The source was detected in the IBIS/ISGRI mosaic at a significance of 21 sigma in the 20-40 keV energy band and 15 sigma in the 40-80 keV energy range. The corresponding fluxes were 41+/-2 mCrab and 42+/-3 mCrab. The source is also detected by JEM-X at a significance of 18 sigma in the 3-10 keV energy band and 11 sigma in the 10-20 keV energy band. The corresponding fluxes were 52+/-3 mCrab and 46+/-5 mCrab. All uncertainties given on the above fluxes are only statistical and at 1 sigma confidence level.
The combined ISGRI+JEM-X spectrum could be well fit (reduced chi^2/d.o.f.=0.9/17) by using a simple power-law model with the column density fixed at 9.1E22 cm^(-2), as measured by Swift/XRT (see ATel #8649). The measured power-law photon index is 2.7+/-0.2 and the spectral flux evaluated from the fit is of 7.5E-10 ergs/cm^2/s in the 5-20 keV energy band and 8.3E-10 ergs/cm^2/s in the 20-100 keV energy band.
We note that neither IBIS/ISGRI nor JEM-X are able to discriminate between the different X-ray emitting sources currently active in the vicinities of Swift J174540.7-290015 due to the limited angular resolution of these instruments compared to, e.g., Swift/XRT. However, we used previous INTEGRAL observations of the Galactic Bulge to verify that none of the other sources could provide a dominating contribution in the 3-20 keV and 20-100 keV energy bands. Observations carried out in the satellite revolution 1597 (from 2015-10-11 at 05:57 to 09:34 UTC) and 1603 (from 2015-10-27 at 03:57 to08:41 UTC) showed that there is no overlapping source detected by IBIS/ISGRI and only 1A 1742-289 has a position compatible with that of Swift J174540.7-290015 in JEM-X. The latter source is always detected by JEM-X in all the above mentioned revolutions at a flux of ~15 mCrab in the 3-10 keV energy band and ~10 mCrab in the 10-20 keV energy band. We are thus confident that the reported spectrum and fluxes for Swift J174540.7-290015 from IBIS/ISGRI cannot be largely contaminated by any other source currently active around the new Swift transients, while for JEM-X a contamination at a level of 20% is possible.
Further observations of the Galactic Bulge with INTEGRAL are already planned for the coming weeks.
|Publication date||12 Feb 2016|
|Number of pages||1|
|Publication status||Published - 12 Feb 2016|
|Series||The Astronomer's telegram|