INTEGRAL sees transient activity in the Galactic Bulge: XTE J1751-305 and GRS 1741.9-2853 in outburst

INTEGRAL monitoring observations of the Galactic Bulge (e.g. ATels #438 and #1944) have been performed between 2009 Oct 7th 20:29 and 8th 00:11 (UTC) during which transient activity from a few known sources has been recorded with respect to an observation 6 days earlier. The transient low-mass X-ray binary and known X-ray burster GRS 1741.9-2853 (e.g.

Cocchi et al. A&A 346, L45, 1999) is detected by INTEGRAL/JEM-X at an average flux of about 58 mCrab (9.3E-10 erg/cm2/s) and 25 mCrab (2.75E-10 erg/cm2/s) in 3-10 keV and 10-25 keV, respectively. The source showed strong variations with a maximum at 92 +/- 12 mCrab (3-10 keV) and 37 +/-11 mCrab (10-25 keV) during one pointing between 2009 Oct 7 21:01 and 21:31 (UTC), and another at 46 +/- 2 mCrab (3-10 keV) and 13 +/- 4 mCrab (10-25 keV) during a pointing between 23:09 and 23:39 (UTC).

At other times the source intensity stayed below 10 mCrab and 4 mCrab in above-mentioned energy ranges. IBIS/ISGRI did also detect GRS 1741.9-2853 with an average intensity of about 10 mCrab and significance of 6.1 sigma in the 18-40 keV range. The accreting millisecond X-ray pulsar XTE J1751-305 (e.g.

Markwardt et al. ApJ 575, L21, 2002) is detected with JEM-X at 15 +/- 6 mCrab (3-10 keV) and 22 +/- 10 mCrab (10-25 keV), and with IBIS/ISGRI at 9 +/- 2mCrab (18-40 keV) and 14 +/- 3mCrab (40-100 keV). The source was not detected in the previous Bulge observation with 3-sigma upper limits of 4 mCrab and 2 mCrab in the same respective JEM-X energy bands.

On the other hand, the closeby and recently discovered accreting millisecond X-ray pulsar IGR J17511-3057 (e.g. ATels #2196-2199) has dimmed to below the INTEGRAL detection levels, whereas it was still detected 6 days earlier at about 12 mCrab averaged over the above-mentioned energy bands. The 3-sigma upper limits are now about 5 mCrab and about 2 mCrab between 3-10 keV and 10-25 keV, respectively.

We further notice that none of the X-ray bursters in the close source pair SLX 1744-299/300 is significantly detected in JEM-X with 3-sigma upper limits of about 6 mCrab (3-10 keV) and about 3 mCrab (10-25 keV). This seems to indicate an unusual change in their spectral state, for the pair is simultaneously detected by IBIS/ISGRI at 16.5 mCrab with a significance of 10.6 sigma (18-40 keV), while it was 8 mCrab (6 sigma significance), and 20 mCrab in average for JEM-X, in the previous observation.

The next INTEGRAL Galactic Bulge observation is planned for 2009 October 16th.