37 research outputs found
The imprint of pulsar parameters on the morphology of Pulsar Wind Nebulae
The morphology of young Pulsar Wind Nebulae (PWN) is largely determined by
the properties of the wind injected by the pulsar. We have used a recent
parametrization of the wind obtained from Force Free Electrodynamics
simulations of pulsar magnetospheres to simulate nebulae for different sets of
pulsar parameters. We performed axisymmetric Relativistic Magnetohydrodynamics
simulations to test the morphology dependence of the nebula on the obliquity of
the pulsar and on the magnetization of the pulsar wind. We compare these
simulations to the morphology of the Vela and Crab PWN. We find that the
morphology of Vela can be reproduced qualitatively if the pulsar obliquity
angle is alpha ~45deg and the magnetization of the wind is high (sigma_0 ~
3.0). A morphology similar to the one of the Crab Nebula is only obtained for
low magnetization simulations with alpha >~ 45deg. Interestingly, we find that
Kelvin-Helmholtz instabilities produce small scale turbulences downstream of
the reverse shock of the pulsar wind.Comment: 18 pages, 12 figures, accepted by MNRA
Optimal strategies for observation of active galactic nuclei variability with Imaging Atmospheric Cherenkov Telescopes
Variable emission is one of the defining characteristic of active galactic
nuclei (AGN). While providing precious information on the nature and physics of
the sources, variability is often challenging to observe with time- and
field-of-view-limited astronomical observatories such as Imaging Atmospheric
Cherenkov Telescopes (IACTs). In this work, we address two questions relevant
for the observation of sources characterized by AGN-like variability: what is
the most time-efficient way to detect such sources, and what is the
observational bias that can be introduced by the choice of the observing
strategy when conducting blind surveys of the sky. Different observing
strategies are evaluated using simulated light curves and realistic instrument
response functions of the Cherenkov Telescope Array (CTA), a future gamma-ray
observatory. We show that strategies that makes use of very small observing
windows, spread over large periods of time, allows for a faster detection of
the source, and are less influenced by the variability properties of the
sources, as compared to strategies that concentrate the observing time in a
small number of large observing windows. Although derived using CTA as an
example, our conclusions are conceptually valid for any IACTs facility, and in
general, to all observatories with small field of view and limited duty cycle.Comment: 14 pages, 11 figure
A new class of large-amplitude radial-mode hot subdwarf pulsators
Using high-cadence observations from the Zwicky Transient Facility at low Galactic latitudes, we have discovered a new class of pulsating, hot compact stars. We have found four candidates, exhibiting blue colors (g β r β€ β0.1 mag), pulsation amplitudes of >5%, and pulsation periods of 200β475 s. Fourier transforms of the light curves show only one dominant frequency. Phase-resolved spectroscopy for three objects reveals significant radial velocity, T eff, and log(g) variations over the pulsation cycle, which are consistent with large-amplitude radial oscillations. The mean T eff and log(g) for these stars are consistent with hot subdwarf B (sdB) effective temperatures and surface gravities. We calculate evolutionary tracks using MESA and adiabatic pulsations using GYRE for low-mass, helium-core pre-white dwarfs (pre-WDs) and low-mass helium-burning stars. Comparison of low-order radial oscillation mode periods with the observed pulsation periods show better agreement with the pre-WD models. Therefore, we suggest that these new pulsators and blue large-amplitude pulsators (BLAPs) could be members of the same class of pulsators, composed of young β0.25β0.35 M β helium-core pre-WDs.Published versio
Towards Efficient Detection of Small Near-Earth Asteroids Using the Zwicky Transient Facility (ZTF)
We describe ZStreak, a semi-real-time pipeline specialized in detecting
small, fast-moving near-Earth asteroids (NEAs) that is currently operating on
the data from the newly-commissioned Zwicky Transient Facility (ZTF) survey.
Based on a prototype originally developed by Waszczak et al. (2017) for the
Palomar Transient Factory (PTF), the predecessor of ZTF, ZStreak features an
improved machine-learning model that can cope with the data rate
increment between PTF and ZTF. Since its first discovery on 2018 February 5
(2018 CL), ZTF/ZStreak has discovered confirmed new NEAs over a total of
232 observable nights until 2018 December 31. Most of the discoveries are small
NEAs, with diameters less than m. By analyzing the discovery
circumstances, we find that objects having the first to last detection time
interval under 2 hr are at risk of being lost. We will further improve
real-time follow-up capabilities, and work on suppressing false positives using
deep learning.Comment: PASP in pres
A New Class of Changing-Look LINERs
We report the discovery of six active galactic nuclei (AGN) caught "turning
on" during the first nine months of the Zwicky Transient Facility (ZTF) survey.
The host galaxies were classified as LINERs by weak narrow forbidden line
emission in their archival SDSS spectra, and detected by ZTF as nuclear
transients. In five of the cases, we found via follow-up spectroscopy that they
had transformed into broad-line AGN, reminiscent of the changing-look LINER
iPTF 16bco. In one case, ZTF18aajupnt/AT2018dyk, follow-up HST UV and
ground-based optical spectra revealed the transformation into a narrow-line
Seyfert 1 (NLS1) with strong [Fe VII, X, XIV] and He II 4686 coronal lines.
Swift monitoring observations of this source reveal bright UV emission that
tracks the optical flare, accompanied by a luminous soft X-ray flare that peaks
~60 days later. Spitzer follow-up observations also detect a luminous
mid-infrared flare implying a large covering fraction of dust. Archival light
curves of the entire sample from CRTS, ATLAS, and ASAS-SN constrain the onset
of the optical nuclear flaring from a prolonged quiescent state. Here we
present the systematic selection and follow-up of this new class of
changing-look LINERs, compare their properties to previously reported
changing-look Seyfert galaxies, and conclude that they are a unique class of
transients well-suited to test the uncertain physical processes associated with
the LINER accretion state.Comment: Submitted to ApJ, 31 pages, 17 Figures (excluding Appendix due to
file size constraints but will be available in electronic version
Discovery of an intermediate-luminosity red transient in M51 and its likely dust-obscured, infrared-variable progenitor
We present the discovery of an optical transient (OT) in Messier 51,
designated M51 OT2019-1 (also ZTF19aadyppr, AT 2019abn, ATLAS19bzl), by the
Zwicky Transient Facility (ZTF). The OT rose over 15 days to an observed
luminosity of (), in the
luminosity gap between novae and typical supernovae (SNe). Spectra during the
outburst show a red continuum, Balmer emission with a velocity width of
km s, Ca II and [Ca II] emission, and absorption features
characteristic of an F-type supergiant. The spectra and multiband light curves
are similar to the so-called "SN impostors" and intermediate-luminosity red
transients (ILRTs). We directly identify the likely progenitor in archival
Spitzer Space Telescope imaging with a m luminosity of
and a color redder than 0.74 mag, similar
to those of the prototype ILRTs SN 2008S and NGC 300 OT2008-1. Intensive
monitoring of M51 with Spitzer further reveals evidence for variability of the
progenitor candidate at [4.5] in the years before the OT. The progenitor is not
detected in pre-outburst Hubble Space Telescope optical and near-IR images. The
optical colors during outburst combined with spectroscopic temperature
constraints imply a higher reddening of mag and higher
intrinsic luminosity of
() near peak than seen in previous ILRT
candidates. Moreover, the extinction estimate is higher on the rise than on the
plateau, suggestive of an extended phase of circumstellar dust destruction.
These results, enabled by the early discovery of M51 OT2019-1 and extensive
pre-outburst archival coverage, offer new clues about the debated origins of
ILRTs and may challenge the hypothesis that they arise from the
electron-capture induced collapse of extreme asymptotic giant branch stars.Comment: 21 pages, 5 figures, published in ApJ
A New Class of Large-amplitude Radial-mode Hot Subdwarf Pulsators
Using high-cadence observations from the Zwicky Transient Facility at low Galactic latitudes, we have discovered a new class of pulsating, hot compact stars. We have found four candidates, exhibiting blue colors (g β r β€ β0.1 mag), pulsation amplitudes of >5%, and pulsation periods of 200β475 s. Fourier transforms of the light curves show only one dominant frequency. Phase-resolved spectroscopy for three objects reveals significant radial velocity, T_(eff), and log(g) variations over the pulsation cycle, which are consistent with large-amplitude radial oscillations. The mean T_(eff) and log(g) for these stars are consistent with hot subdwarf B (sdB) effective temperatures and surface gravities. We calculate evolutionary tracks using MESA and adiabatic pulsations using GYRE for low-mass, helium-core pre-white dwarfs (pre-WDs) and low-mass helium-burning stars. Comparison of low-order radial oscillation mode periods with the observed pulsation periods show better agreement with the pre-WD models. Therefore, we suggest that these new pulsators and blue large-amplitude pulsators (BLAPs) could be members of the same class of pulsators, composed of young β0.25β0.35 M_β helium-core pre-WDs
Seventeen Tidal Disruption Events from the First Half of ZTF Survey Observations: Entering a New Era of Population Studies
While tidal disruption events (TDEs) have long been heralded as laboratories for the study of quiescent black holes, the small number of known TDEs and uncertainties in their emission mechanism have hindered progress towards this promise. Here present 17 new TDEs that have been detected recently by the Zwicky Transient Facility along with Swift UV and X-ray follow-up observations. Our homogeneous analysis of the optical/UV light curves, including 22 previously known TDEs from the literature, reveals a clean separation of light curve properties with spectroscopic class. The TDEs with Bowen fluorescence features in their optical spectra have smaller blackbody radii, as well as longer rise times and higher disruption rates compared to the rest of the sample. The Bowen fluorescence mechanism requires a high density which can be reached at smaller radii, which in turn yields longer diffusion timescales. Thus, the difference in rise times suggests the pre-peak TDE light curves are governed not by the fallback timescale, but instead by the diffusion of photons through the tidal debris. The small subset of TDEs that show only helium emission lines in their spectra have the longest rise times, the highest luminosities and the lowest rates. We also report, for the first time, the detection of soft X-ray flares from a TDE on day timescales. Based on the fact the flares peak at a luminosity similar to the optical/UV blackbody luminosity, we attribute them to brief glimpses through a reprocessing layer that otherwise obscures the inner accretion flow