Gaia18dcu: a three-magnitude brightness increase from a 50,000K stellar object

ATel #12173; B. Mccollum (American Univ.), S. Laine (Caltech/IPAC), F. C. Bruhweiler (American Univ.)
on 6 Nov 2018; 00:58 UT
Credential Certification: Bruce McCollum (mccollub@cua.edu)

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We call attention to the unusual host object of a blue transient, Gaia18dcu, in the Kepler field.

The object is noteworthy in having a very hot quiescent SED which we find dominates not only the UV but also the visible bandpasses, a quiescent Lbol of at least ~11x Lsun over several years, and a strong H-alpha excess in at least one quiescent epoch.

Gaia reports that it has shown a historical variability of ~0.5 magnitude and a G magnitude of ~20.5. It showed an increase of ~3.5 magnitudes in the outburst detected by Gaia. Variability of ~0.5 magnitude is also seen in two catalog r band values.

We performed SED fitting using catalog data. We used data from the Pan-STARRS Release 1, the Kepler INT Survey (Greiss et al. 2012, AJ, 144, 24), Gaia catalog photometry, and the GALEX NUV Survey (Olmedo et al. 2015, ApJ, 813, 100). It was detected by NEOWISE-R. We obtained additional measurements from archival GALEX FUV and Spitzer IRAC images.

We fitted the visible-UV SED to a single-object blackbody. The Av was allowed to vary as a free parameter from 0.30 to 10, with the lower limit set by the 3-D extinction map of Green et al. (2018; MNRAS, 478, 651). The Teff was varied from 1000 K to 100,000 K in 100 K increments. Only a very poor fit could be obtained for the combined UV-visible-IR SED, but a good fit using UV and optical data, so we excluded IR data from the fitting except as upper limits. Comparison with the blackbody SED suggests an IR excess. Also, there is evidence of variability by a factor of several in across two epochs in the Spitzer and WISE IR photometry.

The best fit was obtained for a blackbody of 50,000 +/- 2000 K with Av = 1.81 +/- 024. The fact that the Av value is much larger than expected from interstellar extinction supports the presence of an IR excess from dust.

The SED and best-fit blackbody may be viewed at

http://spider.ipac.caltech.edu/staff/seppo/gaia-figures/Gaia18dcu_SED.png

and

http://spider.ipac.caltech.edu/staff/seppo/gaia-figures/Gaia18dcu_SED_fit.png

The nearest X-ray or gamma ray source cataloged is 12.8 arcmin +/- 15 arcsec distant. Nondetection by the RASS survey places a very approximate X-ray upper limit of ~10x the GALEX FUV flux, consistent with a ~50,000 K blackbody.

Based on the blackbody SED, we estimate a bolometric luminosity of 11x Lsun to 85x Lsun, where the uncertainty is reflects the Gaia distance uncertainty (1289 to 7355 pc).

The Lbol is a few orders of magnitude less than expected of a 50,000 K stellar source. This along with the H-alpha excess suggests that the optical-UV emission is dominated by an accretion disk.

Gaia18dcu is not in the 2MASS catalog, but from the catalog image we placed an upper limit of Ks ~18 +/- 0.5. This is roughly two magnitudes fainter than the absolute magnitude of a main sequence late M star at ~1200 pc. Thus any cool object in the system is probably not earlier than spectral type M unless it is well beyond the Gaia ~7300 pc one-sigma maximum distance.

We suggest that Gaia18dcu might be a previously unknown interacting binary. Spectroscopy of this object is encouraged.

This research has used the VizieR catalogue access too, CDS, Strasbourg, France. We acknowledge the Pan-STARRS1 Survey (PS1) and the PS1 public science archive. 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 Two Micron All Sky Survey. We acknowledge ESA Gaia, DPAC and the Photometric Science Alerts Team (http://gsaweb.ast.cam.ac.uk/alerts). This publication makes use of data products from the Near-Earth Object Wide-field Infrared Survey Explorer (NEOWISE), which is a project of JPL/Caltech.