Search for an IR counterpart to the newly discovered transient Swift J11822.3-1606

ATel #3502; R. M. Bandyopadhyay (UF), P. W. Lucas (Hertfordshire), T. Maccarone (Southampton)
on 20 Jul 2011; 22:40 UT
Credential Certification: Reba Bandyopadhyay (reba@alum.mit.edu)

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We have searched for potential near-infrared counterparts to the new Galactic Center X-ray transient, Swift J1822.3-1606 (ATEL #3488, #3489, #3490, #3491, #3493, #3495, #3496, #3501) using the UKIDSS Galactic Plane Survey (GPS) Catalog (Lucas et al., 2008, MNRAS, 391, 136). Within the enhanced Swift-XRT error circle (1.8" radius) centered at RA 18:22:18.00, Dec -16:04:26.8 (ATel #3493), there are two infrared stellar sources in the UKIDSS GPS catalog:

 
            RA           Dec         J       H        K    
star 1  18:22:17.9   -16:04:25.9   13.90   12.37   11.65   
star 2  18:22:18:0   -16:04:28.2   16.50   15.44   14.97   

These are the same as "S1" and "S2" identified by Gorosabel et al. (Atel #3496); as they note, star 1 is also detected in the 2MASS catalog. The 2MASS K magnitude for star 1 is consistent with the UKIDSS magnitude above. There are two epochs of UKIDSS GPS data on this field, both obtained in 2006. Between these two epochs, star 2 brightened by approximately ~0.05 magnitudes; in contrast, neither star 1 nor other field stars exhibited significant variability during this interval. The UKIDSS astrometric solution is tied to 2MASS, resulting in an astrometric precision of typically 0.09" (Lucas et al. 2008). UKIDSS photometry is also calibrated to 2MASS (Hodgkin et al., 2009, MNRAS, 394, 675); the photometric calibration is typically good to 2%, though the uncertainty is a little larger in the J band, pending improvements that will be supplied in the 8th Data Release. The colours of both sources indicate that the stars are significantly reddened, with J-K = 2.25 and 1.53 for stars 1 and 2, respectively.

In ATel #3491 Gogus et al. report that the timing analysis of the new Swift source is suggestive of a possible Be X-ray binary nature. We are in agreement with Gorosabel et al. that the close astrometric match between the Swift transient and the presence of two stars within the error circle suggests that a (reddened) Be/X-ray binary scenario may be more likely than an SGR nature. The revised nH = 1.56e22 cm^2 reported by Rea et al. (ATel #3501), corresponding to A_V = 8.67 (Predehl & Schmitt, 1995, A&A, 293, 889), is compatible with the measured optical/IR colours of the two stellar sources. The relation between A_V and IR extinction within the inner Galactic Plane is complex, and a specific determination of the extinction along this line of sight will be required to derive the intrinsic colours of the two candidate stars. As a preliminary estimate, using recent measurements of the slope of the interstellar extinction law (see e.g. Stead & Hoare, 2009, MNRAS, 400, 731 and Fritz et al., 2011, arXiv/1105.2822v2) yields intrinsic colours (J-K) ~ 1.1, (H-K) ~ 0.3 for star 1 and (J-K) ~ 0.4, (H-K) ~ 0.1 for star 2. The typical range of colours for Be high-mass X-ray binaries (HMXB) is 0.1 < (J-K) < 1.1 (Reig, 2011, APSS, 332, 1). Our colour estimates for star 1 and star 2 are thus both nominally consistent with that expected for Be HMXBs; however, we stress that a more careful determination of the extinction in this field is necessary before firm conclusions can be drawn.

Follow-up IR imaging of the candidate counterparts for this transient to search for evidence of photometric variability while it is in an X-ray active phase is strongly encouraged. IR spectroscopy to determine the spectral types for the two candidates and to search for evidence of accretion signatures is also essential for determining the nature of this new Swift transient.