Search

We survey the prospective sensitivities of terrestrial and space-borne atom interferometers to gravitational waves generated by cosmological and astrophysical sources, and to ultralight dark matter. We discuss the backgrounds from gravitational gradient noise in terrestrial detectors, and also binary pulsar and asteroid backgrounds in space-borne detectors. We compare the sensitivities of LIGO and LISA with those of the 100m and 1 km stages of the AION terrestrial AI project, as well as two options for the proposed AEDGE AI space mission with cold atom clouds either inside or outside the spacecraft, considering as possible sources the mergers of black holes and neutron stars, supernovae, phase transitions in the early Universe, cosmic strings and quantum fluctuations in the early Universe that could have generated primordial black holes. We also review the capabilities of AION and AEDGE for detecting coherent waves of ultralight scalar dark matter. [ABSTRACT FROM AUTHOR]

The merging binary black holes (BBHs) have been detected by the ground gravitational-wave observatories, but it is very difficult to detect BBHs before their mergers. Since millisecond pulsars (MSPs) are remarkable precise 'clocks' in Universe, they could be used to detect the hidden BBHs, if they exist in systems of BBHs. Doing so, we construct a triple system which consists of a BBH and a third outer MSP. The theoretical studies have shown that the inner binary could cause additional time residuals (i.e. R |$\ddot{o}$| mer delay) on the orbit of the outer MSP, which can be used to detect inner BBHs. Using N -body simulations, in this paper, we calculate such time residuals, and analyse the periodic signals of them by Fast Fourier Transform, which allow us to explore the effects of the parameters of the inner BBHs. Specifically, we assume that the triple system is coplanar and stable; the distance from the earth is ∼1 kpc; the mass of each black hole is |$10\, {\rm M}_\odot$|⁠. We find that the amplitude of the time residual increases with the semimajor axis and eccentricity of the inner BBH. Moreover, we find that there are several prominent and characteristic periodic signals for different parameters of the inner BBHs. With the help of the analytic results, we find these frequencies are due to different orders of the eccentricities of both the inner and outer binaries. Thus, in principle, we can use these frequencies to determine the parameters of the inner BBHs. [ABSTRACT FROM AUTHOR]

A rotating black hole can be clouded by light bosons via superradiance and thus acquire an atom-like structure. If such a gravitational atom system is accompanied by a pulsar, the pulsar can trigger transitions between energy levels of the gravitational atom, and these transitions can be detected by pulsar timing. We show that in such pulsar–black hole systems, fine and hyperfine structure transitions are more likely to be probed than the Bohr transition. Also, the calculation of these fine and hyperfine structure transitions are under better analytic control. Thus, these fine and hyperfine structure transitions are more ideal probes in the search for gravitational collider signals in pulsar–black hole systems. [ABSTRACT FROM AUTHOR]

We report on gamma-ray orbital modulation of the transitioning MSP binary XSS J12270–4859 detected in the Fermi Large Area Telescope (LAT) data. We use long-term optical data taken with the XMM-Newton OM and the Swift UltraViolet Optical Telescope to inspect radio timing solutions that are limited to relatively short time intervals and find that extrapolation of the solutions aligns well with the phasing of the optical data over 15 yr. The Fermi-LAT data folded on the timing solutions exhibit significant modulation (p = 5 × 10−6) with a gamma-ray minimum at the inferior conjunction of the pulsar. Intriguingly, the source seems to show similar modulation in both the low-mass X-ray binary and the MSP states, implying that mechanisms for gamma-ray emission in the two states are similar. We discuss these findings and their implications using an intrabinary shock scenario. [ABSTRACT FROM AUTHOR]

We present new discoveries and results from long-term timing of 72 pulsars discovered in the Pulsar Arecibo L-band Feed Array (PALFA) survey, including precise determination of astrometric and spin parameters, and flux density and scatter broadening measurements at 1.4 GHz. Notable discoveries include two young pulsars (characteristic ages ∼30 kyr) with no apparent supernova remnant associations, three mode-changing, 12 nulling and two intermittent pulsars. We detected eight glitches in five pulsars. Among them is PSR J1939+2609, an apparently old pulsar (characteristic age ∼1 Gy), and PSR J1954+2529, which likely belongs to a newly emerging class of binary pulsars. The latter is the only pulsar among the 72 that is clearly not isolated: a nonrecycled neutron star with a 931 ms spin period in an eccentric (e = 0.114) wide (Pb = 82.7 days) orbit with a companion of undetermined nature having a minimum mass of ∼0.6 M⊙. Since operations at Arecibo ceased in 2020 August, we give a final tally of PALFA sky coverage, and compare its 207 pulsar discoveries to the known population. On average, they are 50% more distant than other Galactic plane radio pulsars; PALFA millisecond pulsars (MSPs) have twice the dispersion measure per unit spin period than the known population of MSP in the plane. The four intermittent pulsars discovered by PALFA more than double the population of such objects, which should help to improve our understanding of pulsar magnetosphere physics. The statistics for these, rotating radio transients, and nulling pulsars suggest that there are many more of these objects in the Galaxy than was previously thought. [ABSTRACT FROM AUTHOR]

Eccentric millisecond pulsars (eMSPs) with white dwarf companions exhibit orbital eccentricities orders of magnitude larger than predicted by turbulent convection in the white dwarfs' red giant progenitors. The orbital periods of eMSPs cluster around P = 20–30 d, remarkably close to the red giant convective eddy turnover time t eddy. We propose that the anomalously large eccentricities are resonantly driven by convective flows somehow made coherent when the turnover time matches the tidally locked red giant's spin period, which is also the tidal forcing period. Numerical simulations of rotating red giants and magnetic field studies of stars show some evidence for especially ordered flow patterns when the convective Rossby number P / t eddy is of order unity. We show that resonant convection boosts eccentricities by a factor of (t nuc/ P)1/2 ≈ 3 × 103 over the random-walk values that characterize conventional MSPs, in good agreement with observations (t nuc is the giant's nuclear burning time-scale). We also show how variations in the eddy turnover time arising from red giant metallicity variations can reproduce the observed effective width of the resonance, Δ P / P ≈ 0.4. [ABSTRACT FROM AUTHOR]

Supernova explosion and the associated neutron star (NS) natal kicks are important events on a pathway of a binary to become a gravitational wave source, an X-ray binary, or a millisecond radio pulsar. Weak natal kicks often lead to binary survival, while strong kicks frequently disrupt the binary. In this article, we aim to further constrain NS natal kicks in binaries. We explore binary population synthesis models by varying prescription for natal kick, remnant mass, and mass accretion efficiency. We introduce a robust statistical technique to analyse combined observations of different nature. Using this technique, we further test different models using parallax and proper motion measurements for young isolated radio pulsars and similar measurements for Galactic Be X-ray binaries (BeXs). Our best model for natal kicks is consistent with both measurements and contains a fraction of w = 0.2 ± 0.1 weak natal kicks with |$\sigma _1 = 45^{+25}_{-15}$|  km s−1, the remaining natal kicks are drawn from the high-velocity component, same as in previous works: σ2 = 336 km s−1. We found that currently used models for natal kicks of NSs produced by electron capture supernova (ecSN; combination of Maxwellian σ = 265 km s−1 and σ = 30 km s−1 for electron capture) are inconsistent or marginally consistent with parallaxes and proper motions measured for isolated radio pulsars. We suggest a new model for natal kicks of ecSN, which satisfy both observations of isolated radio pulsars and BeXs. [ABSTRACT FROM AUTHOR]

We explore the long-term evolution of the spin period of the high-mass X-ray binary pulsar Vela X-1 over a period of 46 yr. Our analysis indicates nearly periodic variations in the spin period of the pulsar on time-scales of about 5.9 yr. There is suggestion of an overall spin-down behaviour of the pulsar though it is noticed that the source appears to stay near its equilibrium period 283.4 s since MJD 51000, with rather erratic spin-up/spin-down episodes around this value. Our study suggests nearly cyclic turnover in the spin behaviour of the pulsar from spin-up to spin-down regimes on time-scales of about 17–19 yr. To our knowledge, this is the first report of periodic variation in the spin behaviour of a wind-fed accreting pulsar. We also observe erratic episodes of spin-up and spin-down behaviour on relatively shorter time-scales that is a well-known archetype of this wind-fed X-ray pulsar. We investigate whether nearly periodic long-term spin period changes in the pulsar can be explained by using known mechanisms of torque reversals in the accretion powered neutron stars. It appears that changes in the accretion environment of the pulsar using current ideas can probably lead to long-term spin period changes in this X-ray pulsar. [ABSTRACT FROM AUTHOR]

In so-called spider pulsars, the X-ray band is dominated by intrabinary shock (IBS) synchrotron emission. While the double-peaked X-ray light curves from these shocks have been well characterized in several spider systems (both black widows and redbacks), the polarization of this emission is yet to be studied. Motivated by the new polarization capability of the Imaging X-ray Polarization Explorer and the confirmation of highly ordered magnetic fields in pulsar wind nebulae, we model the IBS polarization by employing two potential magnetic field configurations: toroidal magnetic fields imposed by the preshock pulsar wind, and tangential shock-generated fields that follow the postshock flow. We find that if IBSs host ordered magnetic fields, the synchrotron X-rays from spider binaries can display a high degree of polarization (≳50%), while the variation in polarization angle provides a good probe of the binary geometry and the magnetic field structure. Our results encourage observational polarization studies of spider pulsars that can distinguish the proposed magnetic models and that constrain the unique properties of these systems better.

We present 294 pulsars found in GeV data from the Large Area Telescope (LAT) on the Fermi Gamma-ray Space Telescope. Another 33 millisecond pulsars (MSPs) discovered in deep radio searches of LAT sources will likely reveal pulsations once phase-connected rotation ephemerides are achieved. A further dozen optical and/or X-ray binary systems colocated with LAT sources also likely harbor gamma-ray MSPs. This catalog thus reports roughly 340 gamma-ray pulsars and candidates, 10% of all known pulsars, compared to ≤11 known before Fermi. Half of the gamma-ray pulsars are young. Of these, the half that are undetected in radio have a broader Galactic latitude distribution than the young radio-loud pulsars. The others are MSPs, with six undetected in radio. Overall, ≥236 are bright enough above 50 MeV to fit the pulse profile, the energy spectrum, or both. For the common two-peaked profiles, the gamma-ray peak closest to the magnetic pole crossing generally has a softer spectrum. The spectral energy distributions tend to narrow as the spindown power $\dot{E}$ decreases to its observed minimum near 10 ^33 erg s ^−1 , approaching the shape for synchrotron radiation from monoenergetic electrons. We calculate gamma-ray luminosities when distances are available. Our all-sky gamma-ray sensitivity map is useful for population syntheses. The electronic catalog version provides gamma-ray pulsar ephemerides, properties, and fit results to guide and be compared with modeling results.

We present the discovery and timing solutions of four millisecond pulsars (MSPs) discovered in the Arecibo 327 MHz Drift-Scan Pulsar Survey. Three of these pulsars are in binary systems, consisting of a redback (PSR J2055+1545), a black widow (PSR J1630+3550), and a neutron star–white dwarf binary (PSR J2116+1345). The fourth MSP, PSR J2212+2450, is isolated. We present the multiyear timing solutions as well as polarization properties across a range of radio frequencies for each pulsar. We perform a multiwavelength search for emission from these systems and find an optical counterpart for PSR J2055+1545 in Gaia DR3, as well as a gamma-ray counterpart for PSR J2116+1345 with the Fermi-LAT telescope. Despite the close colocation of PSR J2055+1545 with a Fermi source, we are unable to detect gamma-ray pulsations, likely due to the large orbital variability of the system. This work presents the first two binaries found by this survey with orbital periods shorter than a day; we expect to find more in the 40% of the survey data that have yet to be searched.

M71E is a spider pulsar (i.e., a millisecond pulsar with a tight binary companion) with the shortest known orbital period of P = 53.3 minutes, which was discovered by Pan et al. Their favored evolutionary model suggests that it bridges between two types of spider pulsars, namely, it descended from a “redback” and will become a “black widow.” Using Hubble Space Telescope (HST) archival imaging data, we report the first optical identification of its companion COM-M71E. The HST and pulsar timing coordinates are in excellent agreement (within ∼10 mas). If M71E is associated with the globular cluster M71, our measured brightness of COM-M71E ( m _F606W ≈ 25.3) is broadly consistent with the expectation from Pan et al.'s preferred binary evolutionary model of a stripped dwarf companion, while it is also compatible with an ultralow-mass degenerate companion. Future multiwavelength photometric and spectroscopic observations can characterize the companion and test the evolutionary scenarios.

We present timing solutions for 21 pulsars discovered in 350 MHz surveys using the Green Bank Telescope (GBT). All were discovered in the Green Bank North Celestial Cap pulsar survey, with the exception of PSR J0957−0619, which was found in the GBT 350 MHz Drift-scan pulsar survey. The majority of our timing observations were made with the GBT at 820 MHz. With a spin period of 37 ms and a 528 days orbit, PSR J0032+6946 joins a small group of five other mildly recycled wide binary pulsars, for which the duration of recycling through accretion is limited by the length of the companion’s giant phase. PSRs J0141+6303 and J1327+3423 are new disrupted recycled pulsars. We incorporate Arecibo observations from the NANOGrav pulsar timing array into our analysis of the latter. We also observed PSR J1327+3423 with the Long Wavelength Array, and our data suggest a frequency-dependent dispersion measure. PSR J0957−0619 was discovered as a rotating radio transient, but is a nulling pulsar at 820 MHz. PSR J1239+3239 is a new millisecond pulsar (MSP) in a 4 days orbit with a low-mass companion. Four of our pulsars already have published timing solutions, which we update in this work: the recycled wide binary PSR J0214+5222, the noneclipsing black widow PSR J0636+5128, the disrupted recycled pulsar J1434+7257, and the eclipsing binary MSP J1816+4510, which is in an 8.7 hr orbit with a redback-mass companion.

Pulsar distances are notoriously difficult to measure, and play an important role in many fundamental physics experiments, such as pulsar timing arrays. Here, we perform a cross-match between International PTA pulsars (IPTA) and Gaia's Data Release 2 (DR2) and Data Release 3 (DR3). We then combine the IPTA pulsar’s parallax with its binary companion’s parallax, found in Gaia, to improve the distance measurement to the binary. We find seven cross-matched IPTA pulsars in Gaia DR2, and when using Gaia DR3 we find six IPTA pulsar cross-matches but with seven Gaia objects. Moving from Gaia DR2 to Gaia DR3, we find that the Gaia parallaxes for the successfully cross-matched pulsars improved by 53%, and pulsar distances improved by 29%. Finally, we find that binary companions with a

We report the discovery of a 2.11 ms binary millisecond pulsar during a targeted search of the redback optical candidate coincident with the γ -ray source 3FGL J0212.5+5320 using the Robert C. Byrd Green Bank Telescope (GBT) with the Breakthrough Listen backend at L band. Over a seven month period, five pointings were made near inferior conjunction of the pulsar in its 20.9 hr orbit, resulting in two detections, lasting 12 and 42 minutes. The pulsar dispersion measure (DM) of 25.7 pc cm ^−3 corresponds to a distance of 1.15 kpc in the NE2001 Galactic electron density model, consistent with the Gaia parallax distance of 1.16 ± 0.03 kpc for the companion star. We suspect the pulsar experiences wide orbital eclipses, similar to other redbacks, as well as scintillation and DM delays caused by its interaction with its companion and surroundings. Although the pulsar was only detected over ≈3.7% of the orbit, its measured acceleration is consistent with published binary parameters from optical radial velocity spectroscopy and light-curve modeling of the companion star, and it provides a more precise mass ratio and a projected semimajor axis for the pulsar orbit. We also obtained a refined optical photometric orbit ephemeris, and observed variability of the tidally distorted companion over 7 yr. A hard X-ray light curve from NuSTAR shows expected orbit-modulated emission from the intrabinary shock. The pulsar parameters and photometric ephemeris greatly restrict the parameter space required to search for a coherent timing solution including pulsar spin-down rate, either using Fermi γ -rays or further radio pulse detections.

Although neutron star–black hole binaries have been identified through mergers detected in gravitational waves, a pulsar–black hole binary has yet to be detected. While short-period binaries are detectable due to a clear signal in the pulsar’s timing residuals, effects from a long-period binary could be masked by other timing effects, allowing them to go undetected. In particular, a long-period binary measured over a small subset of its orbital period could manifest via time derivatives of the spin frequency incompatible with isolated pulsar properties. We assess the possibility of pulsars having unknown companions in long-period binaries and put constraints on the range of binary properties that may remain undetected in current data, but that may be detectable with further observations. We find that for 35% of canonical pulsars with published higher-order derivatives, the precision of measurements is not enough to confidently reject binarity (period ≳2 kyr), and that a black hole binary companion could not be ruled out for a sample of pulsars without published constraints if the period is >1 kyr. While we find no convincing cases in the literature, we put more stringent limits on orbital period and longitude of periastron for the few pulsars with published higher-order frequency derivatives ( n ≥ 3). We discuss the detectability of candidates and find that a sample pulsar in a 100 yr orbit could be detectable within 5–10 yr.

We report on the discovery of the companion star to the millisecond pulsar PSR J1835−3259B in the Galactic globular cluster NGC 6652. Taking advantage of deep photometric archival observations acquired through the Hubble Space Telescope in near-UV and optical bands, we identified a bright and blue object at a position compatible with that of the radio pulsar. The companion is located along the helium-core white dwarf cooling sequence, and the comparison with binary evolution models provides a mass of 0.17 ± 0.02 M _⊙ , a surface temperature of 11,500 ± 1900 K, and a very young cooling age of only 200 ± 100 Myr. The mass and the age of the companion are compatible with a progenitor star of about 0.87 M _⊙ , which started transferring mass to the primary during its evolution along the subgiant branch and stopped during the early red giant branch phase. Combining together the pulsar mass function and the companion mass, we found that this system is observed at an almost edge-on orbit and hosts a neutron star with a mass of 1.44 ± 0.06 M _⊙ , thus suggesting a highly nonconservative mass accretion phase. The young age of the WD companion is consistent with the scenario of a powerful, relatively young MSP indicated by the earlier detection of gamma-rays from this system.

Of the more than 3000 radio pulsars currently known, only ∼300 are in binary systems, and only five of these consist of young pulsars with massive nondegenerate companions. We present the discovery and initial timing, accomplished using the Canadian Hydrogen Intensity Mapping Experiment (CHIME) telescope, of the sixth such binary pulsar, PSR J2108+4516, a 0.577 s radio pulsar in a 269 day orbit of eccentricity 0.09 with a companion of minimum mass 11 M _⊙ . Notably, the pulsar undergoes periods of substantial eclipse, disappearing from the CHIME 400–800 MHz observing band for a large fraction of its orbit, and displays significant dispersion measure and scattering variations throughout its orbit, pointing to the possibility of a circumstellar disk or very dense stellar wind associated with the companion star. Subarcsecond resolution imaging with the Karl G. Jansky Very Large Array unambiguously demonstrates that the companion is a bright, V ≃ 11 OBe star, EM* UHA 138, located at a distance of 3.26(14) kpc. Archival optical observations of EM* UHA 138 approximately suggest a companion mass ranging from 17.5 M _⊙ < M _c < 23 M _⊙ , in turn constraining the orbital inclination angle to 50.°3 ≲ i ≲ 58.°3. With further multiwavelength follow-up, PSR J2108+4516 promises to serve as another rare laboratory for the exploration of companion winds, circumstellar disks, and short-term evolution through extended-body orbital dynamics.

Results from 11 yr of radio timing for the eclipsing black widow millisecond pulsar (MSP) binary J1544+4937 is presented in this paper. We report a phase-connected timing model for this MSP, using observations with the Giant Metrewave Radio Telescope (GMRT) at multiple frequencies and with the Green Bank Telescope. This is the longest-duration timing study of any galactic-field MSP with the GMRT. While extending the timing baseline from the existing 1.5 yr to about a decade we report the first detection for a significant value of proper motion ( μ _T ∼ 10.14(5) mas yr ^−1 ) for this pulsar. Temporal variations of dispersion measure (ΔDM ∼ 10 ^−3 pc cm ^−3 ) manifested by significant determination of first-, second-, and third-order dispersion measure (DM) derivatives are observed along the line of sight to the pulsar. We also noticed frequency-dependent DM variations of the order of 10 ^−3 pc cm ^−3 , which could arise due to spatial electron density variations in the interstellar medium. This study has revealed a secular variation of the orbital period for this MSP for the first time. We investigated possible causes and propose that variation in the gravitational quadrupole moment of the companion could be responsible for the observed temporal changes in the orbital period.

Binary systems are a well-established subclass of gamma-ray sources. The high mass X-ray binary pulsar 1A 0535+262 has been considered to be a possible gamma-ray emitter for a long time, although former gamma-ray searches using the Fermi Large Area Telescope (LAT) and VERITAS data resulted in upper limits only. We aim at a deep search for gamma-ray emission and pulsations from 1A 0535+262 using more than 13 yr of Fermi-LAT data. The analysis was performed for both the whole Fermi-LAT data set, as well as for the X-ray outbursts that 1A 0535+262 has experienced since the launch of Fermi. Various X‐ray observations have been used to generate the ephemeris for the pulsation search. We also investigate the long-term gamma-ray flux variability and perform orbital phase-resolved analysis for the outbursts. We did not detect any steady or pulsed gamma-ray emission from 1A 0535+262 during the whole Fermi-LAT mission span or its X-ray outbursts. We thus derived the deepest gamma-ray luminosity upper limits to date at the 95% confidence level to be around (2.3–4.7) × 10 ^32 erg s ^−1 depending on different spectral indices assumed, which results in a ratio of L _γ to L _X (2–150 keV) being (1.9–3.9) × 10 ^−6 .

We present timing solutions for 12 pulsars discovered in the Green Bank North Celestial Cap 350 MHz pulsar survey, including six millisecond pulsars (MSPs), a double neutron star (DNS) system, and a pulsar orbiting a massive white dwarf companion. Timing solutions presented here include 350 and 820 MHz Green Bank Telescope data from initial confirmation and follow-up, as well as a dedicated timing campaign spanning 1 ryr PSR J1122−3546 is an isolated MSP, PSRs J1221−0633 and J1317−0157 are MSPs in black widow systems and regularly exhibit eclipses, and PSRs J2022+2534 and J2039−3616 are MSPs that can be timed with high precision and have been included in pulsar timing array experiments seeking to detect low-frequency gravitational waves. PSRs J1221−0633 and J2039−3616 have Fermi Large Area Telescope gamma-ray counterparts and also exhibit significant gamma-ray pulsations. We measure proper motions for three of the MSPs in this sample and estimate their space velocities, which are typical compared to those of other MSPs. We have detected the advance of periastron for PSR J1018−1523 and therefore measure the total mass of the DNS system, m _tot = 2.3 ± 0.3 M _⊙ . Long-term pulsar timing with data spanning more than 1 yr is critical for classifying recycled pulsars, carrying out detailed astrometry studies, and shedding light on the wealth of information in these systems post-discovery.

We have performed a systematic study of the rotational, orbital, and X-ray properties of millisecond pulsars (MSPs) in globular clusters (GCs) and compared their nature with those of the MSPs in the Galactic field (GF). We found that GC MSPs generally rotate slower than their counterparts in the GF. Different from the expectation of a simple recycling scenario, no evidence for the correlation between the orbital period and the rotation period can be found in the MSP binaries in GCs. There is also an indication that the surface magnetic field of GC MSPs is stronger than those in the GF. All these suggest dynamical interactions in GCs can alter the evolution of MSPs/their progenitors, which can leave an imprint on their X-ray emission properties. While the MSPs in both GF and GCs have similar distributions of X-ray luminosity and hardness, our sample supports the notion that these two populations follow different relations between the X-ray luminosity and spin-down power. We discuss this in terms of both the pulsar emission model and the observational bias.

The magnetic inclination angle χ , namely the angle between the spin and magnetic axes of a neutron star, plays a vital role in its observational characteristics. However, there are few systematic investigations of its long-term evolution, especially for accreting NSs in binary systems. Applying the model of Biryukov & Abolmasov and the binary evolution code MESA, we simultaneously simulate the evolution of the accretion rate, spin period, magnetic field, and magnetic inclination angle of accreting NSs in intermediate/low X-ray binaries. We show that the evolution of χ depends not only on the initial parameters of the binary systems, but also on the mass transfer history and the efficiency of pulsar loss. Based on the calculated results we present the characteristic distribution of χ for various types of systems including ultracompact X-ray binaries, binary millisecond pulsars, and ultraluminous X-ray sources, and discuss their possible observational implications.

The article explores the discovery of a mysterious compact astronomical object, situated at the lower edge of the "mass gap" between neutron stars and black holes. It mentions that identified as a companion to the millisecond pulsar PSR J0514−4002E, the object's origin and classification as either the most massive neutron star or the least massive black hole pose intriguing astrophysical questions.

The high stellar density in the central parsecs around the Galactic Centre makes it a seemingly favourable environment for finding relativistic binary pulsars. These include pulsars orbiting other neutron stars, stellar-mass black holes, or the central supermassive black hole, Sagittarius A*. Here, we present multi-epoch pulsar searches of the Galactic Centre at four observing frequencies, |$4.85,\, 8.35,\, 14.6,\, 18.95\, {\rm GHz}$|⁠ , using the Effelsberg 100-m radio telescope. Observations were conducted 1 yr prior to the discovery of, and during monitoring observations of, the Galactic Centre magnetar PSR J1745−2900. Our data analysis features acceleration searches on progressively shorter time series to maintain sensitivity to relativistic binary pulsars. The multi-epoch observations increase the likelihood of discovering transient or nulling pulsars, or ensure orbital phases are observed at which acceleration search methods work optimally. In |${\sim}147\, {\rm h}$| of separate observations, no previously undiscovered pulsars have been detected. Through calibration observations, we conclude this might be due to insufficient instantaneous sensitivity, caused by the intense continuum emission from the Galactic Centre, its large distance, and, at higher frequencies, the aggregate effect of steep pulsar spectral indices and atmospheric contributions to the system temperature. Additionally, we find that for millisecond pulsars in wide circular orbits |$({\lesssim}800\, {\rm d})$| around Sagittarius A*, linear acceleration effects cannot be fully corrected in deep observations |$(9\, {\rm h})$| with existing software tools. Pulsar searches of the Galactic Centre with the next generation of radio telescopes – such as MeerKat, ngVLA, and SKA1-mid – will have improved chances of uncovering this elusive population. [ABSTRACT FROM AUTHOR]

In this short overview, we summarize the current knowledge of the pulsar spectra that exhibit turnovers between 0.5 and 1.5 GHz. We focus on the observations of pulsars, for which the analysis of their spectra allowed a formulation of the hypothesis that external mechanisms are responsible for the high‐frequency turnovers. Among these, we discuss the cases of the binary pulsar B1259‐63, the Sagittarius A* radiomagnetar (SGR J1745‐2900), the variable spectrum of pulsar (PSR) B1800‐21, and PSR J1740 + 1000. [ABSTRACT FROM AUTHOR]

The main goal of pulsar timing array experiments is to detect correlated signals such as nanohertz-frequency gravitational waves. Pulsar timing data collected in dense monitoring campaigns can also be used to study the stars themselves, their binary companions, and the intervening ionized interstellar medium. Timing observations are extraordinarily sensitive to changes in path-length between the pulsar and the Earth, enabling precise measurements of the pulsar positions, distances and velocities, and the shapes of their orbits. Here we present a timing analysis of 25 pulsars observed as part of the Parkes Pulsar Timing Array (PPTA) project over time spans of up to 24 yr. The data are from the second data release of the PPTA, which we have extended by including legacy data. We make the first detection of Shapiro delay in four Southern pulsars (PSRs J1017−7156, J1125−6014, J1545−4550, and J1732−5049), and of parallax in six pulsars. The prominent Shapiro delay of PSR J1125−6014 implies a neutron star mass of Mp = 1.5 ± 0.2 M⊙ (68 per cent credibility interval). Measurements of both Shapiro delay and relativistic periastron advance in PSR J1600−3053 yield a large but uncertain pulsar mass of |$M_p = 2.06^{+0.44}_{-0.41}$|  M⊙ (68 per cent credibility interval). We measure the distance to PSR J1909−3744 to a precision of 10 lyr, indicating that for gravitational wave periods over a decade, the pulsar provides a coherent baseline for pulsar timing array experiments. [ABSTRACT FROM AUTHOR]

In the double pulsar, the Lense–Thirring periastron precession |$\dot{\omega }^\mathrm{LT}$| could be used to measure/constrain the moment of inertia |$\mathcal {I}_\mathrm{A}$| of A. Conversely, if |$\mathcal {I}_\mathrm{A}$| will be independently determined with sufficient accuracy by other means, tests of the Lense–Thirring effect could be performed. Such findings rely upon a formula for |$\dot{\omega }^\mathrm{LT,\, A}$| induced by the spin angular momentum |${\boldsymbol{S}}^\mathrm{A}$| of A, valid if the orbital angular momentum |$\boldsymbol{L}$| and |${\boldsymbol{S}}^\mathrm{A}$| are aligned, and neglecting |$\dot{\omega }^\mathrm{LT,\, B}$| because of the smallness of |${\boldsymbol{S}}^\mathrm{B}$|⁠. The impact on |$\dot{\omega }^\mathrm{LT,\, A}$| of the departures of the |${\boldsymbol{S}}^\mathrm{A}$| – |$\boldsymbol{L}$| geometry from the ideal alignment is calculated. With the current upper bound on the misalignment angle δA between them, the angles |$\lambda _\mathrm{A},\ \eta _\mathrm{A}$| of |${\boldsymbol{S}}^\mathrm{A}$| are constrained within |$85^\circ \lesssim \lambda _\mathrm{A}\lesssim 92^\circ ,\ 266^\circ \lesssim \eta _\mathrm{A} \lesssim 274^\circ$|⁠. In units of the first-order post-Newtonian mass-dependent periastron precession |$\dot{\omega }^\mathrm{GR}=16{_{.}^{\circ}}89 \, \mathrm{yr}^{-1}$|⁠ , a range variation |$\Delta \dot{\omega }^\mathrm{LT,\, A}\doteq \dot{\omega }^\mathrm{LT,\, A}_\mathrm{max} - \dot{\omega }^\mathrm{LT,\, A}_\mathrm{min} = 8\times 10^{-8}\, \omega ^\mathrm{GR}$| is implied. The experimental uncertainty |$\sigma _{\dot{\omega }_\mathrm{obs}}$| in measuring the periastron rate should become smaller by 2028–2030. Then, the spatial orientation of |${\boldsymbol{S}}^\mathrm{B}$| is constrained from the existing bounds on the misalignment angle δB, and |$\dot{\omega }^\mathrm{LT,\, B}\simeq 2\times 10^{-7}\, \dot{\omega }^\mathrm{GR}$| is correspondingly calculated. The error |$\sigma _{\dot{\omega }_\mathrm{obs}}$| should become smaller around 2025. The Lense–Thirring inclination and node precessions |$\dot{I}^\mathrm{LT},\ \dot{\Omega }^\mathrm{LT}$| are predicted to be |${\lesssim} 0.05\, \mathrm{arcsec\, yr^{-1}}$|⁠ , far below the current experimental accuracies |$\sigma _{I_\mathrm{obs}}=0{_{.}^{\circ}}5 , \ \sigma _{\Omega _\mathrm{obs}}=2^\circ$| in measuring |$I,\ \Omega$| over 1.5 yr with the scintillation technique. The Lense–Thirring rate |$\dot{x}_\mathrm{A}^\mathrm{LT}$| of the projected semimajor axis x A of PSR J0737−3039A is |${\lesssim} 2\times 10^{-16}\, \mathrm{s\, s}^{-1}$|⁠ , just two orders of magnitude smaller than a putative experimental uncertainty |$\sigma _{\dot{x}^\mathrm{obs}_\mathrm{A}}\simeq 10^{-14}\, \mathrm{s\, s}^{-1}$| guessed from 2006 data. [ABSTRACT FROM AUTHOR]

The timing of millisecond pulsars has long been used as an exquisitely precise tool for testing the building blocks of general relativity, including the strong equivalence principle and Lorentz symmetry. Observations of binary systems involving at least 1 ms pulsar have been used to place bounds on the parameters of Einstein-ćther theory, a gravitational theory that violates Lorentz symmetry at low energies via a preferred and dynamical time threading of the spacetime manifold. However, these studies did not cover the region of parameter space that is still viable after the recent bounds on the speed of gravitational waves from GW170817/GRB170817A. The restricted coverage was due to limitations in the methods used to compute the pulsar 'sensitivities', which parameterize violations of the strong-equivalence principle in these systems. We extend here the calculation of pulsar sensitivities to the parameter space of Einstein-ćther theory that remains viable after GW170817/GRB170817A. We show that observations of the damping of the period of quasi-circular binary pulsars and of the triple system PSR J0337+1715 further constrain the viable parameter space by about an order of magnitude over previous constraints. [ABSTRACT FROM AUTHOR]

We present the results obtained from timing and spectral studies of Be/X-ray binary pulsar EXO 2030+375 using observations with the Large Area Xenon Proportional Counters and Soft X-ray Telescope of AstroSat, at various phases of its Type-I outbursts in 2016, 2018, and 2020. The pulsar was faint during these observations as compared to earlier observations with other observatories. At the lowest luminosity of 2.5 × 10 35 erg s - 1 in 0.5–30 keV energy range, ≈ 41.3 s pulsations were clearly detected in the X-ray light curves. This finding establishes the first firm detection of pulsations in EXO 2030+375 at an extremely low mass accretion rate to date. The shape of the pulse profiles is complex due to the presence of several narrow dips. Though pulsations were detected up to ∼ 80 keV when the source was brighter, pulsations were limited up to ∼ 25 keV during the third AstroSat observation at lowest source luminosity. A search for quasi-periodic oscillations in 2 × 10 - 4 Hz to 10 Hz yielded a negative result. Spectral analysis of the AstroSat data showed that the spectrum of the pulsar was steep with a power-law index of ∼ 2. The values of photon-indices at observed low luminosities follow the known pattern in sub-critical regime of the pulsar. [ABSTRACT FROM AUTHOR]

Evolution of a large part of low-mass X-ray binaries (LMXBs) leads to the formation of rapidly rotating pulsars with a helium white dwarf (He WD) companion. Observations indicate that some He WDs in binary pulsar systems are ultracool (with the effective temperatures T eff ≲ 4000 K). It is hard to cool down a He WD to such low temperatures within the Hubble time because a thick hydrogen envelope was left behind around the He core after the mass transfer process. A possible mechanism that can accelerate the WD cooling is the evaporative wind mass-loss from the He WD driven by the high-energy radiation from the recycled pulsar. In this paper, we evolve a large number of LMXBs and investigate the influence of the pulsar's high-energy radiation on the WD cooling with different input parameters, including the neutron star's spin-down luminosity, the evaporation efficiency, and the metallicity of the companion star. By comparing our results with observations we note that, for relatively hot He WDs (with T eff > 7000 K), standard WD cooling without evaporation considered is able to reproduce their temperatures, while evaporation is probably required for the He WDs with relatively low temperatures (T eff < 5000 K). [ABSTRACT FROM AUTHOR]

Transitional millisecond pulsars are millisecond pulsars that switch between a rotation-powered millisecond pulsar state and an accretion-powered X-ray binary state, and are thought to be an evolutionary stage between neutron star low-mass X-ray binaries and millisecond pulsars. So far, only three confirmed systems have been identified in addition to a handful of candidates. We present the results of a multiwavelength study of the low-mass X-ray binary NGC 6652B in the globular cluster NGC 6652, including simultaneous radio and X-ray observations taken by the Karl G. Jansky Very Large Array and the Chandra X-ray Observatory, and optical spectroscopy and photometry. This source is the second brightest X-ray source in NGC 6652 (⁠|$L_{\textrm {X}}\sim 1.8 \times 10^{34}{\, \mathrm{erg\, s}^{-1}}$|⁠) and is known to be variable. We observe several X-ray flares over the duration of our X-ray observations, in addition to persistent radio emission and occasional radio flares. Simultaneous radio and X-ray data show no clear evidence of anticorrelated variability. Optical spectra of NGC 6652B indicate variable, broad H α emission that transitions from double-peaked emission to absorption over a time-scale of hours. We consider a variety of possible explanations for the source behaviour, and conclude that based on the radio and X-ray luminosities, short time-scale variability and X-ray flaring, and optical spectra, NGC 6652B is best explained as a transitional millisecond pulsar candidate that displays prolonged X-ray flaring behaviour. However, this could only be confirmed with observations of a change to the rotation-powered millisecond pulsar state. [ABSTRACT FROM AUTHOR]

Extremely low-mass white dwarfs (ELM WDs) are the result of binary evolution in which a low-mass donor star is stripped by its companion leaving behind a helium-core white dwarf (WD). We explore the formation of ELM WDs in binary systems considering the Convection And Rotation Boosted magnetic braking treatment. Our evolutionary sequences were calculated using the Modules for Experiments in Stellar Astrophysics code, with initial masses of 1.0 and 1.2 M⊙ (donor), and 1.4 (accretor), compatible with low-mass X-ray binary (LMXB) systems. We obtain ELM models in the range 0.15–0.27 M⊙ from a broad range of initial orbital periods, 1–25 d. The bifurcation period, where the initial period is equal to the final period, ranges from 20 to 25 d. In addition to LMXBs, we show that ultracompact X-ray binaries (UCXBs) and wide-orbit binary millisecond pulsars can also be formed. The relation between mass and orbital period obtained is compatible with the observational data from He WD companions to pulsars. [ABSTRACT FROM AUTHOR]

We report on a search for pulsars at the positions of eight low-mass white dwarfs and one higher mass white dwarf with the 100-m Effelsberg Radio Telescope. These systems have orbital parameters suggesting that their unseen companions are either massive white dwarfs or neutron stars (NSs). Our observations were performed at 1.36 GHz, reaching sensitivities of 0.1–0.2 mJy. We searched our data accounting for the possible acceleration and jerk of the pulsar signals due to orbital motion, but found no significant pulsar signals. Considering our result jointly with 20 non-detections of similar systems with the Greenbank Radio Telescope, we infer f NS ≤ 0.10, for the fraction of NSs orbiting these white dwarfs. We discuss the sensitivity of this result to the underlying assumptions and conclude with a brief discussion on the prospects of targeted surveys for discovering millisecond pulsars. [ABSTRACT FROM AUTHOR]

Merging supermassive black hole binaries produce low-frequency gravitational waves, which pulsar timing experiments are searching for. Much of the current theory is developed within the plane-wave formalism, and here we develop the more general Fresnel formalism. We show that Fresnel corrections to gravitational wave timing residual models allow novel measurements to be made, such as direct measurements of the source distance from the timing residual phase and frequency, as well as direct measurements of chirp mass from a monochromatic source. Probing the Fresnel corrections in these models will require future pulsar timing arrays with more distant pulsars across our Galaxy (ideally at the distance of the Magellanic Clouds), timed with precisions less than 100 ns, with distance uncertainties reduced to the order of the gravitational wavelength. We find that sources with chirp mass of order 109 M⊙ and orbital frequency ω0 > 10 nHz are good candidates for probing Fresnel corrections. With these conditions met, the measured source distance uncertainty can be made less than 10 per cent of the distance to the source for sources out to ∼100 Mpc, source sky localization can be reduced to sub-arcminute precision, and source volume localization can be made to less than 1 Mpc3 for sources out to 1-Gpc distances. [ABSTRACT FROM AUTHOR]

We present observations of 35 high spin-down energy radio pulsars using the MeerKAT telescope. Polarization profiles and associated parameters are also presented. We derive the geometry for a selection of pulsars which show interpulse emission. We point out that, in several cases, these radio pulsars should also be seen in γ-rays but that improved radio timing is required to aid the high-energy detection. We discuss the relationship between the width of the radio profile and its high-energy detectability. Finally, we reflect on the correlation between the spin-down energy and the radio polarization fraction and the implications this may have for γ-ray emission. [ABSTRACT FROM AUTHOR]

The core–envelope analytic models of Negi et al. (Class. Quantum Gravity 6:1141, 1989) have been found to be consistent with both the mechanisms of glitch generation in pulsars, namely (i) the starquake model and (ii) the vortex unpinning model. In Negi (Astrophys. Space Sci. 364:149, 2019, arXiv:2105.14324), the author has been able to reproduce the observed values of the glitch healing parameter, G h (= I core / I total ; G h represents the fractional moment of inertia of the core component in the starquake mechanism of glitch generation) for the Crab as well as for the Vela pulsar. In another study, by using the models discussed in Negi et al. (Class. Quantum Gravity 6:1141, 1989), the author has obtained the minimum value of fractional moment of inertia of the crust about 7.4% and larger for all the values of masses in the range 1 M ⊙ – 1.96 M ⊙ considered for the Vela pulsar (Negi in J. Eng. Sci. 11:86, 2020a, arXiv:2106.02439). The latter study of the author is found to be consistent with the recent requirement (on the basis of vortex unpinning model of the glitch generation) which reveals that the minimum fractional crustal moment of inertia of the Vela pulsar should be about 7% for a mass higher than about 1 M ⊙ . However, a thorough study which requires investigation of pulsational stability and gravitational binding of the models of Negi et al. (Class. Quantum Gravity 6:1141, 1989) has not been carried out so far. The present paper deals with such a study of the models (Negi et al. in Class. Quantum Gravity 6:1141, 1989) for all permissible values of the compactness parameter u (≡ M / a , the total mass to size ratio in geometrized units) and compressibility factor Q (defined in Tolman's VII solution as x = r 2 / K 2 = r 2 / a 2 Q ). It is seen that the configurations remain pulsationally stable and gravitationally bound for all permissible values of u (≤ 0.25) and Q (0 < Q ≤ 1.2 ). [ABSTRACT FROM AUTHOR]

Supermassive black hole binary systems (SMBHBs) emitting gravitational waves may be traced by periodic light curves. We assembled a catalog of 149 such periodic light curves, and using their masses, distances, and periods, predicted the gravitational-wave strain and detectability of each binary candidate using all-sky detection maps. We found that the International Pulsar Timing Array (IPTA) provides almost uniform sky coverage—a unique ability of the IPTA—and by 2025 will improve NANOGrav's current minimum detectable strain by a factor of 6 and its volume by a factor of 216. Moreover, IPTA will reach detection sensitivities for three candidates by 2025, and 13 by the end of the decade, enabling us to constrain the underlying empirical relations used to estimate supermassive black hole masses. We find that we can in fact already constrain the mass of a binary in Mrk 504 to M < 3.3 × 109M⊙. We also identify 24 high-mass, high-redshift galaxies that, according to our models, should not be able to host SMBHBs. Importantly, the GW detection of even one of these candidates would be an essentially eternal multimessenger system, and identifying common false-positive signals from nondetections will be useful to filter the data from future large-scale surveys such as LSST. [ABSTRACT FROM AUTHOR]

We describe the ongoing Relativistic Binary programme (RelBin), a part of the MeerTime large survey project with the MeerKAT radio telescope. RelBin is primarily focused on observations of relativistic effects in binary pulsars to enable measurements of neutron star masses and tests of theories of gravity. We selected 25 pulsars as an initial high priority list of targets based on their characteristics and observational history with other telescopes. In this paper, we provide an outline of the programme, and present polarization calibrated pulse profiles for all selected pulsars as a reference catalogue along with updated dispersion measures. We report Faraday rotation measures for 24 pulsars, twelve of which have been measured for the first time. More than a third of our selected pulsars show a flat position angle swing confirming earlier observations. We demonstrate the ability of the Rotating Vector Model, fitted here to seven binary pulsars, including the Double Pulsar (PSR J0737–3039A), to obtain information about the orbital inclination angle. We present a high time resolution light curve of the eclipse of PSR J0737–3039A by the companion's magnetosphere, a high-phase-resolution position angle swing for PSR J1141–6545, an improved detection of the Shapiro delay of PSR J1811–2405, and pulse scattering measurements for PSRs J1227–6208, J1757–1854, and J1811–1736. Finally, we demonstrate that timing observations with MeerKAT improve on existing data sets by a factor of, typically, 2–3, sometimes by an order of magnitude. [ABSTRACT FROM AUTHOR]

We report the discovery of a new high-mass X-ray binary pulsar, XMMU J050722.1−684758, possibly associated with the supernova remnant (SNR) MCSNR J0507−6847 in the Large Magellanic Cloud, using XMM–Newton X-ray observations. Pulsations with a periodicity of 570 s are discovered from the Be X-ray binary XMMU J050722.1−684758 confirming its nature as a HMXB pulsar. The HMXB is located near the geometric centre of the SNR MCSNR J0507−6847(0.9 arcmin from the centre) which supports the XRB-SNR association. The estimated age of the SNR is 43–63 kyr years which points to a middle aged to old SNR. The large diameter of the SNR combined with the lack of distinctive shell counterparts in optical and radio indicates that the SNR is expanding into the tenuous environment of the superbubble N103. The estimated magnetic field strength of the neutron star is B ≳ 1014 G assuming a spin equilibrium condition which is expected from the estimated age of the parent remnant and assuming that the measured mass-accretion rate remained constant throughout. [ABSTRACT FROM AUTHOR]

In the currently accepted paradigm, dark matter is hypothesized as an explanation of the flat rotation curves of galaxies under the assumption of virialized orbits. The use of millisecond pulsar timing as a probe of Galactic dark matter content is explored as a means of relaxing this assumption. A method of inference of the Galactic potential using the frequency derivative |$\dot{\nu }$| is produced, and an estimate for a virialized Galactic rotation curve is given through direct observation of acceleration. The data set used includes 210 pulsars with known |$\dot{\nu }$| and astrometric properties, a subset of which also have measured |$\ddot{\nu }$|⁠. In principle, this enables the exploration of kinematic effects, but in practice, |$\ddot{\nu }$| values are found to be too imprecise at present to adequately constrain radial velocities of pulsars. Additionally, surface magnetic field strengths are inferred from |$\dot{\nu }$| and the magnetic spin-down contribution to |$\ddot{\nu }$| is estimated. For several pulsars, the radial velocity is known, and the kinematic contribution to |$\ddot{\nu }$| is estimated accordingly. The binary orbital periods of PSR J1713+0747 and other binary pulsars are also used to constrain Galactic mass density models. [ABSTRACT FROM AUTHOR]

We report on the first pulsar and transient survey of the Galactic Center (GC) with the Atacama Large Millimeter/submillimeter Array (ALMA). The observations were conducted during the Global Millimeter VLBI Array campaign in 2017 and 2018. We carry out searches using time series of both total intensity and other polarization components in the form of Stokes parameters. We incorporate acceleration and its derivative in the pulsar search, and also search in segments of the entire observation to compensate for potential orbital motion of the pulsar. While no new pulsar is found, our observations yield the polarization profile of the GC magnetar PSR J1745−2900 at millimeter wavelength for the first time, which turns out to be nearly 100% linearly polarized. Additionally, we estimate the survey sensitivity placed by both system and red noise, and evaluate its capability of finding pulsars in orbital motion with either Sgr A* or a binary companion. We show that the survey is sensitive to only the most luminous pulsars in the known population and future observations with ALMA in Band-1 will deliver significantly deeper survey sensitivity on the GC pulsar population. [ABSTRACT FROM AUTHOR]

We report the results of eROSITA and NICER observations of the 2020 June outburst of the Be/X-ray binary pulsar RX J0529.8−6556 in the Large Magellanic Cloud, along with the analysis of archival X-ray and optical data from this source. We find two anomalous features in the system's behaviour. First, the pulse profile observed by NICER during maximum luminosity is similar to that observed by XMM–Newton in 2000, despite the fact that the X-ray luminosity was different by two orders of magnitude. In contrast, a modest decrease in luminosity in the 2020 observations generated a significant change in pulse profile. Secondly, we find that the historical optical outbursts are not strictly periodic, as would be expected if the outbursts were triggered by periastron passage, as is generally assumed. The optical peaks are also not coincident with the X-ray outbursts. We suggest that this behaviour may result from a misalignment of the Be star disc and the orbital plane, which might cause changes in the timing of the passage of the neutron star through the disc as it precesses. We conclude that the orbital period of the source remains unclear. [ABSTRACT FROM AUTHOR]

Gravitational waves provide a new probe of the Universe which can reveal a number of cosmological and astrophysical phenomena that cannot be observed by electromagnetic waves. Different frequencies of gravitational waves are detected by different means. Among them, precision measurements of pulsar timing provides a natural detector for gravitational waves with light-year scale wavelengths. In this review, first a basic framework to detect a stochastic gravitational wave background using pulsar timing array is introduced, and then possible interpretations of the latest observational result of 12.5-year NANOGrav data are described. [ABSTRACT FROM AUTHOR]

An explicit realistic model featuring a supercooled phase transition, which allows us to explain the background of gravitational waves recently detected by pulsar timing arrays, is constructed. In this model the phase transition corresponds to radiative symmetry breaking (and mass generation) in a dark sector featuring a dark photon associated with the broken symmetry. The completion of the transition is ensured by a non-minimal coupling between gravity and the order parameter and fast reheating occurs thanks to a preheating phase. Finally, it is also shown that the model leads to primordial black hole production. [ABSTRACT FROM AUTHOR]

Frequency metrology outperforms any other branch of metrology in accuracy (parts in 10−16) and small fluctuations (<10−17). In turn, among celestial bodies, the rotation speed of millisecond pulsars is by far the most stable (<10−18). Therefore, the precise measurement of the time of arrival (TOA) of pulsar signals is expected to disclose information about cosmological phenomena, and to enlarge our astrophysical knowledge. Related to this topic, Pulsar Timing Array projects have been developed and operated for the last decades. The TOAs from a pulsar can be affected by local emission and environmental effects, in the direction of the propagation through the interstellar medium or universally by gravitational waves from super massive black hole binaries. These effects (signals) can manifest as a low-frequency fluctuation over time, phenomenologically similar to a red noise, while the remaining pulsar intrinsic and instrumental background (noise) are white. This article focuses on the frequency metrology of pulsars. From our standpoint, the pulsar is an accurate clock, to be measured simultaneously with several telescopes in order to reject the uncorrelated white noise. We apply the modern statistical methods of time-and-frequency metrology to simulated pulsar data, and we show the detection limit of the correlated red noise signal between telescopes. [ABSTRACT FROM AUTHOR]

Close-orbit low-mass X-ray binaries (LMXBs), radio binary millisecond pulsars (BMSPs) with extremely low-mass helium white dwarfs (ELM He WDs) and ultra-compact X-ray binaries (UCXBs) are all part of the same evolutionary sequence. It is therefore of uttermost importance to understand how these populations evolve from one specie to another. Moreover, UCXBs are important gravitational wave (GW) sources and can be detected by future space-borne GW observatories. However, the formation and evolutionary link between these three different populations of neutron star (NS) binaries are not fully understood. In particular, a peculiar fine-tuning problem has previously been demonstrated for the formation of these systems. In this investigation, we test a newly suggested magnetic braking prescription and model the formation and evolution of LMXBs. We compute a grid of binary evolution models and present the initial parameter space of the progenitor binaries which successfully evolve all the way to produce UCXBs. We find that the initial orbital period range of LMXBs, which evolve into detached NS + ELM He WD binaries and later UCXBs, becomes significantly wider compared to evolution with a standard magnetic braking prescription, and thus helps to relieve the fine-tuning problem. However, we also find that formation of wide-orbit BMSPs is prohibited for strong versions of this new magnetic braking prescription, which therefore calls for a revision of the prescription. Finally, we present examples of the properties of UCXBs as Galactic GW sources and discuss their detection by the LISA, TianQin, and Taiji observatories. [ABSTRACT FROM AUTHOR]

We report on a coherent timing analysis of the 163 Hz accreting millisecond X-ray pulsar IGR J17062–6143. Using data collected with the Neutron Star Interior Composition Explorer and XMM-Newton, we investigated the pulsar evolution over a time span of four years. We obtained a unique phase-coherent timing solution for the stellar spin, finding the source to be spinning up at a rate of (3.77 ± 0.09) × 10−15 Hz s−1. We further find that the 0.4–6 keV pulse fraction varies gradually between 0.5% and 2.5% following a sinusoidal oscillation with a 1210 ± 40 day period. Finally, we supplemented this analysis with an archival Rossi X-ray Timing Explorer observation and obtained a phase-coherent model for the binary orbit spanning 12 yr, yielding an orbital period-derivative measurement of (8.4 ± 2.0) × 10−12 s s−1. This large orbital period derivative is inconsistent with a binary evolution that is dominated by gravitational wave emission and is suggestive of highly nonconservative mass transfer in the binary system. [ABSTRACT FROM AUTHOR]

Gravitational-wave (GW) astronomy, together with precise pulsar timing and long baseline interferometry, is changing our ability to perform tests of fundamental physics with astrophysical observations. Some of these tests are based on electromagnetic (EM) probes or electrically charged bodies, and assume an empty Universe. However, the cosmos is filled with plasma, a dilute medium which prevents the propagation of low-frequency, small-amplitude EM waves. We show that the plasma hinders our ability to perform some strong-field gravity tests, in particular: (i) nonlinear plasma effects dramatically quench plasma-driven super-radiant instabilities; (ii) the contribution of EM emission to the inspiral of charged black-hole binaries is strongly suppressed; (iii) EM-driven secondary modes, although present in the spectrum of charged black holes, are excited to negligible amplitude in the GW ringdown signal. The last two effects are relevant also in the case of massive fields that propagate in vacuum and can jeopardize tests of modified theories of gravity containing massive degrees of freedom. [ABSTRACT FROM AUTHOR]