Flare in 1A 1246-588 is an Eddington-limited type I X-ray burst

ATel #837; E. Kuulkers (ESAC/ESA, Spain) & A. M. Levine (MIT, USA)
on 14 Jun 2006; 08:20 UT
Credential Certification: Erik Kuulkers (ekuulker@rssd.esa.int)

Tweet

Prompted by the announcement of a strong flare seen on 2006 May 26 in the low-mass X-ray binary and X-ray burster 1A 1246-588 (ATel #830) we further analyzed the RXTE/ASM data. The X-ray emission during the flare is significantly harder than the X-ray emission before and after the flare. During the decay of the flare the X-ray emission softens. The decay of the flare can be described by an exponential with a decay time of a few minutes.

The X-ray emission during the first, third and fourth dwell (see ATel #830) is consistent with being due to black-body emission. In the second dwell, i.e., at the peak, the emission clearly deviates from that expected from pure black-body emission. Assuming that during all dwells the emission is black-body like, we derive temperatures of about 1.8, 2.8, 1.85 and 1.65 keV, during the four dwells, respectively, with typical statistical errors of about 0.1 keV. This also suggests that during the decay the X-ray emitting object cools.

We further remark that the raw dwell light curve at the peak of the flare has a flat top that suggests a limiting flux was reached, presumably the Eddington limit. Also, the peak flux (1.9 Crab, 1.5-12 keV, ATel #830; see also Piro et al. 1997, IAUC # 6538 ) corresponds to a luminosity of a few times 10^38 erg/s, if the source is at a distance of 5 kpc (see Bassa et al. 2006, A&A 446, L17), and is thus consistent with the source emitting at the Eddington limit.

The above described findings are characteristic for type I X-ray bursts. It thus suggest the flare to be such an event, where during maximum the Eddington limit was reached. We finally note that, in general, spectra obtained during the photospheric radius-expansion phase of Eddington-limited type I X-ray bursts usually deviate from a precise black-body form. This may indicate such a phase being present during the second dwell, at the peak of the flare.