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ATEL # 1961; P. Romano (INAF-IASF Palermo), L. Sidoli (INAF-IASF Milano), H.A. Krimm (CRESST/GSFC/USRA), G. Cusumano (INAF-IASF Palermo), E. Kuulkers (ESA/ESAC, Spain; on behalf of the Galactic bulge monitoring team), S. Vercellone (INAF-IASF Palermo), D.N. Burrows (PSU), N. Gehrels (GSFC), J.A. Kennea (PSU), V. La Parola, V. Mangano (INAF-IASF Palermo)
on 11 Mar 2009; 15:07 UT
Distributed as an Instant Email Notice (Request for Observations)
Password Certification: Pat Romano (firstname.lastname@example.org)
Subjects: X-ray, Gamma Ray, Request for Observations, Transients
On 2009 March 10 at 18:39:55.43 UT, the Swift Burst Alert Telescope (BAT) triggered on the supergiant fast X-ray transient XTE J1739-302/IGRJ17391-3021 (trigger=346069). Swift did not slew to the source at once, so no NFI data were collected immediately.
Since this was an image trigger, the Swift/BAT light curve does not show any significant structure. The rate remained roughly constant during the 1600 s observation when the source was in the field of view (18:18:35 - 18:45:15 UT). The average rate for the observation was 0.0195 +/- 0.0025 counts/s/cm2 (15-50 keV), which corresponds to 85 mCrab. The rate increased somewhat to ~100 mCrab toward the end of the observation (the trigger interval). Using the Swift/BAT data set from T-239 to T+963 sec from the recent telemetry downlink, we find that the time-averaged spectrum (T+0.0 to T+320.0 s) is best fit by a simple power-law model with a photon index of 3.02 +- 0.75. The fluence in the 15-150 keV band is 3.6 +- 1.4 x 10^-07 erg/cm2. All the quoted errors are at the 90% confidence level.
The Swift/XRT light curve obtained between 2009-03-10 20:07:35 and 2009-03-10T21:35:54 (1.9ks net exposure) shows a mean count rate of 0.4-1 counts/s. The mean XRT/PC spectrum (T+5257s to T+10538s since the trigger) can be fit with an absorbed powerlaw with a photon index of Gamma=1.2+-0.4 and an absorbing column density of NH=(4+-1)E+22 cm-2 (chi^2_red=0.95 for 25 dof). The mean flux is 8E-11 erg/cm2/s (unabsorbed, 2-10 keV), which translates into a luminosity of 7E+34 erg/s (assuming the optical counterpart distance of 2.7 kpc, Rahoui et al. 2008, A&A, 484, 801). More observations were performed on 2009-03-11 05:52:47 to 07:19:57 (1.9ks net exposure), and the XRT count rate was down to (1.9 +/- 0.4)E-02 counts/s (4.8 sigma detection).
INTEGRAL observations taken on 2009-02-21 09:56 - 13:38, 2009-02-25 13:21 - 17:02, 2009-03-09 11:16 - 14:57 did not reveal the source with a typical 3 sigma upper limit of about 6 mCrab (18-40 keV) per observation. A 4.5 sigma detection was obtained during observations taken on 2009-03-03 11:42 - 15:24, at with an average flux of 7+/-1.5 mCrab (18-40 keV).
The previous outbursts that triggered Swift/BAT occurred on 2008 Aug 13 (ATel #1659) and 2008 Apr 8 (ATel #1466).
Further Swift observations are being carried out. We encourage multifrequency observations.