Constraints on particle acceleration in SS433/W50 from MAGIC and H.E.S.S. observations

TL;DR

This study used MAGIC and H.E.S.S. observations to search for very high-energy gamma-ray emission from SS 433/W50, constraining particle acceleration models and finding no significant gamma-ray signals, thus limiting the energy transfer to relativistic particles.

Contribution

First combined analysis of MAGIC and H.E.S.S. data on SS 433/W50, providing new upper limits on gamma-ray emission and constraining particle acceleration models.

Findings

No evidence of VHE gamma-ray emission detected.

Constraints on the fraction of jet power transferred to relativistic protons.

Magnetic fields >10 μG inferred at the jet-medium interface.

Abstract

The large jet kinetic power and non-thermal processes occurring in the microquasar SS 433 make this source a good candidate for a very high-energy (VHE) gamma-ray emitter. Gamma-ray fluxes have been predicted for both the central binary and the interaction regions between jets and surrounding nebula. Also, non-thermal emission at lower energies has been previously reported. We explore the capability of SS 433 to emit VHE gamma rays during periods in which the expected flux attenuation due to periodic eclipses and precession of the circumstellar disk periodically covering the central binary system is expected to be at its minimum. The eastern and western SS433/W50 interaction regions are also examined. We aim to constrain some theoretical models previously developed for this system. We made use of dedicated observations from MAGIC and H.E.S.S. from 2006 to 2011 which were combined for the first time and accounted for a total effective observation time of 16.5 h. Gamma-ray attenuation does not affect the jet/medium interaction regions. The analysis of a larger data set amounting to 40-80 h, depending on the region, was employed. No evidence of VHE gamma-ray emission was found. Upper limits were computed for the combined data set. We place constraints on the particle acceleration fraction at the inner jet regions and on the physics of the jet/medium interactions. Our findings suggest that the fraction of the jet kinetic power transferred to relativistic protons must be relatively small to explain the lack of TeV and neutrino emission from the central system. At the SS433/W50 interface, the presence of magnetic fields greater 10$\mu$G is derived assuming a synchrotron origin for the observed X-ray emission. This also implies the presence of high-energy electrons with energies up to 50 TeV, preventing an efficient production of gamma-ray fluxes in these interaction regions.