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The correct optical counterpart to IGR J11435-6109

ATel #1239; Ignacio Negueruela, José Miguel Torrejón (University of Alicante), Vanessa McBride (University of Southampton)
on 14 Oct 2007; 16:28 UT
Credential Certification: Ignacio Negueruela (ignacio@dfists.ua.es)

Subjects: Optical, X-ray, Binary, Neutron Star, Star, Pulsar

Even though an initial analysis (ATel #370) suggested that the Be star USNO-B1.0 0288-0337948 was the likely counterpart to the 162-s X-ray pulsar IGR J11435-6109 (ATel #350, #362), a deeper search of the error circle using the method described in Negueruela & Schurch (2007, A&A 461, 631) detected a fainter candidate emission-line object, namely 2MASS J11440030-6107364 = USNO-B1.0 0288-0337502, which has been confirmed as the correct counterpart by a Chandra localisation (ATel #1231).

We observed USNO-B1.0 0288-0337502 on 2007 April 30, using the Unit Spectrograph on the SAAO 1.9-m telescope, equipped with grating #7. The source is too faint to give any detectable flux shortwards of 5000A. The 5000-7800A spectrum shows a strong H-alpha emission line on top of a reddened continuum. The measured EW = -26A and lack of any other obvious stellar features at this resolution is very strongly suggestive of an obscured Be star. This classification is in full agreement with the position of IGR J11435-6109 in the Corbet diagram (Ps=162 s, Porb=52.4d), typical of a Be/X-ray binary (ATel #377).

Photometry of the field taken in 2004 December with EMMI on the NTT gives the following magnitudes: B=17.71, V=16.43, R=15.44. For a typical early B star, this implies a colour excess E(B-V)~1.6, some of which must be of circumstellar origin. With these colours, even a B2Ve counterpart should be located at >6 kpc and a typical B0Ve would be at >10 kpc. Therefore this is likely to be a very distant Be/X-ray binary.