Transient Survey Research Articles

Automated Transient Detection with Shapelet Analysis in Image-subtracted Data

We present a method for characterizing image-subtracted objects based on shapelet analysis to identify transient events in ground-based time-domain surveys. We decompose the image-subtracted objects onto a set of discrete Zernike polynomials and use their resulting coefficients to compare them to other point-like objects. We derive a norm in this Zernike space that we use to score transients for their point-like nature and show that it is a powerful comparator for distinguishing image artifacts, or residuals, from true astrophysical transients. Our method allows for a fast and automated way of scanning overcrowded, wide-field telescope images with minimal human interaction and we reduce the large set of unresolved artifacts left unidentified in subtracted observational images. We evaluate the performance of our method using archival intermediate Palomar Transient Factory and Dark Energy Camera survey images. However, our technique allows flexible implementation for a variety of different instruments and data sets. This technique shows a reduction in image subtraction artifacts by 99.95% for surveys extending up to hundreds of square degrees and has strong potential for automated transient identification in electromagnetic follow-up programs triggered by the Laser Interferometer Gravitational Wave Observatory-Virgo Scientific Collaboration.

A Forward Modeling Approach to AGN Variability--Method Description and Early Applications

Abstract We present a numerical framework for the variability of active galactic nuclei (AGNs), which links the variability of AGNs over a broad range of timescales and luminosities to the observed properties of the AGN population as a whole, and particularly the Eddington ratio distribution function. We implemented our framework on GPU architecture, relying on previously published time-series-generating algorithms. After extensive tests that characterize several intrinsic and numerical aspects of the simulations, we describe some applications used for current and future time-domain surveys and for the study of extremely variable sources (e.g., “changing-look” or flaring AGNs). Specifically, we define a simulation setup that reproduces the AGN variability observed in the (intermediate) Palomar Transient Factory survey and use it to forward model longer light curves of the kind that may be observed within the Large Synoptic Survey Telescope (LSST) main survey. Thanks to our efficient implementations, these simulations are able to cover, for example, over 1 Myr with a roughly weekly cadence. We envision that this framework will become highly valuable to prepare for, and best exploit, data from upcoming time-domain surveys, such as, for example, LSST.

Multi-tracer and hydrogeophysical investigation of the hydraulic connectivity between coal seam gas formations, shallow groundwater and stream network in a faulted sedimentary basin

Characterization of geological fault networks in sedimentary basins targeted for coal seam gas development typically use deep-penetrating seismic data. Such methods can reveal the presence of numerous fault zones at depths greater than 100 m but not necessarily show if these extend to the surface or provide a significant degree of hydraulic connectivity between deep and shallower geological formations. Our combined multi-tracer and hydrogeophysical study used transient electromagnetic surveys to image the subsurface in a sedimentary coal seam gas basin (Gloucester Basin, NSW, Australia) and indicated that deep faults (greater than 200 m) from seismic analysis may extend to within metres of the ground surface. Analysis of environmental tracers in surface water and groundwater samples near fault traces showed the presence of hotspots of deeper groundwater from the coal seams discharging into the stream network via the alluvial aquifer. The isotopic composition of methane was extremely variable in groundwater (δ13C-CH4 from –81.6 to –29.2‰ V-PDB) and surface water (δ 13C-CH4 from –56.0 to –19.9‰ V-PDB), suggesting several sources of methane being recycled in the connected alluvial aquifer and stream network. A 4He hotspot (1.67 × 10−6 cm3 STP g−1) in the alluvial aquifer close to a fault line showed high methane (463 µg L−1) with a thermogenic signature (δ 13C-CH4 of −62.4‰ and δ2H-CH4 of −174.7‰). The Gloucester Basin, which has experienced multiple episodes of complex restructuring, displays high dilation tendency of fractures close to fault damage zones with fracture orientations parallel to the orientation of maximum horizontal stress. As a result, there is increased permeability for wells close to the fault damage zone and with increasing depth. These findings resulted in a robust hydrogeological conceptual model of a faulted basin, which can be used in a monitoring and modelling framework for improved evaluation of risk associated with unconventional gas developments.

The Rate of iPTF 14gqr like Ultra-stripped Supernovae and Binary Evolution Leading to Double Neutron Star Formation

Abstract Double neutron star (DNS) systems are produced from massive binaries. A supernova (SN) explosion of an extremely stripped star is expected to occur at the final stage of DNS formation. This type of SN is called an ultra-stripped SN (USSN). Recent research revealed that a type Ic SN, iPTF 14gqr (SN 2014ft), has low ejecta mass (≈0.2 M ⊙) and its progenitor has a helium envelope with mass ∼0.01 M ⊙. This SN is interpreted as a USSN, and thus this is the first discovery of a USSN. Furthermore, the observation of iPTF 14gqr provides us with some information about its formation history. Here, we perform rapid population synthesis calculations so as to estimate the detection rate of iPTF 14gqr like USSNe with optical transient surveys: the intermediate Palomar Transient Factory (iPTF), the Zwicky Transient Facility (ZTF), and the Large Synoptic Survey Telescope (LSST). We find that iPTF, ZTF, and LSST can observe iPTF 14gqr like USSNe at rates of 0.3, 10, and 1 yr−1, respectively. The iPTF can detect 1 iPTF 14gqr like USSN during its four year observation. We also investigate effects of mass-loss efficiency during Roche-lobe overflow on formation channels.

Deep Learning for Image Sequence Classification of Astronomical Events

We propose a new sequential classification model for astronomical objects based on a recurrent convolutional neural network (RCNN) which uses sequences of images as inputs. This approach avoids the computation of light curves or difference images. This is the first time that sequences of images are used directly for the classification of variable objects in astronomy. The second contribution of this work is the image simulation process. We generate synthetic image sequences which take into account the instrumental and observing conditions, obtaining a realistic, unevenly sampled, and variable noise set of movies for each astronomical object. The simulated data set is used to train our RCNN classifier. This approach allows us to generate data sets to train and test our RCNN model for different astronomical surveys and telescopes. Moreover, using a simulated data set is faster and more adaptable to different surveys and classification tasks. We aim to build a simulated data set whose distribution is close enough to the real data set, so that fine tuning could match the distributions. To test the RCNN classifier trained with the synthetic data set, we used real-world data from the High cadence Transient Survey (HiTS), obtaining an average recall of 85%, improved to 94% after performing fine tuning with 10 real samples per class. We compare the results of our RCNN model with those of a light curve random forest classifier. The proposed RCNN with fine tuning has a similar performance on the HiTS data set compared to the light curve random forest classifier, trained on an augmented training set with 10 real samples per class. The RCNN approach presents several advantages in an alert stream classification scenario, such as a reduction of the data pre-processing, faster online evaluation, and easier performance improvement using a few real data samples. The results obtained encourage us to use the proposed method for astronomical alert broker systems that will process alert streams generated by new telescopes such as the Large Synoptic Survey Telescope.

HSC16aayt: A Slowly Evolving Interacting Transient Rising for More than 100 Days

We report our observations of HSC16aayt (SN 2016jiu), which was discovered by the Subaru/Hyper Suprime-Cam (HSC) transient survey conducted as part of Subaru Strategic Program (SSP). It shows very slow photometric evolution and its rise time is more than 100 days. The optical magnitude change in 400 days remains within 0.6 mag. Spectra of HSC16aayt show a strong narrow emission line and we classify it as a Type IIn supernova. The redshift of HSC16aayt is 0.6814 +/- 0.0002 from the spectra. Its host galaxy center is at 5 kpc from the supernova location and HSC16aayt might be another example of isolated Type IIn supernovae, although the possible existence of underlying star forming activity of the host galaxy at the supernova location is not excluded.

Probing the black hole merger history in clusters using stellar tidal disruptions

The dynamical assembly of binary black holes (BBHs) in dense star clusters (SCs) is one of the most promising pathways for producing observable gravitational wave (GW) sources, however several other formation scenarios likely operate as well. One of the current outstanding questions is how these different pathways may be distinguished apart. In this paper we suggest a new multi-messenger observable that can be used to constrain the formation of BBH mergers originating from SCs: the electromagnetic signal from tidal disruptions (TDs) of stars by BBHs. Such TDs will show variability in their light curve from the orbital motion of the disruptive BBHs, and can therefore be used to map the BBH orbital period distribution, and thereby also the dynamical mechanisms that eventually drive the BBHs to merger. Using an analytical approach including General Relativistic effects, we find that the orbital period distribution of BBHs within globular clusters peaks on timescales of days, which we argue is unique to this assembly pathway. We propose that the search for variable TDs in current and future EM transient surveys might be used to constrain the merger history of BBHs in SCs.

Tidal Disruptions of Stars by Black Hole Remnants in Dense Star Clusters

Abstract In a dense stellar environment, such as the core of a globular cluster (GC), dynamical interactions with black holes (BHs) are expected to lead to a variety of astrophysical transients. Here we explore tidal disruption events (TDEs) of stars by stellar-mass BHs through collisions and close encounters. Using state-of-the-art cluster simulations, we show that these TDEs occur at significant rates throughout the evolution of typical GCs and we study how their relative rates relate to cluster parameters such as mass and size. By incorporating a realistic cosmological model of GC formation, we predict a BH–main-sequence-star TDE rate of approximately 3 Gpc−3 yr−1 in the local universe (z < 0.1) and a cosmological rate that peaks at roughly 25 Gpc−3 yr−1 for redshift 3. Furthermore, we show that the ejected mass associated with these TDEs could produce optical transients of luminosity ∼1041−1044 erg s−1 with timescales of about a day to a month. These should be readily detectable by optical transient surveys such as the Zwicky Transient Facility. Finally, we comment briefly on BH–giant encounters and discuss how these events may contribute to the formation of BH–white-dwarf binaries.

CRTS J035010.7 + 323230, a new eclipsing polar in the cataclysmic variable period gap

ABSTRACT We report the discovery of a new eclipsing polar, CRTS J035010.7+323230 (hereafter CRTS J0350+3232). We identified this cataclysmic variable (CV) candidate as a possible polar from its multiyear Catalina Real-Time Transient Survey (CRTS) optical light curve. Photometric monitoring of 22 eclipses in 2015 and 2017 was performed with the 2.1-m Otto Struve Telescope at McDonald Observatory. We derive an unambiguous high-precision ephemeris. Strong evidence that CRTS J0350 + 3232 is a polar comes from optical spectroscopy obtained over a complete orbital cycle using the Apache Point Observatory 3.5-m telescope. High velocity Balmer and He ii λ4686Å emission-line equivalent width ratios, structures, and variations are typical of polars and are modulated at the same period, 2.37 h (142.3 min), as the eclipse to within uncertainties. The spectral energy distribution and luminosity is found to be comparable to that of AM Herculis. Pre-eclipse dips in the light curve show evidence for stream accretion. We derive the following tentative binary and stellar parameters assuming a helium composition white dwarf and a companion mass of 0.2 M⊙: inclination i = 74.68° ± 0.03°, semimajor axis a = 0.942 ± 0.024 R⊙, and masses and radii of the white dwarf and companion, respectively: M1 = 0.948 $^{+0.006}_{-0.012}$ M⊙, R1 = 0.00830 $^{+0.00012}_{-0.00006}$ R⊙, and R2 = 0.249 ± 0.002 R⊙. As a relatively bright (V ∼ 17–19 mag), eclipsing, period-gap polar, CRTS J0350 + 3232 will remain an important laboratory for the study of accretion and angular momentum evolution in polars.

Comparing Multiclass, Binary, and Hierarchical Machine Learning Classification schemes for variable stars

ABSTRACTUpcoming synoptic surveys are set to generate an unprecedented amount of data. This requires an automatic framework that can quickly and efficiently provide classification labels for several new object classification challenges. Using data describing 11 types of variable stars from the Catalina Real-Time Transient Survey (CRTS), we illustrate how to capture the most important information from computed features and describe detailed methods of how to robustly use information theory for feature selection and evaluation. We apply three machine learning algorithms and demonstrate how to optimize these classifiers via cross-validation techniques. For the CRTS data set, we find that the random forest classifier performs best in terms of balanced accuracy and geometric means. We demonstrate substantially improved classification results by converting the multiclass problem into a binary classification task, achieving a balanced-accuracy rate of ∼99 per cent for the classification of δ Scuti and anomalous Cepheids. Additionally, we describe how classification performance can be improved via converting a ‘flat multiclass’ problem into a hierarchical taxonomy. We develop a new hierarchical structure and propose a new set of classification features, enabling the accurate identification of subtypes of Cepheids, RR Lyrae, and eclipsing binary stars in CRTS data.

Discovery of the Magdalena ore body. The leading role of geophysical methods for the exploration of a deep volcanogenic massive sulphide deposits (VMS) in the Iberian Pyrite Belt

The Magdalena volcanogenic massive sulphide (VMS) deposit, located in the province of Huelva (Spain) and discovered by Minas de Aguas Teñidas S.A.U. (MATSA) in 2013 constitutes the largest discovery to be made in the Iberian Pyrite Belt (IPB) in recent decades. During a first phase of exploration, the deposit was discovered by drill testing a conductivity anomaly that was first detected during an airborne (helicopter) transient electromagnetic survey (VTEM) in 2011. Drill-hole MA-01 intersected the massive sulphide body at a depth of between 225 m and 248 m. Throughout the drilling campaign of the sulphide body later named “Masa 1”, the drill holes were surveyed using Downhole TEM and Mise-à-la-Masse geophysical methods and during this work the Downhole TEM results from drill hole MA-21, which was originally drilled to 320 m, detected the presence of a deeper and larger sulphide body. The appearance of an off-hole, down-hole TEM anomaly in hole MA-21 was fundamental in the discovery of “Masa 2” which was made once the drill rig was recollared on the original MA-21 pad and drilled to a depth of 530 m. This second sulphide lens (“Masa 2”) was not detected by the initial VTEM survey because it was beyond the range for the system and was subsequently drill tested at depths between 350 and 1,000 m. The discovery of the Magdalena deposit has demonstrated the crucial role of certain geophysical methods for the detection of deep VMS deposits and confirms the prospectivity of the IPB as a metallogenic area of undoubted potential as far as the existence of similar deposits are concerned.

The Hyper Suprime-Cam SSP transient survey in COSMOS: Overview

Abstract We present an overview of a deep transient survey of the COSMOS field with the Subaru Hyper Suprime-Cam (HSC). The survey was performed for the 1.77 deg2 ultra-deep layer and 5.78 deg2 deep layer in the Subaru Strategic Program over six- and four-month periods from 2016 to 2017, respectively. The ultra-deep layer reaches a median depth per epoch of 26.4, 26.3, 26.0, 25.6, and 24.6 mag in g, r, i, z, and y bands, respectively; the deep layer is ∼0.6 mag shallower. In total, 1824 supernova candidates were identified. Based on light-curve fitting and derived light-curve shape parameter, we classified 433 objects as Type Ia supernovae (SNe); among these candidates, 129 objects have spectroscopic or COSMOS2015 photometric redshifts and 58 objects are located at z > 1. Our unique data set doubles the number of Type Ia SNe at z > 1 and enables various time-domain analyses of Type II SNe, high-redshift superluminous SNe, variable stars, and active galactic nuclei.

Correcting airborne gravity data for overburden thickness: a case study from the Nechako interior plateau, British Columbia

Large areas of bedrock in Canada, such as in the interior plateau of British Columbia, are covered by a thick glacial overburden. Lateral variations in overburden thickness can create spurious anomalies in gravity data. These anomalies can be of a size and amplitude similar to those associated with mineral bodies and can be mistaken for them. A methodology is introduced that corrects gravity data for changes in overburden thickness through the use of a bedrock topography map created by integrating information from a helicopter transient electromagnetic survey with geological survey data, well water data, and gravel pit locations. The approach is tested for a 68 km × 38 km area in the prospective Nechako interior plateau of British Columbia, Canada. The methodology extends the traditional Bouguer corrections by taking into account the gravitational contribution of the overburden. Results show that the capability of an airborne survey to detect a change in overburden thickness depends primarily on survey line spacing and to a lesser extent on the level of random noise in the gravity data. The bedrock topography correction has the capability of removing the gravitational attraction of overburden for the purpose of revealing, through interpretation, geological structures in the gravity data that originate from the bedrock and are otherwise concealed.

The Energy Sources of Superluminous Supernovae

Supernovae (SNe) are the most brilliant optical stellar-class explosions. Over the past two decades, several optical transient survey projects discovered more than ∼ 100 so-called superluminous supernovae (SLSNe) whose peak luminosities and radiated energy are ≳ 7 × 1043 erg s−1 and ≳ 1051 erg respectively, at least an order of magnitude larger than those of normal SNe. According to their optical spectra features, SLSNe have been split into two broad categories of type I that are hydrogen-deficient and type II that are hydrogen-rich. Investigating and determining the energy sources of SLSNe would be of outstanding importance for understanding their stellar evolution and explosion mechanisms. The energy sources of SLSNe can be determined by analyzing their light curves (LCs) and spectra. The most prevailing models accounting for the SLSN LCs are the 56Ni cascade decay model, the magnetar spin-down model, the ejecta-circumstellar medium interaction model and the jet-ejecta interaction model. In this review, we present several energy-source models and their different combinations.

Searches for Population III pair-instability supernovae: Predictions for ULTIMATE-Subaru and WFIRST

Abstract ULTIMATE-Subaru (Ultra-wide Laser Tomographic Imager and MOS with AO for Transcendent Exploration on Subaru) and WFIRST (Wide Field Infra-Red Survey Telescope) are the next generation of near-infrared instruments that have a large field-of-view. They allow us to conduct deep and wide transient surveys in the near-infrared. Such a near-infrared transient survey enables us to find very distant supernovae that are redshifted to the near-infrared wavelengths. We have performed mock transient surveys with ULTIMATE-Subaru and WFIRST to investigate their ability to discover Population III pair-instability supernovae. We found that a five-year 1 deg2K-band transient survey with a point-source limiting magnitude of 26.5 mag with ULTIMATE-Subaru may find about two Population III pair-instability supernovae beyond the redshift of 6. A five-year 10 deg2 survey with WFIRST reaching 26.5 mag in the F184 band may find about seven Population III pair-instability supernovae beyond the redshift of 6. We also find that the expected numbers of the Population III pair-instability supernova detections increase by about a factor of 2 if the near-infrared transient surveys are performed towards clusters of galaxies. Other supernovae, such as Population II pair-instability supernovae, would also be detected in the same survey. This study demonstrates that these future wide-field near-infrared instruments allow us to investigate the explosions of first-generation supernovae by performing deep and wide near-infrared transient surveys.

High-speed photometry of faint cataclysmic variables – IX. Targets from multiple transient surveys

Abstract We present high-speed photometric observations of 25 cataclysmic variables detected by the All Sky Automated Search for Super-Novae, the Mobile Astronomical System of the TElescope-Robot, and the Catalina Real-Time Transient Survey. From these observations we determine 16 new orbital periods and 1 new superhump period. Two systems (ASASSN-14ik and ASASSN-14ka) have outburst periods of approximately 1 month, with a third (ASASSN-14hv) having outbursts approximately every 2 months. Included in the sample are 11 eclipsing systems, one probable intermediate polar (ASASSN-15fm), 1 SW Sex-type star (MLS 0720+17), 1 WZ Sge-type star (ASASSN-17fz), and one system showing different photometric and spectroscopic periods (ASASSN-15kw).

Seismic Exploration of the River Sand Mining Area

Engineering geophysical exploration plays an important role in the geotechnical field. It is based on the difference in physical properties of rock, ore (or stratum) and surrounding rock, magnetization, conductivity, and radioactivity, and thus the distribution of various physical fields on the earth. Observations of changes and their changes are currently used in shallow seismic exploration and transient electromagnetic surveys. Based on the exploration demand of a river sand mining area, this paper studies the shallow seismic exploration method and transient electromagnetic method which can carry out geophysical exploration in the sand mining area, and analyse its conditions of using, mechanism principle, results analysis and reliable detection. The difference between the two is that the two have their own advantages and disadvantages. However, for the exploration requirements of the river sand mining area, the shallow seismic exploration has the advantages of lower cost, simpler operation and stronger anti-interference ability. Through actual operation, shallow seismic exploration not only detects the specific collapse area of the sand mining area, but also detects the influence range and depth of sand mining, and provides a data basis for site stability evaluation.

Identifying transient and variable sources in radio images

Abstract With the arrival of a number of wide-field snapshot image-plane radio transient surveys, there will be a huge influx of images in the coming years making it impossible to manually analyse the datasets. Automated pipelines to process the information stored in the images are being developed, such as the LOFAR Transients Pipeline, outputting light curves and various transient parameters. These pipelines have a number of tuneable parameters that require training to meet the survey requirements. This paper utilises both observed and simulated datasets to demonstrate different machine learning strategies that can be used to train these parameters. We use a simple anomaly detection algorithm and a penalised logistic regression algorithm. The datasets used are from LOFAR observations and we process the data using the LOFAR Transients Pipeline; however the strategies developed are applicable to any light curve datasets at different frequencies and can be adapted to different automated pipelines. These machine learning strategies are publicly available as Python tools that can be downloaded and adapted to different datasets ( https://github.com/AntoniaR/TraP_ML_tools ).

Multiple Outbursts of Asteroid (6478) Gault*

Abstract Main-belt asteroid (6478) Gault unexpectedly sprouted two tails in late 2018 and early 2019, identifying it as a new active asteroid. Here we present observations obtained by the 1.2 m Zwicky Transient Facility survey telescope that provide detailed time-series coverage of the onset and evolution of Gault’s activity. Gault exhibited two brightening events, with the first one starting on 2018 October 18 ± 5 days and a second one starting on 2018 December 24 ± 1 days. The amounts of mass released are 2 × 107 kg and 1 × 106 kg, respectively. Based on photometric measurements, each event persisted for about a month. Gault’s color has not changed appreciably over time, with a pre-outburst color of g PS1 − r PS1 = 0.50 ± 0.04 and g PS1 − r PS1 = 0.46 ± 0.04 during the two outbursts. Simulations of dust dynamics shows that the ejecta consists of dust grains of up to 10 μm in size that are ejected at low velocities below regardless of particle sizes. This is consistent with non-sublimation-driven ejection events. The size distribution of the dust exhibits a broken power law, with particles at 10–20 μm following a power law of −2.5 to −3.0, while larger particles follow a steeper slope of −4.0. The derived properties can be explained by either rotational excitation of the nucleus or a merger of a near-contact binary, with the latter scenario to be statistically more likely.

Tidal disruption events: Past, present, and future

The tidal disruption of a star by a nuclear supermassive black hole was predicted in the 1970s and first confirmed by soft X‐ray flares seen from quiescent galaxies in the ROSAT all‐sky survey. We report here on the discovery of a new candidate tidal disruption event, 1RXS J075908.8 + 074835, from the ROSAT bright source catalog, which faded by a factor > 100 in 20 years. Similar events have continued to be found in XMM‐Newton and Chandra data and more recently, flares in large‐area hard X‐ray, ultraviolet (UV) and optical transient surveys have also been attributed to the same phenomenon. Each of these spectral components, sometimes complemented by radio and IR activity, have their own timescale and possibly physical mechanism. Current models and simulations revolve around trying to explain how the energy released by the fall back of stellar debris in the gravitational well of the black hole is converted into radiation in each wave band. We look at the connection between optical, UV, and X‐ray emission and illustrate a dichotomy between X‐ray and optically dominated tidal disruption event (TDE), which is currently unexplained. We look at the observations that are needed to make further breakthroughs in the field and assess the optimum follow‐up strategies needed for events discovered in future large survey missions such as extended Roentgen Survey with an Imaging Telescope Array (eROSITA), the Einstein Probe, and Large Synoptic Survey Telescope (LSST). HIgh‐energy LIght‐curve GeneraTor (HILIGT), a useful new tool for comparing fluxes and upper limits between missions, is presented.