift/BAT hard X-ray (15-50 keV) light curve, which shows an increase in flux since April 2 by a factor of 2 to up to about 80 mCrab on April 4.

Further pointed observations at various wavelengths are encouraged.

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R. E. Rutledge , Editor
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