AstroNote 2020-208

ATLAS is a twin 0.5m telescope system on Haleakala and Mauna Loa which is robotically surveying the sky above declination -40 with a cadence of 2 days (Tonry et al. 2018, PASP,130:064505). Two filters are used, cyan and orange (denoted c and o; all mags quoted are in the AB system). While carrying out the primary mission for Near Earth Objects, we search for and publicly report stationary transients to the IAU Transient Name Server.  Data processing is carried out at Queen's University which combines automated source parameter filtering, machine learning image recognition, and spatial cross-matching with astronomical catalogues (Smith et al. 2020, PASP, 132:085002). More information is on the ATLAS homepage. We are submitting AstroNotes for transients that are either within 100 Mpc, or have some other interesting feature to bring to the community's attention, such as bright nuclear transients, slowly rising or rapidly fading objects.

We report the discovery and characterisation of a very long lived transient source offset fromt the galaxy WISEA J041345.99-222607.4 (z = 0.0949). The first discovery of ATLAS19bepo was on MJD = 58845 (2019-12-28) at magnitude c=19.68 +/- 0.18 and was reported to the TNS as AT2019yxk   [ 2019TNSTR2726....1T ]. ATLAS19bepo is offset by 0.2 arcsec south, 5.0 arcsec east (8.8 Kpc) from WISEA J041345.99-222607.4, which is at z = 0.0949 or d = 417 Mpc (from NED), implying an absolute magnitude of M = -19.0 (assuming m-M = 38.10 and A_r = 0.07 and A_i = 0.06).

It remained visible, with some flux variation until the end of the observing season on 58919. It has since remained at approximately o = 19.2 during the second season, with our last detection on 59147 (2020-10-25) at o = 19.3. The light curve shows a very long plateau for 270 days (in rest-frame) and no clear change in the colours, as measured from the ATLAS c and o bands, with undulations in the well sampled o-band at the  level of +/- 0.4 mag.  ATLAS19bepo could be an analogue of PTF14hls - the very long lived transient that has type II spectra (Arcavi et al. 2017, Natur, 551, 210). Another ATLAS discovery,  ATLAS20jaf (SN 2020faa; Tonry et al. [ 2020TNSTR.869....1T] , Yang et al. 2020 arXiv:2009.07270)  also shares properties of iPTF14hls.

A finder and forced photometry plot are attached, followup observations are encouraged.  

This work has made use of data from the Asteroid Terrestrial-impact Last Alert System (ATLAS) project. ATLAS is primarily funded to search for near earth asteroids through NASA grants NN12AR55G, 80NSSC18K0284, and 80NSSC18K1575; byproducts of the NEO search include images and catalogs from the survey area.  The ATLAS science products have been made possible through the contributions of the University of Hawaii Institute for Astronomy, the Queen's University Belfast, and the Space Telescope Science Institute.