Further INTEGRAL results on SAX J1753.5-2349 and IGR J17375-3022

ATel #1810; Marion Cadolle Bel, Erik Kuulkers (ESA/ESAC, Spain), Jerome Chenevez (DTU/NSI, Denmark), Volker Beckmann, Simona Soldi (ISDC, Geneva University)
on 24 Oct 2008; 13:01 UT
Credential Certification: Erik Kuulkers (ekuulker@rssd.esa.int)

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The IBIS/ISGRI instrument aboard INTEGRAL confirms the brief outburst of the X-ray burster SAX J1753.5-2349, detected by RXTE/PCA and Swift (ATels #1799, #1809), which was in the field of view during the public TOO observations on H 1743-322 (ATels #1779, #1798).

Between UT 2008, Oct 10, 05:19 and 11:20 the source was not detected by ISGRI with a 3 sigma upper limit of about 6 mCrab (18-40 keV). It was clearly visible between UT 2008 Oct 10, 21:35 and Oct 11 21:58 with average fluxes of about 10.5 mCrab (18-40 keV; 14 sigma) and about 10 mCrab (40-100 keV; 8 sigma). The best-fit position found by IBIS/ISGRI is RA, Dec (J2000) = 268.408 degrees, -23.799 degrees with a 90% confidence error of 3 arcmin. This is consistent with that reported from Swift/BAT (ATel #1799; 1.3 arcmin away) and XRT (ATel #1809; 1.9 arcmin away).

Emission consistent with coming from SAX J1753.5-2349 was also seen between UT 2008, Oct 17, 21:00 and Oct 19, 10:17, with average fluxes of about 7 mCrab (18-40 keV; 12 sigma) and 6 mCrab (40-100 keV; 6 sigma), confirming the decline in flux reported from the RXTE/PCA observations (ATel #1799).

SAX J1753.5-2349 was badly covered by the INTEGRAL/JEM-X instrument during the observations performed between Oct 10 and Oct 19. The effective exposure was only 3800 sec and the source was not detected with flux upper limits of about 5 mCrab (3-10 keV) and 2 mCrab (10-25 keV).

To investigate further the nature of IGR J17536-2339 (the source reported to be close to SAX J1753.5-2349, see ATel #790), we re-analysed the INTEGRAL Galactic Bulge monitoring observations performed between UT 2006, April 3, 16:52 and 20:33, based on which the IGR source was reported, using the latest INTEGRAL analysis software (OSA 7). We do not detect the IGR source in this observation; the brightest pixel at the position of the IGR source is at the level of 3.4 sigma. We, therefore, regard the source IGR J17536-2339 as being spurious.

We further note that the recently reported new hard X-ray transient IGR J17375-3022 (ATels #1781, #1783), also in the field of the INTEGRAL TOO, is not detected anymore in the Oct 17-19 observations. We derive upper limits of about 3 mCrab (18-40 keV), suggesting the source is quiescent again.