The First Detection of a Pulsar with ALMA

Abstract

Although there is a general consensus on the fact that pulsars’ radio emission is coherent in nature, whereas the emission from the optical to high-energy γ -rays is due to incoherent processes, it has not been established yet at which wavelengths the transition occurs, which is key information for all emission models of pulsar magnetospheres. Of course, to address this issue, covering the spectral region between the GHz radio frequencies and the mid-infrared (IR) is crucial. We used the Atacama Large Millimeter/submillimeter Array (ALMA) to observe the Vela pulsar (PSR B0833−45), one of the very few observed in radio and from the mid-IR up to the very high-energy γ -rays. We detected Vela at frequencies of 97.5, 145, 233, and 343.5 GHz, which makes it the first pulsar ever detected with ALMA. Its energy density spectrum follows a power law of spectral index α  = −0.93 ± 0.16. This corresponds to very high brightness temperatures—from 10 17 to 10 15 K—suggesting that a coherent radiative process still contributes to the millimeter/submillimeter emission. Therefore, this is the first indication of coherent emission in pulsars extending to the submillmeter range. At the same time, we identified an extended structure, preliminarily detected in ground-based near-IR observations, at a distance of ∼1.″4 from the pulsar, possibly interpreted as a counter-jet protruding from the pulsar.