Discovery of a likely-period of 185 days in AX J1749.1-2733

ATel #1035; J. A. Zurita Heras, S. Chaty, J. Rodriguez (AIM Service d'Astrophysique CEA-Saclay & Université Paris 7)
on 29 Mar 2007; 23:23 UT
Credential Certification: Sylvain Chaty (S.Chaty@open.ac.uk)

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The unclassified transient X-ray source AX J1749.1-2733 was observed at a very low luminosity level during the ASCA galactic centre survey between 1993 and 1999 (Sakano et al. 2002). A flare of this source has been observed by INTEGRAL on Sept. 9, 2003 lasting 1.3 days (Sguera et al. 2006). Due to similarities with other supergiant fast X-ray transients (SFXT), they proposed this source as a candidate SFXT.

Analyzing all the INTEGRAL public data within Feb. 2003 and Oct. 2005, we report 5 significant outbursts lasting between 2 and 12 days. The 5 detections are separated by a period of approximately 185 days. The 22--50 keV outburst flux varies between 5 and 10 mCrab. As reported by Sguera et al. (2006), the source showed a short bright flare on Sept. 9, 2003 that reached 68 mCrab within 1 pointing (~1 h) of the satellite. Besides these detections, deep mosaics (exposure > 400 ksec) of the source field show no other detections with 3-sigma upper limit fluxes of 0.9 to 1.7 mCrab.

As shown in the public page of the INTEGRAL Galactic Bulge program (Kuulkers et al., 2007, http://isdc.unige.ch/Science/BULGE/ ), the source has been detected on several observations on March 19 and 25, 2006 (twice 12 ksec exposure) with a 20--60 keV flux of 5.5 to 7 mCrab and during an observation starting on March 25, 2007 and lasting 12 ksec with a 20--60 keV flux of 8 mCrab. Therefore, the source might be currently undergoing an outburst episode that should end in a few days if similar to the previous detected outbursts. These detections confirm the 185 days likely-period.

The nature of this X-ray source is still unknown. This period can be interpreted as the orbital period of a binary system. A high mass X-ray binary with Be companion fits well with the observational features (recurrent outbursts, large orbital period and long outbursts of several days). Due to the observation gaps with INTEGRAL, the orbital period could be shorter by a factor of 2 or 3, the 185 days likely-period being an harmonic due to the INTEGRAL observation schedule.

Observations at other wavelengths are encouraged.

The detailed analysis will be presented in Zurita Heras et al. (2007, in preparation). We acknowledge the use of the public web page of the INTEGRAL Galactic Bulge program of Kuulkers and collaborators.

INTEGRAL Detections log:
2003 Sept. 7--19
2004 March 14--16
2004 Sept. 18--20
2005 March 17--25
2005 Sept. 18--26

Bulge Program Detections log:
2006 March 19 and 25
2007 March 25