A Cosmic Lighthouse: The Discovery of ASKAP J1745-5051
A New Kind of Radio Beacon
ASKAP J1745-5051 was first detected during a systematic search for circularly polarized radio sources within the Rapid ASKAP Continuum Survey. Its position was later refined using follow-up observations with the MeerKAT telescope.
Optical Identification
An optical counterpart was identified in the Gaia DR3 catalog, with an apparent magnitude of m_G = 19.45. Spectroscopy conducted with the SOAR and Magellan telescopes revealed a flat spectrum with a pronounced blue excess and strong, narrow emission lines of Hydrogen and Helium. These traits are characteristic of magnetic cataclysmic variables (CVs).
Orbital Properties
Radial velocity measurements established an orbital period of P_orb = 1.368 ± 0.053 hours, placing it near the canonical minimum for CVs. The radio pulse period is P_radio = 1.34497 hours, consistent with the spectroscopic period. Radio bursts are observed to occur near orbital conjunctions.
The source exhibits a radio pulse period remarkably close to its orbital period, a key clue to its nature.
Emission Characteristics
The source produces elliptically polarized radio pulses with complex morphology, narrowband structure, and intermittency. The pulses show frequency drifts and modulation lanes, a phenomenon similar to Jupiter-Io decametric emission. Notably, the radio emission is highly luminous compared to typical CVs.
X-ray and UV Emission
Swift and Einstein Probe observations detected coincident X-ray and UV emission. The X-ray flux varies by more than an order of magnitude and is strongly modulated at the orbital period, peaking at phase 0.89. The X-ray luminosity is consistent with accretion-generated emission in CVs.
System Interpretation
The system is likely a polar or asynchronous polar, with a low-mass M dwarf companion. Masses and radii derived from the orbital period suggest an M6 companion. The inclination is estimated at 14 ± 3 degrees, indicating a face-on orientation.
This system's edge-on view may be key to understanding its unusual radio behavior.
Emission Mechanism
The coherent, polarized radio emission likely arises from electron cyclotron maser emission driven by the interaction between the white dwarf's magnetosphere and its companion. The detection of modulation lanes suggests local plasma acting as an interference screen.
Significance
ASKAP J1745-5051 confirms a link between magnetic CVs and long-period radio transients. Its accretion-driven X-ray and radio emission suggests that accreting CVs form at least part of the LPT population.