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An extraordinary binary system

Submitter: Cees Bassa
Description: PSR J1024-0719 (FIRST radio image) is a seemingly ordinary radio millisecond pulsar (MSP) while 2MASS J10243869-0719190 (DSS2 optical image) is a seemingly ordinary main sequence star. Together, they make an extraordinary binary system (Bassa et al. 2016).

Binary evolution predicts that MSPs are spun up by mass transfer from a binary companion. The end result is a pulsar spinning at millisecond spin periods, typically orbiting a white dwarf in a day to year long orbit. Some MSPs are isolated, probably because their energetic radiation evaporated the companion.

Ever since its discovery in 1994, PSR J1024-0719 has been classified as an isolated MSP. Already in 2001, it was noticed that 2MASS J10243869-0719190 was on almost the same position in the sky as the pulsar. However, this positional coincidence was attributed to chance, as a main-sequence star could not be the companion of the pulsar.

It took 22 years of very high precision timing with the Effelsberg, Lovell, Nancay and Westerbork telescopes to show that the pulsar was ever so slightly accelerated towards us. Furthermore, that acceleration seemed to vary with time. Optical images and spectra of 2MASS J10243869-0719190 taken with the VLT in 2001 and 2015 showed that besides the positional coincidence, its proper motion and spectroscopic distance were consistent with the proper motion and parallax of PSR J1024-0719, confirming that both objects are associated and gravitationally bound.

The timing observations of the pulsar and properties of the main sequence star show that they must be in an extremely wide orbit, with a period of several hundreds to thousands of years. These properties are unlike any other MSP binary that we know of. An exotic formation scenario for the binary system is required, either evolving from a triple system, or being ejected from a dense star cluster.
Copyright: Cees Bassa, Gemma Janssen
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