HESS upper limits on very high energy gamma-ray emission from the microquasar GRS 1915+105

Context: High energy particles reside in the relativistic jets of microquasars, making them possible sources of very high energy radiation (VHE, >100 GeV). Detecting this emission would provide a new handle on jet physics.
Aims: Observations of the microquasar GRS 1915+105 with the HESS telescope array were undertaken in 2004-2008 to search for VHE emission.
Methods: Stereoscopic imaging of Cherenkov radiation from extensive air showers is used to reconstruct the energy and direction of the incident gamma rays.
Results: There is no evidence for a VHE gamma-ray signal either from the direction of the microquasar or its vicinity. An upper limit of 6.1 × 10^-13 ph cm^-2 s^-1 (99.9% confidence level) is set on the photon flux above 410 GeV, equivalent to a VHE luminosity of ∼ 10³⁴ erg s^-1 at 11 kpc.
Conclusions: The VHE to X-ray luminosity ratio in GRS 1915+105 is at least four orders of magnitude lower than the ratio observed in gamma-ray binaries. The VHE radiative efficiency of the compact jet is less than 0.01% based on its estimated total power of 10³⁸ erg s^-1. Particle acceleration in GRS 1915+105 is not efficient at high energies and/or the magnetic field is too strong. It is also possible that VHE gamma-rays are produced by GRS 1915+105, but the emission is highly time-dependent.