Rotating radio transient

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Rotating radio transients (RRATs) are sources of short, moderately bright, radio pulses, which were first discovered in 2006. [1] RRATs are thought to be pulsars, i.e. rotating magnetised neutron stars 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 little more than a label (of how they are discovered) and does not represent a distinct class of objects from pulsars. As of March 2015 over 100 have been reported. [2]

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General characteristics

Pulses from RRATs are short in duration, lasting from a few milliseconds. The pulses are comparable to the brightest single pulses observed from pulsars with flux densities of a few Jansky at 1.4 GHz. [1] 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." [3] 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. [1]

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. [4]

The neutron star nature of RRATs was further confirmed when X-ray observations of the RRAT J1819-1458 were made using the space-based Chandra X-ray Observatory. [5] Cooling neutron stars have temperatures of order 1 million kelvins 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.

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. [6] The second defining characteristic of pulsar signals is the dispersion in frequency 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." [6]

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. [6] 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. [1]

As of March 2015 over 100 have been reported, with dispersion measures up to 764 cm−3pc. [2]

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 valley," a theoretical area in pulsar pulsar period—period derivative 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, [7] the RRATs with known periods and period derivatives do not lie near canonical death regions. [6] Another suggestion is that asteroids 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. [8] Since most RRATs have large dispersion measures that indicate larger distances, combining with the similar emission properties, some RRATs could be due to the telescope detection threshold. Nevertheless, the possibility that RRATs share the similar emission mechanism with those pulsars with so called "giant pulses" can neither be excluded. [9] To fully understand the emission mechanisms of RRATs would require directly observing the debris surrounding a neutron star, which is not possible now, 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.

See also

Related Research Articles

<span class="mw-page-title-main">Neutron star</span> Collapsed core of a massive star

A neutron star is the collapsed core of a massive supergiant star. It results from the supernova explosion of a massive star—combined with gravitational collapse—that compresses the core past white dwarf star density to that of atomic nuclei. Except for black holes, neutron stars are the smallest and densest known class of stellar objects. They have a radius on the order of 10 kilometers (6 mi) and a mass of about 1.4 M☉. Stars that collapse into neutron stars have a total mass of between 10 and 25 solar masses (M☉), or possibly more for those that are especially rich in elements heavier than hydrogen and helium.

<span class="mw-page-title-main">Magnetar</span> Type of neutron star with a strong magnetic field

A magnetar is a type of neutron star with an extremely powerful magnetic field (~109 to 1011 T, ~1013 to 1015 G). The magnetic-field decay powers the emission of high-energy electromagnetic radiation, particularly X-rays and gamma rays.

<span class="mw-page-title-main">X-ray binary</span> Class of binary stars

X-ray binaries are a class of binary stars that are luminous in X-rays. The X-rays are produced by matter falling from one component, called the donor, to the other component, called the accretor, which is either a neutron star or black hole. The infalling matter releases gravitational potential energy, up to 30 percent of its rest mass, as X-rays. The lifetime and the mass-transfer rate in an X-ray binary depends on the evolutionary status of the donor star, the mass ratio between the stellar components, and their orbital separation.

Stellar radio sources, radio source stars or radio stars are stellar objects that produce copious emissions of various radio frequencies, whether constant or pulsed. Radio emissions from stars can be produced in many varied ways.

X-ray pulsars or accretion-powered pulsars are a class of astronomical objects that are X-ray sources displaying strict periodic variations in X-ray intensity. The X-ray periods range from as little as a fraction of a second to as much as several minutes.

An astronomical radio source is an object in outer space that emits strong radio waves. Radio emission comes from a wide variety of sources. Such objects are among the most extreme and energetic physical processes in the universe.

<span class="mw-page-title-main">Pulsar</span> Rapidly rotating neutron star

A pulsar is a highly magnetized rotating neutron 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.

<span class="mw-page-title-main">Crab Pulsar</span> Pulsar in the constellation Taurus

The Crab Pulsar is a relatively young neutron star. The star is the central star in the Crab Nebula, a remnant of the supernova SN 1054, which was widely observed on Earth in the year 1054. Discovered in 1968, the pulsar was the first to be connected with a supernova remnant.

GCRT J1745−3009 is a Galactic Center radio transient (GCRT), or bursting low-frequency radio source which lies in the direction of the Galactic Center.

<span class="mw-page-title-main">Astropulse</span> BOINC based volunteer computing SETI@home subproject

Astropulse is a volunteer computing project to search for primordial black holes, pulsars, and extraterrestrial intelligence (ETI). Volunteer resources are harnessed through Berkeley Open Infrastructure for Network Computing (BOINC) platform. In 1999, the Space Sciences Laboratory launched SETI@home, which would rely on massively parallel computation on desktop computers scattered around the world. SETI@home utilizes recorded data from the Arecibo radio telescope and searches for narrow-bandwidth radio signals from space, signifying the presence of extraterrestrial technology. It was soon recognized that this same data might be scoured for other signals of value to the astronomy and physics community.

<span class="mw-page-title-main">Radio-quiet neutron star</span> Neutron star that does not emit radio waves

A radio-quiet neutron star is a neutron star that does not seem to emit radio emissions, but is still visible to Earth through electromagnetic radiation at other parts of the spectrum, particularly X-rays and gamma rays.

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<span class="mw-page-title-main">PSR B1937+21</span> Pulsar in the constellation Vulpecula

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.

<span class="mw-page-title-main">Astrophysical X-ray source</span> Astronomical object emitting X-rays

Astrophysical X-ray sources are astronomical objects with physical properties which result in the emission of X-rays.

RRAT J1819-1458 is a Milky Way neutron star and the best studied of the class of rotating radio transients (RRATs) first discovered in 2006.

PSR J1614–2230 is a pulsar in a binary system with a white dwarf in the constellation Scorpius. 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.

<span class="mw-page-title-main">PALFA Survey</span>

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.

<span class="mw-page-title-main">SGR 1935+2154</span> Soft gamma repeater in the constellation Vulpecula

SGR 1935+2154 is a soft gamma repeater (SGR) that is an ancient stellar remnant, in the constellation Vulpecula, originally discovered in 2014 by the Neil Gehrels Swift Observatory. Currently, the SGR-phenomena and the related anomalous X-ray pulsars (AXP) are explained as arising from magnetars. On 28 April 2020, this remnant about 30,000 light-years away in our Milky Way galaxy was observed to be associated with a very powerful radio pulse known as a fast radio burst or FRB, and a related x-ray flare. The detection is notable as the first FRB detected inside the Milky Way, and the first to be linked to a known source. Later in 2020, SGR 1935+2154 was found to be associated with repeating fast radio bursts.

A central compact object (CCO) is an x-ray source found near the center of a young, nearby supernova remnant (SNR). Given the observed x-ray flux and spectra observed from these objects, the almost certain conclusion is that CCOs are the remnant neutron stars which resulted from the recent supernova. Unlike most pulsars, CCOs generally lack pulsed radio emission or variation in the observed x-rays due to such phenomena being either nonexistent or difficult to detect. The weaker magnetic fields than most other detected neutron stars means that most of the detected x-rays are due to blackbody radiation. Confirmation that the CCO is associated with the past supernova can be done using the kinematics of the objects and matching them to the age and kinematics of the host SNR.

ASKAP J1935+2148 is a neutron star/magnetar candidate located in the constellation Vulpecula, approximately 15,800 light-years away. With a rotation period of 53.8 minutes, it would be the slowest spinning neutron star ever discovered.

References

  1. 1 2 3 4 McLaughlin, M. A.; et al. (2006). "Transient radio bursts from rotating neutron stars". Nature. 439 (7078): 817–820. arXiv: astro-ph/0511587 . Bibcode:2006Natur.439..817M. doi:10.1038/nature04440. PMID   16482150. S2CID   4402381.
  2. 1 2 RRATALOG table
  3. ↑ Bjorn Carey (2007-02-15). "Astronomers Discover Peek-A-Boo Stars". Space.com .
  4. ↑ Burgay, M.; McLaughlin, M. A.; Reynolds, S. P. (2007). "On the debated nature of Rotating RAdio Transients". The multicolored landscape of compact objects and their explosive origins. AIP Conference Proceedings. Vol. 924. AIP Conference Proceedings. pp. 607–612. Bibcode:2007AIPC..924..607B. doi:10.1063/1.2774917.
  5. ↑ Gaensler, B. M.; et al. (2007). "Chandra smells a RRAT: X-ray Detection of a Rotating Radio Transient". Astrophysics and Space Science. 308 (1–4): 95–99. arXiv: astro-ph/0608311 . Bibcode:2007Ap&SS.308...95G. doi:10.1007/s10509-007-9352-8. S2CID   118787389.
  6. 1 2 3 4 McLaughlin, Maura (2009). "Rotating Radio Transients". In Warner Becker (ed.). Neutron Stars and Pulsars. Berlin: Springer. pp. 41–66.
  7. ↑ Zhang, B.; Gil, J.; Dyks, J. (2007). "On the origins of part-time radio pulsars". Monthly Notices of the Royal Astronomical Society . 374 (3): 1103–1107. arXiv: astro-ph/0601063 . Bibcode:2007MNRAS.374.1103Z. doi: 10.1111/j.1365-2966.2006.11226.x . S2CID   5995242.
  8. ↑ Cordes, J. M.; Shannon, R. M. (2008). "Rocking the Lighthouse: Circumpulsar Asteroids and Radio Intermittency". The Astrophysical Journal. 682 (2): 1152–1165. arXiv: astro-ph/0605145 . Bibcode:2008ApJ...682.1152C. doi:10.1086/589425. S2CID   17602731.
  9. ↑ Hu, H.-D.; Esamdin, A; Yuan, J.-P.; Liu, Z.-Y.; Xu, R.-X.; Li, J.; Tao, G.-C.; Wang, N. (2011). "Strong pulses detected from a rotating radio transient J1819-1458". Astronomy and Astrophysics . 530: A67. arXiv: 1104.3256 . Bibcode:2011A&A...530A..67H. doi:10.1051/0004-6361/201015953. S2CID   53482941.
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