LHAASO J2108+5157: Galactic PeVatron Candidate

• LHAASO J2108+5157 is an unidentified ultra-high-energy gamma-ray source detected above 100 TeV, prompting debate over its leptonic versus hadronic origin.
• Survey data and follow-up observations reveal robust detections, spatial extension, and strong molecular cloud associations that support its PeVatron candidate status.
• Multiwavelength campaigns and varied spectral modeling highlight uncertainties in its acceleration mechanisms and the lack of clear lower-energy counterparts.

LHAASO J2108+5157 is an unidentified Galactic ultra-high-energy gamma-ray source discovered by the Large High Altitude Air Shower Observatory and now treated as a prominent PeVatron candidate. It was first reported as a UHE source without a prior VHE identification, with emission detected above $100$ TeV and no secure counterpart at lower energies; subsequent catalog and follow-up work has established it as a persistent, compact-to-moderately extended gamma-ray source whose interpretation remains divided between leptonic TeV-halo or pulsar-wind-nebula scenarios and hadronic illumination of nearby molecular material by an unseen accelerator (Collaboration, 2021, Cao et al., 2023, Kumar, 3 Aug 2025, Fuente et al., 2023).

1. Discovery and survey characterization

The discovery analysis used 308.33 live days of LHAASO-KM2A data acquired between 2019 Dec 27 and 2020 Nov 24. Significant excesses of gamma-ray induced showers were reported in both the $25$–$100$ TeV and $>100$ TeV bands, at $9.5\ \sigma$ and $8.5\ \sigma$, respectively. The centroid for $E_{\rm rec}>25$ TeV was given as $$\mathrm{R.A.}=317.22^\circ \pm 0.07^\circ_{\rm stat}$$, $$\mathrm{Dec.}=51.95^\circ \pm 0.05^\circ_{\rm stat}$$ (J2000), and the source was described as consistent with a point source, with a $95\%$ upper limit on a symmetric Gaussian extension of $25$0. Its measured spectrum from $25$1 to $25$2 TeV was fit by a single power law,

$25$3

with $25$4 TeV, $25$5, and $25$6. The same work emphasized that Fermi-LAT upper limits below $25$7 TeV require a harder spectrum at GeV energies than $25$8 (Collaboration, 2021).

The first LHAASO catalog revised the source characterization using 933 days of KM2A data and 508 days of WCDA data. In that catalog, 1LHAASO J2108+5157 was significantly detected by both instruments, with KM2A $25$9 and WCDA $100$0. Both components were reported as significantly extended, with $100$1 for KM2A and $100$2 for WCDA. The catalog spectra were also split by band: above $100$3 TeV, KM2A found $100$4 at $100$5 TeV with $100$6; in the $100$7–$100$8 TeV WCDA band, $100$9 at $>100$0 TeV with $>100$1. The catalog further reported $>100$2 ($>100$3) above $>100$4 TeV, reinforcing the source’s UHE status (Cao et al., 2023).

Taken together, these survey results show that the source is robustly established in the UHE regime, while its apparent morphology depends on instrument and exposure. This suggests that the earliest point-like characterization was not the final word on source extension.

2. High-energy and multiwavelength follow-up

Follow-up campaigns have concentrated on closing the gap between the LHAASO detections above tens of TeV and the absence of obvious lower-energy counterparts.

Facility	Reported result	Reference
LST-1	$>100$5 h of good-quality data; $>100$6 hint over $>100$7–$>100$8 TeV; $>100$9 excess at $9.5\ \sigma$0 TeV; $9.5\ \sigma$1 C.L. upper limits quoted	(Abe et al., 2022)
VERITAS	$9.5\ \sigma$2 h; no significant excess above $9.5\ \sigma$3 GeV; $9.5\ \sigma$4-CL differential upper limits above $9.5\ \sigma$5 GeV	(Kumar, 3 Aug 2025)
HAWC	$9.5\ \sigma$6 days; $9.5\ \sigma$7 excess between $9.5\ \sigma$8 and $9.5\ \sigma$9 TeV; extended source model favored	(Kumar, 3 Aug 2025)
Fermi-LAT	soft nearby GeV source, but no hard $8.5\ \sigma$0 GeV emission coincident with LHAASO J2108+5157 in later analysis	(Kumar, 3 Aug 2025)
XMM-Newton	no source signal in $8.5\ \sigma$1–$8.5\ \sigma$2 keV; only flux upper limits	(Kumar, 3 Aug 2025)

The LST-1 commissioning study analyzed 91 h of raw observations from 2021, reduced to $8.5\ \sigma$3 h after quality selection. Under a point-source assumption, no significant detection was found below $8.5\ \sigma$4 TeV, while the $8.5\ \sigma$5–$8.5\ \sigma$6 TeV interval showed an excess of $8.5\ \sigma$7 events with Li and Ma significance $8.5\ \sigma$8. Over the full range, the significance was only $8.5\ \sigma$9, so the analysis emphasized upper limits. A power-law fit to the marginal signal yielded

$E_{\rm rec}>25$0

with $E_{\rm rec}>25$1 and $E_{\rm rec}>25$2. A joint LST-1 + LHAASO fit using an exponential-cutoff power law gave $E_{\rm rec}>25$3, $E_{\rm rec}>25$4, and $E_{\rm rec}>25$5 TeV (Abe et al., 2022).

A later multi-instrument campaign reported $E_{\rm rec}>25$6 h of VERITAS observations, $E_{\rm rec}>25$7 days of HAWC data, $E_{\rm rec}>25$8 yr of Fermi-LAT data, and $E_{\rm rec}>25$9 ks of XMM-Newton exposure. VERITAS detected no significant emission and placed $$\mathrm{R.A.}=317.22^\circ \pm 0.07^\circ_{\rm stat}$$0-CL differential flux upper limits above $$\mathrm{R.A.}=317.22^\circ \pm 0.07^\circ_{\rm stat}$$1 GeV. HAWC, by contrast, found a $$\mathrm{R.A.}=317.22^\circ \pm 0.07^\circ_{\rm stat}$$2 excess above $$\mathrm{R.A.}=317.22^\circ \pm 0.07^\circ_{\rm stat}$$3 GeV, with best-fit centroid $$\mathrm{R.A.}=317.22^\circ \pm 0.07^\circ_{\rm stat}$$4, $$\mathrm{R.A.}=317.22^\circ \pm 0.07^\circ_{\rm stat}$$5, Gaussian extension $$\mathrm{R.A.}=317.22^\circ \pm 0.07^\circ_{\rm stat}$$6, and a simple power-law spectrum over $$\mathrm{R.A.}=317.22^\circ \pm 0.07^\circ_{\rm stat}$$7–$$\mathrm{R.A.}=317.22^\circ \pm 0.07^\circ_{\rm stat}$$8 TeV with $$\mathrm{R.A.}=317.22^\circ \pm 0.07^\circ_{\rm stat}$$9 TeV, $$\mathrm{Dec.}=51.95^\circ \pm 0.05^\circ_{\rm stat}$$0, and $$\mathrm{Dec.}=51.95^\circ \pm 0.05^\circ_{\rm stat}$$1. The same work found no source signal in a joint MOS2+pn XMM fit and reported a $$\mathrm{Dec.}=51.95^\circ \pm 0.05^\circ_{\rm stat}$$2-CL unabsorbed upper limit $$\mathrm{Dec.}=51.95^\circ \pm 0.05^\circ_{\rm stat}$$3 (Kumar, 3 Aug 2025).

Fermi-LAT analyses have consistently failed to reveal a hard GeV counterpart that connects smoothly to the TeV–UHE spectrum. One 12-year analysis found that 4FGL J2108.0+5155 lies $$\mathrm{Dec.}=51.95^\circ \pm 0.05^\circ_{\rm stat}$$4 from the UHE position and follows a log-parabola above $$\mathrm{Dec.}=51.95^\circ \pm 0.05^\circ_{\rm stat}$$5 GeV, while a distinct hard source at $$\mathrm{Dec.}=51.95^\circ \pm 0.05^\circ_{\rm stat}$$6 dominates above $$\mathrm{Dec.}=51.95^\circ \pm 0.05^\circ_{\rm stat}$$7 GeV but is spatially separate from J2108+5157 (Abe et al., 2022). A later follow-up likewise concluded that the only catalog object within $$\mathrm{Dec.}=51.95^\circ \pm 0.05^\circ_{\rm stat}$$8 is 4FGL J2108.0+5155, that its spectrum is soft and pulsar-like, and that no hard $$\mathrm{Dec.}=51.95^\circ \pm 0.05^\circ_{\rm stat}$$9 GeV emission coincident with LHAASO J2108+5157 is found (Kumar, 3 Aug 2025).

3. Molecular-cloud environment

From the outset, the source’s environmental context pointed to dense gas as a possible gamma-ray target. The discovery paper identified a spatial coincidence with the giant molecular cloud [MML2017]4607, reporting $95\%$0, $95\%$1, distance $95\%$2 kpc, mass $95\%$3, angular radius $95\%$4, and mean density $95\%$5. This spatial correlation was explicitly cited as favoring a hadronic origin (Collaboration, 2021).

Subsequent CO and HI work broadened the list of candidate target clouds. A $95\%$6 study toward the Cygnus OB7 molecular cloud reported a new optically thin cloud, named [FKT-MC]2022, with angular size $95\%$7, distance $95\%$8 kpc, nucleon density $95\%$9, and total nucleon mass $25$00. The same study confirmed that Kronberger 82 is a molecular clump with angular size $25$01, nucleon density $25$02, and mass $25$03, but judged [FKT-MC]2022 more likely to be associated with the sub-PeV emission because it is located closer to the gamma-ray source (Fuente et al., 2023).

Higher-resolution $25$04 mapping with the Nobeyama 45 m telescope strengthened the gas association. That work identified a cloud component at $25$05, measured an angular size $25$06, and derived $25$07 kpc using the Bayesian kinematic-distance calculator of Reid et al. (2019). Combining CO with archival $25$08 cm HI data gave $25$09, $25$10, total nucleon density $25$11, and total mass $25$12. The authors reported that the morphology of the spatial distribution of the CO emission is strikingly similar to that of the Fermi-LAT excess gamma-ray emission above $25$13 GeV and described the association of molecular gas with the PeVatron candidate as unambiguous (Fuente et al., 2023).

These cloud studies constrain the target material but do not, by themselves, identify the accelerator. That distinction is important: a molecular cloud can explain where hadronic gamma rays are produced without establishing which nearby object supplied the parent cosmic rays.

4. Emission scenarios and model space

Published modeling has concentrated on two broad classes of explanation: inverse-Compton emission from relativistic electrons and $25$14-decay emission from relativistic protons interacting with dense gas.

In the leptonic class, the 2022 multiwavelength study modeled the source with an electron distribution

$25$15

and found best-fit parameters $25$16 and $25$17 TeV, with total electron energy $25$18 erg for $25$19 GeV. The target photon fields were the CMB and FIR. The same work concluded that synchrotron-IC consistency requires $25$20 from the X-ray upper limits, described this as consistent with other TeV halos, and noted that the absence of a known pulsar remains a challenge to the PWN or TeV-halo interpretation (Juryšek et al., 2022). A later HAWC–VERITAS–Fermi-LAT–XMM-Newton synthesis adopted a one-zone PWN model with

$25$21

and obtained a benchmark fit with $25$22, $25$23 TeV, $25$24 erg, together with the constraint $25$25 (Kumar, 3 Aug 2025).

In the hadronic class, one-zone fits have yielded substantially different parameter sets. The LST-1/XMM/Fermi study parameterized the proton spectrum as

$25$26

and found $25$27, $25$28 TeV, and $25$29 erg for a molecular cloud with $25$30 at $25$31 kpc, while stressing that the required proton spectrum is unusually hard (Juryšek et al., 2022). In contrast, the high-resolution molecular-cloud study used a hadronic $25$32-decay model implemented via Naima and found $25$33 erg above $25$34 GeV with proton cutoff $25$35 TeV, reproducing the sub-PeV gamma-ray emission for the denser $25$36 cloud (Fuente et al., 2023). The original discovery paper had already shown that a NAIMA hadronic fit, assuming proton index $25$37 and exponential cutoff, gives $25$38 TeV and $25$39 erg (Collaboration, 2021).

Time-dependent and transport-based hadronic models extend beyond one-zone fits. One paper proposed that shock-accelerated electrons and protons were injected into the local environment from past explosions that happened thousands of years ago, and argued that the observed UHE photon flux can be explained with secondary gamma rays produced by the time-evolved relativistic electron and proton spectra (Kar et al., 2021). Another old-SNR interpretation modeled escaped protons and electrons from an SNR interacting with a neighboring molecular cloud, obtaining $25$40 yr, $25$41 yr, $25$42 PeV, $25$43 erg, $25$44 erg, and $25$45, while predicting neutrino fluxes below IceCube and IceCube-Gen2 detection thresholds (Sarkar, 2023). A different illuminated-cloud calculation instead concluded that, if an SNR is responsible, it must be young ($25$46 kyr) and located within $25$47–$25$48 pc of the cloud, with a low Sedov time preferred and a maximum proton energy of $25$49 PeV assumed; that study also argued that the Galactic CR sea is insufficient by $25$50 to explain the observed flux and that no currently known SNR matches the required properties (Mitchell, 2023).

The coexistence of these results indicates that the source is not yet constrained to a unique physical picture. The main empirical tension is that hadronic models benefit from the molecular-cloud association, whereas leptonic models benefit from the lack of X-ray emission if the magnetic field is extremely low.

5. Counterpart searches and disputed associations

The absence of an obvious counterpart has been a central property of LHAASO J2108+5157 since discovery. Early studies reported no obvious X-ray, radio, or TeV counterpart, no catalogued 4FGL point source within the original $25$51 positional uncertainty, and no known pulsar, PWN, or SNR within $25$52 (Collaboration, 2021, Cao et al., 2023).

One proposed radio counterpart emerged from upgraded GMRT observations at $25$53 MHz. That work found an extended source, GMRT$25$54, within the LHAASO positional uncertainty circle, with a “disk–jet” morphology, overall angular size $25$55, integrated flux density $25$56 mJy, peak brightness $25$57, and radio spectral index $25$58 for $25$59. The authors judged a microquasar interpretation plausible and proposed the source as a candidate accelerator for the PeV gamma rays (Mahanta et al., 2024).

A later dedicated near-infrared study challenged that interpretation. Using CAHA/OMEGA2000 $25$60 imaging and narrow-band observations targeting the $25$61 line, the study reported no evidence of shocked emission, extended nebular structures, or an accreting compact-object signature in the covered field. It further found that the GMRT radio source has $25$62 mag, is marginally resolved in IRAC 1 and clearly resolved in IRAC 2, and shows near-infrared properties incompatible with both a Galactic microquasar and a nearby radio galaxy, discouraging an association with the gamma-ray emission. The analysis concluded that the GMRT jet source is almost certainly an unrelated background radio galaxy with a very faint core and that no convincing counterpart consistent within the positional uncertainty was found (Martí et al., 11 Feb 2026).

Other candidate counterparts have likewise weakened under scrutiny. A dedicated study of Kronberger 80 and Kronberger 82 reported cluster radii of $25$63 and $25$64, ages $25$65–$25$66 Myr and $25$67 Myr, and poor stellar content in massive O-type stars in both cases, concluding that it is unlikely that either region is a PeVatron associated with LHAASO J2108+5157 (Gupta et al., 21 Apr 2025). In parallel, the SNR-illumination scenario remains observationally incomplete because no SNR matching the required age and separation has yet been identified (Mitchell, 2023).

6. PeVatron status, statistical tests, and remaining uncertainties

LHAASO J2108+5157 is widely described as a PeVatron candidate, but the literature does not yet support a definitive classification. The case in favor is straightforward: the source is securely detected above $25$68 TeV, the discovery paper found no sign of a cutoff up to $25$69 TeV, and dense molecular gas overlaps the gamma-ray emission (Collaboration, 2021, Fuente et al., 2023). The case against certainty is equally clear: different spectral fits yield different cutoff energies, and statistical tests do not yet discriminate sharply between sub-PeV and PeV parent-particle populations.

A dedicated Pevatron Test Statistic analysis treated the parent proton population as

$25$70

and found that, for LHAASO J2108+5157, the best-fit proton index is $25$71 with cutoff unconstrained when using the LHAASO points alone. The reported significance was $25$72, with a $25$73 C.L. lower limit on $25$74 of $25$75 TeV. Adding LST-1 upper limits changed the result to $25$76, still far from a $25$77 rejection or confirmation. The conclusion was that no statistically significant decision can yet be drawn regarding whether the source is a true hadronic PeVatron (Angüner et al., 2023).

This statistical ambiguity is mirrored in broadband modeling. The 2022 LST-1/XMM/Fermi study explicitly concluded that LHAASO J2108+5157 is not yet established as a PeVatron because the hadronic fit gave $25$78 TeV and no evidence for gamma rays beyond $25$79 TeV was found (Juryšek et al., 2022). By contrast, the high-resolution molecular-cloud paper obtained $25$80 TeV and judged the source to be “indeed a PeVatron candidate” (Fuente et al., 2023). The later HAWC follow-up described the broadband SED as showing a pronounced hardening above $25$81 TeV and a cutoff-like turnover around $25$82 TeV, which is suggestive of structure in the parent-particle distribution but not, by itself, a unique discriminator between leptonic and hadronic origins (Kumar, 3 Aug 2025).

A more speculative theoretical proposal, based on the Gluon Condensation model, suggested that LHAASO J2108+5157 might contribute to the second excess of electron and fit the source’s gamma-ray data with a GC threshold $25$83 TeV and $25$84 (Wu et al., 2024). This suggests that the source has also become a test case for nonstandard interpretations of correlated electron, gamma-ray, and neutrino spectra, although such ideas remain outside the core observational debate.

The central unresolved issue is therefore not whether LHAASO J2108+5157 exists as a UHE gamma-ray source, but what object accelerates the parent particles and in which channel the gamma rays are produced. Current evidence supports three statements simultaneously: the source is real and persistent; dense molecular material provides a plausible hadronic target; and no secure accelerator counterpart has yet been identified.