Evidence of Past Variability of PGIR20emh/AT 2020rjf
ATel #13977; B. McCollum (American Univ.), S. Laine (IPAC/Caltech)
on 29 Aug 2020; 00:34 UT
Credential Certification: Bruce McCollum (mccollub@cua.edu)
Subjects: Infra-Red, Optical, Microlensing Event
PGIR20emh/AT2020rjf was reported as a candidate microlensing event by Karambelkar et al. (ATel #13969), associated with 2MASS 17304004-2748475 which they suggested is a red dwarf. Tinyanont et al. (ATel # 13973) found near-IR spectral features of a cool star after the event. The Gaia distance (Bailer-Jones et al. 2018) of ~3900 pc along with its historical brightness implies that it is probably not a dwarf star.
Catalog photometry shows significant past variability:
Survey J Magnitude H Magnitude
2MASS 11.77 +/- 0.09
DENIS 12.51 +/- 0.06 11.23 +/- 0.07
VPHAS 15.93 +/- 0.01
VVV DR2 12.84 +/- 0.003 12.29 +/- 0.03
2MASS flags its J and Ks magnitudes as low quality, so they are excluded.
The Spitzer GLIMPSE survey found [3.6] = 9.89 +/- 0.53 mJy and [4.5]= 5.46 +/- 0.38 mJy, while the unWISE catalog (Schafly et al. 2019) reports W1 ~12 mJy and W2 = 6.1 mJy with uncertainties < 1%. (Note that unWISE discards variability information.)
The CMC catalog reports Sloan r' = 15.524 +/- 0.1, while the VPHAS survey reports r = 16.37 +/- 0.01. Pan-STARRS DR1 reports g = 18.501 +/- 0.01. Karambelkar et al. (ATel #13969) report g ~ 19 on UT and r ~ 16 on 2020-05-18, returning to g = 15.40 +/- 0.05 and r = 13.49 +/- 0.05 on UT 2020-08-24.
We performed SED fitting using catalog photometry in bands g, r', I, z, Z, y, J, H, W1, W2, W3, and the Spitzer IRAC channels. To account for possible circumstellar extinction, we allowed Av to vary as a free parameter. Not all data points could be well fitted, but most of the points can fall on or near a reddening-corrected SED. The best fit to the BT-Settl-CIFIST model grid (Baraffe et al. 2015) is for Teff = 6000 and Av = 5.2 +/- 0.1. If we fit only the IR data (y-band and longer), the best fit is Teff = 2800 with Av = 1.21 +/- 0.18. The expected Av is ~3 (Bayestar19 map, Green et al. 2019). Although because of the near-IR variability the quality of the fit is not good enough to be precise about the spectral type, such a large difference dependent on whether visible wavelengths are ignored is consistent with what would be expected from a binary containing a red giant and a hotter companion.
We suggest that this object may be a cataclysmic variable.
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 research has made use of the VizieR catalogue tool, CDS, Strasbourg, France. This publication makes use of VOSA, developed under the Spanish Virtual Observatory project supported from the Spanish MINECO through grant AyA2017-84089. This publication makes use of data products from the 2MASS Survey, which is a joint project of the University of Massachusetts and the IPAC/Caltech, funded by NASA and the National Science Foundation. 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 JPL/Caltech, funded by NASA.
