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BeppoSAX-WFC observations of IGR J11435-6109 and J00291+5934

ATel #362; Jean in 't Zand & John Heise (SRON)
on 8 Dec 2004; 21:20 UT
Credential Certification: Jean in' t Zand (jeanz@sron.nl)

Subjects: X-ray, Binary, Neutron Star, Transient

Referred to by ATel #: 370, 377, 531, 1231, 1239

The BeppoSAX Wide Field Cameras database entails 225 useful X-ray observations of the recently identified X-ray transient IGR J11435-6109 (ATel 350) obtained from July 1996 to April 2002. The net exposure time is 3.6 Msec. The data exhibit detections on 5 separate occasions: 21/22-jul-1996, 10/11-sep-1996, 11-apr-1997, 31-dec-2000/06-jan-2001 and 10-jan-2002. The highest flux on a time scale of one day is 7 mCrab (2-10 keV).

We analyzed in some detail the longest dataset (Dec 00/Jan 01), entailing 212 ksec of net exposure time. The source has a significance of 20. The position is R.A. = 11h44m00.4s, Decl. = -61d07'16" (J2000.0) with a 99%-confidence error circle radius of 1.4'. This is 2.0' from the 2.5'-accurate INTEGRAL position and 0.4' from the ROSAT source 1RXS J114358.1-610736. The spectrum can be described by an absorbed power law with NH=(9+/-2)E22 cm-2 and photon index 1.9+/-0.2. The average 2-10 keV flux is 1.3E-10 erg s-1 cm-2 (not corrected for absorption). Fourier analysis confirms the tentative pulsar detection by Swank & Markwardt (ATel 359). We find a pulse period of 161.76 +/- 0.01 sec. The pulse profile is broad with a 2-6 keV pulsed fraction of roughly 50%.

The outburst recurrence times are consistent with a 52.5 d orbital period as would be expected for a Be X-ray binary. However, the source was observed but not detected during ~10 intermediate outburst epochs.

We note that this source appears to have a long X-ray history. Einstein detected 2E 1141.6-6050 (at 0.4' from the ROSAT position) on Feb 5, 1980 (Thompson et al. 1996; AJ 115, 258).

Regarding the recently by Eckert et al. discovered accretion-powered ms pulsar IGR J00291+5934 (ATel 352; pulsar detection by Markwardt et al. ATel 353), the WFCs made 188 useful observations with a net exposure time of 2.9 Msec. The source was not detected, in persistent emission nor bursts. The detections reported by RXTE/ASM data by Remillard (ATel 357) were not covered by the WFCs. The first was covered 16 days after the last ASM detection and the second 11 days.