Counterparts Of Gravitational Waves Research Articles

Searches after Gravitational Waves Using ARizona Observatories (SAGUARO): Observations and Analysis from Advanced LIGO/Virgo's Third Observing Run

Abstract With the conclusion of the third observing run for Advanced LIGO/Virgo (O3), we present a detailed analysis of both triggered and serendipitous observations of 17 gravitational-wave (GW) events (7 triggered and 10 purely serendipitous) from the Searches After Gravitational-waves Using ARizona Observatories (SAGUARO) program. We searched a total of 4935 deg2 down to a median 5σ transient detection depth of 21.1 AB mag using the Mt. Lemmon 1.5 m telescope, the discovery engine for SAGUARO. In addition to triggered events within 24 hr, our transient search encompassed a time interval following GW events of <120 hr, providing observations on ∼1/2 of the events accessible to the Mt. Lemmon 1.5 m telescope. We covered 2.1%–86% of the LVC total probability (P total) for individual events, with a median P total ≈ 8% within <120 hr. Following improvements to our pipeline and the addition of serendipitous observations, we find a total of seven new optical candidates across five GW events, which we are unable to rule out after searching for additional information and comparing to kilonova models. Using both publicly available and our own late-time data, we investigated a total of 252 optical candidates for these 17 events, finding that only 65% were followed up in some capacity by the community. Of the total 252 candidates, we are able to rule out an additional 12 previously reported counterpart candidates. In light of these results, we discuss lessons learned from the SAGUARO GW counterpart search. We discuss how community coordination of observations and candidate follow-up, as well as the role of archival data, are crucial to improving the efficiency of follow-up efforts and preventing unnecessary duplication of effort with limited electromagnetic resources.

Introduction of the scientific application system of GECAM

The direct detection of gravitational waves has started a new era of gravitational wave astronomy. As an important method for studying the counterparts of gravitational waves, the observation and search of the multi-wavelength radiation of the electromagnetic counterpart of gravitational waves has become the research focus in the field of astronomy. With the advancement of gravitational wave detectors, the number of gravitational wave samples has dramatically increased and the detection of gravitational wave electromagnetic counterparts has become very promising. The Gravitational wave high-energy Electromagnetic Counterpart All-sky Monitor (GECAM) satellite is a full-sky monitoring telescope designed for detecting high-energy electromagnetic counterparts of gravitational wave events. The main reason for designing a scientific application system of GECAM satellites, which is proposed and designed with an emphasis on discovery, is to ensure the scientific operation of the payload of satellites and achieve the production, storage, processing, calibration, release, and analysis of scientific data. It is also designed to provide technical support and services to scientific users. Based on the exploration and research of the gravitational wave electromagnetic counterpart, this article introduces an innovative monitoring method design of the GECAM scientific application system for the gravitational electromagnetic counterpart, namely, the scheme design of the GECAM scientific application system. The design covers system architecture, interface control, main technical processes, functional composition, and software planning and design. The GECAM satellite is scheduled for launch by the end of 2020. By then, the scientific application system will be able to support the satellite’s in-orbit scientific operation and research.

GWOPS: A VO-technology Driven Tool to Search for the Electromagnetic Counterpart of Gravitational Wave Event

The search and follow-up observation of electromagnetic (EM) counterparts of gravitational waves (GW) is a current hot topic of GW cosmology. Due to the limitation of the accuracy of the GW observation facility at this stage, we can only get a rough sky-localization region for the GW event, and the typical area of the region is between 200 and 1500 square degrees. Since GW events occur in or near galaxies, limiting the observation target to galaxies can significantly speedup searching for EM counterparts. Therefore, how to efficiently select host galaxy candidates in such a large GW localization region, how to arrange the observation sequence, and how to efficiently identify the GW source from observational data are the problems that need to be solved. International Virtual Observatory Alliance has developed a series of technical standards for data retrieval, interoperability and visualization. Based on the application of VO technologies, we construct the GW follow-up Observation Planning System (GWOPS). It consists of three parts: a pipeline to select host candidates of GW and sort their priorities for follow-up observation, an identification module to find the transient from follow-up observation data, and a visualization module to display GW-related data. GWOPS can rapidly respond to GW events. With GWOPS, the operations such as follow-up observation planning, data storage, data visualization, and transient identification can be efficiently coordinated, which will promote the success searching rate for GWs EM counterparts.

Combining neutrino experimental light-curves for pointing to the next galactic core-collapse supernova

The multi-messenger observation of the next galactic core-collapse supernova will shed light on the different physical processes involved in these energetic explosions. Good timing and pointing capabilities of neutrino detectors would help in the search for an electromagnetic or gravitational-wave counterparts. An approach for the determination of the arrival time delay of the neutrino signal at different experiments using a direct detected neutrino light-curve matching is discussed. A simplified supernova model and detector simulation are used for its application. The arrival time delay and its uncertainty between two neutrino detectors are estimated with chi-square and cross-correlation methods. The direct comparison of the detected light-curves offers the advantage to be model-independent. Millisecond time resolution on the arrival time delay at two different detectors is needed. Using the computed time delay between different combinations of currently operational and future detectors, a triangulation method is used to infer the supernova localisation in the sky. The combination of IceCube, Hyper-Kamiokande, JUNO and KM3NeT/ARCA provides a 90% confidence area of 140pm 20,hbox {deg}^2. These low-latency analysis methods can be implemented in the SNEWS alert system.

Using machine learning for transient classification in searches for gravitational-wave counterparts

ABSTRACT The large sky localization regions offered by the gravitational-wave interferometers require efficient follow-up of the many counterpart candidates identified by the wide field-of-view telescopes. Given the restricted telescope time, the creation of prioritized lists of the many identified candidates becomes mandatory. Towards this end, we use astrorapid, a multiband photometric light-curve classifier, to differentiate between kilonovae, supernovae, and other possible transients. We demonstrate our method on the photometric observations of real events. In addition, the classification performance is tested on simulated light curves, both ideally and realistically sampled. We show that after only a few days of observations of an astronomical object, it is possible to rule out candidates as supernovae and other known transients.

Optimizing multitelescope observations of gravitational-wave counterparts

ABSTRACT The ever-increasing sensitivity of the network of gravitational-wave detectors has resulted in the accelerated rate of detections from compact binary coalescence systems in the third observing run of Advanced LIGO and Advanced Virgo. Not only has the event rate increased, but also the distances to which phenomena can be detected, leading to a rise in the required sky volume coverage to search for counterparts. Additionally, the improvement of the detectors has resulted in the discovery of more compact binary mergers involving neutron stars, revitalizing dedicated follow-up campaigns. While significant effort has been made by the community to optimize single telescope observations, using both synoptic and galaxy-targeting methods, less effort has been paid to coordinated observations in a network. This is becoming crucial, as the advent of gravitational-wave astronomy has garnered interest around the globe, resulting in abundant networks of telescopes available to search for counterparts. In this paper, we extend some of the techniques developed for single telescopes to a telescope network. We describe simple modifications to these algorithms and demonstrate them on existing network examples. These algorithms are implemented in the open-source software gwemopt, used by some follow-up teams, for ease of use by the broader community.

Strategies for the Follow-up of Gravitational Wave Transients at Very High-Energy Gamma Rays with the Cherenkov Telescope Array

With the observation of the first electromagnetic counterpart of Gravitational Wave (GW) transient GW170817, the potential of multimessenger astronomy has been clearly demonstrated. In its full configuration, the Cherenkov Telescope Array (CTA) observatory will be capable of rapidly covering the regions localized by future GW observations with sufficient sensitivity at very high-energy gamma rays. In view of the forthcoming deployment of its first telescopes, we identify some general strategies for GW follow-up that will improve the CTA contribution to multimessenger discoveries.

Searches after Gravitational Waves Using ARizona Observatories (SAGUARO): System Overview and First Results from Advanced LIGO/Virgo’s Third Observing Run

Abstract We present Searches After Gravitational-waves Using ARizona Observatories (SAGUARO), a comprehensive effort dedicated to the discovery and characterization of optical counterparts to gravitational-wave (GW) events. SAGUARO utilizes ground-based facilities ranging from 1.5 to 10 m in diameter, located primarily in the Northern Hemisphere. We provide an overview of SAGUARO’s telescopic resources, its pipeline for transient detection, and its database for candidate visualization. We describe SAGUARO’s discovery component, which utilizes the 5 deg2 field of view optical imager on the Mt. Lemmon 1.5 m telescope, reaching limits of ≈21.3 AB mag while rapidly tiling large areas. We also describe the follow-up component of SAGUARO, used for rapid vetting and monitoring of optical candidates. With the onset of Advanced LIGO/Virgo’s third observing run, we present results from the first three SAGUARO searches following the GW events S190408an, S190425z and S190426c, which serve as a valuable proof-of-concept of SAGUARO. We triggered and searched 15, 60, and 60 deg2 respectively, 17.6, 1.4, and 41.8 hr after the initial GW alerts. We covered 7.8%, 3.0%, and 5.1% of the total probability within the GW event localizations, reaching 3σ limits of 19.8, 21.3, and 20.8 AB mag, respectively. Although no viable counterparts associated with these events were found, we recovered six known transients and ruled out five potential candidates. We also present Large Binocular Telescope spectroscopy of PS19eq/SN2019ebq, a promising kilonova candidate that was later determined to be a supernova. With the ability to tile large areas and conduct detailed follow-up, SAGUARO represents a significant addition to GW counterpart searches.

National Aures Observatory: A new multimessenger facility

Algeria has taken upon itself to build a new astronomical observatory in the Aliness region in the Aures Mountains in Eastern Algeria to serve the aspirations of the Algerian astronomical community and also to open up for international projects. This Observatory will be mainly dedicated to multimessenger astronomy and the study of transient astronomical phenomena (optical counterparts of gravitational waves, GRBs, asteroid tracking, …), but it will also have other secondary objectives such as the study of early phases of super novae, Space Situational Awareness like surveillance programs, variable stars, exoplanets … In this paper, we describe the main scientific objectives of the National Aures Observatory as well as the rational for the choice of the Aliness region as the main candidate site for setting up the observatory. We will also present statistical cloud coverage from the analysis of EUMETSAT cloud data and will conclude with the present statute of the Observatory and the different stages for its construction

Optimal Search Strategy for Finding Transients in Large-sky Error Regions under Realistic Constraints

In order to identify the rapidly-fading, optical transient counterparts of gravitational wave (GW) sources, an efficient follow-up strategy is required. Since most ground-based optical observatories aimed at following-up GW sources have a telescope with a small field-of-view (FOV) as compared to the GW sky error region, we focus on a search strategy that involves dividing the GW patch into tiles of the same area as the telescope FOV to strategically image the entire patch. We present an improvement over the optimal telescope-scheduling algorithm outlined in Rana et al. (2016), by combining the tiling and galaxy-targeted search strategies, and factoring the effects of the source airmass and telescope slew, along with setting constraints, into the scheduling algorithm in order to increase the chances of identifying the GW counterpart. We propose two separate algorithms: the airmass-weighted algorithm, a specific solution to the Hungarian algorithm that maximizes probability acquired, while minimizing the image airmass, and the slew-optimization algorithm that minimizes the overall slew angle covered between images for the given probability acquired by the optimal telescope-scheduling algorithm in Rana et al. (2016). Using the observatory site of the GROWTH-India telescope as an example, we generate 100s of skymaps to test the performance of our algorithms. Our results indicate that slew-optimization can reduce the cumulative slew angle in the observing schedule by 100s of degrees, saving several of minutes of observing time without the loss of tiles and probability. Further, we demonstrate that as compared to the greedy algorithm, the airmass-weighted algorithm can acquire up to 20 % more probability and 30 sq. deg. more in areal coverage for skymaps of all sizes and configurations.

Probing the Fermi-LAT GeV Excess with Gravitational Waves.

If the gamma-ray excess towards the inner Galaxy (GCE) detected in Fermi-LAT data is due to millisecond pulsars (MSPs), one expects an associated gravitational wave (GW) signal, whose intensity exceeds the disk MSP population emission by an order of magnitude. We compute the expected GW counterpart of the bulge MSP population based on fits of the GCE, and estimate the sensitivity reach of current and future terrestrial GW detectors. The bounds on the average population ellipticity ϵ are competitive with the existing ones derived by LIGO/Virgo towards known MSPs. With a 10-yr data taking in current LIGO/Virgo configuration, one would detect a signal at the level ϵ≃10^{-7}, while ϵ≃10^{-8} would be attainable with a similar data taking period with a third generation GW detector. This sensitivity should be sufficient for crucial diagnostics on the GCE interpretation in terms of MSPs.

2900 Square Degree Search for the Optical Counterpart of Short Gamma-Ray Burst GRB 180523B with the Zwicky Transient Facility

There is significant interest in the models for production of short gamma-ray bursts (GRBs). Until now, the number of known short GRBs with multi-wavelength afterglows has been small. While the Fermi GRB Monitor detects many GRBs relative to the Neil Gehrels Swift Observatory, the large localization regions makes the search for counterparts difficult. With the Zwicky Transient Facility (ZTF) recently achieving first light, it is now fruitful to use its combination of depth (mAB ∼ 20.6), field of view (≈47 square degrees), and survey cadence (every ∼3 days) to perform Target of Opportunity observations. We demonstrate this capability on GRB 180523B, which was recently announced by the Fermi GRB Monitor as a short GRB. ZTF imaged ≈2900 square degrees of the localization region, resulting in the coverage of 61.6% of the enclosed probability over two nights to a depth of mAB ∼ 20.5. We characterized 14 previously unidentified transients, and none were found to be consistent with a short GRB counterpart. This search with the ZTF shows it is an efficient camera for searching for coarsely localized short GRB and gravitational-wave counterparts, allowing for a sensitive search with minimal interruption to its nominal cadence.

Performance of a wide field-of-view atmospheric Cherenkov telescope prototype based on a refractive lens

A wide field-of-view (FoV) is one of the most important goals for future ground-based γ-ray astronomy, and it is especially crucial for detailed studies of temporal and spectral characteristics of solitary events, such as γ-ray bursts (GRBs), highly variable sources like blazars, or potential very high energy (VHE) electromagnetic counterparts of gravitational waves (GWs). Atmospheric Cherenkov Telescope (ACT) based on refractive optics and operated at a very high altitude observatory may be an appropriate solution. In the present study, we report the characteristics and performance of a large FoV ACT prototype based on a refractive convex water lens. A reconstruction method for estimating the arrival direction of the air shower was developed by combining the water-lens telescope and the Extensive Air Shower (EAS) array. The Cherenkov images of the air showers were obtained by a water-lens prototype with 48 pixels, an FoV of 15°×13°, and a median angular resolution of ∼0.9°.

INTEGRAL search for GW counterparts and the GRB170817A/GW170817 detection

The INTernational Gamma-ray Astrophysics Laboratory (INTEGRAL) has detected the Short Gamma-Ray Burst (SGRB), GRB170817A with a signal-to-noise ratio of 4.6 and demonstrated its association with the binary neutron star merging event GW170817 detected by the LIGO and Virgo gravitational wave observatories. The association was immediately evident due to the timing and positional coincidence of this event with the initial error box, derived from gravitational wave (GW) measurements and the contemporaneous detection of a similar gamma-ray signal by Fermi/GBM. This SGRB was detected by the INTEGRAL SPI ACS about 1.7 s after the end of the GW emission, with a fluence of ($$1.4 \pm 0.4$$) $$\times 10^{-7} \text { erg cm}^{-2}$$ in the 75–2000 keV energy range. After the serendipitous detection of the short prompt GRB, INTEGRAL continued the planned observation for about 20 h, and then performed a targeted follow-up ToO observation lasting several days. This ToO observation provided a stringent upper limit on any electromagnetic signal in a very broad energy range, from 3 keV to 8 MeV, in particular constraining the soft gamma-ray afterglow flux to $$< 7.1 \times ^{-11} \text { erg cm}^{-2} \text { s}^{-1}$$ in the range 80–300 keV. Exploiting the unique capabilities of INTEGRAL, we constrained the gamma-ray line emission intensity from radioactive decays expected to be the principal source of the energy behind a kilonova event following a NS–NS coalescence. Finally, we put a stringent upper limit on any delayed bursting activity, for example, from a newly formed magnetar. The INTEGRAL prompt detection and the subsequent continuous observations at all wavelengths have provided important constraints on the high energy emission of the resulting kilonova and the post inspiral object: NS, BH, or a new exotic object.

The origin of polarization in kilonovae and the case of the gravitational-wave counterpart AT 2017gfo

The Gravitational Wave (GW) event GW 170817 was generated by the coalescence of two neutron stars (NS) and produced an electromagnetic transient, labelled AT 2017gfo, that was target of a massive observational campaign. Polarimetry, a powerful diagnostic tool for probing the geometry and emission processes of unresolved sources, was obtained for this event. The observed linear polarization was consistent with being mostly induced by intervening dust, suggesting that the intrinsic emission was weakly polarized ($P < 0.4-0.5$ %). In this paper, we present and discuss a detailed analysis of the linear polarization expected from a merging NS binary system by means of 3D Monte Carlo radiative transfer simulations assuming a range of possible configurations, wavelengths, epochs and viewing angles. We find that polarization originates from the non-homogeneous opacity distribution within the ejecta and can reach levels of $P\sim1$ % at early times (1$-$2 days after the merger) and in the optical R band. Smaller polarization signals are expected at later epochs and/or different wavelengths. From the viewing-angle dependence of the polarimetric signal, we constrain the observer orientation of AT 2017gfo within $\sim$65$^\circ$ from the polar direction. The detection of non-zero polarization in future events will unambiguously reveal the presence of a lanthanide-free ejecta component and unveil its spatial and angular distribution.

Improving science yield for NASA Swift with automated planning technologies

The Neil Gehrels Swift Observatory is a uniquely capable mission, with three on-board instruments and rapid slewing capabilities. It serves as a fast-response satellite observatory for everything from gravitational-wave counterpart searches to cometary science. Swift averages 125 different observations per day, and is consistently over-subscribed, responding to about one-hundred Target of Oportunity (ToO) requests per month from the general astrophysics community, as well as co-pointing and follow-up agreements with many other observatories. Since launch in 2004, the demands put on the spacecraft have grown consistently in terms of number and type of targets as well as schedule complexity. To facilitate this growth, various scheduling tools and helper technologies have been built by the Swift team to continue improving the scientific yield of the Swift mission. However, these tools have been used only to assist humans in exploring the local pareto surface and for fixing constraint violations. Because of the computational complexity of the scheduling task, no automation tool has been able to produce a plan of equal or higher quality than that produced by a well-trained human, given the necessary time constraints. In this proceeding we formalize the Swift Scheduling Problem as a dynamic fuzzy Constraint Satisfaction Problem (DF-CSP) and explore the global solution space. We detail here several approaches towards achieving the goal of surpassing human quality schedules using classical optimization and algorithmic techniques, as well as machine learning and recurrent neural network (RNN) methods. We then briefly discuss the increased scientific yield and benefit to the wider astrophysics community that would result from the further development and adoption of these technologies.

Photon-graviton scattering: A new way to detect anisotropic gravitational waves?

Gravitons are the quantum counterparts of gravitational waves in low-energy theories of gravity. Using Feynman rules one can compute scattering amplitudes describing the interaction between gravitons and other fields. Here, we consider the interaction between gravitons and photons. Using the quantum Boltzmann equation formalism, we derive fully general equations describing the radiation transfer of photon polarization, due to the forward scattering with gravitons. We show that the Q and U photon linear polarization modes couple with the V photon circular polarization mode, if gravitons have anisotropies in their power-spectrum statistics. As an example, we apply our results to the case of primordial gravitons, considering models of inflation where an anisotropic primordial graviton distribution is produced. Finally, we evaluate the effect on cosmic microwave background (CMB) polarization, showing that in general the expected effects on the observable CMB frequencies are very small. However, our result is promising, since it could provide a novel tool for detecting anisotropic backgrounds of gravitational waves, as well as for getting further insight on the physics of gravitational waves.

A low-latency pipeline for GRB light curve and spectrum using Fermi/GBM near real-time data

Rapid response and short time latency are very important for Time Domain Astronomy, such as the observations of Gamma-ray Bursts (GRBs) and electromagnetic (EM) counterparts of gravitational waves (GWs). Based on near real-time Fermi/GBM data, we developed a low-latency pipeline to automatically calculate the temporal and spectral properties of GRBs. With this pipeline, some important parameters can be obtained, such as T90 and fluence, within ∼ 20 min after the GRB trigger. For ∼ 90% of GRBs, T90 and fluence are consistent with the GBM catalog results within 2σ errors. This pipeline has been used by the Gamma-ray Bursts Polarimeter (POLAR) and the Insight Hard X-ray Modulation Telescope (Insight-HXMT) to follow up the bursts of interest. For GRB 170817A, the first EM counterpart of GW events detected by Fermi/GBM and INTEGRAL/SPI-ACS, the pipeline gave T90 and spectral information 21 min after the GBM trigger, providing important information for POLAR and Insight-HXMT observations.

Strategies for the follow-up of gravitational wave transients with the Cherenkov Telescope Array

The observation of the electromagnetic counterpart of gravitational-wave (GW) transient GW170817 demonstrated the potential in extracting astrophysical information from multimessenger discoveries. The forthcoming deployment of the first telescopes of the Cherenkov Telescope Array (CTA) observatory will coincide with Advanced LIGO/Virgo's next observing run, O3, enabling the monitoring of gamma-ray emission at E > 20 GeV, and thus particle acceleration, from GW sources. CTA will not be greatly limited by the precision of GW localization as it will be be capable of rapidly covering the GW error region with sufficient sensitivity. We examine the current status of GW searches and their follow-up effort, as well as the status of CTA, in order to identify some of the general strategies that will enhance CTA's contribution to multimessenger discoveries.

First Results of the SkyMapper Transient Survey

AbstractThe SkyMapper Transient survey (SMT) is exploring variability in the southern sky by performing (a) a rolling search to discover and study supernovæ, and (b) a Target of Opportunity programme that uses the robotic SkyMapper Telescope at Siding Spring Observatory. The supernova survey is obtaining a non-targeted sample of Type Ia supernovæ (SNe Ia) at low redshifts, z &lt; 0.1, and studying other interesting transients found with the search strategy. We have a Target of Opportunity programme with an automatic response mechanism to search for optical counterparts to gravitational-wave and fast radio-burst events; it benefits from SkyMapper’s large field of view of 5.7 sq. deg. and a rapid data reduction pipeline.We present first results of the SMT survey. The SMT pipeline can process and obtain potential candidates within 12 hours of observation. It disentangles real transients from processing artefacts using a machine-learning algorithm. To date, SMT has discovered over 60 spectroscopically confirmed supernovæ, several peculiar objects, and over 40 SNe Ia including one (SNIa 2016hhd) which was found within the first few days of explosion. We have also participated in searches for optical counterparts of gravitational waves, fast radio bursts and other transients, and have published observations of the optical counterpart of the gravitational-wave event GW170817. We also participate in coordinated observations with the Deeper Wider Faster programme, and the Kepler K2 cosmology project.