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Swift observations of a new outburst of the SFXT SAX J1818.6-1703

ATel #5980; J. A. Kennea (PSU), H. A. Krimm (CRESST/GSFC/USRA), P. Romano (INAF-IASFPA), P. A. Evans (U Leicester), S. D. Barthelmy (GSFC), D. N. Burrows (PSU), V. Mangano (PSU), B. Sbarufatti (PSU), P. Esposito (INAF-IASFMI), N. Gehrels (GSFC), S. Vercellone (INAF-IASFPA)
on 14 Mar 2014; 14:44 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Jamie A. Kennea (kennea@astro.psu.edu)

Subjects: X-ray, Transient

At 08:28:20 UT on 2014 March 13, the Swift Burst Alert Telescope (BAT) triggered (trigger=591551) and located an outburst from SAX J1818.6-1703. The BAT detection was a sub-threshold image trigger with a significance of 6.1 sigma. Swift slewed swiftly to the location. The BAT on-board calculated location is RA, Dec 274.617, -17.046 which is
RA(J2000) = 18h 18m 28s
Dec(J2000) = -17d 02' 45"

with an uncertainty of 3 arc-minutes (radius, 90% containment, including systematic uncertainty). This position is 2.5 arc-minutes from SAX J1818.6-1703. The BAT mask-weighted light curve from T-239 to ~T+610 sec, when a pre-planned slew took the source out of the field of view, shows no significant features. The time-averaged spectrum from 0.0 to 328.0 sec is best fitted by a thermal bremsstrahlung model. This fit gives kT = 25.0 (+14.3/-8.4) keV and a normalization of 0.36 (+0.21/-0.12) (chi squared 53.95 for 58 d.o.f.). For this model the total fluence in the 15-150 keV band is 5.2 +/- 1.5 x 10-7 erg cm-2.

The XRT began observing the field at 08:35:08.7 UT, 408.5 seconds after the BAT trigger. Observations are split over 2 orbits, the first with a ~120s exposure time, and the second, beginning at 09:32UT, contained a ~2.5ks exposure. Utilizing 1.9ks of Photon Counting mode data we find a bright, fading point source inside the BAT error circle at the following position, which is corrected for astrometric errors using the methods described by Goad et al (2007, A&A, 476, 1401) and Evans et al. (2009, MNRAS, 397, 1177): RA/Dec(J2000) = 274.65778, -17.04653, which is equivalent to:

RA(J2000) = 18h 18m 37.87s,
Dec(J2000) = -17d 02' 47.5''

With an error radius of 1.7 arc-seconds (90% confidence). This position lies 0.5 arc-seconds from center of the Chandra error circle of SAX J1818.6-1703 reported by in't Zand et al. (2006, ATEL #915), and therefore is consistent with being that object.

The XRT light curve shows a fading behavior, during the first orbit the source is at 4.0 +/- 0.7 XRT count/s, and during the second orbit, this has dropped to 1.0 +/- 0.1 XRT count/s.

The time averaged XRT spectrum can be fit by an absorbed power-law model with N_H = 3.2 +/- 1.1 x 1023 cm-2, and a photon index of 0.1 +/- 0.6. The time averaged flux for the two orbits is 2.9 +/- 0.3 x 10-10 erg cm-2 s-1 (0.5 -10 keV), uncorrected for absorption. Correcting for absorption the time averaged flux is ~5 x 10-10 erg cm-2 s-1 (0.5 - 10 keV).

Previously, Swift observed bright flares from this source on 2007 October 16 (Romano, et al. 2014, A&A, 562, A2), 2008 March 15 (Barthelmy et al. 2008, GCN 7419), 2009 May 06 (Sidoli et al. 2009, MNRAS, 400, 258), 2009 September 05 (Romano et al. 2012, Mem. SAIt S., 21, 210), and 2009 November 04 (Romano et al. 2009, ATel #2279). The historical light curve from the BAT hard X-ray transient monitor (Krimm et al, 2013, ApJS, 209, 14; 15-50 keV) can be found at
http://swift.gsfc.nasa.gov/results/transients/weak/SAXJ1818.6-1703/ .