 The PALFA survey logo | |
| Survey type | astronomical survey  |
|---|---|
| Target | pulsar |
| Observations | Arecibo Telescope |
| Wavelength | 21 centimetre |
| Website | www2 |
 Related media on Wikimedia Commons | |
PALFA is a large-scale survey for radio pulsars at 1.4 GHz using the Arecibo 305-meter telescope and the ALFA multibeam receivers. It is the largest and most sensitive survey of the Galactic plane to date. [1]
Most of the advances in pulsar astronomy were due to the discovery of new objects. A major increase in search sensitivity has already started a new era of discovery at the Arecibo Observatory.
This increase in search sensitivity is due first and foremost by the ALFA receiver and the pulsar surveys it makes possible, which are now being carried by the Pulsar Consortium using the Arecibo 305 m radio telescope. Preliminary estimates (see below) indicate that the Arecibo Galactic plane survey using ALFA could find many hundreds of new pulsars. As of November 2019, the survey had already discovered a total of 192 new pulsars.
The survey is targeting low Galactic latitudes (|b|≤5°) in the Galactic longitude ranges accessible by the Arecibo telescope (32°<l<77°) and 168°<l<214°). Upon completion, it is expected to find hundreds of new pulsars. Highlights from the survey include the discoveries of the second most relativistic binary pulsar known, PSR J1906+0746, [2] and the first eccentric binary millisecond pulsar in the Galactic plane, PSR J1903+0327 [3] In addition, PALFA has produced the first pulsar discovery by volunteer computing, PSR J2007+2722, through the Einstein@Home distributed computing project. [4] The discoveries include all known varieties of rotation-powered neutron stars: millisecond pulsars, [5] [6] relativistic binaries, mildly recycled, [4] normal, and energetic young pulsars. [7] Fourteen of the ALFA objects have been identified through their intermittent single pulses and are likely rotating radio transients (RRATs). [8] The rest have been discovered via blind periodicity searches. Follow-up radio timing observations are carried out through a coordinated effort between Arecibo, Jodrell Bank, Greenbank, and Nançay to obtain phase-connected timing solutions for these pulsars.
The first fast radio burst to be discovered by a telescope other than the Parkes radio telescope was identified in a PALFA pointing in the Galactic anti-center. [9] This was the first repeating fast radio burst ever discovered.
The PALFA survey is carried out by an international consortium of scientists from the United States, Canada, Germany, Netherlands, United Kingdom, Australia, and France. The Principal Investigator of the PALFA survey is Prof. Victoria Kaspi of McGill University.
With ALFA, 47 pointings are needed to cover one square degree, compared to about 330 pointings needed to cover one square degree with similar density with a single-pixel feed. Initially, the survey used the Wideband Arecibo Pulsar Processors (WAPPs) to detect the signal from ALFA's seven beams. These cover 100 MHz of band (with dual polarization capability), initially centered at 1420 MHz and now at 1440 MHz. For search purposes, 256-channel spectra are produced every 64 microseconds. In 2009, the survey transitioned to new and improved back-ends, the Mock polyphase filterbank spectrometers, which are capable of covering 300 MHz (from 1225 MHz to 1525 MHz, the bandwidth covered by ALFA) for each of the seven beams (see detailed technical specifications here). This has led to greatly increased search sensitivity and better means to deal with all the radio frequency interference.
Many of the detections to date have been made with a quick reduction package that allows us to find pulsars almost in real time. This is made possible by reducing the spectral and time resolution by a factor of 16, and using a computer cluster, the Arecibo Signal Processor to search for pulsars in the data. This is a nice and quick way of detecting slow pulsars, but the sensitivity to fast pulsars is severely degraded. Re-processing these data with full resolution is, computationally, a very challenging task but is essential for detecting many fast (both young and recycled) pulsars so far hidden by Galactic plasma.
It is expected that, over the next several years, this survey will generate over 1000 Terabytes of data. The data is stored at the Cornell University Center for Advanced Computing. The full-resolution raw data is processed independently by three software pipelines.
The Cornell University pipeline has conducted standard periodicity search and single-pulse search without doing an acceleration search. It has been run on all WAPP data archived at the Cornell University Center for Advanced Computing and has provided 2.5 million signal candidates. Winnowing of this vast set of candidates is currently under way.
The second pipeline is based on PRESTO, a large suite of pulsar search and analysis software developed by Scott Ransom. It employs a Fourier-domain acceleration search technique, which compensates for the loss of detection sensitivity in a traditional periodicity search due to a rapidly changing frequency of the periodic pulsar signal. Such frequency modulation can occur, for instance, due to a pulsar's orbital motion in a compact binary. This approach thus significantly boosts sensitivity to binary pulsars. The PRESTO pipeline is run on dedicated clusters at several institutions that participate in the ALFA survey, producing over 3 million signal candidates. Over the past two years, the Guillimin supercomputer, managed by McGill University as part of CLUMEQ, has been processing most of the PALFA data with PRESTO.
Since March 2009, part of the Einstein@Home computing power is used to analyze PALFA data. The Einstein@Home algorithm is particularly sensitive to radio pulsars in tight binary systems (as short as 11 minutes), with a phase-space coverage that is complementary to that of the PRESTO pipeline. To date, it has re-detected 123 previously known radio pulsars as well as several previously unknown pulsars.
The data processed thus far has revealed that the radio frequency interference (RFI) environment at Arecibo significantly affects the detection threshold of the survey, creating unforeseen challenges in identifying the many weak pulsars that are likely lurking in the data. To address this, the PALFA consortium is actively developing novel techniques for identification, mitigation, and excision of RFI. We are also implementing a variety of heuristics as well as machine learning algorithms for identifying real pulsars among the millions of signal candidates, most of which appear to be due to RFI. The inevitable growth in the incidence and variety of man-made RFI suggests that this problem will likely be important for all future radio pulsar surveys.
The Arecibo Remote Command Center (ARCC) at the University of Texas at Brownsville, the University of Wisconsin–Milwaukee, and Franklin & Marshall College is currently engaged in searching for radio pulsars in PALFA data. ARCC is an integrated research/education facility that allows students at the high school and undergraduate level to be directly involved with the research at the Arecibo telescope. Web based tools have been developed so that students can rank the pulsar candidates created by the PRESTO analysis.
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PSR B1257+12, previously designated PSR 1257+12, alternatively designated PSR J1300+1240, is a millisecond pulsar located 2,300 light-years from the Sun in the constellation of Virgo, rotating at about 161 times per second. It is also named Lich, after a powerful, fictional undead creature of the same name.
Messier 15 or M15 is a globular cluster in the constellation Pegasus. It was discovered by Jean-Dominique Maraldi in 1746 and included in Charles Messier's catalogue of comet-like objects in 1764. At an estimated 12.5±1.3 billion years old, it is one of the oldest known globular clusters.
Einstein@Home is a volunteer distributed computing project that searches for signals from spinning neutron stars in data from gravitational-wave detectors, from large radio telescopes, and from a gamma-ray telescope. Neutron stars are detected by their pulsed radio and gamma-ray emission as radio and/or gamma-ray pulsars. They also might be observable as continuous gravitational wave sources if they are rapidly spinning and non-axisymmetrically deformed. The project was officially launched on 19 February 2005 as part of the American Physical Society's contribution to the World Year of Physics 2005 event.
A pulsar is a highly magnetized rotating compact star that emits beams of electromagnetic radiation out of its magnetic poles. This radiation can be observed only when a beam of emission is pointing toward Earth, and is responsible for the pulsed appearance of emission. Neutron stars are very dense and have short, regular rotational periods. This produces a very precise interval between pulses that ranges from milliseconds to seconds for an individual pulsar. Pulsars are one of the candidates for the source of ultra-high-energy cosmic rays.
A millisecond pulsar (MSP) is a pulsar with a rotational period less than about 10 milliseconds. Millisecond pulsars have been detected in radio, X-ray, and gamma ray portions of the electromagnetic spectrum. The leading theory for the origin of millisecond pulsars is that they are old, rapidly rotating neutron stars that have been spun up or "recycled" through accretion of matter from a companion star in a close binary system. For this reason, millisecond pulsars are sometimes called recycled pulsars.
The Hulse–Taylor binary is a binary star system composed of a neutron star and a pulsar which orbit around their common center of mass. It is the first binary pulsar ever discovered.
A binary pulsar is a pulsar with a binary companion, often a white dwarf or neutron star. Binary pulsars are one of the few objects which allow physicists to test general relativity because of the strong gravitational fields in their vicinities. Although the binary companion to the pulsar is usually difficult or impossible to observe directly, its presence can be deduced from the timing of the pulses from the pulsar itself, which can be measured with extraordinary accuracy by radio telescopes.
Palomar 5 is a globular cluster discovered by Walter Baade in 1950. It was independently found again by Albert George Wilson in 1955. After the initial name of Serpens, it was subsequently catalogued as Palomar 5.
NGC 6760 is a globular cluster in the constellation Aquila. It may have contributed to the formation of the open cluster Ruprecht 127 during NGC 6760's passage through the galactic disk 71 million years ago.
PSR J1903+0327 is a millisecond pulsar in a highly eccentric binary orbit.
PSR B1937+21 is a pulsar located in the constellation Vulpecula a few degrees in the sky away from the first discovered pulsar, PSR B1919+21. The name PSR B1937+21 is derived from the word "pulsar" and the declination and right ascension at which it is located, with the "B" indicating that the coordinates are for the 1950.0 epoch. PSR B1937+21 was discovered in 1982 by Don Backer, Shri Kulkarni, Carl Heiles, Michael Davis, and Miller Goss.
PSR J0108−1431 is a solitary pulsar located at a distance of about 130 parsecs (424 light-years) in the constellation Cetus. This pulsar was discovered in 1994 during the Parkes Southern Pulsar Survey. It is considered a very old pulsar with an estimated age of 166 million years and a rotation period of 0.8 seconds. The rotational energy being generated by the spin-down of this pulsar is 5.8 × 1023 W and the surface magnetic field is 2.5 × 107 T. As of 2008, it is the second faintest known pulsar.
PSR J2007+2722 is a 40.8-hertz isolated pulsar in the Vulpecula constellation, 5.3 kpc (17,000 ly) distant in the plane of the Galaxy, and is most likely a disrupted recycled pulsar (DRP).
PSR J1614–2230 is a pulsar in a binary system with a white dwarf. It was discovered in 2006 with the Parkes telescope in a survey of unidentified gamma ray sources in the Energetic Gamma Ray Experiment Telescope catalog. PSR J1614–2230 is a millisecond pulsar, a type of neutron star, that spins on its axis roughly 317 times per second, corresponding to a period of 3.15 milliseconds. Like all pulsars, it emits radiation in a beam, similar to a lighthouse. Emission from PSR J1614–2230 is observed as pulses at the spin period of PSR J1614–2230. The pulsed nature of its emission allows for the arrival of individual pulses to be timed. By measuring the arrival time of pulses, astronomers observed the delay of pulse arrivals from PSR J1614–2230 when it was passing behind its companion from the vantage point of Earth. By measuring this delay, known as the Shapiro delay, astronomers determined the mass of PSR J1614–2230 and its companion. The team performing the observations found that the mass of PSR J1614–2230 is 1.97 ± 0.04 M☉. This mass made PSR J1614–2230 the most massive known neutron star at the time of discovery, and rules out many neutron star equations of state that include exotic matter such as hyperons and kaon condensates.
The North American Nanohertz Observatory for Gravitational Waves (NANOGrav) is a consortium of astronomers who share a common goal of detecting gravitational waves via regular observations of an ensemble of millisecond pulsars using the Green Bank and Arecibo radio telescopes. This project is being carried out in collaboration with international partners in the Parkes Pulsar Timing Array in Australia and the European Pulsar Timing Array as part of the International Pulsar Timing Array.
PSR J1311–3430 is a pulsar with a spin period of 2.5 milliseconds. It is the first millisecond pulsar found via gamma-ray pulsations. The source was originally identified by the Energetic Gamma Ray Experiment Telescope as a bright gamma ray source, but was not recognized as a pulsar until observations with the Fermi Gamma-ray Space Telescope discovered pulsed gamma ray emission. The pulsar has a helium-dominated companion much less massive than itself, and the two are in an orbit with a period of 93.8 minutes. The system is explained by a model where mass from the low mass companion was transferred on to the pulsar, increasing the mass of the pulsar and decreasing its period. These systems are known as Black Widow Pulsars, named after the original such system discovered, PSR B1957+20, and may eventually lead to the companion being completely vaporized. Among systems like these, the orbital period of PSR J1311–3430 is the shortest ever found. Spectroscopic observations of the companion suggest that the mass of the pulsar is 2.7 . Though there is considerable uncertainty in this estimate, the minimum mass for the pulsar that the authors find adequately fits the data is 2.15 , which is still more massive than PSR J1614−2230, the previous record holder for most massive known pulsar.
PSR J0348+0432 is a pulsar-white dwarf binary system. It was discovered in 2007 with the National Radio Astronomy Observatory, Green Bank's Robert C. Byrd Green Bank Telescope in a drift-scan survey.
In radio astronomy, a fast radio burst (FRB) is a transient radio pulse of length ranging from a fraction of a millisecond to a few milliseconds, caused by some high-energy astrophysical process not yet understood. Astronomers estimate the average FRB releases as much energy in a millisecond as the Sun puts out in 3 days. While extremely energetic at their source, the strength of the signal reaching Earth has been described as 1,000 times less than from a mobile phone on the Moon. The first FRB was discovered by Duncan Lorimer and his student David Narkevic in 2007 when they were looking through archival pulsar survey data, and it is therefore commonly referred to as the Lorimer Burst. Many FRBs have since been recorded, including several that have been detected to repeat in seemingly irregular ways. Nonetheless, one FRB has been detected to repeat in a regular way: particularly, FRB 180916 seems to pulse every 16.35 days. Most FRBs are extragalactic, but the first Milky Way FRB was detected by the CHIME radio telescope in April 2020. In June 2021, astronomers reported over 500 FRBs from outer space detected.
In radio astronomy perytons are short radio signals having a duration of a few milliseconds, detected only by the 64-meter Parkes radio telescope in Australia since 1998. They are named after the Peryton, a mythical creature by Jorge Luis Borges.
Ingrid Stairs is a Canadian astronomer currently based at the University of British Columbia. She studies pulsars and their companions as a way to study binary pulsar evolution, pulsar instrumentation and polarimetry, and Fast Radio Bursts (FRBs). She was awarded the 2017 Rutherford Memorial Medal for physics of the Royal Society of Canada, and was elected as a Fellow of the American Physical Society in 2018.
