Evidence of a B Star Counterpart to Swift J1845.7-0037

ATel #13211; B. McCollum (American Univ.), S. Laine (Caltech/IPAC)
on 21 Oct 2019; 18:52 UT
Credential Certification: Bruce McCollum (mccollub@cua.edu)

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We performed SED fitting of catalog photometry of 2MASS 18455462-0039341, which was suggested (Krimm et al., Atel #4130) as the IR counterpart to the X-ray transient Swift J1845.7-0037 / MAXI J1847-004 (ATel #4130, #13191, #13189, #13195, #13208). This object is suggested by the authors of those telegrams to be a Be X-ray binary on the basis of its X-ray characteristics. The bandpasses we used were Pan-STARRS y and z, 2MASS J, H, and K, and the four WISE bandpasses (3.5, 4.6, 12, and 22 microns).

To obtain guidance as to the range of temperatures to use in detailed stellar model fitting, we first performed a fit to a simple blackbody curve, with the temperature allowed to vary from 3000 to 30,000 K, and Av allowed to vary from 0.1 to 20. The best blackbody fit is given by Teff = 24,000 +/- 1300 K and Av = 18.0 +/-0.67.

We next 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 15,000 K to 35,000 K in 1000 K intervals. The log g was varied as a free parameter from 1.75 to 4.75. The Av was varied as a free parameter from 0.1 to 20. A standard Galactic extinction law (R = 3.1) was assumed.

The best fit to TLUSTY models is obtained with Teff = 28,000 +/- 1060 K, Av = 18.04 +/- 0.75, and log g = 2.75 +/- 0.22, corresponding to an early B giant. Thus the fit is consistent with Swift J1845.7-0037 being a Be X-ray binary. The fit also confirms the unusually large extinction suggested by Krimm et al. (ATel #4130) and Doroshenko et al. (ATel #13208). The fit suggests an IR flux excess in the WISE wavelengths relative to the stellar model SED at approximately a three-sigma confidence level. The excess increases toward longer wavelengths, from roughly ten percent in the 3.5 micron bandpass to approximately 30 percent in the 22 micron bandpass.

The SED fit may be viewed at the following link.

http://spider.ipac.caltech.edu/staff/seppo/plots/Swift_J1845.7-0037_SED_fit.jpg

This publication makes use of data products from the Wide-field Infrared Survey Explorer, which is a joint project of the University of California, Los Angeles, and the Jet Propulsion Laboratory/California Institute of Technology, funded by the National Aeronautics and Space Administration. This publication makes use of data products from the Two Micron All Sky Survey, which is a joint project of the University of Massachusetts and the Infrared Processing and Analysis Center/California Institute of Technology, funded by NASA and the National Science Foundation. 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. The Pan-STARRS1 Surveys (PS1) and the PS1 public science archive have been made possible through contributions by the Institute for Astronomy, the University of Hawaii, the Pan-STARRS Project Office, the Max-Planck Society and its participating institutes, the Max Planck Institute for Astronomy, Heidelberg and the Max Planck Institute for Extraterrestrial Physics, Garching, The Johns Hopkins University, Durham University, the University of Edinburgh, the Queen's University Belfast, the Harvard-Smithsonian Center for Astrophysics, the Las Cumbres Observatory Global Telescope Network Incorporated, the National Central University of Taiwan, the Space Telescope Science Institute, the National Aeronautics and Space Administration under Grant No. NNX08AR22G issued through the Planetary Science Division of the NASA Science Mission Directorate, the National Science Foundation Grant No. AST-1238877, the University of Maryland, Eotvos Lorand University (ELTE), the Los Alamos National Laboratory, and the Gordon and Betty Moore Foundation.