Swift observations of GRB 130807A/SWIFT J1759.2-2736

ATel #5268; A. Melandri (INAF/OAB), D. N. Burrows (PSU), K. Page (U Leicester), J. A. Kennea (PSU), H. A. Krimm (CRESST/GSFC/NASA), P. Romano (INAF/IASFPA), S. D. Barthelmy (NASA/GSFC), D. Malesani (DARK/NBI), V. D'Elia (ASI-ASDC), G. Tagliaferri (INAF-OAB), N. Gehrels (NASA/GSFC)
on 13 Aug 2013; 00:29 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Jamie A. Kennea (kennea@astro.psu.edu)

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At 10:25:43 UT on 2013 August 7th Swift/BAT triggered on and located a transient source, initially named GRB 130807A (trigger=565651; Melandri et al., GCN #15082). The best available position for this source is

RA (J2000): 17h 59m 12.16s
Dec (J2000): -27d 36' 57.5"

with an uncertainty of 3.6 arcsec (radius, 90% confidence). This location does not correspond with any known cataloged object and is close to the Galactic center (3.3 degrees from the center).

The mask-weighted BAT light curve shows an initial FRED-like peak starting at T-10 sec, peaking at T=0 sec and decaying to background by T+50 sec. This was followed by at least two more weaker peaks, one from roughly T+90 sec to T+130 sec and the second beginning at T+160 sec and continuing until the data cut off at T+243 seconds (Markwardt et al., GCN #15083). The two later peaks are coincident with flares seen in the Swift/XRT data. From the BAT point of view, the detection is consistent either with a long, moderately soft burst, or with a Galactic source.

The XRT light curve showed initially some flare activity between ~10 and ~100 cts/s during the first 300 s. At late times (t>3000s) the source became much fainter, rising from a count rate of ~0.015 c/s after ~3 ks to a maximum of ~0.06 c/s at ~10 ks (with slope alpha of -1.23 (+ 0.44, -0.27), F ∝ t^-α). After that it decayed with a slope of 1.21 (+ 0.30, -0.24) reaching the count rate of ~0.012 c/s at ~80 ks. Then between ~1 and ~5 days the X-ray count rate became roughly constant, decaying with an index of 0.3 (+0.3, -0.6) and consistent with the value of ~0.003 cts/s. This is rather unusual behavior for a GRB lightcurve, raising again the issue of the nature of this source.

The BAT and XRT spectral indices are similar to those observed for GRB 060124 (BAT spectrum gamma = 1.89 +/- 0.19, XRT-WT spectrum gamma = 1.40+/-0.01, XRT-PC spectrum gamma = 2.1+/-0.1; Romano et al., 2006, A&A, 456, 917) and for the transient Tidal Disruption Event Swift J1644+57 (Burrows et al., 2011, Nature, 476, 421), but inconsistent with what generally observed for a Supergiant Fast X-ray Transient. The dynamical range of GRB 130807A (~3300) is similar to that observed for GRB 060124 or for a typical SFXT, but very different from what was observed for Swift J1644+57.

The source was not detected in the BAT hard X-ray transient monitor (15-50 keV) in one-day averages over the period from 2013 August 1-11. This tells us that there is no significant emission in this energy range either before or after the trigger interval. Events like Swift J1644+57 or SFXTs are usually detected in X-rays for several days before the trigger, and events like Swift J1644+57 are detected for multiple days after the trigger. We note that due to its proximity to the Galactic center the extinction towards this object is quite high (E(B-V) = 2.03, Schlegel et al., 1998), there is no detection of an optical counterpart by UVOT with an 3 sigma upper limit of v>19 (De Pasquale & Melandri, GCN #15086).

The nature of this event is still ambiguous: trigger 565651 might be a somewhat unusual GRB, a Galactic transient or even an extragalactic transient, in spite of its proximity to the Galactic center.

Further observations at all wavelengths are strongly encouraged.