Infrared observations of IGR J17497-2821: 3 candidate counterparts

ATel #897; S. Chaty (AIM CEA Saclay/University Paris 7, France), Y. Matsuoka (Inst. Astronomy, Univ. Tokyo, Japan), T. Nagata and Y. Ueda (Dept. Astronomy, Kyoto Univ., Japan)
on 25 Sep 2006; 21:19 UT
Credential Certification: Sylvain Chaty (S.Chaty@open.ac.uk)

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We have obtained infrared (IR) observations in J, H and Ks bands of the field of IGR J17497-2821 (atel #885) at 1.4m IRSF at South African Astronomical Observatory on 3 consecutive nights, respectively starting at 2006-09-22 (UTC 17:47, 18:39 and 20:17), 2006-09-23 (UTC 18:49) and 2006-09-24 (UTC 17:33). The 3-band images were obtained simultaneously with the IR camera SIRIUS, from 5s x 10 dither integrations. The pixel scale is 0.45", giving a field of view of 7.7arcmin x 7.7arcmin. We used standard method to reduce images, including removal of dark, flat-field and median IR sky.

We first point out that there has been some confusion about the Swift coordinates: Walter et al. 2006 (atel #889) give a SWIFT XRT refined position of RA=17:49:37 DEC=-28:21:28 (J2000), with an error of 5". However they look for possible candidate counterparts in the 2MASS catalogue near RA=17:49:37 DEC=-28:21:18 (J2000). Then, Laycock et al. 2006 (atel #895) look for optical counterparts around the position RA=17:49:37.8 DEC=-28:21:18 (J2000), which is indeed the right SWIFT position (J. Zurita, priv. comm.).

Furthermore, by inspecting 2MASS images, we see that the two 2MASS sources which are in the Swift 5" error circle (2MASS 17493780-2821181 and 2MASS 17493774-2821173, see atels #889 and #895) are blended in the 2MASS image. The given magnitudes of both objects in the 2MASS catalogue are therefore probably contaminated by the other source. For instance, 2MASS 17493780-2821181 is much redder and its J magnitude in 2MASS PSC seems to be affected by the other source. The closest to the swift position is 2MASS 17493780-2821181, at only 0.167"; 2MASS 17493774-2821173 is located at 0.989" of the Swift position.

We thereafter inspected our new IRSF/SIRIUS images and compared them with 2MASS images. 2MASS 17493780-2821181 itself appears to be also a blended object in the IRSF K image, and the object located at the East of this blended object is more reddened. We derived the position of this object (called candidate 3 in the following), thanks to 2MASS stars of the field of view: galactic coordinates (l,b) ~ (267.4077, -28.3550). Finally, we computed the magnitudes of the 3 candidate counterparts, using stars of the 2MASS catalogue, present in the field of view. We give the results of this photometry in the Table below, but we point out that the magnitudes of candidate 3 are preliminary because they might be contaminated by 2MASS 17493780-2821181.

  
%%%  
2006-09-22  
Object                  J     err  H    err K    err  
2MASS 17493780-2821181: x     x    13.2 0.3 12.5 0.2  
2MASS 17493774-2821173: 14.0  0.3  13.5 0.2 13.0 0.2  
candidate 3:            >18.8 0.5  13.4 0.4 12.9 0.2  
  
2006-09-23  
Object                  J     err  H    err K    err  
2MASS 17493780-2821181: x     x    13.3 0.2 12.5 0.2  
2MASS 17493774-2821173: 14.3  0.1  13.4 0.2 13.0 0.3  
candidate 3:            >18.7 0.5  x    x   12.5 0.2  
  
2006-09-24  
Object                  J     err  H    err K    err  
2MASS 17493780-2821181: 14.1  0.1  13.3 0.1 12.5 0.2  
2MASS 17493774-2821173: 14.1  0.1  13.3 0.1 12.8 0.2  
candidate 3:            >18.6 0.5  13.2 0.1 12.5 0.2  
  
(The "x" indicate a source too blended to be separated).  
  
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These observations suggest that candidate 3 is the real counterpart of the source, because of the variation of the source, and its reddening, which would be consistent with the position of IGR J17497-2821 in the galactic bulge, as suggested from the high value of its column density (Kuulkers et al. atel #888). Further high resolution IR observations are encouraged, in order to look for variations of these 3 candidate counterparts, and confirm (or not) candidate 3 as the counterpart of IGR J17497-2821.