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Swift/XRT observations of the X-ray transient in NGC6440

ATel #7106; E. Bozzo (ISDC, Switzerland), E. Kuulkers (ESA/ESAC), C. Ferrigno (ISDC, Switzerland)
on 19 Feb 2015; 07:14 UT
Distributed as an Instant Email Notice Transients
Credential Certification: E. Bozzo (enrico.bozzo@unige.ch)

Subjects: X-ray, Transient

Referred to by ATel #: 7136, 7183

Following the discovery of the new INTEGRAL X-ray transient IGR J17488-2018 undergoing an outburst in the direction of the Globular Cluster NGC 6440 (Atel #7098), a 2 ks observation with Swift/XRT was promptly carried out in Photon Counting mode to characterize the source in the soft X-ray domain. A single bright X-ray source was found in the XRT field of view.

A preliminary analysis of the XRT data gave the best determined source position at (we used the XRT on-line tool at http://www.swift.ac.uk ; see Evans et al. 2009, MNRAS, 397, 1177):
(J2000), with an associated uncertainty of 3.5'' (90% c.l.)
This position is consistent, within the uncertainties, with that of IGR J17488-2018 reported in Atel #7098.

Given the estimated position above, the most likely counterpart to IGR J17488-2018 is the low mass X-ray binary SAX J1748.8-2021. We thus suggest that IGR J17488-2018 = SAX J1748.8-2021. The transient source SAX J1748.8-2021 is known to host a 442 Hz accreting millisecond X-ray pulsar (Altamirano et al. 2008, ApJ, 674, 45).

The XRT PC data are heavily piled-up due to the high count-rate of the source. A tentative spectral analysis performed by correcting for pile-up issues gave an absorption column density of 1.1E22 cm^(-2), and a power-law photon index of 1.2+/-0.1. The estimated 0.3-10 keV X-ray flux is 7E-10 erg/cm^2/s (not corrected for absorption).

We thank the Swift team for the prompt scheduling of the reported observations. Additional Swift/XRT observations have been planned.