Swift Detection of an Intermediate Duration Burst from SAX J1712.6-3739
ATel #3663; David Palmer (LANL) on behalf of the Swift team
on 27 Sep 2011; 03:42 UT
Distributed as an Instant Email Notice Transients
Credential Certification: David M. Palmer (palmer@lanl.gov)
Subjects: X-ray, Gamma Ray, Neutron Star, Transient
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).