ALMA Observations of PSR B1259-63/LS 2883 in an Inactive Period: Variable Circumstellar Disk?

We report Atacama Large Millimeter/submillimeter Array (ALMA) observations of the binary system containing the pulsar PSR B1259-63 orbiting around a massive star LS 2883 in an inactive period between the 2017 and 2021 periastron passages. We detected radio continuum emission from the binary system at 97 GHz (Band 3) and 343 GHz (Band 7). Compared with our previous ALMA observations performed soon after the 2017 periastron passage, the fluxes have decreased by an factor of six at 97 GHz and two at 343 GHz. The flux at 343 GHz is large relative to that at 97 GHz and appears to be thermal emission from the circumstellar disk around LS 2883. The decrease of the 343 GHz flux may indicate that the disk had shrunk since our previous observations independently of disk passage of the pulsar. The flux at 97 GHz is consistent with that expected from the pulsed emission from the pulsar, which indicates that the unpulsed emission that was produced through pulsar-disk or pulsar-stellar wind interaction had disappeared. The image of the system is consistent with a point source and shows no sign of ejecta.