The Case for TeV Halos and MSPs
The cosmos isn’t a quiet, perfect clock. It’s a noisy, energetic place where tiny beacons—pulsars—spin out radiation that travels across the galaxy. In the last decade, astronomers have been mapping a new kind of glow around certain middle-aged pulsars: TeV halos. These are extended pockets of very-high-energy gamma rays that appear larger than the pulsar’s immediate surroundings, a hint that electrons are being accelerated to extreme energies and then scattering light from the cosmos into TeV photons. The classic example is Geminga, a nearby pulsar whose halo has become a touchstone for thinking about how pulsars seed their neighborhoods with energetic particles.
But not all pulsars behave the same. Millisecond pulsars (MSPs)—old, rapidly spinning neutron stars that have been spun up by mass transfer from a companion—look, in some pictures, like the quiet cousins of their middle-aged kin. They’re bright in radio and GeV gamma rays, yet whether they also wear TeV halos has been a matter of debate. If MSPs could generate TeV halos as efficiently as some isolated, middle-aged pulsars, they could punch a big hole in our ideas about the sources of very-high-energy gamma rays and the diffuse glow that pervades our galaxy. That question lies at the heart of a study conducted by the High Altitude Water Cherenkov Observatory, or HAWC, a collaboration led by several institutions around the world, with A. U. Abeysekara of the University of Utah among the lead authors.
In broad strokes, the researchers asked: do MSPs, taken as a population, produce a TeV halo signal similar to what we see around Geminga and its peers? To answer, they mined almost seven and a half years of tailed data from HAWC and pressed a meticulous, population-wide analysis into service. It’s a bit like testing whether a city’s many faint streetlamps collectively produce a detectable halo—only here the lamps are MSPs and the halo is TeV gamma rays spread across the sky. The result, as the authors describe, challenges a neat assumption: MSPs may not be as efficient at creating TeV halos as the more typical middle-aged pulsars.
How the HAWC Search Was Built
To hunt for extended TeV halos around MSPs, the team turned to HAWC’s vast dataset. HAWC sits high on a Mexican plateau, a forest of water-filled detectors that detect flashes of Cherenkov light produced by gamma-ray showers in the atmosphere. It’s a natural telescope for teetering on the edge of the TeV world, continuously surveying a large slice of the sky. For this study, the data spanned 2565 days, with energy coverage from roughly 0.3 GeV up to 100 TeV—a window wide enough to catch both compact pulsars and any faint, spread-out halos around them.
The MSPs under the microscope come from several catalogs—the ATNF, Fermi-LAT’s 3PC catalog, WVU, and the LOFAR Tied-Array survey—filtered through a careful set of criteria. The sources had to be observable by HAWC, situated away from the Galactic plane to reduce confusion, bright enough to be detectable given HAWC’s sensitivity, and reasonably isolated from known TeV sources. In total, 57 candidates made the cut, with a mix of radio-loud and radio-quiet MSPs, some gamma-ray bright and others dim in radio wavelengths. The selection is important: if you want to detect a halo, you must first ensure you’re not chasing ordinary, point-like emission or nearby sources that masquerade as halos.
From a modeling perspective, the analysis treated a TeV halo as a two-dimensional Gaussian glow around each MSP, with the extension scaled from Geminga’s halo angular size. The flux was modeled as a power-law in energy, with a common spectral index drawn from the typical TeV halo population. Crucially, the analysis didn’t just look at individual MSPs in isolation; it also stacked the signals from many MSPs under two plausible physical mappings: one that ties halo power to the pulsar’s spin-down luminosity (Lsd) and another that ties it to the MSP’s GeV gamma-ray emission measured by Fermi-LAT. These two weighting schemes are designed to capture the two leading ideas about where the TeV electrons might come from and how efficiently MSPs might convert their power into TeV photons.
Results: No TeV Halos Around MSPs
What the team found was quietly, seriously definitive. After checking individual MSPs for any significant halo signal and then stacking the entire population under both weighting schemes, the data did not reveal a TeV halo excess above the background. In other words, MSPs as a group did not appear to light up TeV gamma rays in a halo-like fashion, at least not in a way that would stand out above the random fluctuations of the gamma-ray sky. The result held across the energy range they studied, from sub-TeV to tens of TeV, and even when the analysis allowed for different halo sizes within reasonable astrophysical expectations.
Takeaway: MSPs do not appear to be as efficient at driving TeV halos as the middle-aged pulsars that have already shown such halos around them. The absence of a clear halo signal from MSPs, especially when dozens of candidates are combined, suggests that the mechanisms that seed and confine TeV electrons near pulsars may be significantly different for MSPs versus isolated, older pulsars. The paper’s authors quantify this by comparing MSP halo efficiencies to those of known TeV halos like Geminga, noting the MSPs’ efficiencies fall short—particularly above 10 TeV.
The authors also explored a second line of evidence: a stacking analysis of potential point-like emission from MSPs. Here the result was again consistent with the null hypothesis, with only marginal hints that did not reach robust statistical significance. In short, MSPs as a population do not glow in TeV halos the way some other pulsars do, at least within the sensitivity and energy window of HAWC’s observations. The non-detection is not a fluke of a single MSP’s behavior; it’s a statement about the population as a whole.
Beyond the headline result, the team carefully assessed systematics—from uncertainties in spectral shape to the instrument response. While those factors can nudge flux limits a bit, they do not overturn the central conclusion: MSPs lack the pronounced, halo-scale TeV emission seen around certain middle-aged pulsars. The study also demonstrates the power of stacking: when you combine many faint sources, you can test whether a hidden population radiates in a coordinated way, or whether the emission is simply too weak to reveal itself amid the sky’s background hum.
Implications for Galactic Gamma-ray Emission
The non-detection of MSP TeV halos reverberates through several long-standing debates in high-energy astrophysics. A prominent thread concerns the Galactic diffuse gamma-ray emission and a feature known as the Galactic Center GeV excess. Some theorists have proposed that a population of unresolved MSPs could explain parts of this excess, especially in the inner galaxy. If MSPs were efficient TeV halo engines, those same electrons that light up the GeV band could also paint a halo in the TeV range. That would tie together MSPs as a source of both GeV glow and TeV halos, providing a clean, testable narrative.
What this study adds is a piece of the puzzle that pulls the ladder away from that narrative—at least for MSPs. Since MSPs do not seem to produce Geminga-like TeV halos, the MSP contribution to the TeV sky is unlikely to be as large as some models required. This weakens the idea that a sizable population of MSPs could be a primary contributor to the diffuse TeV glow or to the Galactic Center TeV budget. It also means that the TeV part of the Galactic diffuse emission remains, at least in part, a story told by other sources—likely the halos of middle-aged pulsars concentrated along the Galactic Plane, where their collective light could still creep into the diffuse background we detect at TeV energies.
The authors make a careful point about the Galactic Center GeV excess. Some prior arguments used TeV halo physics as a lever to connect MSPs to the GeV feature. With MSP TeV halos largely ruled out, the degeneracy between a population of MSPs and other astrophysical explanations gains a new counselor: the absence of MSP halos pushes the GeV mystery toward alternative scenarios, though it doesn’t rule out MSPs at all. In other words, the GeV excess could still be explained by unresolved MSPs, but the TeV halo linkage that would make that case stronger is weaker now. The result forces theorists to refine the bridge between MSP populations, their emission physics, and the gamma-ray sky we observe.
What Comes Next for Pulsars and TeV Astronomy
Science rarely ends with a single definitive result. This one, while negative in the sense of not finding TeV halos around MSPs, is exceedingly informative about the physics of particle acceleration near pulsars and the environments that shape how gamma rays propagate. The authors discuss several plausible reasons MSPs might fail to produce prominent TeV halos. It could be that MSPs simply accelerate electrons less efficiently, perhaps because the magnetosphere or any intrabinary interactions between the pulsar and its companion operate differently than in middle-aged pulsars. Another possibility is that the region where electrons diffuse and shine in the TeV range is smaller around MSPs—confined by stronger local magnetic fields or different wind dynamics—so the halo, while present in principle, stays compact or faint beyond what HAWC can detect.
Subtle physics is also at work in how electrons propagate away from pulsars. The “slow diffusion” regions inferred near some pulsars—areas where energetic particles linger due to turbulence or other processes—could be less extended around MSPs. If so, MSP halos would be harder to detect as extended structures. The study’s results therefore do not just close a door; they illuminate the hallways of the pulsar neighborhood, suggesting different rooms for MSPs and older pulsars and inviting theoretical work to map the diffusion, confinement, and acceleration in these distinct systems.
What does this mean for the next generation of observations? The paper’s discussion points toward next-generation instruments and analyses that could push the sensitivity and angular resolution needed to pick up fainter or more compact halos. The Boulder-to-TeV bridge may be crossed more clearly with the Cherenkov Telescope Array (CTA) as well as continued HAWC observations with refinements in event reconstruction and background modeling. If MSPs do host TeV halos at a presence below current sensitivity, the door is left ajar for discovery with better technology. If not, the astrophysical community will have a cleaner separation between MSPs’ GeV emission and any TeV halo population, sharpening our interpretations of the gamma-ray sky and the Galactic engines that power it.
In the end, the study is a reminder of a scientific truth: absence can be as informative as presence. By showing that MSPs do not light up TeV halos as readily as their middle-aged cousins, the HAWC collaboration helps recalibrate models of Galactic gamma-ray production. It also preserves MSPs as a plausible, if not dominant, piece of the Galactic Center GeV excess story. The universe isn’t withholding truth so much as revealing which flavors of truth are likely—and which ones still require a deeper, more patient harvest of data and theory.
Institutional backbone of the study: This work was conducted by the High Altitude Water Cherenkov (HAWC) Observatory collaboration, with the leadership and analysis led by A. U. Abeysekara of the University of Utah and collaborators spanning the Universidad Nacional Autónoma de México (UNAM), Penn State, Wisconsin–Madison, and many other institutions around the world. The paper reflects a broad, multinational effort to understand how the galaxy’s oldest, fastest-spinning stellar corpses shape the high-energy sky we observe today.
