VERITAS Observations of High-Mass X-Ray Binary SS 433

Abstract: Despite decades of observations across all wavebands and dedicated theoretical modelling, the SS 433 system still poses many questions, especially in the high-energy range. SS 433 is a high-mass X-ray binary at a distance of $\sim 5.5$ kpc, with a stellar mass black hole in a $13$ day orbit around a supergiant $\sim$A7Ib star. SS 433 is unusual because it contains dual relativistic jets with evidence of high-energy hadronic particles. X-ray emission is seen from the central source as well as the jet termination regions, where the eastern and western jets interact with the surrounding interstellar medium. Very-high-energy gamma-ray emission is predicted both from the central source and multiple smaller regions in the jets. This emission could be detectable by current generation imaging atmospheric-Cherenkov telescopes like VERITAS. VERITAS has observed the extended region around SS 433 for $\sim 70$ hours during 2009-2012. No significant emission was detected either from the location of the black hole or the jet termination regions. We report 99\% confidence level flux upper limits above 600 GeV for these regions in the range $(1-10) \times 10^{-13} \mathrm{cm^{-2} \ s^{-1}}$. A phase resolved analysis also does not reveal any significant emission from the extended SS 433 region.