The Zwicky Transient Facility Public Alert Stream
ATel #11685; F. Masci, S. R. Kulkarni, M. Graham, T. Prince, G. Helou (Caltech) on behalf of the Zwicky Transient Facility collaboration
on 5 Jun 2018; 16:14 UT
Credential Certification: Frank Masci (fmasci@ipac.caltech.edu)
Subjects: Optical, AGN, Asteroid, Cataclysmic Variable, Comet, Gamma-Ray Burst, Supernovae, Transient, Variables, Tidal Disruption Event
The Zwicky Transient Facility (ZTF; ATel #11266) announces the start of
public alerts. These alerts will originate from the ZTF public surveys
(Bellm & Kulkarni 2017; Nature Astronomy 1, 71) as described at
www.ztf.caltech.edu/page/msip
Alerts are generated by the ZTF Science Data System housed at IPAC-Caltech
(www.ipac.caltech.edu) using a realtime image-subtraction pipeline
(Masci et al. 2018; www.ztf.caltech.edu/page/technical). Newly acquired images
are differenced against a reference image to reveal candidate events
from flux transients, variable stars, eclipsing binaries, AGN, and Solar System
objects. The candidates are lightly filtered to remove easily
identifiable instrumental artifacts. They are then packetized into files
in the Apache Avro format -- a serialized binary format for efficient
distribution. Each packet contains metadata on the single event
that triggered the alert. This consists of a unique object identifier;
a machine-learned reliability score; source and image metrics to facilitate
further analysis and vetting; the nearest known Solar System object with
metadata; the nearest Pan-STARRS (PS1) sources with star-galaxy classification
scores and metadata inherited from the PS1 catalogs; metadata for previous
positionally-coincident alerts going back 30 consecutive nights if found,
otherwise, flux upper-limits if the alert position was covered during this
period; and ~ 1' x 1' full-resolution cutouts from the new, reference, and
subtraction images. Content details, suggestions for further filtering, and
cautionary notes are described in the Data System documentation at
www.ztf.caltech.edu/page/technical. Documentation on packet usage, examples,
and software is available at github.com/ZwickyTransientFacility/ztf-avro-alert
We expect anywhere from a thousand to >~ 100 thousand public alerts per
night, where numbers (and astrophysical types) depend on the region of sky
visited, i.e., galactic and/or ecliptic latitude range covered. Numbers also
depend on the available reference image coverage. Alerts are generated and
delivered approximately 10 to 20 minutes following observation.
During commissioning of the ZTF alert infrastructure, 185 supernovae were
discovered. Some of them were known and recovered in the alert packets.
Four known supernovae from three nights are listed below. Their selection
is described in ATel #11615.
Survey name | IAU name | Discovery mag (r) | Discovery date
ZTF18aajpjdi SN2018bfx 20.0 2018-04-14.31
ZTF18aajtlbf SN2018bbz 18.2 2018-04-14.42
ZTF18aamfrvy SN2018ahe 19.0 2018-04-21.36
ZTF18aaovbiy SN2018bdo 19.5 2018-04-24.38
Alert packets are transmitted from IPAC-Caltech to the University of Washington
in realtime for redistribution to community brokers. These brokers will
provide value-added services and are responsible for public access. The brokers
who have agreed to support ZTF are ALERCE, ANTARES, and LASAIR. These brokers
are presently concluding development and commissioning, and science users
should consult their websites for available services and timelines. Until the
broker services become available, users can download a tar file of each night's
alerts the following day from ztf.uw.edu/alerts/public/. This mechanism does
not currently support realtime access. Furthermore, all alert packets are
archived at IPAC-Caltech. This archive will be publicly accessible following
the first ZTF Public Data Release, which is anticipated in the Spring of 2019.
ZTF is a project led by PI S. R. Kulkarni at Caltech (see ATEL #11266), and
includes IPAC;Â WIS, Israel;Â OKC, Sweden;Â JSI/UMd, USA; UW,USA; DESY, Germany;
NRC, Taiwan; UW Milwaukee, USA and LANL USA. ZTF acknowledges the generous
support of the NSF under AST MSIP Grant No 1440341.