摘要：Context. The radio pulsar and rotating radio transient populations are
only known in and near the Milky Way. Investigating such populations in other galaxies
requires deep pulsar and transient searches. We performed 4-h radio observations of nearby
galaxies M 33, M 81 and M 82 with LOFAR.
Aims. Our main purpose was to characterise the bright end of the pulsar
population in other galaxies, and compare it to that of the Milky Way.
Methods. We searched for extragalactic radio pulsars through a
periodic-pulse search, and for sporadic fast radio transients through a single-pulse
search. We coherently combined at most 23 LOFAR Core High-Band Antenna (HBA) stations and
covered M 33, M 81, and M 82 in their entirety using multiple tied-array beams.
Results. No pulsating sources or single pulses were found. We have,
therefore established stricter limits on the extragalactic pulsar flux density at lower
frequencies than those obtained in previous Arecibo, GBT, and WSRT searches.
Conclusions. We conclude that in nearby galaxies M 33, M 81, and M 82
there are no pulsars shining toward Earth with pseudo luminosities greater than a few
times that of the brightest pulsars in our Milky Way.
关键词：stars: neutron;pulsars: general;galaxies: ISM;galaxies: individual: M 33;galaxies: individual: M 81;galaxies: individual: M 82
