Optical/NIR imaging of IGR J17098-3628 and IGR J16283-4838

ATel #478; D. Steeghs (CfA), M. A.P. Torres (CfA), P. G.Jonker (SRON/CfA), J. Miller (CfA), P. Green (CfA), C. Rakowski (CfA)
on 3 May 2005; 20:53 UT
Credential Certification: Danny Steeghs (dsteeghs@cfa.harvard.edu)

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We report on optical and infrared observations acquired with the 6.5m Magellan-Baade telescope of the fields containing the INTEGRAL transients IGR J17098-3628 (ATEL#444) and IGR J16283-4838 (ATEL#456) for which SWIFT localizations have been reported (ATEL#476,#459).

IGR J17098-3628: optical I-band images of the field were obtained using the IMACS imaging spectrograph on 2005 April 9 9:40-9:50 UT. Seeing was 0.8" sampled at 0.2" per pixel with the 8Kx8K CCD mosaic. The images were tied to the 2MASS astrometric frame and revealed a large number of sources within the 5" SWIFT error circle reported in ATEL#476. Our images indicate that the counterpart candidate reported by Kong in ATEL#477 is actually a blend containing several sources within a few arcseconds. A comparison between the SuperCOSMOS sky survey image and our 360s IMACS image is available at: http://hea-www.cfa.harvard.edu/~dsteeghs/IGRJ17098-3628.jpg (SWIFT error circle plotted in red).

IGR J16283-4838: K band images were acquired on 2005 April 21 02:58-03:11 UT using the PANIC camera. The frames were obtained with a seeing of 0.7 arcsec and a projected pixel size of 0.125 arcsec on the 1024x1024 Hawaii detector. The images were tied to the 2MASS astrometric frame and show again a group of point sources within the SWIFT error circle (ATEL#459). We remark that the 2MASS candidate reported in ATEL#460 is a blend of point sources. Preliminary relative photometry with respect to four 2MASS nearby stars yields K~14.1 for the brightest source resolved in our images, consistent with the K=14 2MASS source. A comparison between the 2MASS field and our 300s PANIC K-band image is available at: http://hea-www.cfa.harvard.edu/~dsteeghs/IGRJ16283-4838.jpg (SWIFT error circle in red).

We plan to acquire additional imaging of these fields to identify the correct optical/NIR counterparts through their variability.

We caution that given the stellar densities in these low Galactic latitude fields, the chance of finding a non-related field star within a SWIFT X-ray error box is considerable (see linked images), even when using relatively shallow surveys such as DSS and 2MASS. Deeper imaging (to counter any significant reddening) with good spatial resolution at several epochs is crucial to separate the many field stars from the true and time-variable counterpart.