AIMS Provenance: Zwicky Transient Facility

Publisher source

The Zwicky Transient Facility (ZTF) is a time-domain survey that publishes alert packets for difference-image detections. A packet contains the current candidate, historical context, forced-photometry history, and image cutouts; the ZTF project publishes the corresponding Avro schema documentation.

AIMS uses the alert stream as a source of candidate moving detections. It does not ingest all ZTF images, alert-packet content, or source measurements.

What AIMS selects

AIMS queries a public BigQuery mirror of ZTF alerts and selects the upstream alert-table version appropriate for each date. The supported source versions currently include alerts_v1_8, alerts_v3_0, alerts_v3_1, alerts_v3_3, and alerts_v4_02.

A candidate is retained when:

candidate.rb > 0.5, using ZTF’s RealBogus quality score;
candidate.ndethist < 5, limiting sources with repeated spatially coincident alert detections;
its alert id is the deterministic retained row when duplicate records share one candid.

The RealBogus and history cuts favor plausible transient or moving detections. They are not object classifications and do not define a complete ZTF moving-object sample.

Coverage and observatory

AIMS coverage starts 2018-06-01 and is continuing.
AIMS observatory code: I41.

Normalization into AIMS

AIMS field	ZTF source or derivation
`observation_id`	`candid` converted to string.
`exposure_id`	Deterministic `ztf_exp_<mjd_midpoint>` value because the selected alert fields lack a stable exposure id.
`dataset_id`	Constant `ztf`.
`observatory_code`	Constant `I41`.
Observation time	`candidate.jd` converted to UTC MJD and shifted by half the exposure duration to the AIMS midpoint.
`exposure_duration_s`	`candidate.exptime`; older source versions without this field use the operational 30-second exposure value.
`filter`	`candidate.fid` mapped as `1 -> g`, `2 -> r`, `3 -> i`.
`ra_deg`, `dec_deg`	`candidate.ra`, `candidate.dec`.
`ra_sigma_deg`, `dec_sigma_deg`	Unavailable from this AIMS source path and stored as null.
`mag`, `mag_sigma`	`candidate.magpsf`, `candidate.sigmapsf`.
`healpixel`	Derived from RA/Dec at nside=128 NESTED.

Published AIMS products

In the AIMS BigQuery dataset, normalized ZTF rows are written to aims_ztf, copied into aims_detections_hot, and exposed through the aims_detections_unified view with dataset_id = ztf.

The same normalized rows are exported to Google Cloud Storage through the daily aims_detections_parquet product. AIMS also derives aims_frames_unified BigQuery rows and aims_frames_parquet search indexes from the deterministic exposure grouping. Complete detection/frame pairs are listed in aims_unified_parquet_catalog/catalog.json for consumers such as Precovery. These products do not contain the full ZTF alert packet or cutouts.

Scientific validity and limitations

The retained positions and PSF photometry preserve the selected alert-candidate measurements apart from documented time, filter, and indexing conversions.
The rb and ndethist cuts make the product useful for search but introduce an explicit selection function. AIMS ZTF is not an unbiased transient catalog or a complete ZTF data release.
The deterministic exposure id is an AIMS grouping value, not a native ZTF exposure identifier.
The 30-second duration for older schemas is a source-version accommodation. AIMS treats candidate.jd as exposure start and adds half the duration; consumers should regard that midpoint convention as an AIMS source interpretation, not a separately published ZTF timing guarantee.
This path does not provide per-detection astrometric covariance. Both uncertainty fields remain null, and consumers must not treat the absence as zero uncertainty.
AIMS does not retain the full alert history, classifications, forced photometry, or image cutouts in the common detection row.