Spectral Type of the Be Star Component of SWIFT J004427.3-734801

ATel #13714; B. McCollum (American Univ.), S. Laine (Caltech/IPAC)
on 8 May 2020; 03:09 UT
Credential Certification: Bruce McCollum (mccollub@cua.edu)

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We performed SED fitting of catalog photometry of 2MASS J00442806-7348031, which is suggested by Coe et al. (ATel # 13626) and Haberl et al. (ATel #13709) to be the counterpart to the X-ray transient SWIFT J004427.3-734801. These authors propose that this object is a Be/X-ray binary in the SMC. Precise photometry is available of this object in several catalogs. A total of 23 measurements were used for the fitting, including U, B, g, g-prime, V, r, I, Ic, J, H, Ks, WISE W1 and W2, and the four Spitzer IRAC bands from 3.6 to 8 um, with upper limits given in the WISE 12 and 22 um channels.

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 15,000 K to 30,000 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 2.0. A standard Galactic extinction law (R = 3.1) was assumed.

No variability is seen within the photometric uncertainties when measurements in identical filters (B, V, J, and H) from more than one epoch are available. Measurements in three epochs are available in U, two of which differ by 7% which is only marginally within the photometric uncertainties. Because this is probably an accreting system, variability in the U band would not be unexpected and may represent flux which is not photospheric that would create an error in the SED fitting.

Including an average of the U-band measurements in the fitting produces a best fit of Teff = 18,000 +/- 864 K. Excluding the U fluxes results in a best fit of 15,000 K. When the U-band flux is excluded then the reported fluxes appear as a UV excess to the 15,000 K model. The U-band extinction law in the SMC can be significantly different than in the Galaxy (e.g., Prevot et al. 1984, A&A, 132, 389), especially in the Bar, but this object is in the outer SMC. The formal chi-square of the 18,000 K fit is only 4% larger than for the 15,000 K fit and the returned log g values and Av are virtually the same. However, we think that excluding the U band from the fitting should give a more accurate fit.

Excluding the U magnitudes gives a best fit to TLUSTY models of Teff = 15,000 +/- 703 K, Av = 0.77 +/- 0.10, log g = 2.25 +/- 0.41, and metallicity Z = 0.5.

Metallicities in the SMC are typically ~0.1. If we restrict the fitting to only Z values of 0, 0.1, and 0.2, the best fit is for Teff = 15,000 +/- 832 K, Av = 0.77 +/- 0.09, and Z = 0.2 with a chi-square a few percent greater than for Z = 0.5, while the log g becomes 2.5 +/- 0.37. The fitting shows a flux excess of ~40% in the Spitzer 5.8 um and 8 um bands, and a possible smaller excess in the 3.6 and 4.5 um bands.

The SED fit may be viewed at

http://web.ipac.caltech.edu/staff/seppo/plots/SWIFT_J004427.3-734801_SED.png

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 NASA. This work is based in part on observations made with the Spitzer Space Telescope, which was operated by the Jet Propulsion Laboratory, California Institute of Technology under a contract with NASA. 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.