Revised SED of the Probable Infrared Counterpart to Swift J1845.7-0037

ATel #13222; B. McCollum (American Univ.), S. Laine (IPAC/Caltech)
on 23 Oct 2019; 20:44 UT
Credential Certification: Bruce McCollum (mccollub@cua.edu)

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We report revised SED fitting of the probable IR counterpart to Swift J1845.7-0037 (ATel #4130, #13191, #13189, #13195, #13208, #13218, #13219, #13220). Krimm et al. (ATel #4130) suggested 2MASS 18455462-0039341 as the IR counterpart. Steele (ATel #13218) reported that the 2MASS object can be resolved in the H band into two objects separated by ~1.5 arcsec. Banerjee (ATel #1322) noted that the UKIDSS DR-6 survey (Lucas 2012, MNRAS 391, 136L), which has a pixel scale of 0.4 arcsec, appears to resolve the objects in J, H, and K. UKIDSS reports objects at 18:45:54.65472 -00:39:34.29 (designated UGPS J184554.65-003934.2) with J, H, and K magnitudes 16.778 +/- 0.17, 15.132 +/- 0.007, and 13.887 +/- 0.005 respectively, and at 18:45:54.57408 -00:39:33.93 (UGPS J184554.57-003933.9) with J, H, and K magnitudes 17.432 +/- 0.029, 15.529 +/- 0.009, and 14.643 +/- 0.011 respectively. Kennea (Atel #13219) reported a refined SWIFT position of 18:45:54.68 -00:39:34.8, which is significantly closer to the position of UGPS J184554.65-003934.2 than to UGPS J184554.57-003933.9. Also, Steele (Atel #13218) reported H magnitudes of 15.0 and 15.3, and his coordinates for the brighter object are closer to those of UGPS J184554.65-003934.2. We therefore performed fitting using the UKIDSS photometry of UGPS J184554.65-003934.2.

The bandpasses used in the new fitting were Pan-STARRS y and z, UKIDSS J, H, and K, and the four Spitzer IRAC bandpasses (3.6, 4.5, 5.7, and 8 microns). (For the fitting reported in Atel #13211, we had used IRAC magnitudes, not WISE magnitudes as was stated in that telegram.) The fact that the J, H, and K magnitudes of UGPS J184554.65-003934.2 are approximately half a magnitude brighter than for the presumed contaminant suggests that the fitted object's flux contribution may also dominate in y, z, and the IRAC bandpasses. Nevertheless, because Pan-STARRS and Spitzer do not resolve the two objects, for the new fitting we treated those measurements as upper limits. Also, for the previous fitting we had noted but not stated that catalog magnitudes in R and B are available, but because they are ~1-2 orders of magnitude brighter in flux than Pan-STARRS y and z they offer no significant constraint on the SED.

We fitted the SED to the TLUSTY O and B star grids (Hubeny & Lanz 1995, ApJ, 439, 875; Lanz & Hubeny 2003, ApJS, 146, 417; Lanz & Hubeny 2007, ApJS, 169, 83). The Teff was varied as a free parameter from 25,000 K to 35,000 K. The log g was varied as a free parameter from 2.0 to 4.0. The Av was varied as a free parameter from 0.1 to 20 using a standard Galactic extinction law (R = 3.1). The best fit to TLUSTY models is now Teff = 32,500 +/- 2182 K, Av = 18.04 +/- 0.48, and log g = 3.25 +/- 0.31 (1-sigma uncertainties). The actual uncertainty of the best-fit log g value may be greater than the formal uncertainty reported by the fitting routine because generally the fitting is relatively insensitive to the value of log g (Bayo et al. 2008, A&A 492,277B) even in the case of a larger number of firm data points than we can use in this case. Thus the best fit remains consistent with Swift J1845.7-0037 being a Be X-ray binary with a large extinction, although the new fit does not suggest an IR excess because the IRAC magnitudes are now treated as upper limits.

This work is based in part on data obtained as part of the UKIRT Infrared Deep Sky Survey.This work is based in part on observations made with the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory, California Institute of Technology under a contract with NASA. This publication makes use of VOSA, developed under the Spanish Virtual Observatory project supported from the Spanish MINECO through grant AyA2017-84089. This research has made use of the VizieR catalogue access tool, CDS, Strasbourg, France. This work is based in part on the Pan-STARRS1 Surveys (PS1) and the PS1 public science archive.