AstroNote 2021-77

We draw attention to the fast fading lightcurve of AT2021den from the combined Pan-STARRS and ATLAS survey data and encourage rapid spectroscopic classification. 

Pan-STARRS discovered AT2021den (PS21aed; see Chambers et al. 2016, arXiv:1612.05560, and http://pswww.ifa.hawaii.edu) during the "Pan-STARRS Search for Kilonovae" program (see Smartt et al. AstroNote 2019-48 for details) on  2021-02-16.453 (MJD = 59261.330). AT2021den is likely associated with the nearby galaxy PGC 024745, offset by 1.4" N, 5.1" E or a projected distance of 3 Kpc, assuming a distance to PGC 024745 of 126 Mpc (from NED). If so, it would be M_w = -15.0 at peak. 

AT2021den was discovered at w = 20.5 and subsequently followed up by Pan-STARRS six days later at w = 21.2 on 2021-02-23.396 (MJD = 59267.396). We measure a fading of 0.12 mag per day in the PS1 w-band. A screenshot of the Pan-STARRS lightcurve and finder are included below.

The latest non-detection at the position of AT2021den by Pan-STARRS in the i-band occurred 44 days ago on 2021-01-03.547 (MJD = 59217.547). Running the transient position through the ATLAS forced photometry server (see Shingles et al. AstroNote 2021-7 for details) revealed no obvious detection of a point source during the Pan-STARRS gap, suggesting the transient is likely intrinsically faint and not an old supernova. The lack of ZTF detections (via https://lasair.roe.ac.uk/) also indicates it is not an old supernova. 

The luminosity and decline rate are consistent with the locus of viable kilonova models from Nicholl et al. (2021 arXiv:2102.02229). Using one-quarter of these models (those with chirp mass between 0.7 and 1 Solar masses), we measured the decline rates for ugrizy, PS1-w and ATLAS-co passbands and transform them onto a contour probability function of decline rate versus absolute magnitude by applying a Gaussian Kernel Density Estimation. AT2021den falls within the limits of these kilonova model contours.  It's luminosity and decline rate are also similar to some very bright Luminous Blue Variable eruptions.  

Spectroscopic classification is required and/or near-infrared imaging to check for excess in these bands. 

The discoveries from this program are a byproduct of the Pan-STARRS NEO survey observations. Operation of the Pan-STARRS1 and Pan-STARRS2 telescopes is primarily supported by the National Aeronautics and Space Administration under Grant No. NNX12AR65G and Grant No. NNX14AM74G issued through the SSO Near-Earth Object Observations Program. Data are processed at Queen's University Belfast enabled through the STFC grants ST/P000312/1 and ST/T000198/1. 

This work has made use of data from the Asteroid Terrestrial-impact Last Alert System (ATLAS) project. The Asteroid Terrestrial-impact Last Alert System (ATLAS) project is primarily funded to search for Near-Earth asteroids through NASA grants NN12AR55G, 80NSSC18K0284, and 80NSSC18K1575; byproducts of the NEO search include images and catalogues from the survey area. This work was partially funded by Kepler/K2 grant J1944/80NSSC19K0112 and HST GO-15889, and STFC grants ST/T000198/1 and ST/S006109/1. The ATLAS science products have been made possible through the contributions of the University of Hawaii Institute for Astronomy, the Queen’s University Belfast, the Space Telescope Science Institute, the South African Astronomical Observatory, and The Millennium Institute of Astrophysics (MAS), Chile.