HESS J1507-622: an unique unidentified source off the Galactic Plane

Galactic very high energy (VHE, > 100 GeV) gamma ray sources in the inner Galaxy H.E.S.S. survey tend to cluster within 1 degree in latitude around the Galactic plane. HESS J1507-622 instead is unique, since it is located at latitude of ~3.5 degrees. HESS J1507-622 is slightly extended over the PSF of the instrument and hence its Galactic origin is clear. The search for counterparts in other wavelength regimes (radio, infrared and X-rays) failed to show any plausible counterparts; and given its position off the Galactic plane and hence the absorption almost one order of magnitude lower, it is very surprising to not see any counterparts especially at X-rays wavelengths (by ROSAT, XMM Newton and Chandra). Its latitude implies that it is either rather close, within about 1 kpc, or is located well off the Galactic plane. And also the models reflect the uniqueness of this object: a leptonic PWN scenario would place this source due to its quite small extension to multi-kpc distance whereas a hadronic scenario would preferentially locate this object at distances of < 1 kpc where the density of target material is higher.

💡 Research Summary

The paper presents a detailed investigation of HESS J1507‑622, a very‑high‑energy (VHE, > 100 GeV) gamma‑ray source that stands out from the population discovered in the H.E.S.S. Galactic Plane Survey. While the majority of VHE sources cluster within ±1° of the Galactic plane, J1507‑622 lies at a Galactic latitude of ≈ 3.5°, making it the only known high‑latitude, unidentified VHE emitter in the survey. The source is marginally extended beyond the instrument’s point‑spread function (≈ 0.07°), indicating a true physical size of roughly 5 pc × (d/kpc), where d is the distance. This extension, together with its clear Galactic origin, rules out an extragalactic interpretation.

A comprehensive multi‑wavelength search was conducted using radio surveys (MGPS‑2, NVSS), infrared data (WISE, Spitzer), and X‑ray observations (ROSAT, XMM‑Newton, Chandra). No convincing counterpart was found in any band. The lack of an X‑ray counterpart is especially striking because the line‑of‑sight absorption at this latitude is an order of magnitude lower than in the plane, which should make even faint X‑ray emission detectable. The non‑detection therefore implies either an extremely low synchrotron luminosity or an environment with an exceptionally low ambient density.

The latitude translates into a vertical height above the plane of ≈ 600 pc × (d/kpc) sin 3.5°, i.e. ≈ 60 pc for d ≈ 1 kpc and ≈ 300 pc for d ≈ 5 kpc. Consequently two broad distance regimes are possible: (1) a relatively nearby object (≤ 1 kpc) still within the thin disk, or (2) a more distant source located well above the disk. These regimes map onto two competing emission scenarios.

In a leptonic pulsar‑wind‑nebula (PWN) model, relativistic electrons up‑scatter ambient photon fields (cosmic‑microwave background, infrared dust emission) via inverse‑Compton (IC) scattering to produce the observed VHE gamma rays. The modest angular size can be reproduced if the source lies at a multi‑kiloparsec distance, because the electron cooling length then matches the observed extension. The required magnetic field (B ≈ 3–10 µG) and photon energy density (u ≈ 0.5 eV cm⁻³) are typical of the Galactic halo, and the absence of X‑ray synchrotron emission is naturally explained by the low B‑field. The measured spectrum (power‑law index Γ ≈ 2.5 with an exponential cutoff around 10 TeV) is slightly steeper than canonical PWNe, but can be accommodated by a cooled electron population.

In a hadronic scenario, high‑energy protons interact with ambient gas (HI/H₂) to produce neutral pions that decay into gamma rays. This mechanism requires a sufficiently dense target (n ≳ 1 cm⁻³) to achieve the observed luminosity. Such densities are only plausible if the source is nearby (d ≲ 1 kpc), where the line‑of‑sight traverses the Galactic disk. However, CO and HI maps show no prominent molecular cloud or dense atomic gas coincident with J1507‑622, weakening the hadronic case. Moreover, a high proton‑to‑electron ratio would normally generate detectable secondary X‑ray emission, which is not observed.

Both models face challenges. The leptonic model demands a relatively distant, low‑magnetic‑field environment and a pulsar that has either become radio‑quiet or is heavily beamed away from us. The hadronic model requires a nearby, dense target that is not evident in existing gas surveys. The paper therefore suggests that HESS J1507‑622 may represent a new class of VHE source—perhaps an aged, relic particle reservoir in the Galactic halo, an atypical PWN with suppressed synchrotron output, or a yet‑unidentified type of accelerator.

The authors conclude that definitive discrimination between scenarios will require next‑generation VHE instruments (CTA, HAWC‑South) to obtain finer morphology and spectral curvature, deeper X‑ray observations to push limits on synchrotron emission, and high‑resolution molecular line studies to map any hidden gas. Until such data are available, HESS J1507‑622 remains a unique laboratory for studying particle acceleration under conditions markedly different from the dense, plane‑bound environments that dominate the known Galactic VHE source population.