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\title{\vspace{-0.5in}
       Late Science with PTF \&\ Early Science with  ZTF}
\author{S.\ R.\ Kulkarni}
\date{} % leave blank
\maketitle




\section{Background}
\label{sec:Background}

As noted by Astro-2010,  Time Domain Astronomy (TDA) is a frontier
field with great capacity for discovery.  Indeed, there has been
tremendous progress in supernovae of all sorts, relativistic
explosions, events associated with nuclear black holes and a variety
of coalescences.

The Zwicky Transient Facility (ZTF) is the successor of the Palomar
Transient Factory (PTF).  PTF,  PS-1 \& ATLAS have a field-of-view
(FoV) of $\sim 8\,{\rm deg^2}$.  The 47\,deg$^2$ FoV of ZTF is
achieved using a mosaic of 16 large format CCDs mounted at the focus
of the Palomar 48-inch Oschin telescope [EB17].  We expect to achieve
first light of ZTF in July. Following a month of engineering
commissioning the science commissioning will begin.  We have set
aside 4--6 months to this end. The 3-year survey is expected to
start during the December-January period.


For COO there is much riding on the success of ZTF. First, ZTF, a
public-private survey, is now firmly on the LSST roadmap.\footnote{At
the Grapevine AAS Tony Tyson, Chief Scientist of LSST declared ``In
TDA ZTF is the pre-cursor survey for LSST".}   IMHO, the current
standing of the Palomar Observatory is in large part due to the TDA
programs.\footnote{PTF since inception has resulted in 188 published
papers to date. See \url{http://www.ptf.caltech.edu/iptf}. CRTS,
which like PTF, uses Palomar for classification and Keck for followup
has also been very productive.}

This, my only proposal for Palomar \& Keck time, has two goals. The
first is P200 support for the commissioning of ZTF (\S\ref{sec:Mechanics},
\S\ref{sec:TuningMLClassification})  and Keck for undertaking early
science (\S\ref{sec:EarlyScience}; Keck). During the PTF/iPTF era
we primarily focused on finding young events and then following
them  up. An entirely different approach to TDA is to start with
densely sampled light curves, classify (using the light curve and
contextual data such as coincidence with host galaxy) and then
undertake follow up of interesting sources. Indeed, this approach
will  likely be the dominant mode in the LSST era. Furthermore,
this will likely be the mode through the commissioning period.
Motivated thus, I now propose a pilot program based on time series
data (\S\ref{sec:UniqueTransients}, \S\ref{sec:M31}).

\section{The Mechanics of a TDA Survey}
\label{sec:Mechanics}

The basic approach of a TDA survey is simple: take an image tonight
(``new"), subtract it from a ``reference" image (stack of images
obtained in the past) and search for any remaining source(s).  The
process of subtraction produces a significant number of artifacts
(arising from variable PSF, detector artifacts; hereafter false
negatives, FN). The artifacts are easily recognized  by a visual
inspection of the subtracted images by a trained observer.  Next,
one must eliminate ``false positives" (FP).  With large FoV surveys
FP can easily overwhelm a given project.  For those interested in
extragalactic explosive transients the FP  are active Galactic stars
(dwarf novae, flares from M dwarfs are particularly challenging)
and burbling AGN.

An innovation for PTF was the early implementation of  a Machine
Learning (ML) pipeline, \texttt{RealBogus}, to securely identify
transients [BR+12].  For PTF we initially relied on the \texttt{Galaxy
Zoo} group\footnote{The \texttt{Galaxy Zoo} is one of the earliest
Citizen Science initiatives in which volunteers help a science
project. In our case, citizen scientists went through our candidates
and sorted the candidates into ``genuine" or ``artifact".} to filter
out false positives and build up an initial training set for
``transient identification." The next step was to ascertain that
the candidates thus identified were actually explosive events in
the sky. To this end, we undertook an extensive ``spectral
classification" campaign which not only verified the candidates but
also tagged them (not just desirable events but also FP). The metric
``Receiver Operating Characteristic" (ROC) captures the FP \&\ FN
performance of any ML pipeline.


\section{Building the ML Classification Pipeline: \texttt{RealBogus}}
\label{sec:TuningMLClassification}

ZTF will detect transients $12\times$ faster than PTF.  Thus the
\texttt{RealBogus} (RB) pipeline has to have a better ROC.   The
ZTF camera with struts in the focal plane will have different kinds
of instrumental artifacts.  During commissioning we will observe a
few areas repeatedly to ensure we understand and eliminate such
camera and instrument specific artifacts. An entirely new sample
will have to be generated for ZTF to train on.  We plan to use
advanced ML methods\footnote{We will augment the Random Forest (RF)
technique used for iPTF by more active learning on a regular basis
as well as deep learning for non-stellar objects.}
 not just to separate the real candidates from the bogus
ones but also to start exploring separating various real classes
for prioritization.  As noted above, in order to generate a meaningful
training sample, we will need an initial labeled set --whence the
need of DBSP (for $>18.5\,$mag events).\footnote{The training set
should include the full range of brightness and particularly those
at the limiting magnitude. For brighter sources we will use SEDM}
In addition,  we  propose to obtain discriminating spectra especially
for confusing sources  through this proposal to reduce later
dependence on spectroscopy. As we progress we will lower the RB
score (to get more transients; the training set has to adapt).


\section{Early Science}
\label{sec:EarlyScience}

The commissioning will (roughly) be as follows: (1) Choose a field
and dither and go for 2 hours and then another field etc. This will
help us understand the systematics within the instrument. (2) The
next phase will be on finding real transients and thus start training
the ML pipeline. While ML work is being done we will do high cadence
(e.g. sit on M31 for may be a week; Orion region for  weeks etc).
(3)  Increase the number of fields (to say 30 per night). This is
when we will start detecting a large number of SNe (easily a dozen
per night). This phase is well suited to detecting young SNe and
fast rising transients. We will then end with a cadence that is
relevant to the MSIP (the fastest survey) and partnership surveys
(moderate cadence). Each of these two modes will lead to early
science.

High cadence observations with PTF was very productive: an exotic
nuclear source [CB+12], a possible detection of a dirty fireball
(cosmological explosions with, relative to GRBs, a moderate Lorentz
factor; [CK+13] and an untriggered classical GRB [CU+15]. We have
had a number of successes with young SNe.  PTF started the cottage
industry of ``flash spectroscopy" to diagnose the circumstellar
medium of core-collapse SNe [GA+14]. Early studies of Ia shed key
insight into the {\it origins} of type Ia SNe [NS+11],[CK+15]). Beyond
this record (of the bounties of Nature and the track record of the
PI team) and noting the $\times 12$ speed of ZTF I cannot add any
more justification for early science.


\begin{figure}[h]
\centering
\includegraphics[width=0.45\textwidth]{SkyMapClassified.eps}
\caption{\scriptsize Sky locations of classified events (top: all
sky; bottom: M31 fields).  The inclined cross indicates the major
and minor axis of M31.  Most of the classified transients are novae.}
\label{fig:SkyDistribution1}
\end{figure}


\section{Post-PTF:  A Search for Unique Transients}
\label{sec:UniqueTransients}

We divide transients into those whose nature is known  (almost
always via spectroscopic classification) or ``classified" and those
for which we have light curves but which lack spectra and thus are
unknown (``unclassified").  As noted in \S\ref{sec:Background}
during the PTF/iPTF era we undertook spectral classification for
all (save a handful) strong candidates. Only after spectral
classifications were other observations (or thinking) undertaken.
Hereafter, we will refer to such events as ``classified." Our current
holdings are 3132 classified  events (see Fig.~\ref{fig:SkyDistribution1}
for sky map) and $[9087, 2715]$ unclassified events  with $[>10,
>100]$ photometric points (see Fig.~\ref{fig:SkyDistribution2}).
PTF, relative to PS-1, was notable for having a very high rate of
spectral classifications. Nonetheless, the ratio of unstudied sources
to studied is much larger than unity. For ZTF and LSST this ratio
will skyrocket up by one and two orders of magnitude, respectively.

Our plan of attack is as follows: [1] Apply clustering techniques,
based on  features derived from light-curves (including upper-limits)
as well as  ancillary information such as association with a putative
host or a radio source, to unclassified events.  Partition the set
into likely abundant classes, possibly rare classes, and outliers.
[2] We will then overlay the classified sources using the same
parameter space to ``seed" the space. We expect the unclassified
sources to mimic the distribution of the classified ones (e.g. SNIa
will likely far exceed other sources based on the selection criteria,
followed by SNII, SNIIn, CVs, dwarf novae, AGN and others). [3] We
will further make use of discriminators such as the methodology of
Q-M plane [CS+14] and separate various types like Quasi-periodic
sources, periodic dippers, bursters (very useful in weeding out
Galactic interlopers in the apparent vicinity of a background
galaxy).  [4] We will use cross-matching with recent observations
from other surveys like CRTS, Gaia, and Pan-STARRS before making a
final selection.

It is our rough estimate that about 1\% of the sources will be
``interesting".  The primary goals are (1) determine if the source
is extragalatic and (2) determine the energy scale of the phenomena.
This requires spectroscopy of the host or quiescent source.  Such
a study will  help in the future to better understand the parameter
space of transients, and add to our understanding of additional
types besides alerting us to possible biases that went into originally
neglecting these spectra.

\begin{figure}[h]
\centering
\includegraphics[width=0.5\textwidth]{SkyMapNotClassified.eps}
\caption{\scriptsize  
Sky locations of unclassified events. As in Fig.~\ref{fig:SkyDistribution1}
the top panel is all sky and the bottom is the M31 and vicinity.
Only recently we acquired the capacity to process the M31 data on
a routine basis. This explains the paucity of abundance of events
in the vicinity of M31.  
\label{fig:SkyDistribution2}} 
\end{figure}


\section{Transients in the M31 fields}
\label{sec:M31}

During PTF/iPTF Andromeda (M31) was observed at least twice during
every night it was up.  Occasionally intensive campaigns were
carried out (tens to hundreds of observations per night). The M31
field is clearly the most observed region of sky by  PTF/iPTF.
Furthermore Andromeda has been extensively observed by HST, Subaru/HSC
and PS-1.  These data sets add to make the PTF M31 data set  truly
unique.

The PTF field of view is 7.2 deg$^2$. M31 measures approximately
$1.5^\circ\times 1^\circ$.  Thus along with M31 a good fraction of
the ``background sky" was also observed. This special data set will
thus let us explore, in the sense of \S\ref{sec:UniqueTransients},
the phase space of extragalactic transients on timescales ranging
from tens of minutes to  eight years (2009--2016).

A specific project is a search for quasars within and in the close
outskirts of M31.  These quasars will be very useful for probing
the interstellar medium (via reddening, optical and UV spectroscopy)
in the disk and halo of M31. We are aware of the LAMOST key project
for quasars in the vicinity of M31 [HL+15]. However, here, using
variability we can probe quasars within the disk and the halo (inner
and outer) of M31. In contrast, the LAMOST search relied on photometric
selection and is thus well suited to finding quasars {\it away}
from crowded regions.

Two years ago we carried out a pilot project. We identified bright
($<18\,$mag) variable quasar candidates (as  part of a search for
R Cor Bor stars in M31; [TB+13]).  We confirmed the candidates with
DBSP. The quasar areal density, for $B<20.5$\,mag, is 26 deg$^{-2}$
[BF+87]. Thus we can expect to find nearly two hundred quasars.

On behalf of the ZTF commissioning team I request nine nights of
DBSP, five nights of LRIS and four TOOs. These are justified in the
next section.

{\it 
Inserted here to purely test  the algorithm
[EB17], [TB+13], [BF+87]
[CK+15]}
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\newpage

\section{Technical Justification}


\subsection{Telescope \& Instrument Justification}

The first goal is spectral classification of events to train our
ML classifier.  The Spectral Energy Distribution Machine (SEDM),
mounted at the Cassegrain focus of the robotic  Palomar 60-inch
(P60) telescope, is suitable for spectral classification of  bright
transients: at 18\,mag, 15\, minutes. We need DBSP for spectral
classification for fainter sources or sources in crowded regions
(the SEDM suffers from scattered light that we are not able to model
properly).  Typical exposure time for $>19$\,mag objects with DBSP
is 15\,min (all inclusive). Under median conditions the SNR is
sufficient to distinguish the class (and also give good estimates
of redshift).



While the exact number of objects that we will need to target for
spectral classification will depend on how clean the data are, we
expect to need several tens to a few hundred spectra per class for
optimal separation of the classes. Since these are science targets
of varied types, we will be achieving two aims with this single
program: science, and ML that reduces the future requirement of
spectroscopy.


Assuming typical Fall/Winter weather I request 9 nights of DBSP for
spectral classification. I request two LRIS nights (September \&
October) for the M31 project (hunt for quasars and for unique
transients in this densely sampled field). The quasars are guaranteed
returns and the search for unique transients is an exploration and
likely will involve spectroscopy of faint host galaxies (which
likely will be the most interesting transients). I request  three
nights of LRIS for early science (Nov, Dec and Jan). Given the
expected target throughput of ZTF and given the large number of
sources for the Unique Transient project a  larger request can be
justified. However, this request will enable us to undertake
significant early science and make a good foray into the Unique
Transient project.



\subsection{Exposure Time Justification}

See above. 


\subsection{Lunation Justification}


The ZTF 5-sigma limit is 20.5 (under median conditions). As with
many surveys this means that most of the action will be close to
the threshold. Under good seeing conditions spectral classification
at the threshold can be undertaken with DBSP/P200 but does require
dark/gray time. Keck is used to study objects as they fade. Again
this requires dark/gray time.


\subsection{Target Accessibility}

Not an issue for this program.

\subsection{Scheduling Flexibility/Date-Specific Observing}

Not applicable.


\subsection{Cadence/TOO Program Justification}

I request four TOOs each of 30 minute duration with LRIS or DEIMOS
for rapid spectroscopy of fast-fading extra-galactic transients
(this is the pilot project for Anna Ho's thesis).  These transients
are likely to arise from classical GRBs, orphan GRBs or dirty
fireball.  The high cadence observations are particularly well
suited to this project. With luck she may find one event. However,
as  is now clear from her pilot study with PTF/iPTF the false
positive rate is high. She has developed algorithms to cut down the
FPR. Nonetheless, one must be prepared for a few failures -- whence
the request of four TOOs.  Since this is an exploratory effort I
do not seek (valuable) partnership TOOs.




\newpage

\section{Supplementary Information}


\subsection{Backup Program (for Keck proposals only)}

We have sources that range in brightness. If the night is terrible
then we will call our stellar astronomy colleagues (Prince, Kupfer)
and undertake studies of bright binaries (there is an infinite
supply of these objects).



\subsection{Response to CIT TAC Feedback}

Not relevant since both programs proposed here are new.


\subsection{Future Applications}

ZTF will run for 3 years. The production of transients by ZTF can
easily consume all available time on P200 and Keck. However, we
will refine our science to suit the available time (telescope and
personal). I envisage a steady request, per semester, of six nights
of Keck and twelve nights of P200.


\subsection{Keck Archive Observations}

Not applicable. We are studying transients.


\newpage

\section{Target List}

We have sources all over the sky. The only special request is for
M31 (0h+40d) and that is why we specifically seek 1 night each in
September and October.

\end{document}
