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Transitional millisecond pulsars (tMSPs) are neutron star systems that alternate between a rotation-powered radio millisecond pulsar state and an accretion disc-dominated low-mass X-ray binary (LMXB)-like state on multi-year time-scales. During the LMXB-like state, the X-ray emission from tMSPs switches between 'low' and 'high' X-ray brightness modes on a time-scale of seconds to minutes (or longer), while the radio emission shows variability on time-scales of roughly minutes. Coordinated Very Large Array (VLA) and Chandra observations of the nearby tMSP PSR J1023+0038 uncovered a clear anticorrelation between radio and X-ray luminosities such that the radio emission consistently peaks during the X-ray low modes. In addition, there are sometimes also radio/X-ray flares that show no obvious correlation. In this paper, we present simultaneous radio and X-ray observations of a promising tMSP candidate system, 3FGL J1544.6 |$-$| 1125, which shows optical, |$\gamma$| -ray, and X-ray phenomena similar to PSR J1023+0038, but which is challenging to study because of its greater distance. Using simultaneous VLA and Chandra observations, we find that the radio and X-ray emission are consistent with being anticorrelated in a manner similar to PSR J1023+0038. We discuss how our results help in understanding the origin of bright radio emission from tMSPs. The greater sensitivity of upcoming telescopes such as the Square Kilometre Array will be crucial for studying the correlated radio/X-ray phenomena of tMSP systems. [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]

Aql X-1 is one of the best-studied neutron star low-mass X-ray binaries. It was previously targeted using quasi-simultaneous radio and X-ray observations during at least seven different accretion outbursts. Such observations allow us to probe the interplay between accretion inflow (X-ray) and jet outflow (radio). Thus far, these combined observations have only covered one order of magnitude in radio and X-ray luminosity range; this means that any potential radio–X-ray luminosity correlation, L R ∝  L Xβ, is not well constrained (β ≈ 0.4–0.9, based on various studies) or understood. Here we present quasi-simultaneous Very Large Array and Swift -XRT observations of Aql X-1's 2016 outburst, with which we probe one order of magnitude fainter in radio and X-ray luminosity compared to previous studies (6 × 1034 erg s−1 < L X <3 × 1035 erg s−1, i.e. the intermediate to low-luminosity regime between outburst peak and quiescence). The resulting radio non-detections indicate that Aql X-1's radio emission decays more rapidly at low X-ray luminosities than previously assumed – at least during the 2016 outburst. Assuming similar behaviour between outbursts, and combining all available data in the hard X-ray state, this can be modelled as a steep β = |$1.17^{+0.30}_{-0.21}$| power-law index or as a sharp radio cut-off at L X ≲ 5 × 1035 erg s−1 (given our deep radio upper limits at X-ray luminosities below this value). We discuss these results in the context of other similar studies. [ABSTRACT FROM AUTHOR]

IGR J17591−2342 is a new accreting millisecond X-ray pulsar that was recently discovered in outburst in 2018. Early observations revealed that the source's radio emission is brighter than that of any other known neutron star low-mass X-ray binary (NS–LMXB) at comparable X-ray luminosity, and assuming its likely ≳6 kpc distance. It is comparably radio bright to black hole LMXBs at similar X-ray luminosities. In this work, we present the results of our extensive radio and X-ray monitoring campaign of the 2018 outburst of IGR J17591−2342. In total, we collected 10 quasi-simultaneous radio (VLA, ATCA) and X-ray (Swift –XRT) observations, which make IGR J17591−2342 one of the best-sampled NS–LMXBs. We use these to fit a power-law correlation index |$\beta = 0.37^{+0.42}_{-0.40}$| between observed radio and X-ray luminosities (L R ∝  L Xβ). However, our monitoring revealed a large scatter in IGR J17591−2342's radio luminosity (at a similar X-ray luminosity, L X ∼1036 erg s−1, and spectral state), with L R ∼ 4 × 1029 erg s−1 during the first three reported observations, and up to a factor of 4 lower L R during later radio observations. None the less, the average radio luminosity of IGR J17591−2342 is still one of the highest among NS–LMXBs, and we discuss possible reasons for the wide range of radio luminosities observed in such systems during outburst. We found no evidence for radio pulsations from IGR J17591−2342 in our Green Bank Telescope observations performed shortly after the source returned to quiescence. None the less, we cannot rule out that IGR J17591−2342 becomes a radio millisecond pulsar during quiescence. [ABSTRACT FROM AUTHOR]

The discovery of radio pulsars over a half century ago was a seminal moment in astronomy. It demonstrated the existence of neutron stars, gave a powerful observational tool to study them, and has allowed us to probe strong gravity, dense matter, and the interstellar medium. More recently, pulsar surveys have led to the serendipitous discovery of fast radio bursts (FRBs). While FRBs appear similar to the individual pulses from pulsars, their large dispersive delays suggest that they originate from far outside the Milky Way and hence are many orders-of-magnitude more luminous. While most FRBs appear to be one-off, perhaps cataclysmic events, two sources are now known to repeat and thus clearly have a longer lived central engine. Beyond understanding how they are created, there is also the prospect of using FRBs—as with pulsars—to probe the extremes of the Universe as well as the otherwise invisible intervening medium. Such studies will be aided by the high-implied all-sky event rate: there is a detectable FRB roughly once every minute occurring somewhere on the sky. The fact that less than a hundred FRB sources have been discovered in the last decade is largely due to the small fields-of-view of current radio telescopes. A new generation of wide-field instruments is now coming online, however, and these will be capable of detecting multiple FRBs per day. We are thus on the brink of further breakthroughs in the short-duration radio transient phase space, which will be critical for differentiating between the many proposed theories for the origin of FRBs. In this review, we give an observational and theoretical introduction at a level that is accessible to astronomers entering the field. [ABSTRACT FROM AUTHOR]

We present quasi-simultaneous radio (VLA) and X-ray (Swift) observations of the neutron star low-mass X-ray binary (NS-LMXB) 1RXS J180408.9-342058 (J1804) during its 2015 outburst. We found that the radio jet of J1804 was bright (232 ± 4 μJy at 10 GHz) during the initial hard X-ray state, before being quenched by more than an order of magnitude during the soft X-ray state (19 ± 4 μJy). The source then was undetected in radio (<13 μJy) as it faded to quiescence. In NS-LMXBs, possible jet quenching has been observed in only three sources and the J1804 jet quenching we show here is the deepest and clearest example to date. Radio observations when the source was fading towards quiescence (LX = 1034-35 erg s-1) show that J1804 must follow a steep track in the radio/X-ray luminosity plane with β > 0.7 (where LR ∝ LβX). Few other sources have been studied in this faint regime, but a steep track is inconsistent with the suggested behaviour for the recently identified class of transitional millisecond pulsars. J1804 also shows fainter radio emission at LX < 1035 erg s-1 than what is typically observed for accreting millisecond pulsars. This suggests that J1804 is likely not an accreting X-ray or transitional millisecond pulsar. [ABSTRACT FROM AUTHOR]

In outburst, neutron star X-ray binaries produce less powerful jets than black holes at a given X-ray luminosity. This has made them more difficult to study as they fade towards quiescence. To explore whether neutron stars power jets at low accretion rates (LX ≲ 1036 erg s-1), we investigate the radio and X-ray properties of three accreting millisecond X-ray pulsars (IGR J17511-3057, SAX J1808.4-3658 and IGR J00291+5934) during their outbursts in 2015, and of the non-pulsing neutron starCen X-4 in quiescence (2015) and in outburst (1979). We did not detect the radio counterpart of IGR J17511-3057 in outburst or of Cen X-4 in quiescence, but did detect IGR J00291+5934 and SAX J1808.4-3658, showing that at least some neutron stars launch jets at low accretion rates. While the radio and X-ray emission in IGR J00291+5934 seem to be tightly correlated, the relationship in SAX J1808.4-3658 is more complicated.We find that SAX J1808.4-3658 produces jets during the reflaring tail, and we explore a toy model to ascertain whether the radio emission could be attributed to the onset of a strong propeller. The lack of a universal radio/X-ray correlation, with different behaviours in different neutron star systems (with various radio/X-ray correlations; some being radio faint and others not), points at distinct disc-jet interactions in individual sources, while always being fainter in the radio band than black holes at the same X-ray luminosity. [ABSTRACT FROM AUTHOR]

We report on unusually very hard spectral states in three confirmed neutron-star lowmass X-ray binaries (1RXS J180408.9-342058, EXO 1745-248 and IGR J18245-2452) at a luminosity between ∼1036 and 1037 erg s-1. When fitting the Swift X-ray spectra (0.5-10 keV) in those states with an absorbed power-law model, we found photon indices of Γ ∼ 1, significantly lower than the Γ = 1.5-2.0 typically seen when such systems are in their so called hard state. For individual sources, very hard spectra were already previously identified, but here we show for the first time that likely our sources were in a distinct spectral state (i.e. different from the hard state) when they exhibited such very hard spectra. It is unclear how such very hard spectra can be formed; if the emission mechanism is similar to that operating in their hard states (i.e. up-scattering of soft photons due to hot electrons), then the electrons should have higher temperatures or a higher optical depth in the very hard state compared to those observed in the hard state. By using our obtained Γ as a tracer for the spectral evolution with luminosity, we have compared our results with those obtained by Wijnands et al. Our sample of sources follows the same track as the other neutron star systems inWijnands et al., confirming their general results. However, we do not find that the accreting millisecond pulsars are systematically harder than the non-pulsating systems. [ABSTRACT FROM AUTHOR]

It is thought that neutron stars in low-mass binary systems can accrete matter and angular momentum from the companion star and be spun-up to millisecond rotational periods. During the accretion stage, the system is called a low-mass X-ray binary, and bright X-ray emission is observed. When the rate of mass transfer decreases in the later evolutionary stages, these binaries host a radio millisecond pulsar whose emission is powered by the neutron star's rotating magnetic field. This evolutionary model is supported by the detection of millisecond X-ray pulsations from several accreting neutron stars and also by the evidence for a past accretion disc in a rotation-powered millisecond pulsar. It has been proposed that a rotation-powered pulsar may temporarily switch on during periods of low mass inflow in some such systems. Only indirect evidence for this transition has hitherto been observed. Here we report observations of accretion-powered, millisecond X-ray pulsations from a neutron star previously seen as a rotation-powered radio pulsar. Within a few days after a month-long X-ray outburst, radio pulses were again detected. This not only shows the evolutionary link between accretion and rotation-powered millisecond pulsars, but also that some systems can swing between the two states on very short timescales. [ABSTRACT FROM AUTHOR]

Einstein@Home aggregates the computer power of hundreds of thousands of volunteers from 193 countries, to search for new neutron stars using data from electromagnetic and gravitational-wave detectors. This paper presents a detailed description of the search for newradio pulsars using PulsarALFAsurvey data from the Arecibo Observatory. The enormous computing power allows this search to cover a new region of parameter space; it can detect pulsars in binary systems with orbital periods as short as 11 minutes. We also describe the first Einstein@Home discovery, the 40.8 Hz isolated pulsar PSR J2007+2722, and provide a full timing model. PSR J2007+2722's pulse profile is remarkably wide with emission over almost the entire spin period. This neutron star is most likely a disrupted recycled pulsar, about as old as its characteristic spin-down age of 404 Myr. However, there is a small chance that it was born recently, with a low magnetic field. If so, upper limits on the X-ray flux suggest but cannot prove that PSR J2007+2722 is at least ~100 kyr old. In the future, we expect that the massive computing power provided by volunteers should enable many additional radio pulsar discoveries. [ABSTRACT FROM AUTHOR]

Some radio pulsars show clear "drifting subpulses", in which subpulses are seen to drift in pulse longitude in a systematic pattern. Here we examine how the drifting subpulses of PSR B0809+74 evolve with time and observing frequency. We show that the subpulse period (P3) is constant on timescales of days, months and years, and between 14-5100 MHz. Despite this, the shapes of the driftbands change radically with frequency. Previous studies have concluded that, while the subpulses appear to move through the pulse window approximately linearly at low frequencies (<500 MHz), a discrete step of 180 in subpulse phase is observed at higher frequencies (>820 MHz) near to the peak of the average pulse profile. We use LOFAR, GMRT, GBT, WSRT and Effelsberg 100-m data to explore the frequency-dependence of this phase step.We show that the size of the subpulse phase step increases gradually, and is observable even at low frequencies. We attribute the subpulse phase step to the presence of two separate driftbands, whose relative arrival times vary with frequency - one driftband arriving 30 pulses earlier at 20 MHz than it does at 1380 MHz, whilst the other arrives simultaneously at all frequencies. The drifting pattern which is observed here cannot be explained by either the rotating carousel model or the surface oscillation model, and could provide new insight into the physical processes happening within the pulsar magnetosphere. [ABSTRACT FROM AUTHOR]

We present the results of the most sensitive and comprehensive survey yet undertaken for radio pulsars and fast transients in the Andromeda galaxy (M31) and its satellites, using the Westerbork Synthesis Radio Telescope (WSRT) at a central frequency of 328 MHz. We used the WSRT in a special configuration called 8gr8 (eight-grate) mode, which provides a large instantaneous field of view, about 5 deg2 pointing−1, with good sensitivity, long dwell times (up to 8 h pointing−1) and good spatial resolution (a few arcminutes) for locating sources. We have searched for both periodicities and single pulses in our data, aiming to detect bright, persistent radio pulsars and rotating radio transients (RRATs) of either Galactic or extragalactic origin. Our searches did not reveal any confirmed periodic signals or bright single bursts from (potentially) cosmological distances. However, we do report the detection of several single-pulse events, some repeating at the same dispersion measure, which could potentially originate from neutron stars in M31. One in particular was seen multiple times, including a burst of six pulses in 2000 s, at a dispersion measure of 54.7 pc cm−3, which potentially places the origin of this source outside our Galaxy. Our results are compared to a range of hypothetical populations of pulsars and RRATs in M31 and allow us to constrain the luminosity function of pulsars in M31. They also show that, unless the pulsar population in M31 is much dimmer than in our Galaxy, there is no need to invoke any violation of the inverse square law of the distance for pulsar fluxes. [ABSTRACT FROM AUTHOR]

Dispersion in the interstellar medium is a well known phenomenon that follows a simple relationship, which has been used to predict the time delay of dispersed radio pulses since the late 1960s. We performed wide-band simultaneous observations of four pulsars with LOFAR (at 40-190 MHz), the 76-m Lovell Telescope (at 1400 MHz) and the Effelsberg 100-m Telescope (at 8000 MHz) to test the accuracy of the dispersion law over a broad frequency range. In this paper we present the results of these observations which show that the dispersion law is accurate to better than 1 part in 105 across our observing band. We use this fact to constrain some of the properties of the interstellar medium along the line-of-sight and use the lack of any aberration or retardation effects to determine upper limits on emission heights in the pulsar magnetosphere. We also discuss the effect of pulse profile evolution on our observations, and the implications that it could have for precision pulsar timing projects such as the detection of gravitational waves with pulsar timing arrays. [ABSTRACT FROM AUTHOR]

PSR J1903+0327, a millisecond pulsar in an eccentric () 95-d orbit with an companion poses a challenge to our understanding of stellar evolution in binary and multiple-star systems. Here we describe optical and radio observations which rule out most of the scenarios proposed to explain formation of this system. Radio timing measurements of three post-Keplerian effects yield the most precise measurement of the mass of a millisecond pulsar to date: solar masses (99.7 per cent confidence limit). This rules out some equations of state for superdense matter; furthermore, it is consistent with the spin-up of the pulsar by mass accretion, as suggested by its short spin period and low magnetic field. Optical spectroscopy of a proposed main-sequence counterpart shows that its orbital motion mirrors the pulsar's 95-d orbit; being therefore its binary companion. This finding rules out a previously suggested scenario which proposes that the system is presently a hierarchical triple. Conventional binary evolution scenarios predict that, after recycling a neutron star into a millisecond pulsar, the binary companion should become a white dwarf and its orbit should be nearly circular. This suggests that if PSR J1903+0327 was recycled, its present companion was not responsible for it. The optical detection also provides a measurement of the systemic radial velocity of the binary; this and the proper motion measured from pulsar timing allow the determination of the systemic 3D velocity in the Galaxy. We find that the system is always within 270 pc of the plane of the Galaxy, but always more than 3 kpc away from the Galactic Centre. Thus an exchange interaction in a dense stellar environment (like a globular cluster or the Galactic Centre) is not likely to be the origin of this system. We suggest that after the supernova that formed it, the neutron star was in a tight orbit with a main-sequence star and the present companion was a tertiary farther out. The neutron star then accreted matter from its evolving inner companion, forming a millisecond pulsar. The inner companion then disappeared, either due to a chaotic three-body interaction with the outer star (caused by the expansion of the inner orbit that necessarily results from mass transfer), or in the case of a very compact inner system, due to ablation/accretion by the newly formed millisecond pulsar. We discuss in detail the possible evolution of such a system before the supernova. [ABSTRACT FROM AUTHOR]

Neutron stars are composed of the densest form of matter known to exist in our Universe, the composition and properties of which are still theoretically uncertain. Measurements of the masses or radii of these objects can strongly constrain the neutron star matter equation of state and rule out theoretical models of their composition. The observed range of neutron star masses, however, has hitherto been too narrow to rule out many predictions of 'exotic' non-nucleonic components. The Shapiro delay is a general-relativistic increase in light travel time through the curved space-time near a massive body. For highly inclined (nearly edge-on) binary millisecond radio pulsar systems, this effect allows us to infer the masses of both the neutron star and its binary companion to high precision. Here we present radio timing observations of the binary millisecond pulsar J1614-2230 that show a strong Shapiro delay signature. We calculate the pulsar mass to be (1.97 ± 0.04)M⊙, which rules out almost all currently proposed hyperon or boson condensate equations of state (M⊙, solar mass). Quark matter can support a star this massive only if the quarks are strongly interacting and are therefore not 'free' quarks. [ABSTRACT FROM AUTHOR]

We present the discovery of five millisecond pulsars (MSPs) from the PALFA Galactic plane survey using Arecibo. Four of these (PSRs J0557+1551, J1850+0244, J1902+0300, and J1943+2210) are binary pulsars whose companions are likely white dwarfs, and one (PSR J1905+0453) is isolated. Phase-coherent timing solutions, ranging from ∼1 to ∼3 yr in length, and based on observations from the Jodrell Bank and Arecibo telescopes, provide precise determinations of spin, orbital, and astrometric parameters. All five pulsars have large dispersion measures (>100 pc cm–3, within the top 20% of all known Galactic field MSPs) and are faint (1.4 GHz flux density ≲0.1 mJy, within the faintest 5% of all known Galactic field MSPs), illustrating PALFA's ability to find increasingly faint, distant MSPs in the Galactic plane. In particular, PSR J1850+0244 has a dispersion measure of 540 pc cm–3, the highest of all known MSPs. Such distant, faint MSPs are important input for accurately modeling the total Galactic MSP population. [ABSTRACT FROM AUTHOR]

We present radio and γ-ray observations, which, along with concurrent X-ray observations, reveal that the binary millisecond pulsar (MSP)/low-mass X-ray binary transition system PSR J1023+0038 has undergone a transformation in state. Whereas until recently the system harbored a bright millisecond radio pulsar, the radio pulsations at frequencies between 300 to 5000 MHz have now become undetectable. Concurrent with this radio disappearance, the γ-ray flux of the system has quintupled. We conclude that, though the radio pulsar is currently not detectable, the pulsar mechanism is still active and the pulsar wind, as well as a newly formed accretion disk, are together providing the necessary conditions to create the γ-ray increase. This system is the first example of a compact, low-mass binary which has shown significant state changes accompanied by large changes in γ-ray flux; it will continue to provide an exceptional test bed for better understanding the formation of MSPs as well as accretion onto neutron stars in general. [ABSTRACT FROM AUTHOR]