Swift/XRT observation of Terzan 1

ATel #13991; E. Bozzo (ISDC, Switzerland), J. Wilms (ECAP, Germany), E. Kuulkers (ESA/ESTEC, Netherlands - on behalf of the Galactic bulge monitoring program team), L. Sidoli (INAF-IASF, Italy), C. Sanchez-Fernandez (ESA/ESAC, Spain), A. Paizis (INAF-IASF, Italy), J. Chenevez (DTU, Denmark), C. Ferrigno, V. Savchenko (ISDC, Switzerland), L. Ducci (ISDC, Switzerland; IAAT, Germany)
on 4 Sep 2020; 19:42 UT
Credential Certification: E. Bozzo (enrico.bozzo@unige.ch)

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Following the report of hard X-ray activity from the direction of the Globular Cluster Terzan 1 by INTEGRAL (ATel #13983), a short follow-up observation was triggered with Swift/XRT in order to determine the position of the source down to the arc-second accuracy and confirm the eventual association of the INTEGRAL transient with XB 1732-304.

The Swift/XRT observation was carried out on 2020 September at 16:39 UT for a total effective exposure time of 0.67 ks. A single X-ray source is found within the XRT field of view. The best determined position of the source is at RA=263.9471, DEC=-30.4819 (J2000), with an associated accuracy of 2.2 arcsec at 90% confidence level (we used the XRT on-line tool; see Evans et al. 2009, MNRAS, 397, 1177).

The source spectrum could be well described by a power-law with a photon index of 2+/-0.4 and a 0.5-10 keV flux of (3.2+/-0.4)E-10 erg/cm^2/s, roughly in agreement with the value expected by extrapolating the previously reported INTEGRAL results in the soft energy domain (ATel #13983). The absorption column density in the direction of the source estimated by Swift/XRT is 2.7 (+0.8, -0.7) E22 cm^-2

The position determined from XRT is compatible with the previous localizations of XB 1732-304 in both the X-ray (Johnston et al. 1995, A&A, 298, 21) and radio domain (Marti et al. 1998, A&A, 332, 45). Therefore, the XRT results support the association of the INTEGRAL transient with the previously known transient binary XB 1732-304. Being the source located in a Globular Cluster, a Chandra prompt follow-up observation could be used to further enhance the accuracy of the localization to the sub-arcsec level and eventually look for the optical counterpart. Previous Chandra observations could not firmly identify the position of XB 1732-304 during quiescence (Cackett et al. 2006, MNRAS, 369, 407), although we noticed that the Swift source is only 0.73 arcsec away from the Chandra source CXOGLB J173547.2-302855 (CX2 in Cackett et al. 2006, the closest Chandra source to the radio counterpart).

We thank the Swift team for the rapid planning and execution of our requested follow-up observation.