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Pulsars are rapidly rotating neutron stars that emit regular radio wave pulses and beams of magnetic radiation, which can sometimes be detected from Earth. These pulsating stars are dense remnants of massive stars whose life terminated in a supernova explosion.

Some astronomers have recently observed regions of extended gamma-ray emission at tera-electron volt (TeV) energies around middle-aged pulsars, which are between 100,000 and 1 million years old. These regions, referred to as TeV halos, were found to be significantly larger than their corresponding pulsar's wind nebulae (i.e., clouds of charged particles and magnetic fields generated by the neutron stars' rotation and magnetic energy).

While the presence of TeV halos around some middle-aged pulsars is now well-documented, the processes leading to the formation of these halos remain poorly understood. Moreover, the question of whether these halos are a common or rare occurrence has not yet been definitively answered.

The HAWC collaboration, a large group of researchers based at various universities in Mexico and the United States, recently set out to answer this question, by analyzing data collected at the High-Altitude Water Cherenkov (HAWC) Observatory in Mexico. Their findings, in Â鶹ÒùÔºical Review Letters, suggest that TeV halos are universal features of middle-aged pulsars and could thus be used to identify pulsars that cannot be detected using instruments designed to pick up radio waves or lower-energy gamma rays.

"Since 2017, when extended TeV gamma-ray emission was first detected around the middle-aged pulsars Geminga and Monogem, scientists have been trying to understand the origin of this phenomenon," Sara Coutiño De León, co-author of the paper, told Â鶹ÒùÔº.

"A central question has been whether TeV halos are rare features unique to a few pulsars or if they are a common characteristic of many. That idea inspired us to search for TeV halos using data from the HAWC observatory."

As part of their study, De León and her collaborators analyzed a sky map containing high-energy gamma-ray observations collected over a period of 2,300 days at the HAWC observatory. The HAWC observatory, situated on the slopes of the Sierra Negra volcano in Mexico, specializes in the detection of secondary particles that are produced when high-energy gamma rays originating from space reach Earth's atmosphere.

"We focused on the middle-aged pulsars that fall in the field of view by our detector and examined the likelihood profile for each one, which tells us how well the data fits the presence of extended gamma-ray emission," explained De León. "By stacking these profiles—essentially summing them up—we could assess whether there was a collective signal."

Overall, the results of the researchers' analyses suggest that TeV gamma-ray halos are a common feature of middle-aged pulsars, which suggests that TeV halos could be used to detect pulsars that cannot be observed in other wavelengths (i.e., by searching for their radio wave pulse signals, X-ray or GeV data). This idea could be validated in further studies and could inspire future efforts aimed at detecting and studying these pulsars.

"Our work provides evidence supporting the idea that TeV halos are not rare but are likely a common feature of middle-aged pulsars," added De León. "This suggests that we may need to revisit and revise current models of how high-energy particles travel through the Milky Way. We now plan to conduct detailed studies of individual TeV halos. By doing this, we aim to better understand how particles propagate in and around these sources."

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