Rotating radio transient
Encyclopedia
Rotating radio transients (RRATs) are sources of short, moderately bright, radio
pulses, which were first discovered in 2006. RRATs are thought to be pulsar
s, i.e. rotating magnet
ised neutron star
s which emit more sporadically and/or with higher pulse-to-pulse variability than the bulk of the known pulsars. The working definition of what a RRAT is, is a pulsar which is more easily discoverable in a search for bright single pulses, as opposed to in Fourier domain searches so that 'RRAT' is no more than a label and does not represent a distinct class of objects from pulsars.
s. The pulses are comparable to the brightest single pulses observed from pulsars with flux densities
of a few Jansky
at 1.4 GHz
. Andrew Lyne
, a radio astronomer involved in the discovery of RRATs, "guesses that there are only a few dozen brighter radio sources in the sky." The time intervals between detected bursts range from seconds (one pulse period) to hours. Thus radio emission from RRATs is typically only detectable for less than one second per day.
The sporadic emission from RRATs means that they are usually not detectable in standard periodicity searches which use Fourier techniques. Nevertheless underlying periodicity in RRATs can be determined by finding the greatest common denominator of the intervals between pulses. This yields the maximum period but once many pulse arrival times have been determined the periods which are shorter (by an integer factor) can be deemed statistically unlikely. The periods thus determined for RRATs are on the order of 1 second or longer, implying that the pulses are likely to be coming from rotating neutron stars, and led to the name "Rotating Radio Transient" being given. The periods seen in some RRATs are longer than in most radio pulsars, somewhat expected for sources which are (by definition) discovered in searches for individual pulses. Monitoring of RRATs for the past few years has revealed that they are slowing down. For some of the known RRATs this slow-down rate, while small, is larger than that for typical pulsars, and which is again more in line with that of magnetars.
The neutron star nature of RRATs was further confirmed when X-ray
observations of the RRAT J1819-1458
were made using the Chandra X-ray telescope.
Cooling neutron stars have temperatures of order 1 million Kelvin
and so thermally emit at X-ray wavelengths. Measurement of an x-ray spectrum allows the temperature
to be determined, assuming it is thermal emission from the surface of a neutron star. The resulting temperature for RRAT J1819-1458 is much cooler than that found on the surface of magnetars, and suggests that despite some shared properties between RRATs and magnetars, they belong to different populations of neutron stars. None of the other pulsars identified as RRATs has yet been detected in X-ray observation. This is in fact the only detection of these sources outside of the radio band.
of an individual pulse, due to the frequency dependence of the phase velocity
of an electromagnetic wave that travels through an ionized medium. As the interstellar medium
features an ionized component, waves traveling from a pulsar to Earth
are dispersed, and thus pulsar surveys also focused on searching for dispersed waves. The importance of the combination of the two characteristics is such that in initial data processing from the Parkes
Multibeam Pulsar Survey, which is the largest pulsar survey to date, "no search sensitive to single dispersed pulses was included."
After the survey itself had finished, searches began for single dispersed pulses. About a quarter of the pulsars already detected by the survey were found by searching for single dispersed pulses, but there were 17 sources of single dispersed pulses which were not thought to be associated with a pulsar. During follow-up observations, a few of these were found to be pulsars that had been missed in periodicity searches, but 11 sources were characterized by single dispersed pulses, with irregular intervals between pulses lasting from minutes to hours.
space, where the pulsar emission mechanism is thought to fail but may become sporadic as pulsars approach this region. However although this is consistent with some of the behavior of RRATs, the RRATs with known periods and period derivatives do not lie near canonical death regions. Another suggestion is that asteroid
s might form in the debris of the supernova
that formed the neutron star, and infall of these debris in to the light cone of RRATs and some other types of pulsars might cause some of the irregular behavior observed. To confirm this would require directly observing the debris surrounding a neutron star, which is presently not possible, but may be possible in the future with the Square Kilometer Array. Nevertheless, as more RRATs are detected by observatories such as Arecibo
, the Green Bank Telescope
, and the Parkes Observatory
at which RRATs were first discovered, some of the characteristics of RRATs may become clearer.
Radio waves
Radio waves are a type of electromagnetic radiation with wavelengths in the electromagnetic spectrum longer than infrared light. Radio waves have frequencies from 300 GHz to as low as 3 kHz, and corresponding wavelengths from 1 millimeter to 100 kilometers. Like all other electromagnetic waves,...
pulses, which were first discovered in 2006. RRATs are thought to be pulsar
Pulsar
A pulsar is a highly magnetized, rotating neutron star that emits a beam of electromagnetic radiation. The radiation can only be observed when the beam of emission is pointing towards the Earth. This is called the lighthouse effect and gives rise to the pulsed nature that gives pulsars their name...
s, i.e. rotating magnet
Magnet
A magnet is a material or object that produces a magnetic field. This magnetic field is invisible but is responsible for the most notable property of a magnet: a force that pulls on other ferromagnetic materials, such as iron, and attracts or repels other magnets.A permanent magnet is an object...
ised neutron star
Neutron star
A neutron star is a type of stellar remnant that can result from the gravitational collapse of a massive star during a Type II, Type Ib or Type Ic supernova event. Such stars are composed almost entirely of neutrons, which are subatomic particles without electrical charge and with a slightly larger...
s which emit more sporadically and/or with higher pulse-to-pulse variability than the bulk of the known pulsars. The working definition of what a RRAT is, is a pulsar which is more easily discoverable in a search for bright single pulses, as opposed to in Fourier domain searches so that 'RRAT' is no more than a label and does not represent a distinct class of objects from pulsars.
General characteristics
Pulses from RRATs are short in duration, lasting from a few millisecondMillisecond
A millisecond is a thousandth of a second.10 milliseconds are called a centisecond....
s. The pulses are comparable to the brightest single pulses observed from pulsars with flux densities
Radiative flux
Radiative flux, or radiative flux density, is the amount of power radiated through a given area, in the form of photons or other elementary particles, typically measured in W/m2. It is used in astronomy to determine the magnitude and spectral class of a star...
of a few Jansky
Jansky
The flux unit or jansky is a non-SI unit of spectral flux density equivalent to 10−26 watts per square metre per hertz...
at 1.4 GHz
GHZ
GHZ or GHz may refer to:# Gigahertz .# Greenberger-Horne-Zeilinger state — a quantum entanglement of three particles.# Galactic Habitable Zone — the region of a galaxy that is favorable to the formation of life....
. Andrew Lyne
Andrew Lyne
Andrew G. Lyne FRS is a British physicist. Lyne is Langworthy Professor of Physics in the School of Physics and Astronomy, University of Manchester, as well as an ex-director of the Jodrell Bank Observatory. Despite retiring in 2007 he remains an active researcher within the Jodrell Bank Pulsar...
, a radio astronomer involved in the discovery of RRATs, "guesses that there are only a few dozen brighter radio sources in the sky." The time intervals between detected bursts range from seconds (one pulse period) to hours. Thus radio emission from RRATs is typically only detectable for less than one second per day.
The sporadic emission from RRATs means that they are usually not detectable in standard periodicity searches which use Fourier techniques. Nevertheless underlying periodicity in RRATs can be determined by finding the greatest common denominator of the intervals between pulses. This yields the maximum period but once many pulse arrival times have been determined the periods which are shorter (by an integer factor) can be deemed statistically unlikely. The periods thus determined for RRATs are on the order of 1 second or longer, implying that the pulses are likely to be coming from rotating neutron stars, and led to the name "Rotating Radio Transient" being given. The periods seen in some RRATs are longer than in most radio pulsars, somewhat expected for sources which are (by definition) discovered in searches for individual pulses. Monitoring of RRATs for the past few years has revealed that they are slowing down. For some of the known RRATs this slow-down rate, while small, is larger than that for typical pulsars, and which is again more in line with that of magnetars.
The neutron star nature of RRATs was further confirmed when X-ray
X-ray
X-radiation is a form of electromagnetic radiation. X-rays have a wavelength in the range of 0.01 to 10 nanometers, corresponding to frequencies in the range 30 petahertz to 30 exahertz and energies in the range 120 eV to 120 keV. They are shorter in wavelength than UV rays and longer than gamma...
observations of the RRAT J1819-1458
RRAT J1819-1458
RRAT J1819-1458 is a Milky Way neutron star and the best studied of the class of rotating radio transients first discovered in 2006.-General characteristics:...
were made using the Chandra X-ray telescope.
Cooling neutron stars have temperatures of order 1 million Kelvin
Kelvin
The kelvin is a unit of measurement for temperature. It is one of the seven base units in the International System of Units and is assigned the unit symbol K. The Kelvin scale is an absolute, thermodynamic temperature scale using as its null point absolute zero, the temperature at which all...
and so thermally emit at X-ray wavelengths. Measurement of an x-ray spectrum allows the temperature
Temperature
Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...
to be determined, assuming it is thermal emission from the surface of a neutron star. The resulting temperature for RRAT J1819-1458 is much cooler than that found on the surface of magnetars, and suggests that despite some shared properties between RRATs and magnetars, they belong to different populations of neutron stars. None of the other pulsars identified as RRATs has yet been detected in X-ray observation. This is in fact the only detection of these sources outside of the radio band.
Discovery
After the discovery of pulsars in 1967, searches for more pulsars relied on two key characteristics of pulsar pulses in order to distinguish pulsars from noise caused by terrestrial radio signals. The first is the periodic nature of pulsars. By performing periodicity searches through data, "pulsars are detected with much higher signal-to-noise ratios" than when simply looking for individual pulses. The second defining characteristic of pulsar signals is the dispersion in frequencyFrequency
Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency...
of an individual pulse, due to the frequency dependence of the phase velocity
Phase velocity
The phase velocity of a wave is the rate at which the phase of the wave propagates in space. This is the speed at which the phase of any one frequency component of the wave travels. For such a component, any given phase of the wave will appear to travel at the phase velocity...
of an electromagnetic wave that travels through an ionized medium. As the interstellar medium
Interstellar medium
In astronomy, the interstellar medium is the matter that exists in the space between the star systems in a galaxy. This matter includes gas in ionic, atomic, and molecular form, dust, and cosmic rays. It fills interstellar space and blends smoothly into the surrounding intergalactic space...
features an ionized component, waves traveling from a pulsar to Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...
are dispersed, and thus pulsar surveys also focused on searching for dispersed waves. The importance of the combination of the two characteristics is such that in initial data processing from the Parkes
Parkes Observatory
The Parkes Observatory is a radio telescope observatory, 20 kilometres north of the town of Parkes, New South Wales, Australia. It was one of several radio antennas used to receive live, televised images of the Apollo 11 moon landing on 20 July 1969....
Multibeam Pulsar Survey, which is the largest pulsar survey to date, "no search sensitive to single dispersed pulses was included."
After the survey itself had finished, searches began for single dispersed pulses. About a quarter of the pulsars already detected by the survey were found by searching for single dispersed pulses, but there were 17 sources of single dispersed pulses which were not thought to be associated with a pulsar. During follow-up observations, a few of these were found to be pulsars that had been missed in periodicity searches, but 11 sources were characterized by single dispersed pulses, with irregular intervals between pulses lasting from minutes to hours.
Possible pulse mechanisms
In order to explain the irregularity of RRAT pulses, we note that most of the pulsars which have been labelled as RRATs are entirely consistent with pulsars which have regular underlying emission which is simply undetectable due to the low intrinsic brightness or large distance of the sources. However, assuming that when we do not detect pulses from these pulsars that they are truly 'off', several authors have proposed mechanisms whereby such sporadic emission could be explained. For example, as pulsars gradually lose energy, they approach what is called the pulsar "death death valley," a theoretical area in pulsar pulsar period - period derivativeDerivative
In calculus, a branch of mathematics, the derivative is a measure of how a function changes as its input changes. Loosely speaking, a derivative can be thought of as how much one quantity is changing in response to changes in some other quantity; for example, the derivative of the position of a...
space, where the pulsar emission mechanism is thought to fail but may become sporadic as pulsars approach this region. However although this is consistent with some of the behavior of RRATs, the RRATs with known periods and period derivatives do not lie near canonical death regions. Another suggestion is that asteroid
Asteroid
Asteroids are a class of small Solar System bodies in orbit around the Sun. They have also been called planetoids, especially the larger ones...
s might form in the debris of the supernova
Supernova
A supernova is a stellar explosion that is more energetic than a nova. It is pronounced with the plural supernovae or supernovas. Supernovae are extremely luminous and cause a burst of radiation that often briefly outshines an entire galaxy, before fading from view over several weeks or months...
that formed the neutron star, and infall of these debris in to the light cone of RRATs and some other types of pulsars might cause some of the irregular behavior observed. To confirm this would require directly observing the debris surrounding a neutron star, which is presently not possible, but may be possible in the future with the Square Kilometer Array. Nevertheless, as more RRATs are detected by observatories such as Arecibo
Arecibo
Arecibo may refer to:*Arecibo, Puerto Rico, a municipality located by the Atlantic Ocean*Arecibo Observatory, a very sensitive radio telescope located approximately south-southwest from the city of Arecibo...
, the Green Bank Telescope
Green Bank Telescope
The Robert C. Byrd Green Bank Telescope is the world's largest fully steerable radio telescope and the world's largest land-based movable structure. It is part of the National Radio Astronomy Observatory site at Green Bank, West Virginia, USA. The telescope honors the name of the late Senator...
, and the Parkes Observatory
Parkes Observatory
The Parkes Observatory is a radio telescope observatory, 20 kilometres north of the town of Parkes, New South Wales, Australia. It was one of several radio antennas used to receive live, televised images of the Apollo 11 moon landing on 20 July 1969....
at which RRATs were first discovered, some of the characteristics of RRATs may become clearer.