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Possible IR counterpart of the newly discovered X-ray MSP SWIFT J1756.9-2508

ATel #1132; L. Burderi (Univ. Cagliari), V. Testa, M. T. Menna, G. L. Israel (INAF-OARoma), T. Di Salvo (Univ. Palermo), A. Papitto (INAF-OARoma), A. Riggio (Univ. Cagliari), A. Antonelli (INAF-OARoma), E. Bozzo (INAF-OARoma), M. Burgay (INAF-OACagliari), N. D'Amico (INAF-OACagliari), E. Gallo (UCSB), R. Iaria (Univ. Palermo), A. Possenti (INAF-OACagliari)
on 6 Jul 2007; 17:15 UT
Credential Certification: Alessandro Papitto (papitto@oa-roma.inaf.it)

Subjects: Infra-Red, Optical, Binary, Neutron Star, Transient, Pulsar

We have observed the region of the sky including the X-ray position of the accreting millisecond pulsar SWIFT J1756.9-2508 (ATEL #1105, 1108) with the ESO-VLT Yepun equipped with the adaptive optics near-IR camera NAOS-CONICA (NACO) in two different epochs (June 19, 2007 and July 1, 2007) within the ESO-DDT program 279.D-5032 in the Ks band. About 70 objects have been detected within the 3.5 arcsec error circle reported by Swift/XRT (ATEL #1111). A search for IR flux variability resulted in the identification of only one variable object (Ks = 19.7+\-0.1 during the first observation) which faded to Ks = 21.0+/-0.1 in July 1, that is by about 1.3 mags between the two epochs. We therefore consider it as a good candidate for the IR counterpart of the X-ray MSP SWIFT J1756.9-2508. Its position (J2000) is: RA = 17:56:57.2 and DEC = -25:06:28.1 (position uncertainty of about 0.3 arcsec at 2.5 sigma, given by the accuracy of the astrometric reference catalog USNO B1.0), i.e. at about 2 arcsec from the Swift/XRT position (ATel #1111). No other sources within the error circle have been detected with a significant magnitude variation between the two epochs. We thank the Paranal staff for the excellent support, in particular the UT4 team.

1105, 1108, 1111, 1117, 1128, 1129