VER J2019+368: Extended TeV Source

• VER J2019+368 is a bright, extended TeV source in the Cygnus region, associated with pulsar PSR J2021+3651 and its pulsar wind nebula.
• Multi-instrument analyses by VERITAS, HAWC, and SST-1M reveal an elongated, hard-spectrum morphology with evidence for energy-dependent structural features.
• Spectral studies show a hard power-law behavior with hints of curvature, supporting both leptonic and hadronic models as viable mechanisms for its multi-TeV emission.

Searching arXiv for recent and foundational papers on VER J2019+368, HAWC J2019+368, and related PWN interpretations. I’ll look up arXiv entries relevant to VER J2019+368 and its multi-instrument interpretation. VER J2019+368 is a bright, extended very-high-energy $\gamma$-ray source in the Cygnus region, identified by VERITAS as the dominant TeV component of the Milagro source MGRO J2019+37 and subsequently connected to the HAWC source complex HAWC J2019+368/eHWC J2019+368. Across the literature, it is treated as a hard-spectrum, morphologically elongated emitter embedded in the star-forming environment of Cygnus-X, with the pulsar PSR J2021+3651 and its X-ray pulsar wind nebula G75.2+0.1, the “Dragonfly” nebula, forming the principal counterpart candidate. A central question has been whether VER J2019+368 is predominantly the TeV manifestation of that pulsar wind nebula, a composite region containing multiple accelerators, or a hadronic accelerator capable of producing neutrinos and photons beyond $100$ TeV (Collaboration, 2014, Collaboration et al., 2021).

1. Discovery history and source identity

VER J2019+368 emerged from the resolution of the bright extended Milagro source MGRO J2019+37. Milagro had observed MGRO J2019+37 as a single, very bright and extended source, with a bright inner region of $\sim 1^\circ$ extension, but VERITAS re-observed the field and resolved it into two distinct VHE sources: the point-like VER J2016+371, overlapping CTB 87, and the bright extended VER J2019+368, which likely accounts for the bulk of the Milagro emission (Collaboration, 2014).

In the VERITAS analysis, the centroid of VER J2019+368 was found at $$\alpha_{\rm J2000} = 20^{\rm h}\,19^{\rm m}\,25^{\rm s} \pm 72^{\rm s}_{\rm stat}$$ and $$\delta_{\rm J2000} = +36^\circ\,48'\,14'' \pm 58''_{\rm stat}$$, placing it in the Galactic plane near PSR J2021+3651, Sh 2-104, IGR J20188+3647, and 2FGL J2018.0+3626 (Collaboration, 2014). VERITAS characterized it as a bright extended source on a $\sim 1^\circ$ scale, with a core extension $\sim 0.3^\circ$, and showed that its spectrum is consistent with Milagro measurements at $12$, $20$, and $35$ TeV (Collaboration, 2014).

Subsequent HAWC work decomposed the 2HWC J2019+367 region into HAWC J2019+368 and HAWC J2016+371, thereby reproducing, at HAWC angular resolution, the same two-source structure identified by VERITAS. That analysis explicitly associated HAWC J2019+368 with PSR J2021+3651 and the Dragonfly nebula, and treated VER J2019+368 and HAWC J2019+368 as the same TeV nebula observed by instruments with different PSFs, energy responses, and integration procedures (Collaboration et al., 2021).

At still higher energies, eHWC J2019+368 was identified by HAWC as one of the sources emitting $100$0-rays with energies higher than $100$1 TeV. In that context, VER J2019+368 functions as the lower-energy TeV component of the same source complex, providing the crucial $100$2–$100$3 TeV anchor between GeV pulsar emission and the multi-$100$4 to $100$5 TeV regime (Fang et al., 2020).

2. Morphology and spatial structure

VERITAS measured VER J2019+368 with an asymmetric Gaussian morphology. The fitted $100$6 extensions are $100$7 and $100$8, with position angle $100$9 east of north, implying an elongated source roughly along right ascension, i.e. approximately east–west (Collaboration, 2014). The source was detected with pre-trials significance $\sim 1^\circ$0 and post-trials significance $\sim 1^\circ$1 in the extended-source analysis (Collaboration, 2014).

Morphological complexity became more explicit in the deep Cygnus survey. There, the full VER J2019+368 emission was still fit globally by a single asymmetric Gaussian with centroid at $\sim 1^\circ$2 and $\sim 1^\circ$3, semi-major width $\sim 1^\circ$4, semi-minor width $\sim 1^\circ$5, and position angle $\sim 1^\circ$6 (Collaboration et al., 2018). However, the point-source significance map showed two distinct local maxima, leading to the candidate source labels VER J2018+367* and VER J2020+368* (Collaboration et al., 2018). The data were compatible with such a decomposition, but did not statistically require it; the authors therefore retained the candidate designation (Collaboration et al., 2018).

HAWC later found HAWC J2019+368 to be well described by an elliptical Gaussian. In the preferred log-parabola fit, the semi-major axis is $\sim 1^\circ$7, the eccentricity is $\sim 1^\circ$8, and the rotation angle is $\sim 1^\circ$9 (Collaboration et al., 2021). This corresponds to a minor axis of approximately $$\alpha_{\rm J2000} = 20^{\rm h}\,19^{\rm m}\,25^{\rm s} \pm 72^{\rm s}_{\rm stat}$$0, closely matching the VERITAS elongation (Collaboration et al., 2021). HAWC also performed an energy-dependent morphology study in four reconstructed-energy bands from $$\alpha_{\rm J2000} = 20^{\rm h}\,19^{\rm m}\,25^{\rm s} \pm 72^{\rm s}_{\rm stat}$$1–$$\alpha_{\rm J2000} = 20^{\rm h}\,19^{\rm m}\,25^{\rm s} \pm 72^{\rm s}_{\rm stat}$$2 TeV to $$\alpha_{\rm J2000} = 20^{\rm h}\,19^{\rm m}\,25^{\rm s} \pm 72^{\rm s}_{\rm stat}$$3–$$\alpha_{\rm J2000} = 20^{\rm h}\,19^{\rm m}\,25^{\rm s} \pm 72^{\rm s}_{\rm stat}$$4 TeV and found only a very mild indication that the size decreases with energy, with no statistically significant centroid shift toward the pulsar (Collaboration et al., 2021).

SST-1M observations in 2024 again recovered VER J2019+368 as a single elongated excess. In the $$\alpha_{\rm J2000} = 20^{\rm h}\,19^{\rm m}\,25^{\rm s} \pm 72^{\rm s}_{\rm stat}$$5–$$\alpha_{\rm J2000} = 20^{\rm h}\,19^{\rm m}\,25^{\rm s} \pm 72^{\rm s}_{\rm stat}$$6 TeV range, an asymmetric Gaussian fit yielded centroid $$\alpha_{\rm J2000} = 20^{\rm h}\,19^{\rm m}\,25^{\rm s} \pm 72^{\rm s}_{\rm stat}$$7 and $$\alpha_{\rm J2000} = 20^{\rm h}\,19^{\rm m}\,25^{\rm s} \pm 72^{\rm s}_{\rm stat}$$8, with $$\alpha_{\rm J2000} = 20^{\rm h}\,19^{\rm m}\,25^{\rm s} \pm 72^{\rm s}_{\rm stat}$$9 and $$\delta_{\rm J2000} = +36^\circ\,48'\,14'' \pm 58''_{\rm stat}$$0 (Juryšek et al., 22 Jul 2025). A $$\delta_{\rm J2000} = +36^\circ\,48'\,14'' \pm 58''_{\rm stat}$$1D longitudinal profile gave $$\delta_{\rm J2000} = +36^\circ\,48'\,14'' \pm 58''_{\rm stat}$$2 deg, consistent with VERITAS. The same study did not confirm the VERITAS two-source morphology and did not find statistically significant energy-dependent morphology between $$\delta_{\rm J2000} = +36^\circ\,48'\,14'' \pm 58''_{\rm stat}$$3 and $$\delta_{\rm J2000} = +36^\circ\,48'\,14'' \pm 58''_{\rm stat}$$4 TeV (Juryšek et al., 22 Jul 2025). This suggests that the large-scale elongation is robust, whereas the internal multiplicity remains instrument- and statistics-dependent.

3. Spectral properties from TeV to $$\delta_{\rm J2000} = +36^\circ\,48'\,14'' \pm 58''_{\rm stat}$$5 TeV

The defining spectral property of VER J2019+368 is its hardness. The original VERITAS measurement extracted the spectrum from a circular region of radius $$\delta_{\rm J2000} = +36^\circ\,48'\,14'' \pm 58''_{\rm stat}$$6 and found a simple power law over $$\delta_{\rm J2000} = +36^\circ\,48'\,14'' \pm 58''_{\rm stat}$$7–$$\delta_{\rm J2000} = +36^\circ\,48'\,14'' \pm 58''_{\rm stat}$$8 TeV with photon index

$$\delta_{\rm J2000} = +36^\circ\,48'\,14'' \pm 58''_{\rm stat}$$9

normalization at $\sim 1^\circ$0 TeV

$\sim 1^\circ$1

and fit quality $\sim 1^\circ$2 (Collaboration, 2014). The integrated $\sim 1^\circ$3–$\sim 1^\circ$4 TeV energy flux is

$\sim 1^\circ$5

(Collaboration, 2014). VERITAS found no statistically preferred curvature or cutoff up to $\sim 1^\circ$6 TeV (Collaboration, 2014).

The deeper Cygnus survey recovered a somewhat softer global VER J2019+368 spectrum when using a smaller extraction region, with photon index $\sim 1^\circ$7 over $\sim 1^\circ$8 GeV–$\sim 1^\circ$9 TeV (Collaboration et al., 2018). The same analysis found $\sim 0.3^\circ$0 for VER J2018+367* and $\sim 0.3^\circ$1 for VER J2020+368* (Collaboration et al., 2018). This indicates that the hard-spectrum character is preserved across both the full region and its candidate subcomponents.

MAGIC observed the source using the very large zenith angle technique and reported an integrated flux

$\sim 0.3^\circ$2

for an extended Gaussian source model with $\sim 0.3^\circ$3 and $\sim 0.3^\circ$4 (Zarić et al., 2019). The flux was higher than the earlier VERITAS integral fluxes, but consistent with HAWC when comparable large integration regions were used (Zarić et al., 2019). This difference is therefore attributable to aperture and morphology assumptions rather than an intrinsic spectral inconsistency.

HAWC extended the spectrum far beyond the VERITAS range and found strong evidence for curvature. Relative to a pure power law, a log-parabola is preferred by $\sim 0.3^\circ$5, while a power law with exponential cutoff is preferred by $\sim 0.3^\circ$6; the Bayesian Information Criterion favors the log-parabola by $\sim 0.3^\circ$7 (Collaboration et al., 2021). In the log-parabola model, the normalization is $\sim 0.3^\circ$8 and the curvature parameter is $\sim 0.3^\circ$9 (Collaboration et al., 2021). In the exponential-cutoff power-law form, the cutoff energy is

$12$0

(Collaboration et al., 2021). HAWC showed that, after scaling the VERITAS fluxes to account for the smaller VERITAS extraction region by a factor $12$1, the VERITAS and HAWC data align well (Collaboration et al., 2021).

At the highest energies, eHWC J2019+368 was parameterized in one neutrino-oriented study as a log-parabola

$12$2

with $12$3, $12$4, $12$5, and Gaussian extension $12$6 (Niro, 2020). That study emphasized that eHWC J2019+368 is among the three brightest HAWC sources with significant emission above $12$7 TeV (Niro, 2020). SST-1M subsequently measured a simple power law in the $12$8–$12$9 TeV interval with

$20$0

at $20$1, and reported no evidence for cutoff or curvature in that range (Juryšek et al., 22 Jul 2025). This suggests that the detectability of curvature depends sensitively on energy coverage, morphology, and fit parameterization.

4. Multiwavelength counterparts and environment

The most frequently discussed counterpart is PSR J2021+3651 and its PWN G75.2+0.1, the Dragonfly nebula. PSR J2021+3651 is a young, energetic pulsar with $20$2 ms and spin-down power $20$3 (Kirichenko et al., 2015). Chandra revealed a torus-like PWN with jets, very similar to that of the Vela pulsar, while XMM-Newton and Suzaku showed a larger diffuse nebula extending westward from the pulsar toward the VER J2019+368 emission (Kirichenko et al., 2015, Collaboration, 2014).

X-ray studies resolved the PWN into a major-axis structure aligned with the TeV emission. Suzaku found that the western X-ray PWN has source extent $20$4 and that the major axis is oriented consistently with the TeV morphology (Mizuno et al., 2017). Archival XMM-Newton data showed that the eastern X-ray PWN extends to at least $20$5 along the same axis, yielding a total X-ray extent of more than $20$6 along the major axis (Mizuno et al., 2017). The PWN-west spectrum is fit by a power law with $20$7 and photon index $20$8, while the eastern side has $20$9 and $35$0 (Mizuno et al., 2017). The lack of obvious photon-index variation across the X-ray nebula became a key transport constraint (Mizuno et al., 2017).

A major development for the PWN interpretation was the distance revision to PSR J2021+3651. Optical and X-ray analysis using a red-clump-star extinction–distance relation derived

$35$1

at $35$2 confidence (Kirichenko et al., 2015). This is much smaller than the dispersion-measure estimate of $35$3 kpc and makes the pulsar energetically far more plausible as the power source of VER J2019+368 (Kirichenko et al., 2015). The same work reported no optical counterpart down to $35$4 for the pulsar and $35$5 for the compact X-ray nebula, reinforcing the picture of a system that is optically and X-ray faint relative to its high-energy output (Kirichenko et al., 2015).

Other counterparts remain relevant in the complex Cygnus field. Sh 2-104 lies at the western edge of VER J2019+368 and has been considered a possible hadronic contributor because it is a bubble-like H II region with an O6V star, an ultracompact H II region, an embedded stellar cluster, and surrounding massive CO clouds (Collaboration, 2014). However, the swept-up mass is estimated at $35$6, and no anomalously strong radio or X-ray behavior singles it out as the dominant accelerator (Collaboration, 2014). The hard X-ray transient IGR J20188+3647 lies near the brightest VHE emission, but its transient behavior and lack of a persistent nebular counterpart make it disfavored as the main TeV source (Collaboration, 2014). The Fermi-LAT source 2FGL J2018.0+3626 lies southwest of the TeV centroid and may indicate a second pulsar/PWN contributing to the soft southwestern emission, but this remains speculative (Collaboration, 2014).

The survey decomposition sharpened these associations. VER J2020+368* lies only $35$7 from PSR J2021+3651 and is closely aligned with the westward X-ray/radio extension of G75.2+0.1, strengthening the eastern PWN association (Collaboration et al., 2018). By contrast, VER J2018+367* has a more ambiguous status; PSR J2017+3625 is offset by $35$8, and no X-ray or GeV PWN is established there (Collaboration et al., 2018). This suggests, though does not prove, that the eastern part of the complex is more securely tied to PSR J2021+3651 than the western part.

5. Physical interpretations

The dominant physical framework is leptonic PWN emission. In this picture, PSR J2021+3651 injects relativistic electrons and positrons into the Dragonfly nebula, where they emit synchrotron radiation in radio and X-rays and inverse-Compton-scatter ambient photons into the TeV band (Mizuno et al., 2017, Fang et al., 2020). The very hard TeV index, the elongated offset morphology, and the structural analogy to Vela-X have all been cited in support of this interpretation (Collaboration, 2014).

Suzaku/XMM-based modeling required a mean magnetic field of $35$9 to account for the measured X-ray and reported TeV spectrum (Mizuno et al., 2017). Using a broken power-law electron distribution with $100$00 below $100$01 TeV, $100$02 above that break, and exponential cutoff at $100$03 PeV, the model reproduced the X-ray PWN and explained $100$04 of the reported TeV flux (Mizuno et al., 2017). The same study concluded that the X-ray PWN is a major contributor of VER J2019+368 (Mizuno et al., 2017).

A more formal time-dependent one-zone PWN model was developed for eHWC J2019+368 and VER J2019+368. There, the spin-down power is partitioned between leptons and magnetic field with $100$05, and the particle distribution evolves under synchrotron, inverse Compton, adiabatic, and escape losses (Fang et al., 2020). Two injection prescriptions were studied: a broken power law and a single power law (Fang et al., 2020). The favored solution for simultaneously reproducing the radio, X-ray, VERITAS, and HAWC data used a single power-law injection with index $100$06, age $100$07 yr, $100$08, and maximum particle energy $100$09 PeV, giving a present-day nebular magnetic field

$100$10

(Fang et al., 2020). In that model, inverse Compton emission on CMB and IR photons dominates the TeV band, SSC is negligible, and the system qualifies as a leptonic PeVatron capable of producing the $100$11 TeV photons seen by HAWC (Fang et al., 2020).

HAWC obtained a somewhat different but related PWN solution using GAMERA. In its preferred two-zone model, the source is a $100$12 kyr pulsar and nebula system, with present-day $100$13G, birth period $100$14 ms, conversion efficiency $100$15, and injection cutoff $100$16 TeV (Collaboration et al., 2021). The compact X-ray nebula is attributed to recently injected electrons from the last $100$17–$100$18 kyr, while the larger TeV nebula reflects the full $100$19 kyr electron history (Collaboration et al., 2021). This two-zone framework was introduced specifically because a one-zone fit with $100$20G made the X-ray and TeV morphologies too similar (Collaboration et al., 2021).

Alternative scenarios remain under discussion. VERITAS originally framed the region as a likely composite source, with a dominant leptonic PWN component from PSR J2021+3651 and possible hadronic or additional leptonic contributions from Sh 2-104 or a second pulsar near 2FGL J2018.0+3626 (Collaboration, 2014). The survey paper maintained that agnostic stance, especially for the western hotspot VER J2018+367* (Collaboration et al., 2018). SST-1M later showed that simple one-zone inverse-Compton and hadronic $100$21-decay models can both fit its $100$22–$100$23 TeV SED. In that analysis, the leptonic solution required electrons up to $100$24 TeV, while the hadronic solution required protons up to $100$25 PeV (Juryšek et al., 22 Jul 2025). This suggests that the present spectral data alone do not yet eliminate either class of mechanism.

6. Open issues, neutrino implications, and observational status

One persistent uncertainty is source multiplicity. VERITAS found evidence for two local maxima within the broad VER J2019+368 region, but neither the map fit nor the transsect fit statistically required a two-source decomposition (Collaboration et al., 2018). HAWC and SST-1M each recover a single elongated source at their native resolutions (Collaboration et al., 2021, Juryšek et al., 22 Jul 2025). A plausible implication is that VER J2019+368 is either a single extended accelerator with internal brightness structure or a partially blended composite region whose components are difficult to disentangle consistently across instruments.

A second uncertainty concerns distance and age. The PWN-based models that fit the TeV data favor either $100$26 kpc and $100$27 yr (Fang et al., 2020) or a true age of $100$28 kyr in the HAWC/Suzaku model (Collaboration et al., 2021). The optical/X-ray distance estimate of $100$29 kpc resolves earlier energetic tensions but still has wide bounds (Kirichenko et al., 2015). This suggests that luminosity, physical size, transport coefficients, and conversion efficiencies remain moderately model-dependent.

A third issue is the balance between leptonic and hadronic channels. The dedicated PWN models show that the Dragonfly nebula can, under reasonable assumptions, explain most or all of the VER J2019+368 and eHWC J2019+368 flux (Mizuno et al., 2017, Fang et al., 2020). However, neutrino forecasts have been made under the opposite assumption: that the $100$30-ray emission is hadronic. Using the eHWC log-parabola and the Villante–Vissani formalism, one study predicted that IceCube could achieve about $100$31 significance in roughly $100$32 years and about $100$33 in $100$34 years for the compact eHWC J2019+368 morphology, with an optimized neutrino threshold near $100$35 TeV (Niro, 2020). The same work noted that continued non-detection at the predicted sensitivity would begin to challenge a purely hadronic origin for most of the $100$36 TeV $100$37-ray emission (Niro, 2020).

The source has also become a calibration and methodology target for new Cherenkov systems. MAGIC used it to demonstrate the utility of the very large zenith angle technique for multi-TeV extended sources (Zarić et al., 2019). SST-1M used it to test off-axis performance, showing roughly flat $100$38-ray acceptance out to $100$39 and robust detectability of extended Galactic sources in wobble mode (Juryšek et al., 22 Jul 2025). Such results indicate that VER J2019+368 functions not only as an astrophysical source of interest but also as a benchmark for high-energy instrumentation.

Within current evidence, VER J2019+368 is best described as a hard-spectrum, extended TeV source in Cygnus whose dominant component is very likely the pulsar wind nebula powered by PSR J2021+3651, while the degree of source multiplicity, the contribution of the western side of the complex, and the hadronic fraction of the highest-energy emission remain unsettled. The source’s continued visibility from $100$40 TeV to beyond $100$41 TeV, together with its association with the Dragonfly nebula and its candidacy as a leptonic or hadronic PeVatron, ensure that it remains a central object in studies of Galactic particle acceleration (Collaboration et al., 2021, Juryšek et al., 22 Jul 2025).