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Confirmation of near-infrared counterpart of IGR J17407-2808/CXOU J174042.0-280724

ATel #3695; R. Kaur (U. Amsterdam/UW-Milwaukee), C. Heinke (U. Alberta), R. Kotulla (UW-Milwaukee), A. Eigenbrot (UW-Madison), A. Schechtman-Rook (UW-Madison), D. Kaplan (UW-Milwaukee/Madison)
on 20 Oct 2011; 22:24 UT
Credential Certification: Ramanpreet Kaur (raman.pk@gmail.com)

Subjects: Infra-Red, Optical, X-ray, Binary, Black Hole, Cataclysmic Variable, Neutron Star, Transient, Variables

Following the current outburst of the fast hard X-ray transient IGR J17407-2808 (Romano et al. 2011, Atel #3685), we obtained observations in the near-infrared Ks band on October 19, 2011 at UT 01:12 using the WIYN High-Resolution Infrared Camera (WHIRC) mounted on the WIYN 3.5-m telescope located at Kitt Peak, Arizona. The source was observed for a total exposure time of 270 s, consisting of 27 dithered exposures of 10s each. The observations were obtained at the airmass 2.4 and with a seeing of 1.1". The astrometry of the final frame was done using the 2MASS stars in the field of view with a position accuracy of < 0.1". During our observations, we detected a faint source with a 16.6 +/- 0.3 mag in Ks waveband, consistent with the X-ray position of the source, as given by Heinke et. al. (2009, ApJ, 701, 1627), and with the infrared star suggested as the counterpart by Greiss et al. 2011 (Atel #3688).

We also investigated another observation of this source from the ESO archive, obtained using the near-infrared instrument SOFI on the ESO-New Technology Telescope (NTT) on May 15, 2008. During these observations, we detected the source with 15.3 +/- 0.1 mag in Ks waveband, which is approximately a magnitude brighter than the current observations. We confirmed the present variability by comparing the magnitudes of the nearby stars in NTT/SOFI and WIYN/WHIRC images which varied only within 0.1 mag. Our 2008 Ks magnitude is consistent with the Ks magnitude reported by Greiss et al. 2011 (Atel #3688).

The near-infrared variability from this star is strong evidence supporting its association with CXOU J174042.0-280724, and thus with IGR J17407-2808. To determine the nature of the counterpart, near-infrared spectroscopic observations are required.

We encourage the multi-wavelength observations of the source to find its nature.