153 research outputs found
Microlensing Events in Five Years of Photometry from the Zwicky Transient Facility
Microlensing has a unique advantage for detecting dark objects in the Milky
Way, such as free floating planets, neutron stars, and stellar-mass black
holes. Most microlensing surveys focus towards the Galactic bulge, where higher
stellar density leads to a higher event rate. However, microlensing events in
the Galactic plane are closer, and take place over longer timescales. This
enables a better measurement of the microlensing parallax, which serves as an
independent constraint on the mass of the dark lens. In this work, we
systematically searched for microlensing events in Zwicky Transient Facility
(ZTF) Data Release 17 from 2018--2023 in the Galactic plane region . We find 124 high-confidence microlensing events and 54 possible
events. In the event selection, we use the efficient \texttt{EventFinder}
algorithm to detect microlensing signals, which could be used for large
datasets such as future ZTF data releases or data from the Rubin Observatory
Legacy Survey of Space and Time (LSST). With detection efficiencies of ZTF
fields from catalog-level simulations, we calculate the mean Einstein timescale
to be days, smaller than previous
results of the Galactic plane to within 1.5-. We calculate optical
depths and event rates, which we interpret with caution due to the use of
visual inspection in creating our final sample. With two years of additional
ZTF data in DR17, we have more than doubled the amount of microlensing events
(60) found in the three-year DR5 search and found events with longer Einstein
timescales than before.Comment: 9 figures, 3 tables. Submitted to Ap
An Optically Discovered Outburst from XTE J1859+226
Using the Zwicky Transient Facility, in 2021 February we identified the first known outburst of the black hole X-ray transient XTE J1859+226 since its discovery in 1999. The outburst was visible at X-ray, UV, and optical wavelengths for less than 20 days, substantially shorter than its full outburst of 320 days in 1999, and the observed peak luminosity was 2 orders of magnitude lower. Its peak bolometric luminosity was only 2 × 1035 erg s−1, implying an Eddington fraction of about 3 × 10−4. The source remained in the hard spectral state throughout the outburst. From optical spectroscopy measurements we estimate an outer disk radius of 1011 cm. The low observed X-ray luminosity is not sufficient to irradiate the entire disk, but we observe a surprising exponential decline in the X-ray light curve. These observations highlight the potential of optical and infrared synoptic surveys to discover low-luminosity activity from X-ray transients
Light Curves and Period Changes of Type II Cepheids in the Globular Clusters M3 and M5
Light curves in the B, V, and I_c passbands have been obtained for the type
II Cepheids V154 in M3 and V42 and V84 in M5. Alternating cycle behavior,
similar to that seen among RV Tauri variables, is confirmed for V84. Old and
new observations, spanning more than a century, show that V154 has increased in
period while V42 has decreased in period. V84, on the other hand, has shown
large, erratic changes in period that do not appear to reflect the long term
evolution of V84 through the HR diagram.Comment: 28 pages, 12 figure
Palomar discovery and initial characterization of naked-eye long period comet C/2022 E3 (ZTF)
Long-period comets are planetesimal remnants constraining the environment and
volatiles of the protoplanetary disc. We report the discovery of hyperbolic
long-period comet C/2022 E3 (ZTF), which has a perihelion 1.11 au, an
eccentricity 1 and an inclination 109, from images
taken with the Palomar 48-inch telescope during morning twilight on 2022 Mar 2.
Additionally, we report the characterization of C/2022 E3 (ZTF) from
observations taken with the Palomar 200-inch, the Palomar 60-inch, and the NASA
Infrared Telescope Facility in early 2023 February to 2023 March when the comet
passed within 0.28 au of the Earth and reached a visible magnitude of
5. We measure g-r = 0.700.01, r-i = 0.200.01, i-z =
0.060.01, z-J = 0.900.01, J-H = 0.380.01 and H-K = 0.150.01
colours for the comet from observations. We measure the A(0)f
(0.8~m) in a 6500~km radius from the nucleus of 148340~cm, and CN,
C, and C production of 5.4310~mol/s,
2.0110, and 3.0810~mol/s, similar to
other long period comets. We additionally observe the appearance of jet-like
structures at a scale of 4,000 km in wide-field g-band images, which may
be caused by the presence of CN gas in the near-nucleus coma.Comment: Accepted for publication in MNRAS:L, 9 pages, 6 figures, 2 table
Panic at the ISCO: the visible accretion disks powering optical variability in ZTF AGN
About 3-10% of Type I active galactic nuclei (AGN) have double-peaked broad
Balmer lines in their optical spectra originating from the motion of gas in
their accretion disk. Double-peaked profiles arise not only in AGN, but
occasionally appear during optical flares from tidal disruption events and
changing-state AGN. In this paper we identify 250 double-peaked emitters (DPEs)
amongst a parent sample of optically variable broad-line AGN in the Zwicky
Transient Facility (ZTF) survey, corresponding to a DPE fraction of 19%. We
model spectra of the broad H alpha emission line regions and provide a catalog
of the fitted accretion disk properties for the 250 DPEs. Analysis of power
spectra derived from the 5 year ZTF light curves finds that DPEs have similar
amplitudes and power law indices to other broad-line AGN, but have lower
turnover frequencies. Follow-up spectroscopy of 12 DPEs reveals that ~50%
display significant changes in the relative strengths of their red and blue
peaks over long 10-20 year timescales, indicating that broad-line profile
changes arising from spiral arm or hotspot rotation are common amongst
optically variable DPEs. Analysis of the accretion disk parameters derived from
spectroscopic modeling provides evidence that DPEs are not in a special
accretion state, but are simply normal broad-line AGN viewed under the right
conditions for the accretion disk to be easily visible. We compare the radio
variability properties of the two samples and present radio jet imaging of 3
DPEs with disks of inclination angle 14-35 degrees. We discuss some objects
with notable light curves or unusual broad line profiles which are outliers
amongst the variable DPE population. We include inspiraling SMBH binary
candidate SDSSJ1430+2303 in our analysis, and discuss how its photometric and
spectroscopic variability is consistent with the disk-emitting AGN population
in ZTF.Comment: Submitted to ApJ. 30 pages, 10 figures, 4 tables. Comments welcome
VLASS tidal disruption events with optical flares I: the sample and a comparison to optically-selected TDEs
In this work, we use the Jansky VLA Sky Survey (VLASS) to compile the first
sample of six radio-selected tidal disruption events (TDEs) with transient
optical counterparts. While we still lack the statistics to do detailed
population studies of radio-selected TDEs, we use these events to suggest
trends in host galaxy and optical light curve properties that may correlate
with the presence of radio emission, and hence can inform optically-selected
TDE radio follow-up campaigns. We find that radio-selected TDEs tend to have
faint and cool optical flares, as well as host galaxies with low SMBH masses.
Our radio-selected TDEs also tend to have more energetic, larger radio emitting
regions than radio-detected, optically-selected TDEs. We consider possible
explanations for these trends, including by invoking super-Eddington accretion
and enhanced circumnuclear media. Finally, we constrain the radio-emitting TDE
rate to be Gpc yr.Comment: 26 pages, 5 tables, 11 figures, submitted to Ap
Striking the Right Balance of Intermolecular Coupling for High-Efficiency Singlet Fission
Singlet fission is a process that splits collective excitations, or excitons, into two with unity efficiency. This exciton splitting process, unique to molecular photophysics, has the potential to considerably improve the efficiency of optoelectronic devices through more efficient light harvesting. While the first step of singlet fission has been characterized in great detail, subsequent steps critical to achieving overall highly-efficient singlet-to-triplet conversion are only just beginning to become well understood. One of the most elementary suggestions, which has yet to be tested, is that an appropriately balanced coupling is necessary to ensure overall highly efficient singlet fission; that is, the coupling needs to be strong enough so that the first step is fast and efficient, yet weak enough to ensure the independent behavior of the resultant triplets. In this work, we show how high overall singlet-to-triplet conversion efficiencies can be achieved in singlet fission by ensuring that the triplets comprising the triplet pair behave as independently as possible. We show that side chain sterics govern local packing in amorphous pentacene derivative nanoparticles, and that this in turn controls both the rate at which triplet pairs form and the rate at which they decay. We show how compact side chains and stronger couplings promote a triplet pair that effectively couples to the ground state, whereas bulkier side chains promote a triplet pair that appears more like two independent and long-lived triplet excitations. Our results show that the triplet pair is not emissive, that its decay is best viewed as internal conversion rather than triplet–triplet annihilation, and perhaps most critically that, in contrast to a number of recent suggestions, the triplets comprising the initially formed triplet pair cannot be considered independently. This work represents a significant step toward better understanding intermediates in singlet fission, and how molecular packing and couplings govern overall triplet yields
An Optically-Discovered Outburst from XTE J1859+226
Using the Zwicky Transient Facility, in 2021 February we identified the first
known outburst of the Black Hole X-ray Transient XTE J1859+226 since its
discovery in 1999. The outburst was visible at X-ray, UV, and optical
wavelengths for less than 20 days, substantially shorter than its 320-day full
outburst in 1999, and the observed peak luminosity was two orders of magnitude
lower. Its peak bolometric luminosity was only erg s,
implying an Eddington fraction of about . The source remained
in the hard spectral state throughout the outburst. From optical spectroscopy
measurements we estimate an outer disk radius of 10 cm. The low observed
X-ray luminosity is not sufficient to irradiate the entire disk, but we observe
a surprising exponential decline in the X-ray lightcurve. These observations
highlight the potential of optical and infrared (O/IR) synoptic surveys to
discover low-luminosity activity from X-ray transients.Comment: 12 pages, 6 figures, accepted for publication in Ap
Computationally Aided Design of a High-Performance Organic Semiconductor: The Development of a Universal Crystal Engineering Core
Herein, we describe the design and synthesis of a suite of molecules based on a benzodithiophene “universal crystal engineering core”. After computationally screening derivatives, a trialkylsilylethyne-based crystal engineering strategy was employed to tailor the crystal packing for use as the active material in an organic field-effect transistor. Electronic structure calculations were undertaken to reveal derivatives that exhibit exceptional potential for high-efficiency hole transport. The promising theoretical properties are reflected in the preliminary device results, with the computationally optimized material showing simple solution processing, enhanced stability, and a maximum hole mobility of 1.6 cm2 V−1 s−1
SRGeJ045359.9+622444: A 55-min Period Eclipsing AM CVn Discovered from a Joint SRG/eROSITA + ZTF Search
AM CVn systems are ultra-compact binaries where a white dwarf accretes from a
helium-rich degenerate or semi-degenerate donor. Some AM CVn systems will be
among the loudest sources of gravitational waves for the upcoming Laser
Interferometer Space Antenna (LISA), yet the formation channel of AM CVns
remains uncertain. We report the study and characterisation of a new eclipsing
AM CVn, SRGeJ045359.9+622444 (hereafter SRGeJ0453), discovered from a joint
SRG/eROSITA and ZTF program to identify cataclysmic variables (CVs). We
obtained optical photometry to confirm the eclipse of SRGeJ0453 and determine
the orbital period to be min. We
constrain the binary parameters by modeling the high-speed photometry and
radial velocity curves and find
and . The X-ray spectrum is
approximated by a power-law model with an unusually flat photon index of
previously seen in magnetic CVs with SRG/eROSITA, but verifying
the magnetic nature of SRGeJ0453 requires further investigation. Optical
spectroscopy suggests that the donor star of SRGeJ0453 could have initially
been a He star or a He white dwarf. SRGeJ0453 is the ninth eclipsing AM CVn
system published to date, and its lack of optical outbursts have made it
elusive in previous surveys. The discovery of SRGeJ0453 using joint X-ray and
optical surveys highlights the potential for discovering similar systems in the
near future.Comment: Submitted to Ap
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