Apparently ultra-long period radio sources from self-lensed pulsar-black hole binaries

Abstract

Pulsar-black hole (BH) close binary systems, which have not been found yet, are unique laboratories for testing theories of gravity and understanding the formation channels of gravitational-wave sources. We study the self-gravitational lensing effect in a pulsar-BH system on the pulsar's emission. Because this effect occurs once per orbital period for almost edge-on binaries, we find that it could generate apparently ultra-long period (minutes to hours) radio signals when the intrinsic pulsar signal is too weak to detect. Each of such lensed signals, or 'pulse', is composed of a number of amplified intrinsic pulsar pulses. We estimate that a radio telescope with a sensitivity of $10\mathrm{mJy}$ could detect $\sim$ a few systems that emit such signals in our galaxy. The model is applied to three recently found puzzling long-period radio sources: GLEAM-X J1627, PSR J0901-4046, and GPM J1839-10. To explain their observed signal durations and periods, the masses of their lensing components would be $\sim 10^4{\mathrm{M}}_{\odot }$, $\sim 4{\mathrm{M}}_{\odot }$ and $10^{3-6}{\mathrm{M}}_{\odot }$, respectively, with their binary coalescence times ranging from a few tens to thousands of years. However, the implied merger rates (as high as $\sim 10^{3-4}\mathrm{My}{\mathrm{r}}^{-1}$ per galaxy) and the large period decay rates ($>10^{-8}\mathrm{s}{\mathrm{s}}^{-1}$) tend to disfavour this self-lensing scenario for these three sources. Finally, for a binary containing a millisecond pulsar and a stellar-mass BH, the Shapiro delay effect would cause a $\ge 10\%$ variation of the profile width for the sub-pulses in such lensed signals.

13 pages, 6 figures, 1 table, Accepted for publication in ApJ

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