MAXI/GSC detection of an X-ray outburst probably from SAX J1747.0-2853 and Swift followup observation of the Galactic center region

ATel #5041; H. Negoro (Nihon U.), J. A. Kennea (PSU), S. Ueno, H. Tomida, S. Nakahira, M. Kimura, M. Ishikawa (JAXA), T. Mihara, M. Sugizaki, M. Serino, M. Morii, T. Yamamoto, J. Sugimoto, T. Takagi, M. Matsuoka (RIKEN), N. Kawai, R. Usui, K. Ishikawa, T. Yoshii (Tokyo Tech), A. Yoshida, T. Sakamoto, Y. Nakano, Y. Kawakubo (AGU), H. Tsunemi, M. Sasaki (Osaka U.), M. Nakajima, T. Onodera, K. Fukushima, K. Suzuki (Nihon U.), Y. Ueda, M. Shidatsu, T. Kawamuro (Kyoto U.), Y. Tsuboi, M. Higa (Chuo U.), M. Yamauchi, K. Yoshidome, Y. Ogawa, H. Yamada (Miyazaki U.), K. Yamaoka (Nagoya U.) report on behalf of the MAXI team
on 3 May 2013; 02:28 UT
Credential Certification: Hitoshi Negoro (negoro@phys.cst.nihon-u.ac.jp)

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MAXI/GSC detected an X-ray outburst from the Galactic center region, which started from around April 11th, 2012 (MJD 56393) and reached about 120 mCrab in the 2-20 keV band on April 14th (MJD 56396). After that, the 2-20 keV flux has exponentially decayed with a time constant of 7+-2 days. The brightest part of the enhanced flux on the GSC image locates the position near SAX J1747.0-2853, but MAXI/GSC can not resolve other sources near the Galactic center, including the newly discovered SGR J1745-29 (e.g., ATel #5006, #5009, #5020, #5037). Although the flare seen by MAXI is not consistent (brighter and earlier) than this new SGR. We therefore requested a Swift ToO observation to search from enhanced X-ray emission SAX J1747.0-2853 and other nearby sources.

At 17:39UT on April 26th, 2013 Swift took a 7-point tiling observation of a region including Sgr A*, KS 1741-293 and SAX J1747.0-2853 for a total of 3.5ks. Utilizing XRT PC mode data, SAX J1747.0-2853 was detected in a single tile with total exposure time of 470s at a count rate of ~1 XRT count/s, confirming that the source is in an active state. By comparison, the most recent observations of this source taken between April 29th and June 28th of 2012 show no detection in a combined 1.6ks observation, with a 90% confidence level upper limit of 2.5 x 10^-3 XRT count/s. The spectrum can be well fit by an absorbed power-law model with N_H = 5 +/- 2 x 10^22 cm^-2 and photon index of 2.1 +/- 0.9. The observed flux in the 0.3-10 keV XRT band is ~ 6 x 10^-11 erg/s/cm^2.

SAX J1747.0-2853 is therefore the best candidate of the outburst source detected by MAXI, though the flux observed by Swift is about 0.1-0.3 times the flux expected from the peak flux and the decay constant obtained in the MAXI observation. Swift/BAT data also show that the average count rate from April 11th to April 27th is 1.29 +- 0.46 x 10^-3 c/cm2/sec (http://heasarc.gsfc.nasa.gov/docs/swift/results/transients/weak/SAXJ1747.0-2853/), which strengthens this conclusion. On the other hand, no correlation is found for light curves of KS 1741-293 (AX J1744.8-2921) currently in outburst (ATel #4840).

The GSC spectrum in outburst, especially in the rise phase, tends to be harder than that in the giant outburst in February 2011 (ATel #3123, #3163), but the reliable spectrum could not be obtained due to large contamination from 1A 1742-294 and the Galactic ridge emission. We also note that the previous outburst of SAX J747.0-2853, detected with INTEGRAL/JEM-X on April 14th, 2012 (ATel #3930), was not recognized in the MAXI/GSC light curve.

MAXI public data of SAX J1747.0-2853