Sources In Binary Systems Research Articles

New framework to follow up candidates from continuous gravitational-wave searches

Searches for continuous gravitational waves from unknown neutron stars are limited in sensitivity due to their high computational cost. For this reason, developing new methods or improving existing ones can increase the probability of making a detection. In this paper we present a new framework that uses Markov chain Monte Carlo (MCMC) or nested sampling methods to follow up candidates of continuous gravitational-wave searches. This framework aims to go beyond the capabilities of (which is limited to the sampler), by allowing a flexible choice of sampling algorithm (using as a wrapper) and multidimensional correlated prior distributions. We show that MCMC and nested sampling methods can recover the maximum posterior point for much bigger parameter-space regions than previously thought (including for sources in binary systems), and we present tests that examine the capabilities of the new framework: a comparison between the , , and samplers, the usage of correlated priors, and its improved computational cost. Published by the American Physical Society 2024

Polarization Evolution of Fast Radio Burst Sources in Binary Systems

Recently, some fast radio bursts (FRBs) have been reported to exhibit complex and diverse variations in Faraday rotation measurements (RM) and polarization, suggesting that dynamically evolving magnetization environments may surround them. In this paper, we investigate the Faraday conversion (FC) effect in a binary system involving an FRB source and analyze the polarization evolution of FRBs. For a strongly magnetized high-mass companion binary, when an FRB with ∼100% linear polarization passes through the radial magnetic field of the companion star, the circular polarization (CP) component will be induced and oscillate symmetrically around the point with the degree of CP equal to zero, the rate and amplitude of the oscillation decrease as the frequency increases. The very strong plasma column density in the HMCBs can cause CP to oscillate with frequency at a very drastic rate, which may lead to depolarization. Near the superior conjunction of the binary orbit, the DM varies significantly due to the dense plasma near the companion, and the significant FC also occurs in this region. As the pulsar moves away from the superior conjunction, the CP gradually tends toward zero and then returns to its value before incidence. We also investigate the effect of the rotation of the companion star. We find that a sufficiently significant RM reversal can be produced at large magnetic inclinations and the RM variation is very diverse. Finally, we apply this model to explain some polarization observations of PSR B1744-24A and FRB 20201124A.

Evolution of Neutron Star Magnetic Fields

Neutron stars are natural physical laboratories allowing us to study a plethora of phenomena in extreme conditions. In particular, these compact objects can have very strong magnetic fields with non-trivial origin and evolution. In many respects, its magnetic field determines the appearance of a neutron star. Thus, understanding the field properties is important for the interpretation of observational data. Complementing this, observations of diverse kinds of neutron stars enable us to probe parameters of electro-dynamical processes at scales unavailable in terrestrial laboratories. In this review, we first briefly describe theoretical models of the formation and evolution of the magnetic field of neutron stars, paying special attention to field decay processes. Then, we present important observational results related to the field properties of different types of compact objects: magnetars, cooling neutron stars, radio pulsars, and sources in binary systems. After that, we discuss which observations can shed light on the obscure characteristics of neutron star magnetic fields and their behaviour. We end the review with a subjective list of open problems.

A resampling algorithm to detect continuous gravitational-wave signals from neutron stars in binary systems

A rapidly rotating neutron star (NS) with non-axisymmetric deformations is an interesting type of continuous gravitational-Wave (CW) source for the advanced LIGO–Virgo detectors. Within the sensitivity bands of these detectors, more than half of the known pulsars (i.e. 1200) in our galaxy are in binary systems. All CW signals are modulated by the doppler effect due to Earth’s motion, while for sources in binary systems there is an extra modulation due to the source orbital motion, which further decreases the detectability of the signal, if not properly taken into account. In order to correct for these modulations, one would need to know at least the orbital parameters and source sky location with very high accuracy. For unknown parameters the correction implies an extensive computational burden. In this paper we investigate—for the first time—the application and robustness of binary time-domain corrections to directed narrowband searches through the stroboscopic resampling, which has already been applied in several CW searches for isolated NSs. We also present a 90% confidence-level sensitivity estimation for a Scorpius X-1 directed narrowband search on publicly available data from the second observing advanced LIGO–Virgo run.

Characterizing the sensitivity of isolated continuous gravitational wave searches to binary orbits

Broadband all-sky searches for continuous gravitational waves (CW) from isolated neutron stars typically yield a number of significant candidates that are discarded through hierarchical semi-coherent and coherent follow-up searches. However, these searches target purely isolated sources and do not take into account possible phase modulations to the signals due to companion objects in binary orbits. If the source object is in a binary orbit, the signal-to-noise ratio (SNR) of a signal will diminish due to the mismatch between the signal waveform and the template waveform used in the search. In this paper, we investigate the sensitivity of CW searches to signals from sources in binary systems over an exhaustive range of binary orbits. We use Monte-Carlo simulations to constrain the ranges of binary orbital parameters that leave a CW signal from a source in a binary orbit visible in a search for signals from an isolated source (isolated searches), and consequently tighten the parameter space explorable with dedicated searches for signals from sources in binary systems (binary searches).

Reduced order modelling in searches for continuous gravitational waves – I. Barycentring time delays

The frequencies and phases of emission from extra-solar sources measured by Earth-bound observers are modulated by the motions of the observer with respect to the source, and through relativistic effects. These modulations depend critically on the source's sky-location. Precise knowledge of the modulations are required to coherently track the source's phase over long observations, for example, in pulsar timing, or searches for continuous gravitational waves. The modulations can be modelled as sky-location and time-dependent time delays that convert arrival times at the observer to the inertial frame of the source, which can often be the Solar system barycentre. We study the use of reduced order modelling for speeding up the calculation of this time delay for any sky-location. We find that the time delay model can be decomposed into just four basis vectors, and with these the delay for any sky-location can be reconstructed to sub-nanosecond accuracy. When compared to standard routines for time delay calculation in gravitational wave searches, using the reduced basis can lead to speed-ups of 30 times. We have also studied components of time delays for sources in binary systems. Assuming eccentricities <0.25, we can reconstruct the delays to within 100 s of nanoseconds, with best case speed-ups of a factor of 10, or factors of two when interpolating the basis for different orbital periods or time stamps. In long-duration phase-coherent searches for sources with sky-position uncertainties, or binary parameter uncertainties, these speed-ups could allow enhancements in their scopes without large additional computational burdens.

First all-sky search for continuous gravitational waves from unknown sources in binary systems

We present the first results of an all-sky search for continuous gravitational waves from unknown spinning neutron stars in binary systems using LIGO and Virgo data. Using a specially developed analysis program, the TwoSpect algorithm, the search was carried out on data from the sixth LIGO Science Run and the second and third Virgo Science Runs. The search covers a range of frequencies from 20 Hz to 520 Hz, a range of orbital periods from 2 to ~2,254 h and a frequency- and period-dependent range of frequency modulation depths from 0.277 to 100 mHz. This corresponds to a range of projected semi-major axes of the orbit from ~0.6e-3 ls to ~6,500 ls assuming the orbit of the binary is circular. While no plausible candidate gravitational wave events survive the pipeline, upper limits are set on the analyzed data. The most sensitive 95% confidence upper limit obtained on gravitational wave strain is 2.3e-24 at 217 Hz, assuming the source waves are circularly polarized. Although this search has been optimized for circular binary orbits, the upper limits obtained remain valid for orbital eccentricities as large as 0.9. In addition, upper limits are placed on continuous gravitational wave emission from the low-mass x-ray binary Scorpius X-1 between 20 Hz and 57.25 Hz.

Implementation of the frequency-modulated sideband search method for gravitational waves from low mass x-ray binaries

We describe the practical implementation of the sideband search, a search for periodic gravitational waves from neutron stars in binary systems. The orbital motion of the source in its binary system causes frequency-modulation in the combination of matched filters known as the $\mathcal{F}$-statistic. The sideband search is based on the incoherent summation of these frequency-modulated $\mathcal{F}$-statistic sidebands. It provides a new detection statistic for sources in binary systems, called the $\mathcal{C}$-statistic. The search is well suited to low-mass X-ray binaries, the brightest of which, called Sco X-1, is an ideal target candidate. For sources like Sco X-1, with well constrained orbital parameters, a slight variation on the search is possible. The extra orbital information can be used to approximately demodulate the data from the binary orbital motion in the coherent stage, before incoherently summing the now reduced number of sidebands. We investigate this approach and show that it improves the sensitivity of the standard Sco X-1 directed sideband search. Prior information on the neutron star inclination and gravitational wave polarization can also be used to improve upper limit sensitivity. We estimate the sensitivity of a Sco X-1 directed sideband search on 10 days of LIGO data and show that it can beat previous upper limits in current LIGO data, with a possibility of constraining theoretical upper limits using future advanced instruments.

Semicoherent search strategy for known continuous wave sources in binary systems

We present a method for detection of weak continuous signals from sources in binary systems via the incoherent combination of many "short" coherently-analyzed segments. The main focus of the work is on the construction of a metric on the parameter space for such signals for use in matched-filter based searches. The metric is defined using a maximum likelihood detection statistic applied to a binary orbit phase model including eccentricity. We find that this metric can be accurately approximated by its diagonal form in the regime where the segment length is << the orbital period. Hence correlations between parameters are effectively removed by the combination of many independent observation. We find that the ability to distinguish signal parameters is independent of the total semi-coherent observation span (for the semi-coherent span >> the segment length) for all but the orbital angular frequency. Increased template density for this parameter scales linearly with the observation span. We also present two example search schemes. The first uses a re parameterized phase model upon which we compute the metric on individual short coherently analyzed segments. The second assumes long >> the orbital period segment lengths from which we again compute the coherent metric and find it to be approximately diagonal. In this latter case we also show that the semi-coherent metric is equal to the coherent metric.

An all-sky search algorithm for continuous gravitational waves from spinning neutron stars in binary systems

Rapidly spinning neutron stars with non-axisymmetric mass distributions are expected to generate quasi-monochromatic continuous gravitational waves. While many searches for unknown, isolated spinning neutron stars have been carried out, there have been no previous searches for unknown sources in binary systems. Since current search methods for unknown, isolated neutron stars are already computationally limited, expanding the parameter space searched to include binary systems is a formidable challenge. We present a new hierarchical binary search method called TwoSpect, which exploits the periodic orbital modulations of the continuous waves by searching for patterns in doubly Fourier-transformed data. We will describe the TwoSpect search pipeline, including its mitigation of detector noise variations and corrections for Doppler frequency modulation caused by changing detector velocity. Tests on Gaussian noise and on a set of simulated signals will be presented.

Searching for continuous gravitational wave sources in binary systems

We consider the problem of searching for continuous gravitational wave (cw) sources orbiting a companion object. This issue is of particular interest because the Low mass x-ray binaries (LMXB's), and among them Sco X-1, the brightest x-ray source in the sky, might be marginally detectable with {approximately}2 y coherent observation time by the Earth-based laser interferometers expected to come on line by 2002 and clearly observable by the second generation of detectors. Moreover, several radio pulsars, which could be deemed to be cw sources, are found to orbit a companion star or planet, and the LIGO-VIRGO-GEO600 network plans to continuously monitor such systems. We estimate the computational costs for a search launched over the additional five parameters describing generic elliptical orbits (up to e < {approximately}0.8) using match filtering techniques. These techniques provide the optimal signal-to-noise ratio and also a very clear and transparent theoretical framework. Since matched filtering will be implemented in the final and the most computationally expensive stage of the hierarchical strategies, the theoretical framework provided here can be used to determine the computational costs. In order to disentangle the computational burden involved in the orbital motion of the cw source from the other source parameters (positionmore » in the sky and spin down) and reduce the complexity of the analysis, we assume that the source is monochromatic (there is no intrinsic change in its frequency) and its location in the sky is exactly known. The orbital elements, on the other hand, are either assumed to be completely unknown or only partly known. We provide ready-to-use analytical expressions for the number of templates required to carry out the searches in the astrophysically relevant regions of the parameter space and how the computational cost scales with the ranges of the parameters. We also determine the critical accuracy to which a particular parameter must be known, so that no search is needed for it; we provide rigorous statements, based on the geometrical formulation of data analysis, concerning the size of the parameter space so that a particular neutron star is a one-filter target. This result is formulated in a completely general form, independent of the particular kind of source, and can be applied to any class of signals whose waveform can be accurately predicted. We apply our theoretical analysis to Sco X-1 and the 44 neutron stars with binary companions which are listed in the most updated version of the radio pulsar catalog. For up to {approximately}3 h of coherent integration time, Sco X-1 will need at most a few templates; for 1 week integration time the number of templates rapidly rises to {approx_equal}5{times}10{sup 6}. This is due to the rather poor measurements available today of the projected semi-major axis and the orbital phase of the neutron star. If, however, the same search is to be carried out with only a few filters, then more refined measurements of the orbital parameters are called for{emdash}an improvement of about three orders of magnitude in the accuracy is required. Further, we show that the five NS's (radio pulsars) for which the upper limits on the signal strength are highest require no more than a few templates each and can be targeted very cheaply in terms of CPU time. Blind searches of the parameter space of orbital elements are, in general, completely un-affordable for present or near future dedicated computational resources, when the coherent integration time is of the order of the orbital period or longer. For wide binary systems, when the observation covers only a fraction of one orbit, the computational burden reduces enormously, and becomes affordable for a significant region of the parameter space.« less

Is magnetic reconnection intrinsically transient or steady‐state? The Earth's magnetopause as a laboratory

In terms of universality, few physical processes rival magnetic reconnection. It is thought to be responsible for a wide variety of astronomical, solar, and laboratory plasma phenomena. In astrophysics, for example, transient emissions from compact X ray sources in binary systems are thought by some to originate in magnetic reconnection between a neutron star's magnetosphere and the surrounding accretion disk. In solar physics, it has long been thought that magnetic reconnection is at the root of the enormous energy release found in flares, and may also play an important role in causing coronal mass ejections. At the opposite extreme of size scales, laboratory plasma experiments show that magnetic reconnection is important in the stability, and instability, of reverse field pinches, spheromaks and tokamaks.

The masses of black hole candidates in binary systems

Abstract Recent results concerning the determination of the masses of five black hole candidates are described. It is very significant that there are no pulsars among five massive (Mx > 3M>) X-ray sources in binary systems.

Masses of Neutron Stars and Black Holes in X-Ray Binaries

The determination of masses for compact stars in X-ray binaries is useful for at least four reasons. First, the observed values of the masses of neutron stars provide a unique test of the combined predictions of theories of nuclear matter and of general relativity (or of alternative theories of gravity). Many important recent theoretical investigations have been devoted to sharpening these predictions. Second, the observed masses provide information about the final stages of stellar evolution. The determination of the masses of X-ray sources enables one to learn something about the cores of stars that have passed through the final stages of stellar evolution when neutron stars and black holes are formed. Third, the values of the masses are important for answering questions about the nature of the X-ray source itself. eyg X-l is a good example of this kind of application since the mass inferred for the X-ray star exceeds the standard theoretical limits for the masses of neutron stars and of white dwarfs. Finally, the numerical values of the masses are required for understanding specific phenomena, such as the spin-up rate of the pulsar period, various other processes that depend on mass transfer, and the applicability of the Eddington limit. This review presents a coherent summary of the methods used to determine the masses of X-ray sources in binary systems and describes the presently available results. I have paid special attention to some questions that have not been emphasized in the general literature such as the importance of the assumed geometry (or equipotentials) and the nature of possible systematic distortions that may affect observational parameters.

Masses of Compact X-Ray Sources

The presence of compact x-ray sources in binary systems provides an opportunity to determine observationally the masses of neutron stars and possibly of black holes. These masses are important for equations of state at nuclear densities, for theories of gravitation, and for late stages of stellar evolution. A mass range for each source is determined by finding the sets of orbital elements that are consistent with the available x-ray and optical data. As such analyses are subject to various uncertainties, the resulting mass ranges are rather wide. Nevertheless, significant mass estimates are obtained, accompanied by interesting results on other system parameters.

X-ray sources and planetary nebulae

CURRENT models of X-ray sources in binary systems generally assume accretion on to a neutron star, a black hole or a white dwarf as the mechanism of X-ray production. The accretion takes place from either a stellar wind or the inner Lagrangian point1–5. Here we explore the possible existence of a compact X-ray source in a binary system, where the companion star ejects its envelope to become a planetary nebula.

Empirical relation between interstellar X-ray absorption and optical extinction

An empirical relation between interstellar X-ray absorption and optical extinction is derived from the correlation of measurements made on objects of large intrinsic diameter. The result is Av = 4.5 times 10 to the -22nd power N(H) mag, with the principal error being largely systematic in origin, where N(H) represents the column density of interstellar matter in the Brown and Gould model for the X-ray absorption coefficient. Applying this ratio to optically identified compact sources, it is concluded that sources in binary systems showing pronounced X-ray occultations have an intrinsic absorption equivalent to about 10 to the 22-nd power atoms per sq cm of interstellar matter and that there are a few compact sources where the absorption seems to be primarily interstellar in origin. The interstellar absorption expected in Cyg X-1 from the extinction of its optical counterpart is much greater than that suggested by X-ray spectra, which may be due to a soft X-ray component greater than that predicted by the power law fitted to higher energy data.

Are binary systems involved in supernova explosions always disrupted?

No known pulsar is a member of binary system. Does this mean, as has been suggested1, that pulsars are the result of supernova explosions (hypothesis 1), and explosions which occurred in binary systems have in most cases disrupted the systems (hypothesis 2)? The discovery of two X-ray ‘pulsars’ (Cen X-3 and Her X-1) and other compact X-ray sources in binary systems seems to be an argument against hypothesis 2 if we believe that hypothesis 1 is also valid for compact X-ray sources. Here I show that hypothesis 2 is indeed untenable from the point of view of present theories of close binary evolution.

Identification of 2U0525 – 06 with 2Orionis

BLACK holes may be observed in the X-ray band1 and the discovery of X-ray sources in binary systems with non-luminous massive companions is the most convincing evidence of existence of black holes2–5. We have compared the Trimble and Thorne lists6 of collapsed star candidates with the Uhuru catalogue of X-ray sources7. Trimble and Thorne considered only systems with unseen secondary stars of mass M2 > 1.4 M⊙ in the catalogue examined8. We have found that four of these objects fall within 3 arc deg of X-ray sources (Table 1). υ2 Orionis (Fig. 1) is the only one of the four which is inside the error box of an X-ray source (2U0525–06).