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
Credential Certification: Erik Kuulkers (ekuulker@rssd.esa.int)
Subjects: X-ray, Request for Observations, Binary, Globular Cluster, Neutron Star, 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.
