Electromagnetic Observations Research Articles

GALAXY SURVEY ON THE FLY: PROSPECTS OF RAPID GALAXY CATALOGING TO AID THE ELECTROMAGNETIC FOLLOW-UP OF GRAVITATIONAL WAVE OBSERVATIONS

Galaxy catalogs are essential for efficient searches of the electromagnetic counterparts of extragalactic gravitational-wave (GW) signals with highly uncertain localization. We show that one can efficiently catalog galaxies within a short period of time with 1-2 meter-class telescopes such as the Palomar Transient Factory (PTF) or MDM, in response to an observed GW signal from a compact binary coalescence. We find that a rapid galaxy survey is feasible on the relevant time scale of $\lesssim 1$ week, maximum source distance of $>200$ Mpc and sky area of 100 deg$^2$. With PTF-like telescopes, even 1 day is sufficient for such a survey. This catalog can then be provided to other telescopes to aid electromagnetic follow-up observations to find kilonovae from binary coalescences, as well as other sources. We consider H$\alpha$ observations, which track the star formation rate and are therefore correlated with the rate of compact binary mergers. H$\alpha$ surveys are also able to filter out galaxies that are farther away than the maximum GW source distance. Rapid galaxy surveys that follow GW triggers could achieve $\sim90\%$ completeness with respect to star formation rate, which is currently unavailable. This will significantly reduce the required effort and enhance the immediate availability of catalogs compared to possible future all-sky surveys.

Aberration corrected 1.2-MV cold field-emission transmission electron microscope with a sub-50-pm resolution

Atomic-resolution electromagnetic field observation is critical to the development of advanced materials and to the unveiling of their fundamental physics. For this purpose, a spherical-aberration corrected 1.2-MV cold field-emission transmission electron microscope has been developed. The microscope has the following superior properties: stabilized accelerating voltage, minimized electrical and mechanical fluctuation, and coherent electron emission. These properties have enabled to obtain 43-pm information transfer. On the bases of these performances, a 43-pm resolution has been obtained by correcting lens aberrations up to the third order. Observations of GaN [411] thin crystal showed a projected atomic locations with a separation of 44 pm.

Electromagnetic Observations at Taal Volcano

The electromagnetic (EM) method first applied to monitor the activity of the Taal Volcano from 2005 consists of repeatedly surveying total magnetic intensity (TMF) and continuously measuring EM fields. The EM observation system was greatly improved through the SATREPS project summarized in this paper. The resistivity structure is investigated in the project using the magnetotelluric (MT) method. The MT method revealed a large hydrothermal reservoir just beneath the center of Volcano Island. The existence of this reservoir explains several things – the special features of the devastating 1911 eruption, i.e., that it was magmatic-hydrothermal, together with the geochemical characteristics of Main Crater Lake, especially the absence of SO2gas. The reservoir’s formation process provides useful suggestions for understanding the volcano’s past eruption cycle.

R-mode frequencies of slowly rotating relativistic neutron stars with realistic equations of state

The frequencies of r-mode oscillations of rotating neutron stars can be useful for guiding and interpreting gravitational wave and electromagnetic observations. The frequencies of slowly rotating, barotropic, and non-magnetic Newtonian stars are well known, but subject to various corrections. After making simple estimates of the relative strengths of these corrections we conclude that relativistic corrections are likely to be the most important. For this reason we extend the formalism of K. H. Lockitch, J. L. Friedman, and N. Andersson [Phys. Rev. D 68, 124010 (2003)], who consider relativistic polytropes, to the case of realistic equations of state. This formulation results in perturbation equations which are solved using a spectral method. We find that for realistic equations of state the r-mode frequency ranges from 1.39 to 1.57 times the spin frequency of the star when the relativistic compactness parameter (M/R) is varied over the astrophysically motivated interval 0.11 to 0.31. The results presented here are relevant to the design of gravitational wave and electromagnetic r-mode searches, and following a successful r-mode detection could help constrain the high density equation of state.

Hunting Gravitational Waves with Multi-Messenger Counterparts: Australia’s Role

AbstractThe first observations by a worldwide network of advanced interferometric gravitational wave detectors offer a unique opportunity for the astronomical community. At design sensitivity, these facilities will be able to detect coalescing binary neutron stars to distances approaching 400 Mpc, and neutron star–black hole systems to 1 Gpc. Both of these sources are associated with gamma-ray bursts which are known to emit across the entire electromagnetic spectrum. Gravitational wave detections provide the opportunity for ‘multi-messenger’ observations, combining gravitational wave with electromagnetic, cosmic ray, or neutrino observations. This review provides an overview of how Australian astronomical facilities and collaborations with the gravitational wave community can contribute to this new era of discovery, via contemporaneous follow-up observations from the radio to the optical and high energy. We discuss some of the frontier discoveries that will be made possible when this new window to the Universe is opened.

Improved methods for detecting gravitational waves associated with short gamma-ray bursts

In the era of second generation ground-based gravitational wave detectors, short gamma-ray bursts (GRBs) will be among the most promising astrophysical events for joint electromagnetic and gravitational wave observation. A targeted search for gravitational wave compact binary merger signals in coincidence with short GRBs was developed and used to analyze data from the first generation LIGO and Virgo instruments. In this paper, we present improvements to this search that enhance our ability to detect gravitational wave counterparts to short GRBs. Specifically, we introduce an improved method for estimating the gravitational wave background to obtain the event significance required to make detections; implement a method of tiling extended sky regions, as required when searching for signals associated to poorly localized GRBs from Fermi Gamma-ray Burst Monitor or the InterPlanetary Network; and incorporate astrophysical knowledge about the beaming of GRB emission to restrict the search parameter space. We describe the implementation of these enhancements and demonstrate how they improve the ability to observe binary merger gravitational wave signals associated with short GRBs.

Mapping thunder sources by inverting acoustic and electromagnetic observations

AbstractWe present a new method of locating current flow in lightning strikes by inversion of thunder recordings constrained by Lightning Mapping Array observations. First, radio frequency (RF) pulses are connected to reconstruct conductive channels created by leaders. Then, acoustic signals that would be produced by current flow through each channel are forward modeled. The recorded thunder is considered to consist of a weighted superposition of these acoustic signals. We calculate the posterior distribution of acoustic source energy for each channel with a Markov Chain Monte Carlo inversion that fits power envelopes of modeled and recorded thunder; these results show which parts of the flash carry current and produce thunder. We examine the effects of RF pulse location imprecision and atmospheric winds on quality of results and apply this method to several lightning flashes over the Magdalena Mountains in New Mexico, USA. This method will enable more detailed study of lightning phenomena by allowing researchers to map current flow in addition to leader propagation.

Advances in alternating electromagnetic field data processing for earthquake monitoring in China

The alternating electromagnetic (EM) field is one of the most sensitive physical fields related to earthquakes. There have been a number of publications reporting EM anomalies associated with earthquakes. With increasing applications and research of artificial-source extremely low frequency EM and satellite EM technologies in earthquake studies, the amount of observed data from the alternating EM method increases rapidly and exponentially, so it is imperative to develop suitable and effective methods for processing and analyzing the influx of big data. This paper presents research on the self-adaptive filter and wavelet techniques and their applications to analyzing EM data obtained from ground measurements and satellite observations, respectively. Analysis results show that the self-adaptive filter method can identify both natural- and artificial-source EM signals, and enhance the ratio between signal and noise of EM field spectra, apparent resistivity, and others. The wavelet analysis is capable of detecting possible correlation between EM anomalies and seismic events. These techniques are effective in processing and analyzing massive data obtained from EM observations.

Improved upper limits on the stochastic gravitational-wave background from 2009-2010 LIGO and Virgo data.

Gravitational waves from a variety of sources are predicted to superpose to create a stochastic background. This background is expected to contain unique information from throughout the history of the Universe that is unavailable through standard electromagnetic observations, making its study of fundamental importance to understanding the evolution of the Universe. We carry out a search for the stochastic background with the latest data from the LIGO and Virgo detectors. Consistent with predictions from most stochastic gravitational-wave background models, the data display no evidence of a stochastic gravitational-wave signal. Assuming a gravitational-wave spectrum of Ω_{GW}(f)=Ω_{α}(f/f_{ref})^{α}, we place 95% confidence level upper limits on the energy density of the background in each of four frequency bands spanning 41.5-1726Hz. In the frequency band of 41.5-169.25Hz for a spectral index of α=0, we constrain the energy density of the stochastic background to be Ω_{GW}(f)<5.6×10^{-6}. For the 600-1000Hz band, Ω_{GW}(f)<0.14(f/900 Hz)^{3}, a factor of 2.5 lower than the best previously reported upper limits. We find Ω_{GW}(f)<1.8×10^{-4} using a spectral index of zero for 170-600Hz and Ω_{GW}(f)<1.0(f/1300 Hz)^{3} for 1000-1726Hz, bands in which no previous direct limits have been placed. The limits in these four bands are the lowest direct measurements to date on the stochastic background. We discuss the implications of these results in light of the recent claim by the BICEP2 experiment of the possible evidence for inflationary gravitational waves.

B1-32 熱・電磁気学的調査に基づく薩摩硫黄島火山のマグマ : 熱水系(ラハール・水蒸気爆発・熱水系,口頭発表)

B1-32 熱・電磁気学的調査に基づく薩摩硫黄島火山のマグマ : 熱水系(ラハール・水蒸気爆発・熱水系,口頭発表)

Release timescales of solar energetic particles in the low corona

Aims. We present a systematic study of the timing and duration of the release processes of near-relativistic (NR; >50 keV) electrons in the low corona.Methods. We analyze seven well-observed events using in situ measurements by both the ACE and Wind spacecraft and context electromagnetic observations in soft X-rays, radio, hard X-rays and white light. We make use of velocity dispersion analysis to estimate the release time of the first arriving electrons and compare with the results obtained by using a simulation-based approach, taking interplanetary transport effects into account to unfold the NR electron release time history from in situ measurements.Results. The NR electrons observed in interplanetary space appear to be released during either short ( 30 min) or long (> 2 h) periods. The observation of NR electron events showing beamed pitch-angle distributions (PADs) during several hours is the clearest observational signature of sustained release in the corona. On the other hand, the in situ observation of PADs isotropizing in less than a couple of hours is a clear signature of a prompt release of electrons in the low corona. Short release episodes appear to originate in solar flares, in coincidence with the timing of the observed type III radio bursts. Magnetic connectivity plays an important role. Only type III radio bursts reaching the local plasma line measured at 1 AU are found to be related with an associated release episode in the low corona. Other type III bursts may also have a release of NR electrons associated with them, but these electrons do not reach L1. Long release episodes appear associated with signatures of long acceleration processes in the low corona (long decay of the soft X-ray emission, type IV radio bursts, and time-extended microwave emission). Type II radio bursts are reported for most of the events and do not provide a clear discrimination between short and long release timescales.

Light rings as observational evidence for event horizons: Long-lived modes, ergoregions and nonlinear instabilities of ultracompact objects

Ultracompact objects are self-gravitating systems with a light ring. It was recently suggested that fluctuations in the background of these objects are extremely long-lived and might turn unstable at the nonlinear level, if the object is not endowed with a horizon. If correct, this result has important consequences: objects with a light ring are black holes. In other words, the nonlinear instability of ultracompact stars would provide a strong argument in favor of the "black hole hypothesis," once electromagnetic or gravitational-wave observations confirm the existence of light rings. Here we explore in some depth the mode structure of ultracompact stars, in particular constant-density stars and gravastars. We show that the existence of very long-lived modes -- localized near a second, stable null geodesic -- is a generic feature of gravitational perturbations of such configurations. Already at the linear level, such modes become unstable if the object rotates sufficiently fast to develop an ergoregion. Finally, we conjecture that the long-lived modes become unstable under fragmentation via a Dyson-Chandrasekhar-Fermi mechanism at the nonlinear level. Depending on the structure of the star, it is also possible that nonlinearities lead to the formation of small black holes close to the stable light ring. Our results suggest that the mere observation of a light ring is a strong evidence for the existence of black holes.

CONSTRAINING PARAMETERS OF WHITE-DWARF BINARIES USING GRAVITATIONAL-WAVE AND ELECTROMAGNETIC OBSERVATIONS

The space-based gravitational wave (GW) detector, \emph{evolved Laser Interferometer Space Antenna} (eLISA) is expected to observe millions of compact Galactic binaries that populate our Milky Way. GW measurements obtained from the eLISA detector are in many cases complimentary to possible electro-magnetic (EM) data. In our previous papers, we have shown that the EM data can significantly enhance our knowledge of the astrophysically relevant GW parameters of the Galactic binaries, such as the amplitude and inclination. This is possible due to the presence of some strong correlations between GW parameters that are measurable by both EM and GW observations, for example the inclination and sky position. In this paper, we quantify the constraints in the physical parameters of the white-dwarf binaries, i.e. the individual masses, chirp mass and the distance to the source that can be obtained by combining the full set of EM measurements such as the inclination, radial velocities, distances and/or individual masses with the GW measurements. We find the following $2-\sigma$ fractional uncertainties in the parameters of interest. The EM observations of distance constrains the the chirp mass to $\sim 15-25 %$, whereas EM data of a single-lined spectroscopic binary constrains the secondary mass and the distance with factors of 2 to $\sim 40 %$. The single-line spectroscopic data complemented with distance constrains the secondary mass to $\sim 25-30%$. Finally EM data on double-lined spectroscopic binary constrains the distance to $\sim 30%$. All of these constraints depend on the inclination and the signal strength of the binary systems. We also find that the EM information on distance and/or the radial velocity are the most useful in improving the estimate of the secondary mass,inclination and/or distance.

ON DISCOVERING ELECTROMAGNETIC EMISSION FROM NEUTRON STAR MERGERS: THE EARLY YEARS OF TWO GRAVITATIONAL WAVE DETECTORS

We present the first simulation addressing the prospects of finding an electromagnetic (EM) counterpart to gravitational wave (GW) detections during the early years of only two advanced detectors. The perils of such a search may have appeared insurmountable when considering the coarse ring-shaped GW localizations spanning thousands of square degrees using time-of-arrival information alone. Leveraging the amplitude and phase information of the predicted GW signal narrows the localization to arcs with a median area of only a few hundred square degrees, thereby making an EM search tractable. Based on the locations and orientations of the two LIGO detectors, we find that the GW sensitivity is limited to only two of the four sky quadrants. Thus, the rates of GW events with two interferometers is only ≈40% of the rate with three interferometers of similar sensitivity. Another important implication of the sky quadrant bias is that EM observatories in North America and Southern Africa would be able to systematically respond to GW triggers several hours sooner than Russia and Chile. Given the larger sky areas and the relative proximity of detected mergers, 1 m class telescopes with very wide-field cameras are well-positioned for the challenge of finding an EM counterpart. Identification of the EM counterpart amidst the larger numbers of false positives further underscores the importance of building a comprehensive catalog of foreground stellar sources, background active galactic nucleus and potential host galaxies in the local universe. This initial study is based on a small sample of 17 detected mergers; future works will expand this sample.

Low-latency data analysis for the spherical detector Mario Schenberg

The confrontation of gravitational waves (GWs) with their electromagnetic (EM) counterparts will be rich with information about astrophysical events. Initially, this confrontation will corroborate GW detections; afterwards, when GW detections become more recurrent, it will allow astrophysics to combine information from different channels (GW, EM and also neutrinos). A low-latency data analysis which provides the direction of an incoming GW candidate is required to confront it with fast follow-up EM observations. Until now, no low-latency data analysis has been developed for spherical detectors. One spherical detector alone is capable of determining both the GW direction and polarization. By using this capability, we have developed a low-latency data analysis pipeline for the Mario Schenberg detector. This pipeline is able to retrieve the direction of GW triggers with an average angular resolution from δs ∼ 8° at SNR ∼ 12 to δs ∼ 1° at SNR ∼ 80, in a timespan of a 4 s for 32 s of data being analyzed. We apply a veto which reduces fake events up to 90% when maintaining the GW efficiency above 90% for high SNRs. We provide here a description of the method and its efficiency: resolution on the direction, false alarm rate and computational time.

Implementation and evaluation of prognostic representations of the optical diameter of snow in the SURFEX/ISBA-Crocus detailed snowpack model

Abstract. In the SURFEX/ISBA-Crocus multi-layer snowpack model, the snow microstructure has up to now been characterised by the grain size and by semi-empirical shape variables which cannot be measured easily in the field or linked to other relevant snow properties. In this work we introduce a new formulation of snow metamorphism directly based on equations describing the rate of change of the optical diameter (dopt). This variable is considered here to be equal to the equivalent sphere optical diameter, which is inversely proportional to the specific surface area (SSA). dopt thus represents quantitatively some of the geometric characteristics of a porous medium. Different prognostic rate equations of dopt, including a re-formulation of the original Crocus scheme and the parameterisations from Taillandier et al. (2007) and Flanner and Zender (2006), were evaluated by comparing their predictions to field measurements carried out at Summit Camp (Greenland) in May and June 2011 and at Col de Porte (French Alps) during the 2009/10 and 2011/12 winter seasons. We focused especially on results in terms of SSA. In addition, we tested the impact of the different formulations on the simulated density profile, the total snow height, the snow water equivalent (SWE) and the surface albedo. Results indicate that all formulations perform well, with median values of the RMSD between measured and simulated SSA lower than 10 m2 kg−1. Incorporating the optical diameter as a fully fledged prognostic variable is an important step forward in the quantitative description of the snow microstructure within snowpack models, because it opens the way to data assimilation of various electromagnetic observations.

UTILITY OF GALAXY CATALOGS FOR FOLLOWING UP GRAVITATIONAL WAVES FROM BINARY NEUTRON STAR MERGERS WITH WIDE-FIELD TELESCOPES

The first detections of gravitational waves from binary neutron star mergers with advanced LIGO and Virgo observatories are anticipated in the next five years. These detections could pave the way for multi-messenger gravitational-wave (GW) and electromagnetic (EM) astronomy if GW triggers are successfully followed up with targeted EM observations. However, GW sky localization is relatively poor, with expected localization areas of ~10--100 square degrees; this presents a challenge for following up GW signals from compact binary mergers. Prioritizing wide-field follow-ups based on the probability of successful imaging is important when it is impossible to tile the entire gravitational-wave localization region. Galaxy catalogs were effective at narrowing down regions of the sky to search in initial attempts at joint GW/EM observations. The next generation of GW detectors will have a ten-fold increase in range thereby increasing the expected number of galaxies per unit solid angle a thousand-fold. As an additional complication, catalogs may be highly incomplete. We show how to quantify the advantages of using galaxy catalogs as a function of only two parameters: the three-dimensional volume within the field of view of a telescope after accounting for the GW distance measurement uncertainty, and the fraction of the GW sky localization uncertainty region that can be followed up. We find that the use of galaxy catalogs can improve success rates by ~10% to a factor of 4 relative to follow-up strategies that do not utilize such catalogs for the scenarios we considered. We determine that catalogs with a 75% completeness perform comparably to complete catalogs in most cases, while 33%-complete catalogs can lead to lower follow-up success rates than complete catalogs for small fields of view, though still providing an advantage over strategies that do not use a catalog at all.

New Method of Active Electromagnetic Induction and Seismic Monitoring in Oil Saturated Media

It is provided a comparison of no equilibrium effects by independent hydro dynamical and electromagnetic induction influence on an oil layer and the medium, which it surrounds. It is known, that by drainage and steeps the hysteresis effect on curves of the relative phase permeability in dependence from porous medium water saturation by some cycles of influence: drainage-steep-drainage is observed. In earlier papers the analysis of the seism acoustic monitoring data in regimes of phone radiation, response on the first influence of given frequency and on the second influence is developed. For the analysis of seism acoustic response in time on fixed intervals along the borehole an algorithm of phase diagrams of the state of many phase medium is suggested. On the base of developed algorithm a new algorithm of analyze of space, but integral in time for equal observation periods changing by the method of phase diagram state of many phase medium in the oil layer is developed. The developed method allows on quality level to classify the state of the polyphase medium, which is the oil layer, using data of many cycles influence. In that paper we suggest the algorithm of modeling of 2-d seismic field distribution in the heterogeneous medium with hierarchic inclusions. Using the developed earlier 3-d method of induction electromagnetic frequency geometric monitoring we showed the opportunity of defining of physical and structural features of hierarchic oil layer structure and estimating of water saturating by crack inclusions. That allows managing the process of drainage and steeping by water displacement the oil out of the layer. Thus, the developed methods allow on the quality and quantity levels to make a classification of the many phase medium, which is an oil layer, using data for multiple excitation. For quantitative solution of earlier listed events of no equilibrium and hysteretic interaction of water and oil by out working of the oil layer, it is urgently to add and to further develop the system of seism acoustic and electromagnetic observations.

Challenges in the Measurement of Neutron Star Radii

The recent discovery of neutron stars near two solar masses has placed strong constraints on the properties of cold matter at a few times nuclear saturation density. Even tighter constraints would come from precise and accurate measurements of the radii of neutron stars of known masses, but current inferences are dominated by systematic errors. We summarize the current methods used to estimate neutron star radii and assess the prospects for reliable radii from future electromagnetic and gravitational wave observations.

Three‐dimensional simulation of the electromagnetic fields induced by the 2011 Tohoku tsunami

AbstractThe motion of seawater induces electromotive force of significant intensity due to Faraday's law, and the resulting electromagnetic (EM) field can be recorded by instruments installed on land or on the ocean bottom. However, few studies have successfully made a quantitative interpretation to obtain geophysical information from observations of tsunami‐induced EM signals by an exact and accurate application of Maxwell equations. We built a scheme for three‐dimensional numerical simulation to calculate EM fields due to ocean tidal flow and tested the system's accuracy by using the source currents in the ocean as expected from a Tohoku tsunami simulation. Here we show examples of a comparison of data from one land observatory in the Tohoku district, one island observatory in the Izu‐Bonin arc, and one seafloor station in the northwest Pacific Ocean. The water motion that generates source current in the sea consists of both the primary poloidal and toroidal magnetic modes. Our numerical simulation shows that the field of the primary toroidal magnetic mode can be effective for seafloor observations but only when the seafloor is highly conductive. We examined how the conductivity of the shallower part of the seabed, composed of sediments, can be constrained by the tsunami‐induced EM field observations, which is difficult in case of using ordinary seafloor magnetotelluric signals. We also defined the motional impedance just as the case of ordinary magnetotellurics and showed that both modeled and observed impedances approximately indicate the phase velocity of the long wave as predicted by a simple theory.