The optical and near-infrared counterpart to IGR J17091-3624

ATel #3150; M. A.P. Torres (SRON/CfA), P. G. Jonker (SRON/CfA/RU), D. Steeghs (Warwick/CfA), J. S. Mulchaey (Carnegie)
on 8 Feb 2011; 10:28 UT
Credential Certification: Manuel Torres (mtorres@cfa.harvard.edu)

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We report the detection of the optical and near-infrared counterparts to the X-ray transient IGR J17091-3624 (Kuulkers et al. 2003, ATel #149). This finding is based on optical observations of the currently ongoing outburst (Krimm et al. 2011, ATel #3144, #3148) as well as optical and near-infrared pre-outburst images. All epochs of imaging were acquired with the 6.5m Magellan Baade telescope at LCO.

OPTICAL IMAGING: Our outburst observations consist of two 180s I-band images obtained during 2011 Feb 6 08:47-08:56 UT using the IMACS imaging spectrograph. The frames were acquired with an image quality of 0.7 arcsec and a projected pixel scale of 0.20 arcsec/pixel. The field of IGR J17091-3624 was also imaged with similar integration times, image quality and instrumental set-up on 2005 April 9 UT when following the outburst of the X-ray transient IGR J17098-3628 (see Steeghs et al. 2005, ATel #478, #494). At that time IGR J17091-3624 was in quiescence.

An astrometric calibration of the data sets was performed using 2MASS objects in the field of view. This delivered a positional RMS < 0.1 arcsec. No flux standards were observed during the nights. The instrumental magnitudes derived from PSF-photometry have been calibrated assuming I=17.12 for the USNO B1.0 object 0535-0482649.

We searched for counterpart candidates within the 3.6 arcsec Swift X-ray position for IGR J17091-3624 (Kennea et al. 2007, ATel #1140) by comparing the two epochs of imaging. Our 2011 outburst images show a I = 18.35 +/- 0.03 mag point-like source at R.A (J2000)=17:09:07.62, Dec (J2000)=-36:24:25.35. The 2005 images show this object at I = 20.32 +/- 0.02 (see finding chart). The positional coincidence within the Swift error circle and apparent variability does support this source as the optical counterpart for IGR J17091-3624.

NEAR-INFRARED IMAGING: Our pre-outburst data consist of a series of 15s Ks-band images totalling 225s on source and acquired on 2008 Jun 23 05:16-05:27 UT with the PANIC camera. The observations were performed during good sky conditions with 0.45 arcsec imaging quality. Additional 3s exposures were acquired to derive an astrometric and absolute flux calibration of the data using 2MASS objects in the field of view. This delivered a positional RMS < 0.1 arcsec and a photometric zero point accuracy of ~ 0.1 mag.

Visual inspection shows an infrared source consistent with the optical counterpart and with the Ks=16.65 candidate infrared counterpart 'C2' reported in Chaty et al. (2008,A&A,484,783). PSF-fitting reveals that this object is actually two unresolved point-like sources of similar brightness and separated by 0.4 arcsec. The brighter of these two is the true infrared counterpart to IGR J17091-3624 (it matches the astrometric position reported above) and has Ks=16.98 +/- 0.04. The other source NE of IGR J17091-3624 is likely a field star with Ks=17.19 +/- 0.04.

Our outburst imaging has allowed us to securely identify the optical and near-infrared counterpart to the recurrent transient IGR J17091-3624. The detection in quiescence offers the opportunities to study this X-ray binary in more detail.

Finding Chart