Probable superbursts in 4U 0614+091 and 4U 1608-522
ATel #483; Erik Kuulkers (ISOC, ESA/ESAC, Spain)
on 6 May 2005; 15:27 UT
Credential Certification: Erik Kuulkers (ekuulker@rssd.esa.int)
Subjects: X-ray, Binary, Neutron Star
Referred to by ATel #: 6668
Inspection of the RXTE/ASM database of 4U 0614+091 reveals a recent flare
which occurred on March 12, 2005. The 1.5-12 keV flux increased by a factor of 5-6
up to 0.3 Crab within ~7.5 hours. About 1.5 hours later the flux had dropped
to 0.17 Crab; ~9.5 hours later it had reached the pre-flare flux level
again. The exponential decay time of the flare is about 2.2 hours.
During the peak of the flare the X-ray emission significantly
hardens with respect to the pre- and post-flare level.
These characteristics are similar to those reported for
other hours-long thermonuclear X-ray events, so-called superbursts,
seen in various other type I X-ray burst sources (see, e.g., Strohmayer & Bildsten
astro-ph/0301544, and ATel #68, #482).
Note that the RXTE/ASM light curve of 4U 0614+091 shows more instances with
flux values higher than about 0.25 Crab. Several of them, however, can be attributed
to normal type I X-ray bursts in the field of view of the RXTE/ASM camera's, possibly
from 4U 0614+091. It is interesting to comment that 4U 0614+091 only sporadically
shows normal type I X-ray bursts (Swank et al. 1978, MNRAS 182, 349;
Brandt et al. 1992, A&A 262, L15), i.e., unlike the other superburst sources,
which possibly constrains superburst modeling.
Further to the discovery of a likely superburst in 4U 1608-522 (ATel #482), one may
add that at the peak of that event the X-ray emission is clearly harder
than the persistent X-ray emission before and after the event. During the decay
of the event the X-ray emission is seen to soften. This further strengthens the
association of this event with a superburst.
