A possible IR counterpart to the transient X-ray pulsar IGR J17480-2446 in Terzan 5

ATel #3264; V. Testa (INAF-OAR), T. Di Salvo (Univ. Palermo), L. Burderi (Univ. Cagliari), M. T. Menna (INAF-OAR), T. M. Belloni (INAF-OAB), A. D'Ai' (Univ. Palermo), R. Iaria (Univ. Palermo), S. Motta (INAF-OAB), A. Papitto (Univ. Cagliari), A. Riggio (INAF-OAC)
on 7 Apr 2011; 10:10 UT
Credential Certification: Luciano Burderi (burderi@mporzio.astro.it)

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We observed the center of the globular cluster Terzan 5 around the region including the Chandra position (ATel #2974) of the newly discovered X-ray pulsar IGR J17480-2446 (ATel #2919, #2920, #2922, #2924, #2929, #2932, #2933, #2935, #2937, #2939, #2946, #2952, #2958, #3000, #3044), while it was in outburst, with the ESO-VLT Yepun equipped with the adaptive optics near-IR camera NAOS-CONICA. Observations were executed during the nights of 6 and 7 November 2010 at a low airmass (z=1.9). Despite the low elevation and the non-optimal atmospherical conditions, good Ks-band images were obtained. Two objects were found around the Chandra position within a radius of 0.3 arcsec, that we choose conservatively as our global astrometric uncertainty estimate (95% c.l.). We used a control data set from the ESO archive, obtained with the multi-conjugated AO camera MAD in August 2008, having a limiting magnitude slightly deeper than our images. The brighter object is present in the catalog of Ferraro et al. 2009 (Nature, 462, 483) and shows no sign of variation in magnitude. Its color and magnitude (J-K=1.72, K=13.91) are typical of a Red Giant Branch object, just below the level of the Horizontal Branch. The fainter object in the box is not visible in the MAD images, nor it is present in the published catalog of Ferraro et al. 2009 hence we mark it as a possible candidate for the infrared counterpart to IGR J17480-2446. The estimated position and magnitude of the candidate counterpart are RA=17:48:04.829, DEC=-24:46:49.10 (+/-0.3"), Ks = 16.6+/-0.5, with a 5-sigma limiting magnitude of ~17.3. The separation from the brighter object in the same error circle is 0.11 arcsec. Further analysis is in progress. A map showing the region of the candidate counterpart is available at this URL http://www.oa-roma.inaf.it/testa/IGRJ17480/ We gratefully acknowledge the excellent support from the ESO staff.