AstroNote 2019-23

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, 13, 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. 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 a new transient source, of unknown nature in the galaxy SBS 1436+529A. We discovered ATLAS19mbg (AT2019gsc)  on MJD 58636.36 == 2019-06-02.36, at m_c = 19.7 +/- 0.2. There was no detection by ATLAS on MJD 58630.8 == 2019-05.27.8. ATLAS19mbg is offset by 5.05 arcsec north, 9.35 east from its host galaxy SBS 1436+529A. , which is at z = 0.011 or d = 53 Mpc (from NED), implying an absolute magnitude of M = -14.0 (assuming m-M = 33.64 and A_r = 0.022 and A_i = 0.016). 

While the forced photometry (see attached file) may appear marginal, there are also two ZTF detections (ZTF object ZTF19aawhlcn)  on 58635.24 in g (19.8) and r (19.9), visible in the Lasair broker (Smith et al. 2019) indicating that this is a faint,  blue transient.  The ATLAS forced photometry plot is attached below.  

Followup observations are encouraged of this unusual, intrinscailly faint, blue source. It is unlikely to be heavily reddened supernova given its colours, and is almost certainly quite young, given the ATLAS and ZTF recent limits. 

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.