WD2 - Origin and physics of Soft Gamma-ray Repeaters and Anomalous X-ray Pulsars |
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Speaker |
Vieira Lobato, Ronaldo |
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Coauthors |
Jaziel Coelho, Manuel Malheiro |
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Radio emission in magnetars and the white dwarfs pulsars |
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Abstract |
Recently, an alternative model based on white dwarfs pulsars has been proposed to explain a class of pulsars known as Soft Gamma Repeaters (SGR) and Anomalus X-Ray Pulsars (AXP), usually named as magnetars. In this model, the magnetized white dwarfs can have surface magnetic field B ~ 10^7 - 10^10 G and rotate very fast with angular frequencies ~ 1 rad/s, allowing them to produce large electromagnetic (EM) potentials and generate electron-positron pairs. These EM potentials are comparable with the ones of neutron star pulsars with strong magnetic fields and even larger. In this study we consider two possible processes associated with the particle acceleration, both of them are common used to explain radio emission in neutron star pulsars: in the first process the pair production happens near to the star polar caps, i.e. inside of the light cylinder where magnetic field lines are closed; in the second one the creation of pair happens in the outer magnetosphere, i.e. far away of the star surface where magnetic field lines are open. The analysis of the possibility of radio emission were done for 23 SGRs/AXPs of the McGill Online Magnetar Catalog that contains the current information available on these sources. The results of this work show that the model where the particles production occurs in the outer magnetosphere emission "o2" is the one process compatible with the astronomical observations of absence of radio emission for almost all SGRs/AXPs, when these sources are understoond as white dwarf pulsars. We explicitly show that the radius R of these sources modeled as white dwarfs, increases the polar cap radius and the polar cap angle. In the case of SGRs/AXPS that have long periods P ~1 0 s, the light cylinder radius is too large compared to the neutron star radius, but only around 100 times larger than the radius of a dense white dwarf, essentially the same scale of radio neutron star pulsars. This ratio is also important to explain the absense of radio emission in SGRs/AXPs in the outer magnetosphere emission model, where the dipole magnetic field is calculated on the light cylinder radius, much smaller than its value at the star surface. Furthermore, the polar cap model predict radio emission for all sources represented as neutron star pulsars. Our work is a first attempted - using the white dwarf pulsar model - to find an explanation for the puzzle why for almost all the SGRs/XPS were expected radio emission, but it was observed in only four of them. |
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