Chandra/Very Large Array Follow‐Up of TeV J2032+4131, the Only Unidentified TeV Gamma‐Ray Source

Abstract

The HEGRA Cherenkov telescope array group recently reported a steady and extended unidentified TeV gamma-ray source lying at the outskirts of Cygnus OB2. This is the most massive stellar association known in the Galaxy, estimated to contain ~2600 OB type members alone. It has been previously argued that the large scale shocks and turbulence induced by the multiple interacting supersonic winds from the many young stars in such associations may play a role in accelerating Galactic cosmic rays. Indeed, Cyg OB2 also coincides with the nonvariable MeV-GeV range unidentified EGRET source, 3EG 2033+4118. We report on the near-simultaneous follow-up observations of the extended TeV source region with the CHANDRA X-ray Observatory and the Very Large Array (VLA) radio telescope obtained in order to explore this possibility. Analysis of the CO, HI, and IRAS 100 mm emissions shows that the TeV source region coincides with an outlying sub-group of powerful OB stars which have evacuated or destroyed much of the ambient atomic, molecular and dust material and which may be related to the very high-energy emissions. An interesting SNR-like structure is also revealed near the TeV source region in the CO, HI and radio emission maps. Applying a numerical simulation which accurately tracks the radio to gamma-ray emission from primary hadrons as well as primary and secondary e ± , we find that the broadband spectrum of the TeV source region favors a predominantly nucleonic -rather than electronic -origin of the highenergy flux, though deeper X-ray and radio observations will help confirm this. A very reasonable, ~0.1%, conversion efficiency of Cyg OB2's extreme stellar wind mechanical luminosity to nucleonic acceleration to ~PeV (10 15 eV) energies is sufficient to explain the multifrequency emissions. Corresponding author Y.M.B.

TeVγrays and neutrinos from photodisintegration of nuclei in Cygnus OB2

“…The obvious possibility is an increased density of very hot OB stars in the cell of TeV J2032+4130 which provide more efficient trapping and accelerating conditions, as well as a hotter photon background. Indeed, a recent estimate [44] indicates around 10 O stars in the region of the source, which is a factor of 3 larger than expected on the basis of a uniform population. (This strongly supports our previous choice of R ⋆ A .)…”

“…The difficulty to accommodate the spectrum by conventional electromagnetic mechanisms has been exacerbated by the failure of CHANDRA and VLA to detect X-rays or radiowaves signaling acceleration of any electrons [44].…”

“…We now briefly discuss some models in which accelerated protons and nuclei interacting with a local dense gas cloud [44] or with stellar winds [48], produce photons from π 0 decay. In the case of gas cloud interactions [44], an interstellar density of n tot ∼ 30 cm −3 and an injection efficiency (the ratio of cosmic ray energy to the kinetic energy of the accelerating winds) of 0.08% reproduces well the observed TeV spectrum.…”

“…We now briefly discuss some models in which accelerated protons and nuclei interacting with a local dense gas cloud [44] or with stellar winds [48], produce photons from π 0 decay. In the case of gas cloud interactions [44], an interstellar density of n tot ∼ 30 cm −3 and an injection efficiency (the ratio of cosmic ray energy to the kinetic energy of the accelerating winds) of 0.08% reproduces well the observed TeV spectrum. However, such a large density has been called into question [48] because a re-assessment [49] of the CO-H 2 conversion factor for this type of environment (used for calibration) would imply a considerably lower density, n tot ∼ 0.1 cm −3 .…”

Radio continuum and near-infrared study of the MGRO J2019+37 region

“…To explain steady VHE γ-ray emission, hadronic models have been developed by several authors (e.g., Aharonian & Atoyan 1996;Butt et al 2003;Torres et al 2004;Bordas et al 2009). The electromagnetic radiation produced by both hadronic jets from microquasars and Galactic cosmic rays, and their interaction with the ISM were explored by Bosch-Ramon et al (2005).…”