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Optical amateur observations of the field of IGR J18245-2452 in M28

ATel #4964; L. A.G. Monard (CBA Klein Karoo Observatory, South Africa) & E. Kuulkers (ESA/ESAC, Spain)
on 9 Apr 2013; 18:37 UT
Credential Certification: Erik Kuulkers (ekuulker@rssd.esa.int)

Subjects: Optical, Binary, Neutron Star, Star, Transient, Variables

Referred to by ATel #: 4981, 5031, 5069, 5086

Optical observations of the field around the recently discovered X-ray transient (Atels #4925, #4929) and type I X-ray burster (ATels #4959, #4960, #4961) IGR J18245-2452 in M28 (ATels #4925, #4927) were reported by Monard (2013, vsnet-alert 15587). Images of M28 were taken at the CBA Klein Karoo Observatory one week after the discovery alert by INTEGRAL, as well as one week before the alert. A brightened star was found at (J2000.0) RA, Dec = 18h 24m 32.93s, -24deg 51' 59.6" (with an astrometric uncertainty of less than <0.03" in RA and <0.5" in Dec) as measured against the USNO CCD Astrograph Catalog, UCAC2, possibly being the optical counterpart to IGR J18245-2452 (Monard 2013). A re-analysis of the unfiltered CCD images taken on UT 2013 March 22.14 and April 5.14 show magnitudes of 13.0 and 11.9 CR, respectively, with 90% confidence uncertainties in the CR magnitudes of 0.2 mag.

Further observations on April 6.05 and 8.02 show unfiltered CCD magnitudes of 11.85 and 11.7 CR (+/-0.2), respectively, consistent with the observations from April 5. Using various Bessel filters, the following magnitudes were derived both on April 6.07 and 8.07:
CCD-V: 12.9+/-0.3,
CCD-R: 11.9+/-0.2,
CCD-I: 10.4+/-0.5,
(i.e., no significant change between the observations). Time series photometry on April 6 and 8 did not reveal any obvious modulation.

Given the above optically observed V-band magnitude, the distance to M28 of 5.5 kpc and a reddening of E(B-V) = 0.40 (Harris 1996, AJ, 112, 1487), we infer an absolute visual magnitude of the optical star of about -2, which is in the range of that found for active low-mass X-ray binaries (e.g., van Paradijs & McClintock 1994, A&A 290, 133). However, the difference in the optical magnitude of the star before and after the discovery of the X-ray transient (about 1 mag), is not what is usually seen between quiescence and outburst maximum of X-ray transient low-mass X-ray binaries (> several magnitudes, e.g., Shahbaz & Kuulkers 1998, MNRAS 295, L1). Moreover, the 3 filter magnitudes hint at a somewhat reddish star, whereas active low-mass X-ray binaries are expected to be bluer (see, e.g., van Paradijs 1981, A&A 103, 140).

We note that the optical star is 11.6 arcsec from the X-ray position quoted by Heinke et al. (2013, ATel #4927; see also ATels #4929, #4959), i.e., taking the uncertainties quoted they are not consistent with being the same object. According to Simbad Astronomical Database a long-period variable star, NGC 6626 SAW V21 (Wehlau & Hogg 1984, AJ 89, 1005) is close (0.85" away) to the optically derived position. Although the variable lies just outside the optical error region, the redness of the brightened star and the small amplitude between the optical images in March and April are not inconsistent with such a type of variable. But, we note that various other variables are known within the optical error region (see, e.g., Samus et al. 2009, PASP 121, 1378).

At the exact X-ray position given by Heinke et al. we do not find evidence of any (new) optical object. Note that there is no hint of another star that brightened in the imaged field. Due to the dense region of M28, with bright background causing a firmly reduced resolution in space and contrast, we estimate the optical limit of the unfiltered images around the object of interest to be about 17 CR mag. The true optical counterpart could thus be fainter than this value.

We encourage follow-up observations to shed more light on the possible optical counterpart of IGR J18245-2452.

This research has made use of the SIMBAD database and the VizieR catalogue access tool, operated at CDS, Strasbourg, France. The Center for Backyard Astrophysics (CBA) is a global network of small telescopes dedicated to photometry of cataclysmic variables.