Wave effects are often neglected in microlensing studies; however, for coherent point-like sources, such as pulsars and fast radio bursts (FRBs), wave effects will become important in their gravitational lensing. In this paper, we describe the wave optics formalism, its various limits, and the conditions for which these limits hold. Using the simple point lens as an example, we will show that the frequency dependence of wave effects breaks degeneracies that are present in the usual geometric optics limit, and constructive interference results in larger magnifications further from the lens. This latter fact leads to a generic increase in cross section for microlensing events in the wave-optics regime compared to the geometric optics regime. For realistic percent-level spectral sensitivities, this leads to a relative boost in lensing cross section of more than an order of magnitude. We apply the point-lens model to the lensing of FRBs and pulsars and find that these radio sources will be lensed in the full wave-optics regime by isolated masses in the range of $0.1-100\,M_\oplus$, which includes free-floating planets (FFPs), whose Einstein radius is smaller than the Fresnel scale. More generally, the interference pattern allows an instantaneous determination of lens masses, unlike traditional microlensing techniques which only yield a mass inference from the event timescale.
Indian Institute of Science Education and Research Thiruvananthapuram（印度科学教育研究所特里凡得琅分校）；Department of Physics and Astronomy, University of California, Riverside（加州大学河滨分校物理与天文系）；Department of Astronomy, University of California Berkeley（加州大学伯克利分校天文系）；Max-Planck-Institut für Radioastronomie（马克斯·普朗克射电天文研究所）；Physik-Institut, University of Zurich（苏黎世大学物理学院）
We present four new fast radio bursts discovered in a search of the Parkes 70-cm pulsar survey data archive for dispersed single pulses and bursts. We searched dispersion measures (DMs) ranging between 0 to 5000 pc cm$^{-3}$ with the HEIMDALL and FETCH detection and classification algorithms. All four of the FRBs discovered have significantly larger widths ($> 50$ ms) than almost all of the FRBs detected and cataloged to date. The large pulse widths are not dominated by interstellar scattering or dispersive smearing within channels. One of the FRBs has a DM of 3338 pc cm$^{3}$, the largest mea…
We investigate the possibilities that pulsars act as the lens in gravitational microlensing events towards the galactic bulge or a spiral arm. Our estimation is based on expectant survey and observations of FAST (Five hundred meter Aperture Spherical Telescope) and SKA (Square Kilometer Array), and two different models of pulsar distribution are used. We find that the lensing rate is > 1 event/decade, being high enough to search the real events. Therefore, the microlensing observations focusing on pulsars identified by FAST or SKA in the future are meaningful. As an independent determination o…
Young pulsars and the pulsar wind nebulae (PWNe) or supernova remnants (SNRs) that surround them are some of the most dynamic and high-powered environments in our Universe. With the rise of more sensitive observations, the number of pulsar-SNR and PWN associations (hereafter, SNR/PWN) has increased, yet we do not understand to which extent this environment influences the pulsars' impulsive radio signals. We studied the dispersive contribution of SNRs and PWNe on Galactic pulsars, and considered their relevance to fast radio bursts (FRBs) such as FRB 121102. We investigated the dispersion measu…
We propose a new extragalactic but non-cosmological explanation for fast radio bursts (FRBs) based on very young pulsars in supernova remnants. Within a few hundred years of a core-collapse supernova the ejecta is confined within $\sim$1 pc, providing a high enough column density of free electrons for the observed 375-1600 pc cm$^{-3}$ of dispersion measure (DM). By extrapolating a Crab-like pulsar to its infancy in an environment like that of SN 1987A, we hypothesize such an object could emit supergiant pulses sporadically which would be bright enough to be seen at a few hundred megaparsecs. …
We discuss possible association of fast radio bursts (FRBs) with supergiant pulses emitted by young pulsars (ages $\sim$ tens to hundreds of years) born with regular magnetic field but very short -- few milliseconds -- spin periods. FRBs are extra-Galactic events coming from distances $d \lesssim 100$ Mpc. Most of the dispersion measure (DM) comes from the material in the freshly ejected SNR shell; for a given burst the DM should decrease with time. FRBs are not expected to be seen below $\sim 300 $ MHz due to free-free absorption in the expanding ejecta. A supernova might have been detected y…
Fast radio bursts (FRBs) are a new kind of extragalactic radio transients. Some of them show repeating behaviors. Recent observations indicate that a few repeating FRBs (e.g., FRB 121102) present time--frequency downward drifting patterns and nearly 100$\%$ linear polarization. Following the model of \citet{dai 2016} who proposed that repeating FRBs may originate from a slowly-rotating, old-aged pulsar colliding with an asteroid belt around a stellar-mass object, we focus on the prediction of time--frequency drifting and polarization. In this scenario, the frequency drifting is mainly caused b…
The Parkes Pulsar Timing Array (PPTA) project monitors two dozen millisecond pulsars (MSPs) in order to undertake a variety of fundamental physics experiments using the Parkes 64m radio telescope. Since June 2017 we have been undertaking commensal searches for fast radio bursts (FRBs) during the MSP observations. Here, we report the discovery of four FRBs (171209, 180309, 180311 and 180714). The detected events include an FRB with the highest signal-to-noise ratio ever detected at the Parkes observatory, which exhibits unusual spectral properties. All four FRBs are highly polarized. We discuss…
Evidence for Intermediate-Mass Black Holes From Microlensing Signatures in CHIME/FRB catalog 2
School of Physics and Optoelectronic Engineering, Yangtze University（长江大学物理与光电工程学院）；School of Physics and Astronomy, Beijing Normal University（北京师范大学物理学院）；Institute for Frontiers in Astronomy and Astrophysics, Beijing Normal University（北京师范大学天文与前沿科学研究院）；School of Physics and Technology, Wuhan University（武汉大学物理科学与技术学院）
Bright, short radio bursts are emitted by sources at a large range of distances: from the nearby Crab pulsar to remote Fast Radio Bursts (FRBs). FRBs are likely to originate from distant neutron stars, but our knowledge of the radio pulsar population has been limited to the Galaxy and the Magellanic Clouds. In an attempt to increase our understanding of extragalactic pulsar populations, and its giant-pulse emission, we employed the low-frequency radio telescope LOFAR to search the Andromeda Galaxy (M31) for radio bursts emitted by young, Crab-like pulsars. For direct comparison we also present…
We show that the periodic FRB 180916.J0158+65 can be interpreted by invoking an interacting neutron star binary system with an orbital period of $\sim 16$ days. The FRBs are produced by a highly magnetized pulsar, whose magnetic field is ``combed'' by the strong wind from a companion star, either a massive star or a millisecond pulsar. The FRB pulsar wind retains a clear funnel in the companion's wind that is otherwise opaque to induced Compton or Raman scatterings for repeating FRB emission. The 4 day active window corresponds to the time when the funnel points toward Earth. The interaction a…
We report on the polarization properties of two fast radio bursts (FRBs): 151230 and 160102 discovered in the SUrvey for Pulsars and Extragalactic Radio Bursts (SUPERB) at the Parkes radio telescope. FRB 151230 is observed to be 6 $\pm$ 11% circularly polarized and 35 $\pm$ 13 % linearly polarized with a rotation measure (RM) consistent with zero. Conversely, FRB 160102 is observed to have a circular polarization fraction of 30 $\pm$ 11 %, linear polarization fraction of 84 $\pm$ 15 % for RM = -221(6) rad m$^{-2}$ and the highest measured DM (2596.1 $\pm$ 0.3 pc cm$^{-3}$) for an FRB to date. …
We investigate properties of Galactic microlensing events in which a stellar object is lensed by a neutron star. For an all-sky photometric microlensing survey, we determine the number of lensing events caused by $\sim10^{5}$ potentially-observable radio pulsars to be $\sim0.2\ \rm{yr^{-1}}$ for $10^{10}$ background stellar sources. We expect a few detectable events per year for the same number of background sources from an astrometric microlensing survey. We show that such a study could lead to precise measurements of radio pulsar masses. For instance, if a pulsar distance could be constraine…
Depolarization Induced by Rapid Polarization Angle Swings: A Common Feature of Pulsars and Fast Radio Bursts?
University of Science and Technology of China（中国科学技术大学）；Tsung-Dao Lee Institute & School of Physics and Astronomy, Shanghai Jiao Tong University（上海交通大学李政道理论与计算科学中心及物理与天文学院）；Purple Mountain Observatory, Chinese Academy of Sciences（中国科学院紫金山天文台）
The extremely high brightness temperatures of pulsars and fast radio bursts (FRBs) require their radiation mechanisms to be coherent. Coherent curvature radiation by bunches has been long discussed as the mechanism for radio pulsars and recently for FRBs. Assuming that bunches are already generated in pulsar magnetospheres, we calculate the spectrum of coherent curvature radiation under a three-dimensional magnetic field geometry. Different from previous works assuming parallel trajectories and a mono-energetic energy distribution of electrons, we consider a bunch characterized by its length, …
Star quakes and internal phase transitions within anomalous x-ray pulsars (AXPs) and soft $γ$-ray repeaters (SGRs) can produce mini contractions and pulsar glitches. Shocks break out from their surface following such contractions produce thermal x/$γ$-ray bursts. Highly relativistic dipolar $e^+e^-$ bunches launched from the pulsar polar caps emit fast radio bursts (FRBs) of narrowly beamed coherent curvature radiation, visible if they point in the direction of Earth. Although these surface x/$γ$-ray bursts are isotropic and are many orders of magnitude more energetic than the FRBs, they are d…
The mass of single neutron stars (NSs) can be measured using astrometric microlensing events. In such events, the center-of-light motion of a star lensed by a NS will deviate from the expected non-lensed motion and this deviation can be used to measure the mass of the NS. I search for future conjunctions between pulsars, with measured proper motion, and stars in the GAIA-DR2 catalog. I identified one candidate event of a star that will possibly be lensed by a pulsar during the next ten years in which the expected light deflection of the background star will deviate from the non-lensed motion b…
The canonical pulsar magnetosphere contains a bubble of closed magnetic field lines that is separated from the open lines by current sheets, and different branches of such sheets intersect at a critical line on the light cylinder (LC). The LC is located far away from the neutron star, and the pulsar's intrinsic magnetic field at that location is much weaker than the commonly quoted numbers applicable to the star surface. The magnetic field surrounding supermassive black holes that reside in galactic nuclei is of comparable or greater strength. Therefore, when the pulsar travels inside such reg…
We describe the ongoing `Survey for Magnetars, Intermittent pulsars, Rotating radio transients and Fast radio bursts' (SMIRF), performed using the newly refurbished UTMOST telescope. SMIRF repeatedly sweeps the southern Galactic plane performing real-time periodicity and single-pulse searches, and is the first survey of its kind carried out with an interferometer. SMIRF is facilitated by a robotic scheduler which is capable of fully autonomous commensal operations. We report on the SMIRF observational parameters, the data analysis methods, the survey's sensitivities to pulsars, techniques to m…
Primordial black holes (PBHs) with a mass from $10^{-16}$ to $10^{-11}\,M_\odot$ may comprise 100% of dark matter. Due to a combination of wave and finite source size effects, the traditional microlensing of stars does not probe this mass range. In this paper, we point out that X-ray pulsars with higher photon energies and smaller source sizes are good candidate sources for microlensing for this mass window. Among the existing X-ray pulsars, the Small Magellanic Cloud (SMC) X-1 source is found to be the best candidate because of its apparent brightness and long distance from Earth. We have ana…
To understand the radiative processes of pulsars and fast radio bursts with the FAST
University of Chinese Academy of Sciences（中国科学院大学）；Chinese Academy of Sciences（中国科学院）；School of Physics, Peking University（北京大学物理学院）；School of Physics and Mechanical Electrical & Engineering, Hubei University of Education（湖北教育学院物理与机电工程学院）
Discoveries of rotating radio transients and fast radio bursts (FRBs) in pulsar surveys suggest that more of such transient sources await discovery in archival data sets. Here we report on a single-pulse search for dispersed radio bursts over a wide range of Galactic latitudes (|b| < $60^{\circ}$) in data previously searched for periodic sources by Burgay et al. We re-detected 20 of the 42 pulsars reported by Burgay et al. and one rotating radio transient reported by Burke-Spolaor. No FRBs were discovered in this survey. Taking into account this result, and other recent surveys at Parkes, we c…
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 ap…
Since the unexpected discovery of fast radio bursts (FRBs), researchers have proposed varied theories and models to explain these phenomena. One such model that has recently been developed incorporates the so-called Gertsenshtein-Zel'dovich (GZ) effect, which states that when gravitational waves traverse a pulsar magnetosphere, a portion of the gravitational radiation is transformed into electromagnetic (EM) radiation. The observed properties of FRBs are consistent with the properties of this EM radiation, implying, remarkably, that the GZ effect can account for both repeating and non-repeatin…
We report the discovery of four Fast Radio Bursts (FRBs) in the ongoing SUrvey for Pulsars and Extragalactic Radio Bursts (SUPERB) at the Parkes Radio Telescope: FRBs 150610, 151206, 151230 and 160102. Our real-time discoveries have enabled us to conduct extensive, rapid multi-messenger follow-up at 12 major facilities sensitive to radio, optical, X-ray, gamma-ray photons and neutrinos on time scales ranging from an hour to a few months post-burst. No counterparts to the FRBs were found and we provide upper limits on afterglow luminosities. None of the FRBs were seen to repeat. Formal fits to …
We used a new spectral-fitting technique to identify a subpopulation of 6 narrow-band giant pulses from the Crab pulsar out of a total of 1578. These giant pulses were detected in 77 minutes of observations with the 46-m dish at the Algonquin Radio Observatory at 400-800 MHz. The narrow-band giant pulses consist of both main- and inter-pulses, thereby being more likely to be caused by an intrinsic emission mechanism as opposed to a propagation effect. Fast Radio Bursts (FRBs) have demonstrated similar narrow-band features while only little has been observed in the giant pulses of pulsars. We r…
Observations of the Faraday rotation measure, combined with the dispersion measure, can be used to infer the magnetoionic environment of a radio source. We investigate the magnetoionic environments of FRBs by deriving their estimated average magnetic field strengths along the line of sight $\langle B_{\parallel}\rangle$ in their host galaxies and comparing them with those of Galactic pulsars and magnetars. We find that for those FRBs with RM measurements, the mean $\langle B_{\parallel}\rangle$ is $1.77^{+9.01}_{-1.48}\,\rmμG$ and $1.74^{+14.82}_{-1.55}\,\rmμG$ using two different methods, whi…
With upcoming (continuum) surveys of high-resolution radio telescopes, detection rates of fast radio bursts (FRBs) might approach $10^5$ per sky per day by future extremely large observatories, such as the possible extension of the Square Kilometer Array (SKA) to a phase 2 array. Depending on the redshift distribution of FRBs and using the repeating FRB121102 as a model, we calculate a detection rate of multiply-imaged FRBs with their multiply-imaged hosts caused by the distribution of galaxy-cluster scale gravitational lenses of the order of $10^{-4}$ per square degree per year for a minimum …
Fast radio bursts (FRBs) are an emerging class of short and bright radio transients whose sources remain enigmatic. Within the galactic center, the non-detection of pulsars within the inner $\sim \!10\,{\rm pc}$ has created a missing pulsar problem that has intensified with time. With all reserve, we advance the notion that the two problems could be linked by a common solution: the collapse of neutron stars (NS) due to capture and sedimentation of dark matter (DM) within their cores. Bramante \& Linden (2014), Phys.\ Rev.\ Lett.~19, 191301 showed that certain DM properties allow for rapid NS c…
We report on a search for Fast Radio Bursts (FRBs) with the Green Bank Northern Celestial Cap (GBNCC) Pulsar Survey at 350 MHz. Pointings amounting to a total on-sky time of 61 days were searched to a DM of 3000 pc cm$^{-3}$ while the rest (23 days; 29% of the total time) were searched to a DM of 500 pc cm$^{-3}$. No FRBs were detected in the pointings observed through May 2016. We estimate a 95% confidence upper limit on the FRB rate of $3.6\times 10^3$ FRBs sky$^{-1}$ day$^{-1}$ above a peak flux density of 0.63 Jy at 350 MHz for an intrinsic pulse width of 5 ms. We place constraints on the …
Despite existing constraints, it remains possible that up to $35\%$ of all dark matter is comprised of compact objects, such as the black holes in the 10-100\,M$_\odot$ range whose existence has been confirmed by LIGO. The strong gravitational lensing of transients such as FRBs and GRBs has been suggested as a more sensitive probe for compact dark matter than intensity fluctuations observed in microlensing experiments. Recently ASKAP has reported burst substructure down to $15μ$s timescales in FRBs in the redshift range $0.3-0.5$. We investigate here the implications of this for the detectabil…
Recent observations of fast radio bursts (FRBs) indicate a perplexing, inconsistent picture. We propose a unified scenario to interpret diverse FRBs observed. A regular pulsar, otherwise unnoticeable at a cosmological distance, may produce a bright FRB if its magnetosphere is suddenly "combed" by a nearby, strong plasma stream towards the anti-stream direction. If the Earth is to the night side of the stream, the combed magnetic sheath would sweep across the direction of Earth and make a detectable FRB. The stream could be an AGN flare, a GRB or supernova blastwave, a tidal disruption event, o…
Since their serendipitous discovery, Fast Radio Bursts (FRBs) have garnered a great deal of attention from both observers and theorists. A new class of radio telescopes with wide fields of view have enabled a rapid accumulation of FRB observations, confirming that FRBs originate from cosmological distances. The high occurrence rate of FRBs and the development of new instruments to observe them create opportunities for FRBs to be utilized for a host of astrophysical and cosmological studies. We focus on the rare, and as yet undetected, subset of FRBs that undergo repeated bursts and are strongl…
It is currently not known if repeating fast radio bursts (FRBs) are fundamentally different from those that have not been seen to repeat. One striking difference between repeaters and apparent non-repeaters in the CHIME sample is that the once-off events are typically shorter in duration than sources that have been detected two or more times. We offer a simple explanation for this discrepancy based on a selection effect due to beamed emission, in which highly-beamed FRBs are less easily observed to repeat, but are abundant enough to detect often as once-off events. The explanation predicts tha…
Fast radio bursts (FRBs) are extragalactic radio flashes of unknown physical origin. Their high luminosities and short durations require extreme energy densities, like those found in the vicinity of neutron stars and black holes. Studying the burst intensities and polarimetric properties on a wide range of timescales, from milliseconds down to nanoseconds, is key to understanding the emission mechanism. However, high-time-resolution studies of FRBs are limited by their unpredictable activity levels, available instrumentation and temporal broadening in the intervening ionised medium. Here we sh…
Fast radio bursts (FRBs) can be scattered by ionized gas in their local environments, host galaxies, intervening galaxies along their lines-of-sight, the intergalactic medium, and the Milky Way. The relative contributions of these different media depend on their geometric configuration and the internal properties of the gas. When these relative contributions are well understood, FRB scattering is a powerful probe of density fluctuations along the line-of-sight. The precise scattering measurements for FRB 121102 and FRB 180916 allow us to place an upper limit on the amount of scattering contrib…
Asteroids orbiting into the highly magnetized and highly relativistic wind of a pulsar offer a favourable configuration for repeating fast radio bursts (FRB). The body in direct contact with the wind develops a trail formed of a stationary Alfvén wave, called an \textit{Alfvén wing}. When an element of wind crosses the Alfvén wing, it sees a rotation of the ambient magnetic field that can cause radio-wave instabilities. In the observer's reference frame, the waves are collimated in a very narrow range of directions, and they have an extremely high intensity. A previous work, published in 2014,…
The discovery and study of pulsars and Fast Radio Bursts (FRBs) in time-domain radio data is often hampered by radio frequency interference (RFI). Some of this terrestrial RFI is impulsive and bright, and relatively easy to identify and remove. Other anthropogenic signals, however, are weaker yet periodic, and their persistence can drown out astrophysical signals. Here we show that Fourier-domain excision of periodic RFI is an effective and powerful step in detecting weak cosmic signals. We find that applying the method significantly increases the signal-to-noise ratio of transient as well as …
PSR B0950+08 is a bright non-recycled pulsar whose single-pulse fluence variability is reportedly large. Based on observations at two widely separated frequencies, 55 MHz (NenuFAR) and 1.4 GHz (Westerbork Synthesis Radio Telescope), we review the properties of these single pulses. We conclude that they are more similar to ordinary pulses of radio emission than to a special kind of short and bright Giant Pulses, observed from only a handful of pulsars. We argue that temporal variation of properties of interstellar medium along the line of sight to this nearby pulsar, namely the fluctuating size…
We explore possible effects of a magnetar burst on the radio, X-ray, and gamma-ray flux of a pulsar wind nebula (PWN). We assume that the burst injects electron-positron pairs or powers the magnetic field and explore the total energy at injection and the spectral index needed in order to increase the X-ray flux by about one order of magnitude, as well as its decay time until reaching quiescence. We also explore magnetically powered phenomenology that could explain a temporary increase of the PWN synchrotron emitted flux and perhaps the relatively common lack of PWNe surrounding magnetars. This…
The majority of fast radio bursts (FRBs) are poorly localised, hindering their potential scientific yield as galactic, intergalactic, and cosmological probes. LOFT-e, a digital backend for the U.K.'s e-MERLIN seven-telescope interferometer will provide commensal search and real-time detection of FRBs, taking full advantage of its field of view (FoV), sensitivity, and observation time. Upon burst detection, LOFT-e will store raw data offline, enabling the sub-arcsecond localisation provided by e-MERLIN and expanding the pool of localised FRBs. The high-time resolution backend will additionally …
We analyze plasma dispersion and scattering of fast radio bursts (FRBs) to identify the dominant locations of free electrons along their lines of sight and thus constrain the distances of the burst sources themselves. We establish the average $τ$-DM relation for Galactic pulsars and use it as a benchmark for discussing FRB scattering. Though scattering times $τ$ for FRBs are large in the majority of the 17 events we analyze, they are systematically smaller than those of Galactic pulsars that have similar dispersion measures (DMs). The lack of any correlation between $τ$ and DM for FRBs suggest…
Observation of fast radio bursts (FRBs) are rising very quickly with the advent of specialised instruments and surveys, and it has recently been shown that some of them repeat quasi-periodically. In particular, evidence of a $P=16.35$ day period has been reported for FRB 180916.J0158+65. We seek an explanation within the frame of our orbiting asteroid model, whereby FRBs are produced in the plasma wake of asteroids immersed in the wind of a pulsar or a magnetar. We used the data reported by the CHIME/FRB collaboration in order to infer the orbital characteristics of asteroid swarms, and perfor…
Intense radiation field can modify plasma properties, the corresponding refractive index, and lead to such nonlinear propagation effects as self-focusing. We estimate the corresponding effects in pair plasma, both in unmagnetized and strongly magnetically dominated case. First, in the unmagnetized pair plasma the ponderomotive force does not lead to charge separation, but to density depletion. Second, for astrophysically relevant plasmas of pulsar magnetospheres, (and possible loci of Fast Radio Bursts), where cyclotron frequency $ω_B$ dominates over plasma frequency $ω_p$ and the frequency of…
