Swift J1734.5-3027: Swift discovery of a possible new superbursting transient

ATel #5354; J. A. Kennea, D. N. Burrows (PSU), J. R. Cummings (GSFC/UMBC), C. B. Markwardt, N. Gehrels (GSFC), T. N. Ukwatta (MSU), B. Sbarufatti (INAF-OAB/PSU), D. Malesani (DARK/NBI), C. J. Mountford (Leicester)
on 2 Sep 2013; 04:56 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Jamie A. Kennea (kennea@astro.psu.edu)

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At 09:13:17 UT, the Swift Burst Alert Telescope (BAT) triggered and located possible GRB 130901A or a Galactic source (trigger=569022). Using partial data covering from T-60 to T+63 sec from the trigger time, we report further analysis. The BAT ground-calculated position is RA, Dec = 263.613, -30.454 deg which is

RA(J2000) = 17h 34m 27.2s
Dec(J2000) = -30d 27' 14.1"

with an uncertainty of 3.1 arcmin, (radius, sys+stat, 90% containment). The partial coding was 92%.

Based on only partial coverage as noted above, the BAT light curve shows a gradual rise starting at about T-20 until T+63, beyond which we currently do not have more data.

The time-averaged BAT spectrum from T-5.12 to T+21.47 sec can be fit by a black-body model with temperature kT = 2.5 +/- 0.4 keV.  The spectrum is also consistent with a simple power law with photon index 5.8 +/- 0.7. The fluence in the 15-150 keV band is 1.6 +/- 0.3 x 10^-07 erg/cm2 (based on only partial data). All the quoted errors are at the 90% confidence level.

Swift's X-ray Telescope began observations at 09:15:20.0 UT, 123.0 seconds after the BAT trigger. XRT found a bright, uncatalogued X-ray source located at RA, Dec = 263.6020, -30.3984 which is equivalent to:

RA(J2000) = 17h 34m 24.48s
Dec(J2000) = -30d 23' 54.0"

with an uncertainty of 3.5 arcseconds (radius, 90% containment). We note that a catalogued B=17.2 star lies inside the XRT error circle, which may be the optical counterpart of this X-ray source.

The XRT data in the first orbit are well fit by an absorbed black-body model with average fitted parameters of kT = 1.50 +/- 0.02 keV and absorption N_H = 0.67 +/- 0.05 x 10²² cm^-2. We note that the data show evidence of softening during the initial outburst, suggesting that the spectrum is cooling as it fades, which also follows from the higher temperature fit to earlier BAT data.

The X-ray light-curve shows significant structure, with two apparent broad dips lasting 20-30s in the first ~200 seconds of data. After this period, the light-curve has a featureless decline. We await data from later observations by Swift to constrain the rate of fading of this source.

The X-ray spectrum is extremely soft, and consistent with X-ray emission during a thermonuclear X-ray burst from a neutron star system.  However, the duration is more suggestive of a thermonuclear "superburst". There are no known X-ray bursters in the XRT or BAT error circles.

We note that XRT data from Swift's second snapshot observation of this source appear to be corrupted, and we caution against drawing any conclusions from them until the problems with these data are resolved by the XRT team. Data from the first snapshot and from the 3rd snapshot appear to be uncorrupted.

Swift observations of this source are on-going.