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In this white paper, we review five top considerations for selecting locations of the fields of the Roman High-latitude Time Domain Survey. Based on these considerations, we recommend Akari Deep Field South (ADFS)/Euclid Deep Field South (EDFS) in the Southern Hemisphere has it avoids bright stars, has minimal Milky Way dust, is in Roman Continuous viewing zone, overlaps with multiple past and future surveys, and minimal zodiacal background variation. In the North, Extended Groth Strip (EGS) is good except for its zodiacal variation and Supernova/Acceleration Probe North (SNAP-N) and European Large Area Infrared Space Observatory Survey-North 1 (ELAIS N-1) are good except for their synergistic archival data., Submitted to the Roman Core Community Survey call for white papers

The current reference High-latitude time domain survey increases the completeness of transients with prism temporal time series data by adjusting the ratio of prism-to-imaging time. However, there are two other nobs that allow for a more complete prism coverage: prism cadence and exposure time. In this white paper, we discuss how changes to the prism cadence and exposure time -- in order to increase the fraction of observed transients with spectral time series -- affect supernova cosmology, transient typing and template building, and the study of rare transients., Submitted to the Roman Core Community Survey call for white papers

Compact object binaries (a black hole or a neutron star orbiting a non-degenerate stellar companion) are key to our understanding of late massive star evolution, in addition to being some of the best probes of extreme gravity and accretion physics. Gaia has opened the door to astrometric studies of these systems, enabling geometric distance measurements, kinematic estimation, and the ability to find new previously unknown systems through measurement of binary orbital elements. Particularly puzzling are newly found massive black holes in wide orbits (~AU or more) whose evolutionary history is difficult to explain. Astrometric identification of such binaries is challenging for Gaia, with only two such examples currently known. Roman's enormous grasp, superb sensitivity, sharp PSF and controlled survey strategy can prove to be a game-changer in this field, extending astrometric studies of compact object binaries several mag deeper than Gaia. We propose to use the microlensing Galactic Bulge Time Domain Survey to identify new wide-orbit black hole compact object binaries, determine their prevalence and their spatial distribution, thus opening up new parameter space in binary population studies., Submitted to the Nancy Grace Roman Space Telescope CCS White Paper call (4.5 pages of text + figures, references and a cover sheet). Comments/collaboration welcome

Analysing the available data relative to the anisotropies of the ultra-high-energy cosmic rays (UHECR) at intermediate angular scales, we examine to what extent they can be used to constrain the origin of these particles, and what could be gained from a new generation of observatories with increased exposure. We simulate realistic UHECR sky maps for a wide range of scenarios, with the assumption that the distribution of UHECR sources follows that of matter in the Universe, also considering possible biases. We produce numerous datasets on which we apply similar analyses as those recently used by the Auger and TA collaborations. We find that: i) the investigated scenarios can easily account for the significance of the anisotropies reported by Auger and TA; ii) the direction in which the maximum flux excess is found in the Auger data differs from where it is found in most of our simulations; iii) for datasets simulated with the same astrophysical scenario, the significance with which the isotropy hypothesis is rejected through the Auger likelihood analysis can be largest either when "all galaxies" or when "starburst" galaxies are used to model the signal, depending on which GMF model is used; iv) the study of the energy evolution of the anisotropy patterns can provide new insight about the origin of UHECRs; v) the direction in which the most significant flux excess is found in the Auger dataset above 8 EeV appears to essentially disappear in the dataset above 32 EeV; vi) this appears to be very uncommon in our simulations, which could point to a failure of some generic assumption in the investigated scenarios, such as the predominance of a unique type of sources in the flux above the ankle, with essentially the same composition and spectrum; vii) a meaningful measurement of their energy evolution, from 10 EeV to the highest energies, will require a significant increase in statistics., Comment: 28 pages, 23 figures

We motivate the cosmological science case of measuring Type Ia supernovae with the Nancy Grace Roman Space Telescope as part of the High Latitude Time Domain Survey. We discuss previously stated requirements for the science, and a baseline survey strategy. We discuss the various areas that must still be optimized and point to the other white papers that consider these topics in detail. Overall, the baseline case should enable an exquisite measurement of dark energy using SNe Ia from z=0.1 to z>2, and further optimization should only strengthen this once-in-a-generation experiment.

Type-I Superluminous Supernovae (SLSNe) are an exotic class of core-collapse SN (CCSN) that can be up to 100 times brighter and more slowly-evolving than normal CCSNe. SLSNe represent the end-stages of the most massive stripped stars, and are thought to be powered by the spin-down energy of a millisecond magnetar. Studying them and measuring their physical parameters can help us to better understand stellar mass-loss, evolution, and explosions. Moreover, thanks to their high luminosities, SLSNe can be seen up to greater distances, allowing us to explore how stellar physics evolves as a function of redshift. The High Latitude Time Domain Survey (HLTDS) will provide us with an exquisite dataset that will discover 100s of SLSNe. Here, we focus on the question of which sets of filters and cadences will allow us to best characterize the physical parameters of these SLSNe. We simulate a set of SLSNe at redshifts ranging from z = 0.1 to z = 5.0, using six different sets of filters, and cadences ranging from 5 to 100 days. We then fit these simulated light curves to attempt to recover the input parameter values for their ejecta mass, ejecta velocity, magnetic field strength, and magnetar spin period. We find that four filters are sufficient to accurately characterize SLSNe at redshifts below $z = 3$, and that cadences faster than 20 days are required to obtain measurements with an uncertainty below 10\%, although a cadence of 70 days is still acceptable under certain conditions. Finally, we find that the nominal survey strategy will not be able to properly characterize the most distant SLSNe at $z = 5$. We find that the addition of 60-day cadence observations for 4 years to the nominal HLTDS survey can greatly improve the prospect of characterizing these most extreme and distant SNe, with only an 8\% increase to the time commitment of the survey., Comment: Submitted as part of the Nancy Grace Roman Space Telescope's Core Community Surveys call for white papers

Pair-instability supernovae (PISNe) are explosions of very massive stars that may have played a critical role in the chemical evolution and reionization of the early Universe. In order to quantify their roles, it is required to know the PISN event rate at z > 6. Although Roman Space Telescope has a capability to discover PISNe at z > 6, identifying rare high-redshift PISN candidates among many other transients is challenging. In order to efficiently identify PISN candidates at z > 6, we propose to add sparse F213 observations reaching 26.5 mag (or deeper) every half year in the High Latitude Time Domain Survey. By adding the F213 information, PISNe at z > 6 can be efficiently identified in the color-magnitude diagram., 5 pages, 2 figures, submitted as a Roman Core Community Survey White Paper

Although there are estimated to be 100 million isolated black holes (BHs) in the Milky Way, only one has been found so far, resulting in significant uncertainty about their properties. The Galactic Bulge Time Domain Survey provides the only opportunity in the coming decades to grow this catalog by order(s) of magnitude. This can be achieved if 1) Roman's astrometric potential is fully realized in the observation strategy and software pipelines, 2) Roman's observational gaps of the Bulge are minimized, and 3) observations with ground-based facilities are taken of the Bulge to fill in gaps during non-Bulge seasons. A large sample of isolated BHs will enable a broad range of astrophysical questions to be answered, such as massive stellar evolution, origin of gravitational wave sources, supernova physics, and the growth of supermassive BHs, maximizing Roman's scientific return., Comment: 20 pages. Submitted in response to Nancy Grace Roman Space Telescope white paper call: https://roman.gsfc.nasa.gov/science/ccs_white_papers.html. v2 fixes a typo in Figure 5 axis label (days --> year)

Our limited understanding of the physical properties of matter at ultra-high density, high proton/neutron number asymmetry, and low temperature is presently one of the major outstanding problems in physics. As matter in this extreme state is known to only exist stably in the cores of neutron stars (NSs), complementary measurements from electromagnetic and gravitational wave astrophysical observations of NSs, combined with terrestrial laboratory constraints and further theoretical investigations, hold the promise to provide important insight into the properties of matter in a region of the quantum chromodynamics phase space that is otherwise inaccessible. This multidisciplinary endeavor imposes the following requirements for facilities and resources in the upcoming decade and beyond: * A next generation of gravitational wave detectors to uncover more double NS and neutron star-black hole mergers; * Sensitive radio telescopes to find the most massive and fastest spinning NSs; * Large-area, high-time-resolution and/or high angular resolution X-ray telescopes to constrain the NS mass-radius relation; * Suitable laboratory facilities for nuclear physics experiments to constrain the dense matter equation of state; * Funding resources for theoretical studies of matter in this regime; * The availability of modern large-scale high performance computing infrastructure. The same facilities and resources would also enable significant advances in other high-profile fields of inquiry in modern physics such as the nature of dark matter, alternative theories of gravity, nucleon superfluidity and superconductivity, as well as an array of astrophysics, including but not limited to stellar evolution, nucleosynthesis, and primordial black holes., Submitted to the Proceedings of the US Community Study on the Future of Particle Physics (Snowmass 2021) under Cosmic Frontier (CF07: Cosmic probes of fundamental physics); 30 pages, 8 figures

Physics of the Dark Universe 42, 101267 (2023). doi:10.1016/j.dark.2023.101267, Neutrinos are the Standard Model (SM) particles which we understand the least, often due to how weakly they interact with the other SM particles. Beyond this, very little is known about interactions among the neutrinos, i.e., their self-interactions. The SM predicts neutrino self-interactions at a level beyond any current experimental capabilities, leaving open the possibility for beyond-the-SM interactions across many energy scales. In this white paper, we review the current knowledge of neutrino self-interactions from a vast array of probes, from cosmology, to astrophysics, to the laboratory. We also discuss theoretical motivations for such self-interactions, including neutrino masses and possible connections to dark matter. Looking forward, we discuss the capabilities of searches in the next generation and beyond, highlighting the possibility of future discovery of this beyond-the-SM physics., Published by Elsevier, Amsterdam [u.a.]

We propose to develop a wide-field and ultra-high-precision photometric survey mission, temporarily named "Earth 2.0 (ET)". This mission is designed to measure, for the first time, the occurrence rate and the orbital distributions of Earth-sized planets. ET consists of seven 30cm telescopes, to be launched to the Earth-Sun's L2 point. Six of these are transit telescopes with a field of view of 500 square degrees. Staring in the direction that encompasses the original Kepler field for four continuous years, this monitoring will return tens of thousands of transiting planets, including the elusive Earth twins orbiting solar-type stars. The seventh telescope is a 30cm microlensing telescope that will monitor an area of 4 square degrees toward the galactic bulge. This, combined with simultaneous ground-based KMTNet observations, will measure masses for hundreds of long-period and free-floating planets. Together, the transit and the microlensing telescopes will revolutionize our understandings of terrestrial planets across a large swath of orbital distances and free space. In addition, the survey data will also facilitate studies in the fields of asteroseismology, Galactic archeology, time-domain sciences, and black holes in binaries., 116 pages,79 figures

In Jiang et al., we detected a bright flash (hereafter GN-z11-flash) that appeared as compact continuum emission during our Keck MOSFIRE observations of the galaxy GN-z11 at z ~ 11. We performed a comprehensive analysis of the origin of the flash using all available information and our current understanding of known man-made objects or moving objects in the solar system. We found that GN-z11-flash was likely a rest-frame UV flash associated with a long gamma-ray burst (GRB) from GN-z11. Recently, Steinhardt et al., Michalowski et al., and Nir et al. reported that GN-z11-flash was more likely from a satellite. While one cannot completely rule out the possibility of unknown satellites (or debris), we find that either the chance probabilities of being a satellite estimated by these authors have been largely overestimated or their identified satellites have been ruled out in our original analysis. Our new calculations show that the probability of GN-z11-flash being a satellite is still lower than that of it being a signal originated from GN-z11., Published in Nature Astronomy Matters Arising 5, 998-1000, 2021; Authors' version; Some content slightly overlaps with our previous reply in arXiv:2102.01239

We investigate the implications of the current data regarding large scale anisotropies, and examine to what extent they can be used to constrain the origin of UHECRs and the astrophysical parameters of the source scenarios. We discuss the possibility of observing an associated anisotropy of the composition, the potential benefit of the separation of the different nuclear components and the interest of observing the UHECR sky with larger exposure future observatories. We simulate UHECR sky maps for various astrophysical scenarios satisfying the current observational constraints, taking into account the energy losses of the UHE protons and nuclei and their deflections by intervening magnetic fields. We investigate scenarios in which the UHECR source distribution follows that of the galaxies, varying the source composition and spectrum, the source density and the magnetic field models. We apply similar analyses as those used by the Auger collaboration. We find that: i) reproducing the observed dipole anisotropy and its energy evolution is relatively easy within our assumptions; ii) this agreement can be obtained with different sets of assumptions on the astrophysical parameters, and is thus not, at this stage, very constraining for UHECR source scenarios; iii) the actual reconstructed direction of the dipole appears non natural in essentially all scenarios investigated, and challenges their main assumptions on the source distribution or the assumed magnetic field configuration, especially in the Galaxy; iv) except for protons, the energy range in which the GZK horizon strongly reduces is a key target for anisotropy searches for each given nuclear species; v) The composition anisotropy naturally expected in our models is unlikely to account for that recently reported by Auger unless the observed amplitude is a strong positive statistical fluctuation of an intrinsically weaker signal., Comment: 39 pages, 32 figures

In this paper we explore the scientific synergies between Athena and some of the key multi-messenger facilities that should be operative concurrently with Athena. These facilities include LIGO A+, Advanced Virgo+ and future detectors for ground-based observation of gravitational waves (GW), LISA for space-based observations of GW, IceCube and KM3NeT for neutrino observations, and CTA for very high energy observations. These science themes encompass pressing issues in astrophysics, cosmology and fundamental physics such as: the central engine and jet physics in compact binary mergers, accretion processes and jet physics in Super-Massive Binary Black Holes (SMBBHs) and in compact stellar binaries, the equation of state of neutron stars, cosmic accelerators and the origin of Cosmic Rays (CRs), the origin of intermediate and high-Z elements in the Universe, the Cosmic distance scale and tests of General Relativity and the Standard Model. Observational strategies for implementing the identified science topics are also discussed. A significant part of the sources targeted by multi-messenger facilities is of transient nature. We have thus also discussed the synergy of \textsl{Athena} with wide-field high-energy facilities, taking THESEUS as a case study for transient discovery. This discussion covers all the Athena science goals that rely on follow-up observations of high-energy transients identified by external observatories, and includes also topics that are not based on multi-messenger observations, such as the search for missing baryons or the observation of early star populations and metal enrichment at the cosmic dawn with Gamma-Ray Bursts (GRBs)., Comment: White paper of the multi-messenger Athena synergy excersise

This is a combination of the errata of seven papers published between 2008 and 2016 with Jiang-Tao Li (JTL) as the first author. All the problems are caused by two mistakes in the original scripts written by JTL used to calculate the physical parameters of the hot gas from X-ray spectral analysis with a thermal plasma code. The mistakes will result in an overestimate of some parameters, such as the electron number density and hot gas mass by a factor of $\sqrt{10}\approx3.162$, and an overestimate of the thermal pressure by a factor of $\approx2.725$. JTL apologizes to the community for the inconvenience caused by these mistakes. We present an update on the text, numbers, figures, and tables of all the seven papers affected by these mistakes. Other papers led by JTL or co-authored papers are not affected., 11 pages, 4 figures, 9 tables. Errata of seven papers from 2008 to 2016 with Jiang-Tao Li as the first author

Current daily paper releases are becoming increasingly large and areas of research are growing in diversity. This makes it harder for scientists to keep up to date with current state of the art and identify relevant work within their lines of interest. The goal of this article is to address this problem using Machine Learning techniques. We model a scientific paper to be built as a combination of different scientific knowledge from diverse topics into a new problem. In light of this, we implement the unsupervised Machine Learning technique of Latent Dirichlet Allocation (LDA) on the corpus of papers in a given field to: i) define and extract underlying topics in the corpus; ii) get the topics weight vector for each paper in the corpus; and iii) get the topics weight vector for new papers. By registering papers preferred by a user, we build a user vector of weights using the information of the vectors of the selected papers. Hence, by performing an inner product between the user vector and each paper in the daily Arxiv release, we can sort the papers according to the user preference on the underlying topics. We have created the website IArxiv.org where users can read sorted daily Arxiv releases (and more) while the algorithm learns each users preference, yielding a more accurate sorting every day. Current IArxiv.org version runs on Arxiv categories astro-ph, gr-qc, hep-ph and hep-th and we plan to extend to others. We propose several new useful and relevant implementations to be additionally developed as well as new Machine Learning techniques beyond LDA to further improve the accuracy of this new tool., We are open to new ideas and to scientists and institutions wishing to collaborate and/or partner in further improvements for this service. With this tool the time a paper is sent is irrelevant for its order of appearance

Since the birth of X-ray Astronomy, spectacular advances have been seen in the imaging, spectroscopic and timing studies of the hot and violent X-ray Universe, and further leaps forward are expected in the future. On the other hand, polarimetry is very much lagging behind: after the measurements of the Crab Nebula and Scorpius X-1, obtained by OSO-8 in the 70s, no more observations have been performed in the classical X-ray band, even if some interesting results have been obtained in hard X-rays and in soft gamma-rays. The NASA/ASI mission IXPE, scheduled for the launch in 2021, is going to provide for the first time imaging X-ray polarimetry in the 2-8 keV band thanks to its photoelectric polarimeter, coupled with ~25'' angular resolution X-ray mirrors. Its orders of magnitude improvement in sensitivity with respect to the OSO-8 Bragg polarimeter implies scientifically meaningful polarimetric measurements for at least the brightest specimens of most classes of X-ray sources. In 2027, the Chinese-led mission eXTP should also be launched. In addition to timing and spectroscopic instruments, eXTP will have on board photoelectric polarimeters very similar to those of IXPE, but with a total effective area 2-3 times larger. Building on IXPE results, eXTP will increase the number of sources for which significant polarimetric measurements could be obtained. However, further progresses, such as exploring a broader energy range, considering a larger effective area, improving the angular resolution, and performing wide-field polarization measurements, are needed to reach a mature phase for X-ray polarimetry. In the first part of this White Paper we will discuss a few scientific cases in which a next generation X-ray Polarimetry mission can provide significant advances. In the second part, a possible concept for a medium-class Next Generation X-ray Polarimetry (NGXP) mission will be sketched., White Paper submitted in response to the ESA Voyage 2050 call, 20 pages + title page + references + list of team members, 10 figures

Insight-HXMT is the first Chinese X-ray astronomical mission, launched successfully on June 15, 2017, from China's Jiuquan Satellite Launch Center. Insight-HXMT was designed to have a broad energy coverage in X-rays, from 1-250 keV, with excellent timing and adequate energy resolution at soft X-rays, and the largest effective area at hard X-rays. Here, we present a collection of papers of the Journal of High Energy Astrophysics on the Early Results of China's 1st X-ray Astronomy Satellite Insight-HXMT. These papers cover the in-orbit performance, the background model, and all calibration results, together with several first results on observations of binaries and details on the Galactic plane survey., Comment: Published to introduce a set of 20+ papers from the HXMT collaboration. Most of the referred papers can be accessed in astro-ph already, and all will be avaialble from JHEAp's webpage too, at https://www.sciencedirect.com/journal/journal-of-high-energy-astrophysics/special-issue/10B0FLKPH6C

A recent multi-year Caltech-NRAO Stripe 82 Survey (CNSS) revealed a group of objects that appeared as new radio sources after $>$5--20 years of absence. They are transient phenomena with respect to the Faint Images of the Radio Sky at Twenty Centimeters (FIRST) survey and constitute the first unbiased sample of renewed radio activity. Here we present the follow-up, radio, optical and X-ray study of them. The group consist of 12 sources, both quasars and galaxies with wide redshift ($\rm 0.04 < z < 1.7$) and luminosity ($\rm 2224.5$) distribution. Their radio properties in the first phase of activity, namely the convex spectra and compact morphology, allow them all to be classified as gigahertz-peaked spectrum (GPS) sources. We conclude that the spectral changes are a consequence of the evolution of newly-born radio jets. Our observations show that over the next few years of activity the GPS galaxies keep the convex shape of the spectrum, while GPS quasars rapidly transform into flat-spectrum sources, which may result in them not being recognized as young sources. The wide range of bolometric luminosities, black hole masses and jet powers among the transient sources indicates even greater population diversity in the group of young radio objects. We also suggest that small changes of the accretion disc luminosity (accretion rate) may be sufficient to ignite low-power radio activity that evolves on the scale of decades.

MAAT is proposed as a visitor mirror-slicer optical system that will allow the OSIRIS spectrograph on the 10.4-m Gran telescopio CANARIAS (GTC) the capability to perform Integral Field Spectroscopy (IFS) over a seeing-limited FoV 14.20''x10'' with a slice width of 0.303''. MAAT@GTC will enhance the resolution power of OSIRIS by 1.6 times as compared to its 0.6'' wide long-slit. All the eleven OSIRIS grisms and volume-phase holographic gratings will be available to provide broad spectral coverage with moderate resolution (R=600 up to 4100) in the 3600 - 10000 {\AA} wavelength range. MAAT unique observing capabilities will broaden its use to the needs of the GTC community to unveil the nature of most striking phenomena in the universe well beyond time-domain astronomy. The GTC equipped with OSIRIS+MAAT will also play a fundamental role in synergy with other facilities, some of them operating on the northern ORM at La Palma. This White Paper presents the different aspects of MAAT@GTC - including scientific and technical specifications, outstanding science cases, and an outline of the instrument concept., Comment: 49 pages, 36 figures

We present a sensitive search with the Karl G. Jansky Very Large Array (VLA) for the radio counterpart of the gravitational wave candidate S191216ap, classified as a binary black hole merger, and suggested to be a possible multi-messenger event, based on the detection of a high energy neutrino and a TeV photon. We carried out a blind search at C band (4--8 GHz) over 0.3 deg$^2$ of the gamma-ray counterpart of S191216ap reported by the High-Altitude Water Cherenkov Observatory (HAWC). Our search, spanning three epochs over 130 days post-merger and having mean source-detection threshold of 75$\mu$Jy/beam (4$\sigma$), yielded 5 variable sources associated with AGN activity and no definitive counterpart of S191216ap. We find $, Comment: 9 pages, 5 figures, 2 tables, submitted to ApJ

Current X-ray observations and simulations show that gravitational lensing can be used to infer the structure near the event horizons of black holes, constrain the dynamics and evolution of black-hole accretion and outflows, test general relativity in the strong-gravity regime and place constraints on the evolution of dark matter in the lensing galaxies. These science goals currently cannot be achieved in a statistically large sample of z = 0.5 - 5 lensed quasars due to the limited capabilities of current X-ray telescopes and the relatively low number (~200) of known lensed quasars. The latter limitation will be resolved with the multi-band and wide-field photometric optical survey of LSST that is expected to lead to the discovery of > 4,000 additional gravitationally lensed systems. As we show in this white paper, these science goals can be reached with an X-ray telescope having a spatial resolution of 0.5 m^2 at 1 keV., Submitted to the Astro2020 Decadal Survey call for science white papers on Astronomy and Astrophysics, 7 pages, includes 3 figures

Galactic binaries with orbital periods less than $\approx$1 hr are strong gravitational wave sources in the mHz regime, ideal for the Laser Interferometer Space Antenna (LISA). In fact, theory predicts that \emph{LISA} will resolve tens of thousands of Galactic binaries individually with a large fraction being bright enough for electromagnetic observations. This opens up a new window where we can study a statistical sample of compact Galactic binaries in both, the electromagnetic as well the gravitational wavebands. Using multi-messenger observations we can measure tidal effects in detached double WD systems, which strongly impact the outcome of WD mergers. For accreting WDs as well as NS binaries, multi-messenger observations give us the possibility to study the angular momentum transport due to mass transfer. In this white paper we present an overview of the opportunities for research on Galactic binaries using multi-messenger observations and summarize some recommendations for the 2020 time-frame., 5 pages, 1 figure, submitted to the Astro2020 call for science white papers

We write in response to the call from the 2020 Decadal Survey to submit white papers illustrating the most pressing scientific questions in astrophysics for the coming decade. We propose exploration as the central question for the Decadal Committee's discussions.The history of astronomy shows that paradigm changing discoveries are not driven by well formulated scientific questions, based on the knowledge of the time. They were instead the result of the increase in discovery space fostered by new telescopes and instruments. An additional tool for increasing the discovery space is provided by the analysis and mining of the increasingly larger amount of archival data available to astronomers. Revolutionary observing facilities, and the state of the art astronomy archives needed to support these facilities, will open up the universe to new discovery. Here we focus on exploration for compact objects and multi messenger science. This white paper includes science examples of the power of the discovery approach, encompassing all the areas of astrophysics covered by the 2020 Decadal Survey.

This paper presents the ESA Voyage 2050 White Paper for a concept of TeraHertz Exploration and Zooming-in for Astrophysics (THEZA). It addresses the science case and some implementation issues of a space-borne radio interferometric system for ultra-sharp imaging of celestial radio sources at the level of angular resolution down to (sub-) microarcseconds. THEZA focuses at millimetre and sub-millimetre wavelengths (frequencies above $\sim$300~GHz), but allows for science operations at longer wavelengths too. The THEZA concept science rationale is focused on the physics of spacetime in the vicinity of supermassive black holes as the leading science driver. The main aim of the concept is to facilitate a major leap by providing researchers with orders of magnitude improvements in the resolution and dynamic range in direct imaging studies of the most exotic objects in the Universe, black holes. The concept will open up a sizeable range of hitherto unreachable parameters of observational astrophysics. It unifies two major lines of development of space-borne radio astronomy of the past decades: Space VLBI (Very Long Baseline Interferometry) and mm- and sub-mm astrophysical studies with "single dish" instruments. It also builds upon the recent success of the Earth-based Event Horizon Telescope (EHT) -- the first-ever direct image of a shadow of the super-massive black hole in the centre of the galaxy M87. As an amalgam of these three major areas of modern observational astrophysics, THEZA aims at facilitating a breakthrough in high-resolution high image quality studies in the millimetre and sub-millimetre domain of the electromagnetic spectrum., Comment: White Paper submitted in response to the ESA Call Voyage 2050

This white paper highlights compact object and fundamental physics science opportunities afforded by high-throughput broadband (0.1-60 keV) X-ray polarization observations. X-ray polarimetry gives new observables with geometric information about stellar remnants which are many orders of magnitude too small for direct imaging. The X-ray polarimetric data also reveal details about the emission mechanisms and the structure of the magnetic fields in and around the most extreme objects in the Universe. Whereas the Imaging X-ray Polarimetry Explorer (IXPE) to be launched in 2021 will obtain first results for bright objects, a follow-up mission could be one order of magnitude more sensitive and would be able to use a broader bandpass to perform physics type experiments for representative samples of sources., Comment: White Paper submitted to the Astro2020 Astronomy and Astrophysics Decadal Survey organized by the Board of Physics and Astronomy and the Space Studies Board of the National Academies

This is a white paper submitted in response to the call from the Astro2020 Decadal Survey Committee. We outline the scientific progress that will be made in the next few decades in the study of supernova remnants in the X-ray band, using observatories like Athena, Lynx, and AXIS., Comment: Submitted as an Astro2020 Science White Paper

This paper outlines the importance of understanding jets from compact binaries for the problem of understanding the broader phenomenology of jet production. Because X-ray binaries are nearby and bright, have well-measured system parameters, and vary by factors of $\sim 10^6$ on $\sim$ year timescales, they provide a unique opportunity to understand how various aspects of the jet physics change in response to changes in the accretion flow, giving the possibility of looking for trends within individual systems and testing their universality with other systems, rather than trying to interpret large samples of objects on a statistical basis., Comment: white paper submitted to Astro2020 decadal survey

The blossoming field of joint gravitational wave and electromagnetic (GW-EM) astronomy is one of the most promising in astronomy. The first, and only, joint GW-EM event GW170817 provided remarkable science returns that still continue to this day. Continued growth in this field requires increasing the sample size of joint GW-EM detections. In this white paper, we outline the case for using some percentage of LSST survey time for dedicated target-of-opportunity follow up of GW triggers in order to efficiently and rapidly identify optical counterparts. We show that the timeline for the LSST science survey is well matched to the planned improvements to ground based GW detectors in the next decade. LSST will become particularly crucial in the later half of the 2020s as more and more distant GW sources are detected. Lastly, we highlight some of the key science goals that can be addressed by a large sample of joint GW-EM detections., Comment: 8 pages, 2 figures. Submitted to the Astro 2020 call for white papers

The Large Synoptic Survey Telescope (LSST) will open a discovery frontier for faint and fast transients with its ability to detect variable flux components down to $\sim$24.5 mag in a $\sim$30 second exposure. Spectroscopic follow-up of such phenomena - which are necessary for understanding the physics of stellar explosions - can require a rapid response and several hours with a 8-10m telescope, making it both expensive and difficult to acquire. The future Extremely Large Telescopes (ELTs) would be able to provide not only spectroscopy but capabilities such as spectropolarimetry and high-resolution diffraction-limited imaging that would contribute to future advances in our physical understanding of stellar explosions. In this white paper we focus on several specific scientific impacts in the field of explosive transient astrophysics that will be generated by the combination of LSST's discovery abilities and ELTs' follow-up capacities. First, we map the uncharted frontier of discovery phase-space in terms of intrinsic luminosity and timescales for explosive transients, where we expect the unexpected. We then focus on six areas with open science questions for known transients: the progenitors of thermonuclear supernovae (SNe), mass loss prior to core collapse, asymmetry in stellar explosions, light echoes, high-$z$ transients, and strongly lensed SNe. We conclude with a brief discussion of the practical aspects of ELT & LSST synergy., 8 pages, 4 figures, submitted to Astro2020 call for Science White Papers

Powerful winds driven by supermassive black holes (SMBHs) are likely the main mechanism through which SMBHs regulate their own growth and influence the host galaxy evolution. However, their origin and their capability to impact the large-scale environment are still highly debated. Fundamental results will come from high-energy and spatial resolution X-ray observatories., Science White Paper Submitted for the Astro2020 Decadal Survey on Astronomy and Astrophysics

Clusters of galaxies are the most massive virialized structures in the Universe. The microphysical properties of the intracluster plasma can affect dynamical processes over an enormous range: from the feedback of active galactic nuclei to shock acceleration in merging clusters. All the major cosmological simulations assume the astrophysical plasma to be inviscid. It is critical to measure microphysical properties of the intracluster plasma to truly understand the physical processes that drive the cosmic evolution. Tremendous progress has been made by comparing high spatial resolution X-ray images to (magneto-)hydrodynamic simulations. Future X-ray missions with calorimeters promise a direct measurement of transport coefficients and gas motions, providing a more realistic benchmark for cosmological simulations., Comment: Science white paper submitted to the Astro2020 US Decadal Survey. 7 pages, 3 figures

Black holes in binary star systems are vital for understanding the process of pr oducing gravitational wave sources, understanding how supernovae work, and for p roviding fossil evidence for the high mass stars from earlier in the Universe. At the present time, sample sizes of these objects, and especially of black hole s in binaries, are quite limited. Furthermore, more precise measurements of the binary parameters are needed, as well. With improvements primarily in X-ray an d radio astronomy capabilities, it should be possible to build much larger sampl es of much better measured black hole binaries., Comment: white paper submitted to Astro2020 Decadal Survey

The origin of the highest energy Galactic cosmic rays is still not understood, nor is the transition to EeV extragalactic particles. Scientific progress requires enhancements of existing air-shower arrays, such as: IceCube with its surface detector IceTop, and the low-energy extensions of both the Telescope Array and the Pierre Auger Observatory., Comment: Science White Paper for the Astronomy and Astrophysics Decadal Survey (Astro2020)

Ultra-compact binaries (UCBs) are systems containing compact or degenerate stars with orbital periods less than one hour. Tens of millions of UCBs are predicted to exist within theGalaxy emitting gravitational waves (GWs) at mHz frequencies. Combining GW searches with electromagnetic (EM) surveys like Gaia and LSST will yield a comprehensive, multimessenger catalog of UCBs in the galaxy. Joint EM and GW observations enable measurements of masses, radii, and orbital dynamics far beyond what can be achieved by independent EM or GW studies. GW+EM surveys of UCBs in the galaxy will yield a trove of unique insights into the nature of white dwarfs, the formation of compact objects, dynamical interactions in binaries, and energetic, accretion-driven phenomena like Type Ia superonovae., Comment: 5 pages, 1 figure, submitted to the Astro2020 call for science white papers

The discovery of GeV gamma-rays from classical novae has led to a reassessment of these garden-variety explosions, and highlighted their importance for understanding radiative shocks, particle acceleration, and dust formation in more exotic, distant transients. Recent collaboration between observers and theorists has revealed that shocks in novae are energetically important, and can even dominate their bolometric luminosity. Shocks may also explain long-standing mysteries in novae such as dust production, super-Eddington luminosities, and `flares' in optical light curves. Here, we highlight the multi-wavelength facilities of the next decade that will further test our nova shock model and fulfill the promise of novae as powerful astrophysical laboratories., Comment: Submitted to the Astro2020 Decadal Survey call for science white papers, 7 pages, 2 figures

Interacting binaries containing white dwarfs can lead to a variety of outcomes that range from powerful thermonuclear explosions, which are important in the chemical evolution of galaxies and as cosmological distance estimators, to strong sources of low frequency gravitational wave radiation, which makes them ideal calibrators for the gravitational low-frequency wave detector LISA mission. However, current theoretical evolution models still fail to explain the observed properties of the known populations of white dwarfs in both interacting and detached binaries. Major limitations are that the existing population models have generally been developed to explain the properties of sub-samples of these systems, occupying small volumes of the vast parameter space, and that the observed samples are severely biased. The overarching goal for the next decade is to assemble a large and homogeneous sample of white dwarf binaries that spans the entire range of evolutionary states, to obtain precise measurements of their physical properties, and to further develop the theory to satisfactorily reproduce the properties of the entire population. While ongoing and future all-sky high- and low-resolution optical spectroscopic surveys allow us to enlarge the sample of these systems, high-resolution ultraviolet spectroscopy is absolutely essential for the characterization of the white dwarfs in these binaries. The Hubble Space Telescope is currently the only facility that provides ultraviolet spectroscopy, and with its foreseeable demise, planning the next ultraviolet mission is of utmost urgency., Comment: 5 pages, 2 figures Science White Paper submitted to Astro2020 Decadal Survey

GrailQuest (Gamma Ray Astronomy International Laboratory for QUantum Exploration of Space-Time) is a mission concept based on a constellation (hundreds/thousands) of nano/micro/small-satellites in low (or near) Earth orbits. Each satellite hosts a non-collimated array of scintillator crystals coupled with Silicon Drift Detectors with broad energy band coverage (keV-MeV range) and excellent temporal resolution ( below or equal 100 nanoseconds) each with effective area around 100 cm2. This simple and robust design allows for mass-production of the satellites of the fleet. This revolutionary approach implies a huge reduction of costs, flexibility in the segmented launching strategy, and an incremental long-term plan to increase the number of detectors and their performance: a living observatory for next-generation, space-based astronomical facilities. GrailQuest is conceived as an all-sky monitor for fast localisation of high signal-to-noise ratio transients in the X/gamma-ray band, e.g. the elusive electromagnetic counterparts of gravitational wave events. Robust temporal triangulation techniques will allow unprecedented localisation capabilities, in the keV-MeV band, of a few arcseconds or below, depending on the temporal structure of the transient event. The ambitious ultimate goal of this mission is to perform the first experiment, in quantum gravity, to directly probe space-time structure down to the minuscule Planck scale, by constraining or measuring a first order dispersion relation for light in vacuo. This is obtained by detecting delays between photons of different energies in the prompt emission of Gamma-ray Bursts., Comment: White paper submitted in response to the ESA call Voyage 2050 - Submitted to Experimental Astronomy

The X-ray regime is a largely underused resource for constraining interstellar dust grain models and improving our understanding of the physical processes that dictate how grains evolve over their lifetimes. This is mostly due to current detectors' relatively low sensitivity and high background, limiting the targets to the brightest sources. The improved sensitivity of the next generation of X-ray detectors will allow studies of much fainter sources, at much higher angular resolution, expanding our sampled sightlines in both quality and quantity., Comment: Astro2020 decadal survey science white paper submitted to the National Academy of Sciences

The Payload for Ultrahigh Energy Observations (PUEO) long-duration balloon experiment is designed to have world-leading sensitivity to ultrahigh-energy neutrinos at energies above 1 EeV. Probing this energy region is essential for understanding the extreme-energy universe at all distance scales. PUEO leverages experience from and supersedes the successful Antarctic Impulsive Transient Antenna (ANITA) program, with an improved design that drastically improves sensitivity by more than an order of magnitude at energies below 30 EeV. PUEO will either make the first significant detection of or set the best limits on ultrahigh-energy neutrino fluxes., Comment: 40 pages, 17 figures. Version accepted to JINST

Coalescing, massive black-hole (MBH) binaries are the most powerful sources of gravitational waves (GWs) in the Universe, which makes MBH science a prime focus for ongoing and upcoming GW observatories. The Laser Interferometer Space Antenna (LISA) -- a gigameter scale space-based GW observatory -- will grant us access to an immense cosmological volume, revealing MBHs merging when the first cosmic structures assembled in the Dark Ages. LISA will unveil the yet unknown origin of the first quasars, and detect the teeming population of MBHs of $10^4 - 10^7$ solar masses. forming within protogalactic halos. The Pulsar Timing Array, a galactic-scale GW survey, can access the largest MBHs the Universe, detecting the cosmic GW foreground from inspiraling MBH binaries of about 10^9 solar masses. LISA can measure MBH spins and masses with precision far exceeding that from electromagnetic (EM) probes, and together, both GW observatories will provide the first full census of binary MBHs, and their orbital dynamics, across cosmic time. Detecting the loud gravitational signal of these MBH binaries will also trigger alerts for EM counterpart searches, from decades (PTAs) to hours (LISA) prior to the final merger. By witnessing both the GW and EM signals of MBH mergers, precious information will be gathered about the rich and complex environment in the aftermath of a galaxy collision. The unique GW characterization of MBHs will shed light on the deep link between MBHs of $10^4-10^{10}$ solar masses and the grand design of galaxy assembly, as well as on the complex dynamics that drive MBHs to coalescence., Comment: 5 pages, 2 figures submitted to the Astro2020 Decadal

While still in its infancy, the budding field of gravitational-wave astronomy has so far exceeded most expectations, and the achievements that have already been made bode well for the decade to come. While the discoveries made possible by LIGO have captured the imagination of experts and nonexperts alike, it is important when looking ahead to consider those discoveries in the context of the field as a whole. Just as radio, optical, and x-ray radiation probe different physical phenomena occurring on a range of length and energy scales, the future of gravitational-wave astrophysics depends on our ability to open up the entire spectrum. We will describe the scientific prospects for the field of gravitational-wave astronomy as a whole as we enter the coming decade, and we will place the specific contributions from a future space-based gravitational-wave observatory within this context., submission to the 2020-2030 Astronomy and Astrophysics Decadal Survey (Astro2020)

What are the most important conditions and processes governing the growth of stellar-origin compact objects? The identification of compact object type as either black hole (BH) or neutron star (NS) is fundamental to understanding their formation and evolution. To date, time-domain determination of compact object type remains a relatively untapped tool. Measurement of orbital periods, pulsations, and bursts will lead to a revolution in the study of the demographics of NS and BH populations, linking source phenomena to accretion and galaxy parameters (e.g., star formation, metallicity). To perform these measurements over sufficient parameter space, a combination of a wide-field (>5000 deg^2) transient X-ray monitor over a dynamic energy range (~1-100 keV) and an X-ray telescope for deep surveys with, 9 pages, 2 figures. Submitted to the Astro2020 Decadal Survey

GHz radio astronomy has played a fundamental role in the recent dazzling discovery of GW170817, a neutron star (NS)-NS merger observed in both gravitational waves (GWs) and light at all wavelengths. Here we show how the expected progress in sensitivity of ground-based GW detectors over the next decade calls for U.S.-based GHz radio arrays to be improved beyond current levels. We discuss specifically how several new scientific opportunities would emerge in multi-messenger time-domain astrophysics if a next generation GHz radio facility with sensitivity and resolution $10\times$ better than the current Jansky Very Large Array (VLA) were to work in tandem with ground-based GW detectors. These opportunities include probing the properties, structure, and size of relativistic jets and wide-angle ejecta from NS-NS mergers, as well as unraveling the physics of their progenitors via host galaxy studies., Comment: 8 pages, 5 figures, Submitted to the Astro2020 Decadal Survey

The past four years have seen a scientific revolution through the birth of a new field: gravitational-wave astronomy. The first detection of gravitational waves---recognised by the 2017 Nobel Prize in Physics---provided unprecedented tests of general relativity while unveiling a previously unknown class of massive black holes, thirty times more massive than the Sun. The subsequent detection of gravitational waves from a merging binary neutron star confirmed the hypothesised connection between binary neutron stars and short gamma-ray bursts while providing an independent measurement of the expansion of the Universe. The discovery enabled precision measurement of the speed of gravity while shedding light on the origin of heavy elements. At the time of writing, the Laser Interferometer Gravitational-wave Observatory (LIGO) and its European partner, Virgo, have published the detection of eleven gravitational-wave events. New, not-yet-published detections are announced on a nearly weekly basis. This fast-growing catalogue of gravitational-wave transients is expected to yield insights into a number of topics, from the equation of state of matter at supra-nuclear densities to the fate of massive stars. The science potential of 3G observatories is enormous, enabling measurements of gravitational waves from the edge of the Universe and precise determination of the neutron star equation of state. Australia is well-positioned to help develop the required technology. The Mid-term Review for the Decadal plan for Australian astronomy 2016-2025 should consider investment in a scoping study for an Australian Gravitational-Wave Pathfinder that develops and validates core technologies required for the global 3G detector network., Comment: 8 pages, 7 figures

The Advanced Telescope for High Energy Astrophysics (Athena) is the X-ray observatory large mission selected by the European Space Agency (ESA), within its Cosmic Vision 2015-2025 programme, to address the "Hot and Energetic Universe" scientific theme (Nandra et al. 2013), and it is provisionally due for launch in the early 2030s. The Square Kilometer Array (SKA) is the next generation radio observatory and consists of two telescopes, one comprised of dishes operating at mid frequencies (SKA1-MID) and located in South Africa, and the other comprised of Log-Periodic antennas operating at low radio frequencies (SKA1-LOW), which will be located in Australia (Braun et al. 2017). The scientific commissioning of the radio telescope is planned to begin in 2021-2022. The SKA-Athena Synergy Team (SAST) has been tasked to single out the potential scientific synergies between Athena and SKA. The astrophysical community was involved in this exercise primarily through a dedicated SKA-Athena Synergy Workshop, which took place on April 24-25, 2017 at SKAO, Jodrell Bank, Manchester. The final result of the synergy exercise, this White Paper, describes in detail a number of scientific opportunities that will be opened up by the combination of Athena and SKA, these include: 1. the Cosmic Dawn; 2. the Evolution of black holes and galaxies; 3. Active galaxy feedback in galaxy clusters; 4. Non-thermal phenomena in galaxy clusters; 5. Detecting the cosmic web; 6. Black-hole accretion physics and astrophysical transients; 7. Galactic astronomy: stars, planets, pulsars and supernovae., Comment: 79 pages, 28 figures. A high resolution version of the White Paper is available here: https://drive.google.com/open?id=1HpuGUlDGl2oWHV7j7fQPDYnkZZ15af2I

The aim of this work is to use gamma-ray burst supernovae (GRB-SNe) as cosmological probes to measure the Hubble constant, $H_0$, in the local Universe. In the context of the Expanding Photosphere Method (EPM), I use empirically derived dilution factors of a sample of nearby SNe Ic, which were derived in Paper I of a two-paper series, as a proxy for the dilution factors of GRB-SNe. It is seen that the dilution factors as a function of temperature in $VI$ display the least amount of scatter, relative to $BVI$ and $BV$. A power-law function is fit to the former, and is used to derive model dilution factors which are then used to derive EPM distances to GRB-SNe 1998bw and 2003lw: $36.7\pm9.6$ and $372.2\pm137.1$ Mpc, respectively. In linear Hubble diagrams in filters $BVR$, I determine the offset of the Hubble ridge line, and armed with the peak absolute magnitudes in these filters for the two aforementioned GRB-SNe, I find a (weighted average) Hubble constant of $\bar{H_{0,\rm w}} = 61.9\pm12.3$ km s$^{-1}$ Mpc$^{-1}$ for GRB-SNe located at redshifts $z\le0.1$. The 20\% error is consistent with the value of $H_0$ calculated by Planck and SNe Ia within 1$\sigma$. I tested the fitting method on five nearby SNe Ic, and found that their EPM distances varied by 18-50\%, with smaller errors found for those SNe which had more numerous usable observations. For SN 2002ap, its EPM distance was overestimated by 18\%, and if the distance to SN 1998bw was similarly over-estimated by the same amount, the resultant value of the Hubble constant is $H_0 = 72$ km s$^{-1}$ Mpc$^{-1}$, which perfectly matches that obtained using SNe Ia. [abridged], Comment: 11 pages, submitted to MNRAS. Comments welcomed

In this work, the empirically derived dilution/correct factors of a sample stripped-envelope, core-collapse supernovae (SE-SNe), including five SNe IIb, four SNe Ib, six SNe Ic and two relativistic broad-lined type Ic supernovae (SNe IcBL) are presented. The ultimate goal of this project is to derive model-free distances to the host galaxy of one or more gamma-ray burst supernova (GRB-SN), and to exploit their observed luminosity$-$decline relationship by employing them as cosmological probes. In the first part of a two-paper analysis, I present my method for deriving the dilution factors of the SE-SN sample, which were chosen on the basis that cosmological-model-independent distances exist to their host galaxies, and each has a sufficient dataset that allows for host-subtracted, dereddened rest-frame $BVI$ LCs to be constructed, and time-series spectra. A Planck function was fit to the data to derive the blackbody radius and blackbody temperature as a function of time, while the blueshifted velocity of either Si II $��$6355 or Fe II $��$5169 was used a proxy of the photospheric velocity, and hence photospheric radius. The ratio of these empirically derived radii was taken as the dilution/correct factor. I then compared the empirically derived dilution factors with synthetic values obtained from radiative transfer models calculated for SE-SNe arising from binary systems. It is seen that the empirical dilution factors of the SNe Ic and GRB-SNe, the latter which were derived based on luminosity distances calculated from their spectroscopic redshift, are very similar. It is found that the dilution factors of the two relativistic SN IcBL are very different to those of the GRB-SNe, meaning that these engine-driven events may arise from fundamentally different progenitor systems., 28 pages, submitted to MNRAS. Comments welcomed

Telescope slew and settle time markedly reduces the efficiency of wide-field multi-epoch surveys for sensitive interferometers with small fields of view. The overheads can be mitigated through the use of On-the-Fly Mosaicing (OTFM), where the the antennas are driven at a non-sidereal rate and visibilities are recorded continuously. Here we introduce the OTFM technique for the VLA, and describe its implementation for the Caltech-NRAO Stripe 82 Survey (CNSS), a dedicated 5-epoch survey for slow transients at S band (2-4 GHz). We also describe the OTFSim tool for planning dynamically-scheduled OTFM observations on the VLA, the latest imaging capabilities for OTFM in CASA, and present a comparison of OTFM observations with pointed observations. Using the subset of our observations from the CNSS pilot and final surveys, we demonstrate that the wide-band and wide-field OTFM observations with the VLA can be imaged accurately, and that this technique offers a more efficient alternative to standard mosaicing for multi-epoch shallow surveys such as the CNSS and the VLA Sky Survey (VLASS). We envisage that the new OTFM mode will facilitate new synoptic surveys and high-frequency mapping experiments on the VLA., 15 pages, 14 figures, Accepted for publication in ApJ

EUSO-SPB2 is a second generation Extreme Universe Space Observatory (EUSO) on a Super-Pressure Balloon (SPB). This document describes the physics capabilities, the proposed technical design of the instruments, and the simulation and analysis software., Comment: References added