Pulsar radio emission, associated with rapidly spinning neutron stars, enables the estimation of their distances in the Galaxy through dispersion measure (DM). The DM quantifies the dispersive delay experienced by electromagnetic radiation as it traverses ionized matter, such as the interstellar medium (ISM). By measuring this delay across a range of frequencies, astronomers can obtain information about the electron density between the pulsar and the observer. Accurate modeling of free electrons in the Galaxy allows pulsar DMs to be transformed into source distances, which are invaluable for exploring pulsar distributions and characteristics in our Galaxy, as well as the structure and characteristics of the intervening ISM. They can also contribute to our understanding of the Milky Way’s structure and dynamics.
Our research centered on PSR J1720-0534, a binary millisecond pulsar belonging to the “black widow” category. In such systems, a pulsar is accompanied by a low-mass companion star. The pulsar’s powerful particle wind can strip away the outer layers of its companion, gradually reducing it to nothingness, earning it the black widow moniker. According to the most recent electron density model of the Galaxy, PSR J1720-0534’s DM places it a mere 191 parsecs from Earth, making it the nearest known black widow pulsar. Our work utilized an array of Earth- and space-based observatories to search for emission across infrared, optical, X-ray, and gamma-ray wavelengths, all known to arise from the neutron star and its companion in black widow pulsars. Unfortunately, our observations did not yield any significant detections, leading us to dub the source as “the invisible black widow.” Despite this, we were able to establish a minimum distance of PSR J1720-0534 using these non-detections, which surprisingly was more than five times further than the distance determined by its DM. We attribute this significant disparity to the inclusion of a large and dense component in the latest electron density model in the vicinity of PSR J1720-0534. Given our findings and recent parallax-based distances to other pulsars in this direction, we contend that such a local and large component in the electron density model of the Galaxy is unwarranted.