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First superburst observed by INTEGRAL, from SAX J1747.0-2853

ATel #3183; J. Chenevez, S Brandt (DTU Space, Denmark), E. Kuulkers (ESA/ESAC, Spain), J. Alfonso-Garzon (LAEX-CAB/INTA-CSIC, Spain), V. Beckmann (APC, France), T. Bird (Southampton, UK), Th. Courvoisier (ISDC, Switzerland), M. Del Santo (INAF/IASF-Roma, Italy), A. Domingo (LAEX-CAB/INTA-CSIC, Spain), K. Ebisawa (ISAS, Japan), P. Jonker (SRON, The Netherlands), P. Kretschmar (ESA/ESAC, Spain), C. Markwardt (GSFC, USA), T. Oosterbroek (ESA/ESTEC, The Netherlands), A. Paizis (INAF-IASF, Italy), K. Pottschmidt (UMBC/NASA GSFC, USA), C. Sanchez-Fernandez (ESA/ESAC, Spain), and R. Wijnands (UvA, The Netherlands)
on 23 Feb 2011; 09:38 UT
Credential Certification: Jerome CHENEVEZ (jerome@dsri.dk)

Subjects: X-ray, Binary, Neutron Star, Transient

Referred to by ATel #: 3217, 3930, 4622, 6602

A re-analysis of the INTEGRAL Galactic Bulge monitoring observation on February 13 (ATel #3172) shows that the flaring behaviour reported from SAX J1747.0-2853 is in fact due to a superburst.

The event started on February 13, 2011 at 13:01:40 UTC with a 2 minutes spike, but the JEM-X (3-30 keV) source light curve attests that the intensity continued to slowly decay during the remaining two hours of the observation. INTEGRAL slewed away from the Galactic centre region while the source intensity was still half of the maximum intensity level at 780 mCrab, and about three times above the average intensity prior to the flare. A rough extrapolation of the light curve suggests a total duration of four hours, which is consistent with a superburst.
The time-resolved spectral analysis confirms the nature of the event: the average JEM-X spectrum prior to the superburst is best described by an absorbed power law, while an additional black-body component is necessary to describe the average superburst spectra taken during the subsequent four exposures of 1800 sec duration each. The temperature decreases steadily from kT=2.1+/-0.1 keV during the spike down to 1.4+/-0.05 keV during the last exposure of the observation. The unabsorbed 3-30 keV persistent source flux prior to the flare is 3.0e-9 erg/cm2/s. The total unabsorbed 3-30 keV flux at the peak of the superburst is 6.7e-8 erg/cm2/s.
As mentioned in ATel #3172, the whole event was only marginally detected by IBIS/ISGRI, and the 15-25 keV light curve during the entire observation is roughly flat at an average flux of 25 mCrab.

This first superburst ever recorded from SAX J1747.0-2853 is also the first superburst observed by INTEGRAL. It is preceded 30 minutes earlier by a shorter type I X-ray burst. The latter started during a 2 minutes slew of the INTEGRAL satellite, but due to the burst duration of several minutes, the tail was still observable during the next stable pointing. A preliminary analysis of this precursor burst indicates a peak count rate approximately at the same level as the peak of the superburst, as well as a dual exponential decay.
The above-mentioned persistent flux is derived from the average spectrum taken during the 1800 sec exposure prior to the first burst, i.e., less than one hour before the onset of the superburst.

We note that this event is only the 2nd superburst seen from a normal X-ray transient source after the superburst observed from 4U 1608-52 in 2005 (ATel #482).