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 (firstname.lastname@example.org)
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:
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).