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Swift/XRT observation of superburst from IGR J17062-6143/Swift J1706.6-6146

ATel #8253; Wataru Iwakiri (RIKEN), Laurens Keek (NASA/GSFC), Motoko Serino(RIKEN), Tod Strohmayer (NASA/GSFC), Jean in 't Zand (SRON), Erik Kuulkers (ESA/ESAC), Takanori Sakamoto (AGU)
on 7 Nov 2015; 00:39 UT
Credential Certification: Motoko Suzuki (motoko@crab.riken.jp)

Subjects: X-ray, Binary, Neutron Star

Swift/XRT followed up the long X-ray burst from IGR J17062-6143/Swift J1706.6-6146 (ATel #8241) observed by MAXI on November 3 10:29:46 UT (= T0). A series of regular observations was performed, with more observations planned. In the first 40 hours, starting at T0+10 ksec, 16.7 ksec of exposure time were accumulated. The observations were carried out partly in PC mode and partly in WT mode. The data in PC mode suffer from pile-up, for which we correct by excluding the center of the point-spread function. For this reason, the WT mode data are of higher statistical quality.

A blackbody provides a reasonable description of the spectra. Especially in WT mode, however, there is an excess in the high energy part. Modeling it with a power law with a photon index of ~1, yields reduced chi^2 values of unity. The residuals indicate an emission line around 1 keV, which may be similar to the feature seen in the previous long X-ray burst from this source (Degenaar et al. ApJ 767, L37, 2013). Within 12 hours after T0, the blackbody temperature decreases on an exponential timescale of 5.2 ± 0.2 hour from kT=1.02 ± 0.02 keV to kT=0.62 ± 0.01 keV.

Initial comparison with the count rate observed by MAXI suggests that the early decay may have been faster, on a timescale of 44 minutes, whereas the Swift/XRT data alone show the decay to be on a timescale of several hours. A fast decay would categorize the burst as "intermediate duration" instead of "superburst", but more precise cross-calibration is needed to confirm this. Further observations with Swift/XRT are planned to investigate signs of changing burning behavior that can help constrain the nature of this rare burst.