China's Einstein Probe traced a pulsar's X-ray tail out to 42 light-years

Alignment on the sky alone is weak evidence. Two structures can line up in projection while sitting at very different depths. The link here rests on radiation physics: electrons travelling along the tail move through magnetic fields and give off synchrotron radiation, part of which the Einstein Probe registers as X-rays ^[15]. The same electrons run into low-energy photons already present in space and boost them to extreme energies, and those are what LHAASO sees ^[16].

The length is an angle multiplied by an assumed distance. Thirty-two arcminutes is 0.0093 radians, and 0.0093 times 4,600 light-years gives about 43 light-years, close to the 42 reported ^[19]. So the figure inherits the distance error: move 4,600 light-years by 10% and the tail moves by 10% ^[20]. For particles travelling near the speed of light ^[17], crossing that tail takes at least 42 years ^[21].

XMM-Newton had already picked up a tail several light-years long around this pulsar ^[10]. The Einstein Probe's Follow-up X-ray Telescope has a wide field of view and a low background, which suits a search for faint, diffuse structures across large areas of sky ^[11]. About 19 hours of that exposure ^[18] produced the longest X-ray tail of a pulsar wind nebula found so far ^[13]. Part of that record belongs to the instrument rather than to this pulsar. Other middle-aged pulsars may carry comparable tails that narrower observations could not lift out of the background.

For LHAASO's unidentified sources, the useful result is the directionality. If particles leaving a pulsar wind nebula hold a preferred direction for tens of light-years instead of scattering into isotropy as soon as they escape ^[3], then a source with no obvious object near it can still have a parent well outside its own position ^[8][9]. Some of LHAASO's gamma rays come from particles above one petaelectronvolt ^[7]. The paper puts the wider claim no more strongly than a new way to explain some ultrahigh-energy sources that lack clear counterparts ^[4].

Gabriele Ponti and colleagues at Italy's National Institute for Astrophysics published a commentary on the work in the same issue ^[6]. In my view the single-electron-population fit is the firm part of this result; whether it accounts for the counterpart-less sources as a class rests on one pulsar, and the phys.org account of the paper describes the gamma rays it connects as previously "orphan" ^[22].