The Most Interesting Anomalies Discovered in ZTF DR3 from the SNAD-III Workshop

SNAD⁹ is a project devoted to tackling the anomaly detection problem in large astronomical data sets using machine learning methods (Malanchev et al. 2020; Ishida et al. 2019; Kornilov et al. 2019; Pruzhinskaya et al. 2019). The main focus of the third SNAD workshop was anomaly detection in the public Data Release 3 (DR3)¹⁰ of the Zwicky Transient Facility (ZTF, Bellm et al. 2019), using the data from the first 9.4 months of ZTF science operations, dated from 2018 March 17th to December 31st. We analyzed two ∼47 sq. deg fields (fieldid = 795 and fieldid = 807), and a third ∼3 sq. deg. field (fieldid = 695, ccdid = 11) which contains the M31 galaxy. Each light curve was characterized by 42 features (e.g., amplitude, periodogram peak, etc.) which have proven their effectiveness in describing variable starlight curves (Kim et al. 2014; D'Isanto et al. 2016). Four different anomaly detection algorithms were applied to the data: Gaussian Mixture Model (Reynolds 2009), Local Outlier Factor (Breunig et al. 2000), One-Class Support Vector Machine (Khan & Madden 2010), and Isolation Forest (Liu et al. 2008, 2012). The outliers given by the algorithms were summarized in a common table and were ordered in the decreasing order of their anomaly scores. Since not all of the outliers are necessarily interesting astrophysical anomalies (for example, they could be artifacts generated during image pre-processing, etc.), they were also investigated by our team experts.

For each field, we obtained a list with potentially interesting astronomical objects which will be subjected to further analysis. We report some of the interesting non-cataloged anomalies in Table 1. The first column contains the object identifiers from ZTF DR3 (some of the targets also appeared in the ZTF alert stream and, therefore, have an additional identifier). The equatorial coordinates (J2000.0) are given in the second column while the maximum r-magnitudes from ZTF automatic photometry are given in column 3 (the magnitude is not corrected for Milky Way dust extinction). In a few cases, an SDSS galaxy has been identified at the source position. For these objects we estimated the r-band absolute magnitude at maximum light assuming a flat ΛCDM cosmology with Ω_Λ = 0.702 ± 0.022 (Scolnic et al. 2018) and H₀ = 70 km s⁻¹ Mpc⁻¹ (column 4). Candidate host galaxies from SDSS DR16 ¹¹ (if available) and their corresponding photometric redshifts (z_ph, obtained via the KD-tree method) are given in the columns 5 and 6. The last column contains comments about the possible origin of the source.

Among the anomalies listed in Table 1, there are six objects that are likely extragalactic in origin. These are potential supernovae or active galactic nuclei (AGNs) candidates. Three other detected sources possibly belong to the M31 galaxy.

• 1.  
  ZTF DR3 contains only the descending part of the light curve for object 795202100005941. Considering the photometric redshift of the possible host galaxy, its absolute magnitude is ∼−22.4 which makes it a potential superluminous supernova (SLSN) candidate.
• 2.  
  The object 795213200000671 also appeared in Gaia Alerts¹² on 2018 March 12 with an apparent magnitude of 18.83 mag in G filter which was ∼8 days before ZTF first detection (AT 2018afr/Gaia18apj).
• 3.  
  The object 695211100131796 is located near the ionized hydrogen region [AMB2011] H ii 2692 (Azimlu et al. 2011). It was previously detected as an object of unknown nature PSO J011.0457+41.5548 (Lee et al. 2014). We consider it as a candidate for luminous blue variables (LBV) or for variables of the S Doradus type (SDOR).
• 4.  
  According to the Transient Name Server¹³ the object 695211100022045 was first seen on 2017 October 29 as AT 2017ixs. It was detected a second time on 2017 December 15 with 19.5 mag in Clear filter and classified as a possible nova (PNV; Carey 2017). Six months latter, on 2018 June 20, MASTER-Kislovodsk auto-detection system discovered MASTER OT J004355.89+413209.9 with an unfiltered magnitude of 19.0 mag at AT 2017ixs position (Balanutsa et al. 2018). The behavior of its light curve is not typical for dwarf novae or for cataclysmic variables.