Three active neutron star X-ray transients: SAX J1747.0-2853, XTE J1739-285 and GRS 1747-312

Apart from the active black-hole candidate X-ray transient XTE J1817-330 (ATel #714), three known neutron star X-ray transients have been found to be active in the INTEGRAL Galactic Bulge monitoring observations on 9 Feb 2006 (ATel #731). These are SAX J1747.0-2853, XTE J1739-285 and GRS 1747-312. The former two were already active near the end of the second period (26 Oct 2005) of our monitoring observations (ATels #615, #622, #642).

Since there have been no intermediate INTEGRAL observations of the region in between the two periods, due to sun-angle contraints, we cannot say whether they remained active or whether they showed renewed activity. The latter source (located in Terzan 6) is known to show outbursts roughly every 4.5 months (in 't Zand et al. 2003, A&A 406, 233).

Judged from the publicly available RXTE/ASM light curves the last outburst seemed to have indeed occurred in Sep 2005. SAX J1747.0-2853, XTE 1739-285, and GRS 1747-312 were seen with average JEM-X fluxes of 15 mCrab, 66 mCrab and 35 mCrab in the 3-10 keV band, respectively, and of 15 mCrab, 24 mCrab and 18 mCrab in the 10-25 keV band, respectively.

The uncertainty in the JEM-X fluxes in both energy bands is around 1 mCrab. SAX J1747.0-2853 was detected at the 5 sigma level with IBIS/ISGRI with a flux of 8 +/- 2 mCrab, while XTE 1739-285 and GRS 1747-312 were not detected by IBIS/ISGRI with 5 sigma upper limits of typically 10-15 mCrab. Power-law spectral fits to the JEM-X spectra revealed steep power-law indices with values of ~2.8, ~5.7, and ~4.6, for SAX J1747.0-2853, XTE 1739-285, and GRS 1747-312, respectively.

A weak X-ray burst was seen from SAX J147.0-2853 on UTC 11:40, 9 Feb 2006. Further observations are encouraged.