MNRAS 505, 1254–1267 (2021) https://doi.org/10.1093/mnras/stab1344
Advance Access publication 2021 May 12
Year 1 of the ZTF high-cadence Galactic plane survey: strategy, goals, and
early results on new single-mode hot subdwarf B-star pulsatos
Thomas Kupfer,
1,2‹
Thomas A. Prince ,
3
Jan van Roestel,
3
Eric C. Bellm,
4
Lars Bildsten,
5,1
Michael W. Coughlin ,
6
Andrew J. Drake,
3
Matthew J. Graham ,
3
Courtney Klein ,
7
Shrinivas R. Kulkarni,
3
Frank J. Masci,
8
Richard Walters,
9
Igor Andreoni,
3
Rahul Biswas ,
10
Corey Bradshaw,
2
Dmitry A. Duev ,
3
Richard Dekany,
9
Joseph A. Guidry,
11
J. J. Hermes,
12
Russ R. Laher
8
and Reed Riddle
9
1
Kavli Institute for Theoretical Physics, University of California, Santa Barbara, CA 93106, USA
2
Department of Physics and Astronomy, Texas Tech University, PO Box 41051, Lubbock, TX 79409, USA
3
Division of Physics, Mathematics and Astronomy, California Institute of Technology, Pasadena, CA 91125, USA
4
DIRAC Institute, Department of Astronomy, University of Washington, 3910 15th Avenue NE, Seattle, WA 98195, USA
5
Department of Physics, University of California, Santa Barbara, CA 93106, USA
6
School of Physics and Astronomy, University of Minnesota, Minneapolis, MN 55455, USA
7
Department of Physics and Astronomy, University of California, Irvine, CA 92697, USA
8
IPAC, California Institute of Technology, 1200 E. California Blvd, Pasadena, CA 91125, USA
9
Caltech Optical Observatories, California Institute of Technology, Pasadena, CA 91125, USA
10
Department of Physics, The Oskar Klein Center, S tockholm University, AlbaNova, SE-10691 Stockholm, Sweden
11
Department of Astronomy, The University of Texas at Austin, Austin, TX 78712, USA
12
Department of Astronomy, Boston University, 725 Commonwealth Ave., Boston, MA 02215, USA
Accepted 2021 May 5. Received 2021 May 5; in original form 2021 January 14
ABSTRACT
We present the goals, strategy, and first results of the high-cadence Galactic plane survey using the Zwicky Transient Facility
(ZTF). The goal of the survey is to unveil the Galactic population of short-period variable stars, including short-period binaries,
and stellar pulsators with periods less than a few hours. Between 2018 June and 2019 January, we observed 64 ZTF fields resulting
in 2990 deg
2
of high stellar density in the ZTF-r band along the Galactic plane. Each field was observed continuously for 1.5 to
6 h with a cadence of 40 sec. Most fields have between 200 and 400 observations obtained over 2–3 continuous nights. As part
of this survey, we extract a total of ≈230 million individual objects with at least 80 epochs obtained during the high-cadence
Galactic plane survey reaching an average depth of ZTF–r ≈ 20.5 mag. For four selected fields with 2–10 million individual
objects per field, we calculate different variability statistics and find that ≈1–2 per cent of the objects are astrophysically variable
over the observed period. We present a progress report on recent discoveries, including a new class of compact pulsators, the first
members of a new class of Roche lobe filling hot subdwarf binaries as well as new ultracompact double white dwarfs and flaring
stars. Finally, we present a sample of 12 new single-mode hot subdwarf B-star pulsators with pulsation amplitudes between
ZTF–r = 20–76 mmag and pulsation periods between P = 5.8–16 min with a strong cluster of systems with periods ≈6 min. All
of the data have now been released in either ZTF Data Release 3 or Data Release 4.
Key words: surveys – binaries (including multiple): close – stars: oscillations (including pulsations) – white dwarfs.
1 INTRODUCTION
Large-scale optical time-domain surveys have opened a new window
to study the variable sky providing hundreds to thousands of epochs
across the whole sky. Starting with the Sloan Digital Sky Survey
(SDSS; York et al. 2000), a new generation of wide-field optical
surveys has exploited new affordable CCD detectors to open the
frontier of data-intensive astronomy. This allows the study of stellar
variability on different time-scales across the full magnitude range.
In particular, surveys covering also low Galactic latitudes are well
suited to study the Galactic distribution of photometric variable stars.
E-mail: tkupfer@ttu.edu
Ground-based surveys include the Optical Gravitational Lensing
Experiment (OGLE; e.g. Soszy
´
nski et al. 2015),PTF(Lawetal.
2009), the Vista Variables in the Via Lactea (; Saito et al. 2012),
ASAS-SN, (Shappee et al. 2014; Jayasinghe et al. 2018), and most
recently ATLAS (Heinze et al. 2018; Tonry et al. 2018). In particular,
the fastest stellar variabilities on time-scales of minutes to hours are
of great interest for a large number of scientific questions. This
includes ultracompact binaries (UCBs), compact pulsators as well as
fast flaring stars.
UCBs are a class of binary stars with orbital periods less than
about 60 min, consisting of a neutron star (NS)/white dwarf (WD)
primary and a Helium-star (He-star)/WD/NS secondary. These UCBs
are sources of low-frequency gravitational wave signals as probed
C
2021 The Author(s)
Published by Oxford University Press on behalf of Royal Astronomical Society
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ZTF high-cadence Galactic plane survey 1255
by the Laser Interferometer Space Antenna (LISA) and are crucial to
our understanding of compact binary evolution and offer pathways
towards Type Ia and other thermonuclear supernovae. Systems with
orbital periods <20 min will be the strongest Galactic LISA sources
and will be detected by LISA within weeks after its operation begins
(Nelemans, Yungelson & Portegies Zwart 2004;Str
¨
oer & Vecchio
2006;Nissankeetal.2012; Littenberg et al. 2013; Korol et al. 2017;
Kremer et al. 2017; Kupfer et al. 2018; Lamberts et al. 2019; Burdge
et al. 2019, 2020a) and as such are ideal multimessenger sources
(Shah, van der Sluys & Nelemans 2012; Shah & Nelemans 2014;
Baker et al. 2019; Littenberg et al. 2019; Kupfer et al. 2019a)
Short time-scale photometric variations can also originate from
astrophysical changes within the internal structure or atmosphere
of the star. Such sources are flaring stars, pulsating stars, or white
dwarfs. Prominent examples of rapidly pulsating stars are δ Scuti
stars (Breger 2000), SX Phoenicis variables (Nemec & Mateo 1990),
or Ap or Am stars (Renson, Gerbaldi & Catalano 1991), and pulsating
white dwarfs (ZZ Ceti variables; Fontaine & Brassard 2008) as well
as pulsating extremely low-mass white dwarfs (Hermes et al. 2013).
The stars exhibit pulsation amplitudes of less than 1 percent up to
several tens of percent on time-scales of few to tens of minutes.
Recently, a new class of short-period pulsating hot stars known
as Blue Large-Amplitude Pulsators (BLAPs) was discovered by
Pietrukowicz et al. (2017). Their pulsation periods are typically
between 20–40 min. Romero et al. (2018) and Byrne & Jeffery (2018)
proposed that the BLAPs are hot pre-helium white dwarfs that are
cooling and contracting, with masses in the range 0.3–0.35 M
.A
recent study by Meng et al. (2020) suggests that BLAPs could be
the surviving companions of type Ia supernovae. In this scenario
mass transfer from the BLAP progenitor leads to the detonation of a
white dwarf companion, leaving behind a BLAP as single star. Their
pulsation properties can best be explained by fundamental radial
mode pulsators. However, the known sample of BLAPs is small and
in-homogeneous, discovered only in the OGLE survey.
A number of fast cadence ground-based surveys have been
executed to study the variable sky down to a few minutes period. The
main goal of these high-cadence surveys is the discovery of rapid
brightness variations seen in UCBs, compact pulsators as well as fast
flaring stars. The first survey at low Galactic latitudes targeting short-
period systems was the Rapid Temporal Survey (RATS; Ramsay
& Hakala 2005; Barclay et al. 2011) covering a total of 46 deg
2
.
Another more recent survey is the OmegaWhite (OW) survey, which
covers a total of 400 deg
2
at low Galactic latitudes (|b| < 10
◦
)aswell
as in the Galactic Bulge using high-cadence optical observations.
Two neighbouring 1 deg
2
fields are alternatingly observed in 39-s
exposures over an observing duration of 2 h, with an observational
median cadence of ≈2.7 min per field (Macfarlane et al. 2015,
2017a,b; Toma et al. 2016; Kupfer et al. 2017)
As part of the Zwicky Transient Facility (ZTF), the Palomar 48-
inch (P48) telescope images the sky every clear night conducting
several surveys including a Northern Sky Survey with a 3-d cadence,
as well as smaller surveys such as a 1-d Galactic plane survey and
simultaneous observations of the Northern TESS sectors (Graham
et al. 2019; van Roestel et al. 2019; Bellm et al. 2019a,b). As part of
the partnership share of ZTF, we conducted a dedicated high-cadence
Galactic plane survey with a cadence of 40 sec at low Galactic
latitudes aiming to find UCBs and compact pulsators. During that
dedicated survey, we either observed one field or alternated between
two adjacent fields continuously for ≈1.5–3 h on two to three
consecutive nights in the ZTF-r band. Here, we present an overview
of the ZTF high-cadence Galactic plane survey executed in ZTF year
1. We present the observing strategy as well as some survey statistics.
Tab le 1 . Overview of the high-cadence Galactic plane survey.
Period No. of fields Sky coverage Filter
(deg
2
)
06-15-2018–07-31-2018 16 750 ZTF-r
08-03-2018–08-18-2018 14 650 ZTF-r
11-15-2018–01-15-2019 34 1590 ZTF-r
We show a progress report and finalize with some early results from
the survey. In Section 2, we discuss the observing strategy as well
as the field selection and the data processing. In Section 3, we
present results for four representative fields and in Section 4 we give
a progress report of already published results. In Section 5, we show
some new early results from the high-cadence Galactic plane survey
and summarize and conclude in Section 6.
2 DESIGN OF THE HIGH-CADENCE
GALACTIC PLANE SURVEY
ZTF uses a 47 deg
2
camera consisting of 16 individual CCDs each 6k
× 6k covering the full focal plane of the P48 telescope. The ZTF high-
cadence Galactic plane survey covered a total of ≈2990 deg
2
split
in 64 individual ZTF fields observed over three observing blocks;
mid-June–July 2018, two weeks in 2018 August and November 15
to 2019 January 15. All observations of the high-cadence Galactic
plane survey were obtained in ZTF-r band. Each field has ≈200–
400 epochs. The images were processed using ZTF data-processing
pipeline described in full detail in Masci et al. (2019).
During June and July, we observed 16 fields covering ≈750 deg
2
mostly at low Galactic longitudes. In June, we observed every field
continuously for 1 h15 min and in July for 1 h 25 min. All fields
were observed over 2 or 3 consecutive nights. In August, 14 fields
covering ≈650 deg
2
were observed over two weeks (see Table 1).
We alternated between two adjacent fields continuously for 2 h
40min each night. The same fields were repeated the following night.
The observations in June/July and August were done under stable
conditions with an average seeing of ≈2 arcsec. We lost only a total of
five nights due to weather during June/July and August observations.
Between 2018 November 15 and 2019 January 15 high-cadence
Galactic plane observations were scheduled for every night. We
observed an additional 33 fields covering ≈1550 deg
2
with stable
weather conditions (see Table 1). The overall strategy varied during
those two months due to unstable weather. Most fields were observed
alternating with adjacent fields but in particular low declination
fields were observed continuously. Because more time was available
each night most fields were observed for ≈3 h. However, due to
the unstable weather conditions about half of the fields were only
observed in a single night and not repeated in subsequent nights. All
other fields were observed over 2 or 3 nights. Although the seeing
varies strongly between 1.7 and 4 arcsec, each field has a limiting
magnitude of >19.5 mag. A detailed overview of the fields observed
in stable weather conditions is given in Tables A1–A3.
2.1 Field selection
The main science driver for the survey is to find and study UCBs
consisting of fully degenerate or semi-degenerate stars. Hence, we
selected ZTF fields based on the density of objects residing well
below the main sequence, including mostly white dwarfs and hot
subdwarf stars.
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1256 T. Kupfer et al.
Figure 1. Sky density of candidate white dwarfs and hot subdwarfs selected from Gaia DR2 with the selected fields of the ZTF high-cadence Galactic plane
survey observed in ZTF year-1. The squares show individual ZTF fields which have been observed in high-cadence Galactic plane observations. The white lines
correspond to the Galactic equator and |b|=15
o
.
To achieve this, we extracted objects with absolute magnitudes
which placed them below the main-sequence based on Gaia data
release 2 (Gaia Collaboration et al. 2016, 2018). Only objects with de-
clinations >−30
◦
were selected. As the goal was not to extract a clean
sample we used a very relaxed tolerance for the parallax precision
( /σ
> 3) and did not include any additional quality cuts, resulting
in ≈350 000 individual objects (Fig. 2). For each object, the ZTF field
was calculated and the fields with the largest number of individual
objects were selected for our survey. Selected fields with the highest
density have ≈1000 selected objects per deg
2
. The final field selection
was mainly driven by stellar density but also included other aspects
like visibility and weather. Fig. 1 shows the sky density of the selected
sources overplotted with the fields observed as part of the high-
cadence Galactic plane survey. As the stellar density is highest at low
Galactic latitudes most fields are located at Galactic latitudes ≤15
o
.
2.2 Data-processing and light-curve extraction
Data-processing and light-curve generation follows the standard
procedure for ZTF and occurs at the Infrared Processing and Analysis
Center, Caltech. The raw camera image data are first instrumentally
calibrated and astrometric solutions are derived using the Gaia DR1
catalogue. Sources are detected and fluxes measured using both aper-
ture (Bertin & Arnouts 1996) and PSF-fit photometry (Stetson 1987).
Sources are photometrically calibrated using the Pan-STARRS1 DR1
catalogue. The epochal image data are then co-added within their
respective survey fields and camera readout channels to construct
reference images. Each reference image is constructed using a
minimum of 15 and maximum of 40 good-quality epochal images,
yielding depths of ≈2–2.5 mag deeper than the single-epoch images.
The reference image co-adds are archived for use in other down-
stream processing: image differencing and light curve construction.
To support light curve generation, sources are first detected and
Figure 2. Hertzsprung–Russell diagram of the objects used for the field
selection. The grey shaded region corresponds to the underlying Hertzsprung–
Russell diagram showing the position of the main sequence and the red giant
branch. The colour–coded region corresponds to the objects which were
selected for the field selection. The clump around M
G
≈ 5 corresponds to the
hot subdwarfs whereas the region below corresponds to the white dwarfs.
extracted from each reference image using PSF-fit photometry
by running the DAOPhot utility (Stetson 1987). These sources
provide the seeds to facilitate positional cross-matching across all
the single-epoch-based PSF extraction catalogues going back to
the beginning of the survey. PSF-fitting on the single-epoch images
is also performed using DAOPhot. These single-epoch images are
direct single exposure images, not difference images. A fixed source-
match radius of 1.5 arcsec is used for the positional matching. Only
the PSF-fit-derived positions and photometry, along with selected
PSF-fit metrics are retained during the source-matching process.
Aperture photometry is not propagated to the light-curve metadata.
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ZTF high-cadence Galactic plane survey 1257
Figure 3. Sky density of individual objects with at least 80 epochs from the ZTF high-cadence Galactic plane survey observed in ZTF year-1. The squares show
individual ZTF fields, which have been observed in high-cadence Galactic plane observations. The white lines correspond to the Galactic equator and |b|=15
o
.
Further details of the data processing, PSF-fit photometry soft-
ware, light-curve generation, formats, and overall performance on
photometric accuracy are described in Masci et al. (2019). We
extracted the light curves from each object which has at least 80
epochs obtained as part of the high-cadence Galactic plane survey
from the ZTF light curve data base ‘Kowalski’ and extracted a
total of ≈230 million light curves across all observed fields. Fig. 3
shows the sky density of the extracted objects overplotted with
the fields observed as part of the high-cadence Galactic plane
survey.
3 RESULTS FROM REPRESENTATIVE FIELDS
We select four representative fields with different stellar densities and
calculate statistics for each individual light curve, including median
magnitude, reduced χ
2
, interquartile range (IQR), skewness, inverse
von-Neumann, Stetson J, and Stetson K statistics. We refer to table 2
in Coughlin et al. (2020) for a detailed description of the individual
statistics. These statistics are used to evaluate light curve variability
and identify variable objects in the high-cadence Galactic plane data.
The selected fields are FieldID = 331 (l=8.
◦
59, b=8.
◦
69),
FieldID = 538 (l = 44.
◦
64, b = 3.
◦
09), FieldID = 769
((l = 91.
◦
87, b = 2.
◦
38)l = 91.87 deg, b =−2.38 deg) and FieldID =
309 (l = 230.
◦
68, b = 2.
◦
98). Fig. 4 shows the stellar density of the
four selected fields. The different colouring correspond to different
densities. Some structure of lower and higher density regions can
be seen in the fields. The white circles are masked regions due to
saturated stars and the horizontal and vertical gaps are chip gaps
between the 16 CCDs. The two white square areas in FieldID
331 are individual quadrants, which were not processed. FieldID
= 331 was observed in three consecutive nights for 1.5 h each
night. FieldID = 538 was observed for two consecutive nights for
1.5 and 2.5 h. FieldID = 769 was observed for two consecutive
nights for 3 h per night and FieldID = 309 was observed for two
consecutive nights for 1.7 and 3 h. See Tables A1–A3 for more
details. The four selected fields represent a large range of stellar
densities from ≈10 million individual light curves for FieldID =
331, ≈5.8 million for FieldID = 538, ≈3.8 million for FieldID =
769, and ≈2.3 million for FieldID = 309.
Different statistics can be used to evaluate whether an object
is considered variable or not. We decided to use the IQR which
corresponds to the difference between the upper and lower quartile
values. The main advantage of IQR is that it is a robust statistic not
affected by extreme outliers due to individual bad photometry data
points and therefore a better estimate of the general spread around the
median magnitude. When there are outliers in a sample, the median
and IQR are best used to summarize a typical value and the variability
in the sample, respectively. Large variability in the light curve leads
to a larger IQR.
To estimate whether an object is variable in the high-cadence
Galactic plane data, we used the following approach. We calculate the
median ZTF-r band magnitude for each object. The full magnitude
range of all objects in a ZTF field is binned into several bins and each
object is added to a bin based on its median magnitude. We used bin
sizes of 0.5 mag between 12 and 16.5 mag where the IQR is constant
at ≈0.02 mag for non-variable sources. Above 16.5 mag, we used
bin sizes of 0.2 mag because the IQR for non-variable sources is
increasing with increasing magnitudes to ≈0.2 mag at the faint end
at ZTF-r = 20.5 mag because the underlying noise of the sources
is increasing for fainter sources. Each bin contains at least a few
thousand objects.
For each magnitude bin, we then calculate a histogram of IQR
values. If there are only constant sources with a similar noise pattern,
a normal distribution in an IQR histogram is expected. An excess
of variable light curves will result in a deviation from a normal
distribution towards more objects with larger IQR values. To estimate
the excess and number of variable objects, we fit a Gaussian to each
IQR histogram and define an object as variable if, in a given histogram
bin, less than 5 per cent of the sources in that magnitude bin are
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1258 T. Kupfer et al.
Tab le 2 . Photometric properties of the high-gravity-BLAPs.
Object RA (J2000) Dec. (J2000) P A
b
ZTF-r
c
g
a
g − r
a
FieldID
(
h
:
min
:
sec
)(
◦
:
:
) (s) (mmag) (mas) (mag) (mag)
ZTF-sdBV1 18:26:36.09 +10:00:22.3 347.287 ± 0.02 19.9 0.5640 ± 0.0372 14.90 −0.41 538
ZTF-sdBV2 19:13:54.77 −11:05:19.4 360.576 ± 0.02 19.6 0.4568 ± 0.0888 16.24 −0.33 385
ZTF-sdBV3 21:03:08.50 +44:43 14.9 365.822 ± 0.01 53.3 0.3550 ± 0.0769 16.07 −0.31 768
ZTF-sdBV4 08:06:07.05 −20:08 19.1 370.481 ± 0.02 26.0 0.4021 ± 0.0531 16.37 −0.32 311
ZTF-sdBV5 21:49:45.56 +45:58 36.5 375.690 ± 0.02 36.1 0.3112 ± 0.0683 16.55 −0.35 770
ZTF-sdBV6 07:40:00.71 −14:42 02.5 375.782 ± 0.02 21.1 0.5979 ± 0.0394 15.10 −0.36 310
ZTF-sdBV7 19:24:08.62 −12:58 30.9 377.468 ± 0.02 19.3 0.7399 ± 0.0355 14.64 −0.41 385
ZTF-sdBV8 06:54:48.55 −25:22 08.3 435.459 ± 0.02 31.4 0.5458 ± 0.0318 13.93 −0.44 259
ZTF-sdBV9 18:52:49.07 −18:46 08.4 524.829 ± 0.02 73.4 0.1427 ± 0.1538 17.12 −0.38 334
ZTF-sdBV10 19:49:59.34 +08:31 06.1 646.306 ± 0.02 32.8 0.3884 ± 0.0642 16.23 −0.38 541
ZTF-sdBV11 07:18:43.69 −02:29 31.2 688.485 ± 0.02 30.2 0.1571 ± 0.1329 17.52 −0.33 411
ZTF-sdBV12 17:07:41.34 −
15:22 42.9 995.479 ± 0.02 37.3 0.4376 ± 0.0346 14.11 −0.35 330
a
From the Pan-STARRS release 1 (PS1) survey (Chambers et al. 2016) reddening corrected using Green et al. (2019).
b
Amplitude in ZTF-r from the ZTF light curves.
c
Parallaxes from Gaia EDR3 (Gaia Collaboration et al. 2016, 2020).
Figure 4. Sky density of ZTF objects with at least 80 epochs obtained during the high-cadence Galactic plane survey for four selected fields. The white circles
are masked regions due to saturated stars and the horizontal and vertical gaps are chip gaps between the 16 CCDs. The two white square areas in FieldID 331
are individual quadrants, which were not processed.
considered constant based on the Gaussian fit. Fig. 5 shows two
examples of IQR histograms with Gaussian fits of FieldID = 309
and 331 for the magnitude range 15.5–16 mag. The orange curve
corresponds to the Gaussian fit of the underlying histogram and the
red-dashed line corresponds to the IQR limit above which we call an
object a variable.
Using this method, we find 15 per cent variable sources for
FieldID = 331, 9 per cent variable sources for FieldID = 769, and
FieldID = 538 and 5 per cent variable sources for FieldID = 309 (see
Fig. 6). The number of variable sources per field decreases with the
stellar density of the field and it is likely that blending could produce
false positives where the object appears variable due to a close-by
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