GeminiFocus April 2017 | Page 6

Cold Plasma Dispersion
When electromagnetic waves pass through interstellar plasma , the inertia of electrons moving in response to the electric fields causes the lower frequency waves to propagate slower than the higher frequency waves . For non-relativistic , diffuse plasma , the pulse arrival time difference between two frequencies is given by
where the dispersion measure is the integral of the electron density from the source to the observer , ν is the radio frequency and
m e
, e and c are the mass and charge of an electron and the speed of light , respectively . The Milky Way interstellar medium ( ISM ) contribution to the DM along different lines of sight has been characterized using pulsar DM measurements , Hi maps and Galactic models . Any excess in DM would have to be attributed to either excess electrons near the source or the intergalactic medium ( IGM ).
Theoretical Models for FRBs
The simplest , yet unbelievable , explanation is that the source is extragalactic and the excess DM is contributed by the electrons in the intergalactic medium ( IGM ) — placing the source of the Lorimer burst at a redshift of z ~ 0.3 , a distance of ~ 1 billion parsecs . The emitted power at the source would have been 10 42 erg / s , about a billion times more luminous than the brightest radio pulsars ever observed in the Milky Way .
The Population of FRBs
Over the next decade , such radio bursts were detected at multiple radio telescopes
Due to the very short timescale ( few milliseconds ) and the bright , often polarized emission , it is almost necessary to invoke a compact magnetic field to produce an FRB , making some variety of neutron stars an obvious choice for FRB sources . However , the observed energy scales of FRBs are far higher than those of galactic radio pulsars . A plethora of models have been proposed including magnetar giant flares , Crab-like giant pulses from young extra-galactic pulsars , planets in pulsar magnetospheres , asteroids impacting neutron stars , neutron-star mergers and neutron stars collapsing into blackholes , black hole-neutron star mergers , magnetar pulse-wind interactions , flares from nearby stars , quark novae , and axion stars . For a more complete review , please see Katz , 2016 .
— Parkes , Green Bank ( West Virginia ), Arecibo ( Puerto Rico ), and Molonglo ( near Canberra , Australia ) — and came to be known as Fast Radio Bursts ( FRBs ). To date , only 26 bursts have been reported in the literature , but considering the narrow fields-of-view of radio telescopes and the survey durations , the expected sky rate of FRBs is large — 10 3 per sky per day above a peak flux density of 1 Jy at an observing frequency of 1.4 GHz ( Lawrence et al ., 2016 ).
Despite this prodigious rate , we have little knowledge about the sources that emit FRBs and the emission mechanisms that allow such luminous coherent bursts . Until this work , even the distance to any FRB was only estimated from the excess DM . Due to the paucity of observational constraints , there are more theoretical models of FRBs than the total number of observations ( see box at lower left ). In the future , FRBs are projected to serve as excellent cosmological probes of the electron and baryon distribution in the Universe .
The Repeater
FRB 121102 was discovered by the 300-meter Arecibo Observatory during a survey of the Galactic plane with a DM of 557 pc cm -3 ( Spitler et al ., 2014 ). In follow up Arecibo observations conducted in 2015 , eleven more bursts were found at the same location with the same DM ( Spitler et al ., 2016 ), earning FRB 121102 the moniker “ Repeater .” None of the other FRBs , even after several follow up observations of various durations , have yet been observed to repeat .
It is not clear at this time whether the Repeater belongs to a separate population from the rest of the FRBs or whether all FRBs are a homogeneous population — but the much higher sensitivity of Arecibo compared to other radio telescopes allowed Arecibo to detect fainter bursts ; ones that are likely to
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