2011ATel.3663....1P

On September 26, 2011, 20:11:28 UT, BAT detected high flux from a location consistent SAX J1712.6-3739, an LMXB. Swift slewed immediately to the source. The BAT on-board calculated location is

(RA, Dec) = (258.161, -37.649) = (17h 12m 39s, -37d 38' 56") (J2000)

with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty).

Based on downlinked BAT data from T-120s to T+300s, the emission started

at T-75s, rising smoothly to a peak at T+100s, and declining to near zero at T+200s. The counts are almost all in the 15-25 keV band. Over

the time interval T-74s to T+188s the data are well-fit by OTTB (T∼4.7 keV) or Blackbody (T∼2.7 keV) models with fluence above 15 keV of ∼1.7x10^-6 erg/cm^2

The XRT began observing the field at 20:13:25.9 UT, 117.4 seconds after the BAT trigger. XRT found a bright X-ray source located at

(RA, Dec) = (258.1528, -37.6469) = (17h 12m 36.67s, Dec(J2000) = -37d 38' 48.8") (J2000)

with an uncertainty of 5.9 arcseconds (radius, 90% containment). This position is 15.6 arcseconds from the catalogued location of a known X-ray source: 1RXS J171237.1-373834 in the ROSAT RASSBSC catalogue (identified by SIMBAD with SAX J1712.6-3739 and positionally consistent

within the joint ROSAT-XRT uncertainty).

The XRT light curve shows near-constant emission until ∼T+220s, followed

by high-amplitude variation on multiple timescales for at least 7 minutes. Over T+120s to T+800s, the XRT spectrum is well fit by an absorbed black-body, kT=2.18 ± 0.07 keV and N_H = 1.69 ± 0.14 x 10^22 cm^-2. The average absorption-corrected flux is ∼2x10^-8 erg/s/cm^2.

This timescale indicates that event is an intermediate duration burst (typical timescale ∼30 minutes), not a superburst (typical timescale ∼10

hours), and not an ordinary Type I burst (typical timescale ∼10 seconds). (See Cumming et al. 2006, ApJ 646, 429.)

The history of this source includes an ordinary Type I burst seen by BAT in July 2010 (ATel #2717; Strohmeyer & Baumgartner, 2010) and an intermediate duration burst in 1999 found in RXTE/ASM archival data (ATEL #2141; Kuulkers, 2009).