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About two thirds of known milli-second pulsars (MSPs) are in binary systems,
usually with white dwarf companions and with
orbital periods of up to hundreds of days.
Some binary MSPs, however,
have non-degenerate low-mass companions and very short orbital
periods, of less than a day. These are known as "Redbacks",
and have companions with masses of 0.1 ~ 0.4 solar masses which are partially
ablated by irradiation from the pulsar wind.
(Redbacks are related to the "Black Widow" binary systems
which have lighter companions, with masses less than 0.05 solar masses,
that are almost fully ablated.)
The Fermi gamma-ray source 4FGL J2039.5−5617 showed properties consistent
with it being a redback pulsar, and this was recently
confirmed by the discovery of gamma-ray millisecond pulsations with a period of 2.6 milliseconds.
Corongiu et al. observed 4FGL J2039.5−5617 with
the Parkes radio telescope in 2016 and 2019, and have recently
confirmed the 2.6 ms period in the radio data.
The 1.4 GHz data shows clear evidence of eclipses of the radio signal
for about half of the orbit, a characteristic phenomenon in redback systems, which is associated
with the presence of intra-binary gas.
The image above shows a colour map of the radio pulse amplitude as a function of spin and orbital phase (vertial and
horizontal axis, respectively). The occurrence of a signal eclipse
over the orbital phase range 0.1 to 0.5 is apparent.
The paper
has been submitted to Monthly Notices of the Royal Astronomical Society.
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