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A Candidate Optical Counterpart of IGR J17480-2446 in Terzan 5 identified with the Hubble Space Telescope

ATel #3924; A. Patruno (Amsterdam), A. P. Milone (IAC)
on 14 Feb 2012; 16:35 UT
Credential Certification: Alessandro Patruno (a.patruno@uva.nl)

Subjects: Infra-Red, Optical, X-ray, Binary, Globular Cluster, Neutron Star, Transient, Pulsar

We report the identification of a candidate optical counterpart of IGR J17480-2446 in Hubble Space Telescope (HST) archival observations, taken on 2003 Sept. 17. The accreting pulsar IGR J17480-2446 has been observed in outburst in October-November 2010 (Atel #2919, #2939, #2946). The compact object is an 11 Hz pulsar in the globular cluster Terzan 5 orbiting in about 21.3 hours around a donor star with mass M>0.4 Msun (Papitto et al. 2011, A&A, 526, L3). A Chandra observation taken on 2010 October 24 at 19:30:29 UTC allowed the identification of IGR J17480-2446 as the faint source CX25 in the Terzan 5 X-ray source catalog of Heinke et al. (2006, ApJ, 651, 1098).

A VLT/NAOS-CONICA observation taken between 2010 November 6 and 7 revealed a candidate NIR counterpart (Atel #3892) while IGR J17480-2446 was still in outburst. The candidate counterpart was observed to be 0.11 arcsec off a bright object which is probably an unassociated red giant star of the cluster.

Thanks to the high resolving power of the HST, we clearly resolve the red giant object and the faint candidate counterpart and we identify the latter as one candidate optical counterpart of IGR J17480-2446. The object shows F606W = 22.46 and F814W=19.14, where differential reddening corrections have been applied. The counterpart falls in the typical blue-straggles region of the color-magnitude diagram. However, care must be taken when interpreting the colors of Roche lobe filling objects since irradiation and mass loss effects can alter substantially the intrinsic colors of the donor star.

The position of the candidate counterpart is at R.A. = 17:48:04.86 and DEC = -24:46:49.24 (J2000) with a 90% confidence radius of 0.3 arcsec in agreement with the X-ray positions reported in Atel #2974 and #3892. A finding chart of the region and the Color-Magnitude diagram of Terzan 5 can be found at this URL:

http://www.astro.uva.nl/~apatruno/17480.html

We identify other three optical candidates, which fall within the three sigma error-box of the X-ray source Chandra position (Atel #2974) and two (Nr. 1 and 6) which are marginally outside the error box.

Thanks to the precise orbital ephemeris of the binary (see for example Papitto et al. 2011, A&A, 526, L3) we can reconstruct the phase between the donor star and the observer at infinity at the time of the HST observation, with negligible error. The phase of the donor was about 0.8 cycles, where phase 0 is defined as the point zero of true longitude (i.e., the ascending node). During the epoch of the HST observation (note that the source was probably in quiescence at that time), the observer would have seen approximately 10-20% of the donor heated face, assuming an inclination of about 30 degrees (see Patruno et al. 2011 astro-ph/1112.5315 for a discussion of the binary parameters).