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Discovery of very high energy gamma-ray emission coincident with molecular clouds in the W28 (G6.4-0.1) field

HESS Collaboration, F Aharonian

arXiv:0801.3555v3astro-ph

TL;DR

The paper investigates Very High Energy gamma-ray emission associated with molecular clouds in the W28 field. Using H.E.S.S., EGRET, and molecular-cloud observations, it finds positional associations consistent with a possible hadronic origin and requires cosmic-ray densities about 10 to 30 times the local value under the stated distance assumptions.

  • Problem

    The study examines the origin of VHE gamma-ray sources associated with dense molecular clouds in the W28 field.

  • Method

    The authors combine about 40 hours of H.E.S.S. observations, an EGRET data reanalysis, and NANTEN ^12CO(J=1-0) and other CO observations.

  • Results

    13 to 32 times the local cosmic-ray density is required for a hadronic interpretation of the VHE emission under cloud-distance assumptions of 2 and 4 kpc.

  • Takeaways & Limitations

    The VHE/molecular-cloud association could indicate a hadronic origin, with W28 and additional or alternative SNRs, HII regions, and open clusters as possible particle accelerators.

  • Takeaways & Limitations

    Cloud distances, mass estimates, and the hadronic interpretation remain uncertain because of velocity crowding, broad velocity ranges, and limited constraints on electron contributions.

Abstract

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We observed the W28 field (for ~40 h) at Very High Energy (VHE) gamma-ray energies (E>0.1 TeV) with the H.E.S.S. Cherenkov telescopes. A reanalysis of EGRET E>100 MeV data was also undertaken. Results from the NANTEN 4m telescope Galactic plane survey and other CO observations have been used to study molecular clouds. We have discovered VHE gamma-ray emission (HESSJ1801-233) coincident with the northeastern boundary of W28, and a complex of sources (HESSJ1800-240A, B and C) ~0.5 deg south of W28, in the Galactic disc. The VHE differential photon spectra are well fit by pure power laws with indices Gamma~2.3 to 2.7. The NANTEN ^{12}CO(J=1-0) data reveal molecular clouds positionally associating with the VHE emission, spanning a ~15 km s^{-1} range in local standard of rest velocity. The VHE/molecular cloud association could indicate a hadronic origin for HESSJ1801-233 and HESSJ1800-240, and several cloud components in projection may contribute to the VHE emission. The clouds have components covering a broad velocity range encompassing the distance estimates for W28 (~2 kpc), and extending up to ~4 kpc. Assuming a hadronic origin, and distances of 2 and 4 kpc for cloud components, the required cosmic ray density enhancement factors (with respect to the solar value) are in the range ~10 to ~30. If situated at 2 kpc distance, such cosmic ray densities may be supplied by a SNR like W28. Additionally and/or alternatively, particle acceleration may come from several catalogued SNRs and SNR candidates, the energetic ultra compact HII region W28A2, and the HII regions M8 and M20 along with their associated open clusters. Further sub-mm observations would be recommended to probe in detail the dynamics of the molecular clouds at velocites >10 km s^{-1}, and their possible connection to W28.

1. Introduction: W 28 and surroundings

W 28 is an old, interacting SNR whose dense molecular-cloud environment makes it a useful target for distinguishing hadronic and electronic gamma-ray emission. The paper investigates newly discovered VHE emission in the W 28 field and its relationships with molecular clouds and other potential accelerators.

  • Motivation: Dense molecular clouds with n > 10^3 cm^-3 can help distinguish hadronic from electronic TeV emission because radiative losses suppress multi-TeV electrons in older remnants.The paper identifies W 28 as especially suitable because it is old and interacts with dense molecular material.
  • W 28 and surroundings: W 28 is a mixed-morphology SNR measuring 50′x45′, at an estimated distance of 1.8–3.3 kpc and age of 35,000–150,000 years.It is thought to have entered its radiative phase, allowing many cosmic rays to escape into the surrounding interstellar medium.
  • W 28 and surroundings: Molecular-cloud interaction is established along W 28’s northern and northeastern boundaries through 1720 MHz OH masers and very high-density shocked gas.Shell-like radio emission also peaks at these boundaries.
  • Potential accelerators: The W 28 neighborhood contains the HII regions M 8, M 20, and W 28A2, plus additional SNRs and SNR candidates that may provide particle acceleration.M 8 and M 20 are associated with open clusters and represent active massive-star formation in the region.
  • Previous observations: Earlier CANGAROO-I observations found no VHE emission and set upper limits of approximately 0.2–0.5 Crab flux above 1.5 TeV.The limits corresponded to 1.1–2.9×10^-11 erg cm^-2 s^-1 for various regions.

2. Results at VHE and E >100 MeV γ-ray energies

H.E.S.S. observations of W 28 revealed two VHE gamma-ray emission sites, while a dedicated EGRET reanalysis confirmed a pointlike source above 100 MeV. The VHE sources have power-law spectra and include a southern complex with three tentatively distinct components.

  • H.E.S.S. observations: Approximately 42 hours of H.E.S.S. observations covered W 28 during the 2004–2006 observing seasons.Runs were selected using quality criteria for cosmic-ray background rates, camera performance, calibration, and tracking.
  • VHE results: H.E.S.S. detected VHE emission toward the northeastern and southern boundaries of W 28 with post-trial significances of at least 5σ.The sources had pre-trial significances of at least 7σ after accounting for approximately 2.2×10^5 trials.
  • VHE results: The northeastern source is HESS J1801−233, while the southern complex HESS J1800−240 contains components A, B, and C that may not be fully resolved.The components are local peaks approximately 2σ above their surroundings and may relate to distinct multiwavelength counterparts.
  • VHE spectra: Photon indices are approximately 2.5–2.7 over 0.3–5 TeV, with pure power-law fits; all sources except HESS J1800−240C appear extended with intrinsic radii of about 10′.At 2 kpc, source luminosities from 0.3 to 3 TeV are of order 10^33 erg s^-1.
  • EGRET results: The EGRET reanalysis confirmed a pointlike source above 100 MeV, labeled GRO J1801−2320, shifted by approximately 0.2° from the 3EG catalogue position.The analysis used observation cycles 1–6 and finalized instrumental responses.

3. NANTEN and other observations of Molecular Clouds

NANTEN CO observations identify molecular clouds spatially associated with the VHE sources across multiple velocity components, but their distances and physical connections remain uncertain.

  • The NANTEN Galactic Plane Survey provided 12CO(J=1–0) data for the W 28 region with a 4′ grid spacing.
  • Two molecular clouds spatially correspond with the VHE emission and span VLSR ranges of 0–10 and 10–20 km s−1.
  • The northeastern cloud overlapping HESS J1801−233 contains shocked and unshocked gas, with estimated densities of ~10^4 and ~10^3 cm−3, respectively.
  • The southern cloud overlaps all HESS J1800−240 components, with its dominant fraction associated with components A and B and a CO peak within 0.02° of W 28A2.
  • A northeastern-cloud mass of ~5 × 10^4 M⊙ is estimated at d = 2 kpc, but velocity crowding, X-factor applicability, and possible distances up to ~4 kpc limit interpretation.

4. Radio to X-ray views

Multiwavelength views place HESS J1801−233 on W 28’s northeastern shell and show nearby radio, infrared, and X-ray structures associated with the region’s remnants and star-forming objects.

  • HESS J1801−233 overlaps W 28’s northeastern shell and coincides with a strong peak in the 90 cm radio continuum.
  • The radio view also identifies nearby SNRs, SNR candidates, and the non-thermal radio arc G5.71−0.08 overlapping HESS J1800−240C.
  • W 28A2 lies within 0.1° of the centroid of HESS J1800−240.
  • The ROSAT image shows predominantly thermal central X-ray emission with kT ∼0.4 to 2 keV and an X-ray peak at the northeastern boundary.
  • MSX 8.28 µm emission shows heated dust concentrated around W 28A2 and G6.1−0.6.

5. Discussion

The VHE emission associated with dense molecular clouds may have a hadronic origin, requiring enhanced cosmic-ray densities that W 28 or other nearby accelerators could plausibly supply. Distance uncertainties, projection effects, unresolved GeV emission, and alternative leptonic or accelerator scenarios motivate further observations.

  • Hadronic interpretation: The VHE/molecular-cloud association could indicate hadronic gamma-ray production through accelerated protons interacting with dense ambient matter.The expected flux depends on cloud density or mass and on the cosmic rays penetrating the clouds.
  • Cosmic-ray requirements: 13 to 32 times the local cosmic-ray density is required to explain the E > 1 TeV VHE emission under the hadronic assumptions considered.The estimate assumes cloud masses, distances of 2 or 4 kpc, and association of each source’s emission with the relevant cloud component.
  • Cosmic-ray requirements: Slow diffusion from W 28 could produce the required enhancement within 30 pc for up to ∼10^5 years after the supernova explosion.This scenario is discussed for clouds at ∼2 kpc and a diffusion coefficient D_10 ∼10^26 cm^2 s^−1.
  • Alternative accelerators: Several additional or alternative accelerators remain possible, including other SNRs, SNR candidates, M 20, W 28A2, and associated star-formation sites.Unknown distances for some radio SNRs and the multi-peaked morphology of HESS J1800−240 complicate source attribution.
  • Multiwavelength constraints: EGRET’s degree-scale point-spread function cannot exclude unresolved MeV/GeV emission from HESS J1800−240, requiring higher-resolution observations.The EGRET emission peak coincides with HESS J1801−233, but the southern VHE components remain unresolved.

6. Conclusions

H.E.S.S. revealed VHE sources spatially coincident with molecular clouds near W 28, while NANTEN data show cloud components spanning velocities associated with distances from roughly 2 to 4 kpc. A hadronic interpretation requires cosmic-ray densities about 10–30 times the local value, with W 28 and other accelerators remaining possible contributors.

  • 6. Conclusions: H.E.S.S. detected HESS J1801−233 northeast of W 28 and the three-component complex HESS J1800−240 just beyond its southern boundary.The sources positionally coincide well with molecular clouds.
  • 6. Conclusions: NANTEN ^12CO(J=1−0) data reveal molecular clouds spanning V_LSR = 5 to ∼20 km s^−1, encompassing W 28’s estimated distance and allowing components up to ∼4 kpc.If connected at ∼2 kpc, the clouds may form a larger parent cloud disrupted by W 28 or other regional objects.
  • 6. Conclusions: A hadronic interpretation implies cosmic-ray densities ∼10 to ∼30 times the local value for cloud components placed at 2 or 4 kpc.W 28 could supply these densities under slow diffusion, while other accelerators or distant sources remain possible.
  • 6. Conclusions: Further detailed modeling and multiwavelength observations are recommended to clarify the molecular-gas connections, particle accelerators, and nature of the accelerated particles.High-transition sub-mm observations would improve cloud-mass estimates and probe material at velocities above 10 km s^−1.

List of Objects

The list of objects records alternate names and page locations for W 28, nearby H II regions, SNRs, SNR candidates, and gamma-ray or pulsar sources.

  • List of Objects: The index lists W 28A2, G6.1−0.6, 6.225−0.569, GRO J1801−2320, PSR J1801−23, G7.06−0.12, and G5.71−0.08 with their page locations.W 28 itself is indexed on page 1, while W 28A2 is indexed on page 4.
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