A search for very high-energy flares from the microquasars GRS 1915+105, Circinus X-1, and V4641 Sgr using contemporaneous H.E.S.S. and RXTE observations

Context. Microquasars are potential γ-ray emitters. Indications oftransient episodes of γ-ray emission were recently reported in atleast two systems: Cyg X-1 and Cyg X-3. The identification of additionalγ-ray-emitting microquasars is required to better understand howγ-ray emission can be produced in these systems. Aim. Theoreticalmodels have predicted very high-energy (VHE) γ-ray emission frommicroquasars during periods of transient outburst. Observations reportedherein were undertaken with the objective of observing a broadbandflaring event in the γ-ray and X-ray bands. Methods:Contemporaneous observations of three microquasars, GRS 1915+105,Circinus X-1, and V4641 Sgr, were obtained using the High EnergySpectroscopic System (H.E.S.S.) telescope array and the Rossi X-rayTiming Explorer (RXTE) satellite. X-ray analyses for each microquasarwere performed and VHE γ-ray upper limits from contemporaneousH.E.S.S. observations were derived.  Results: No significantγ-ray signal has been detected in any of the three systems. Theintegral γ-ray photon flux at the observational epochs isconstrained to be I(>560 GeV) <7.3 × 10^-13cm-2 s^-1, I(>560 GeV ) <1.2 ×10^-12 cm-2 s^-1, and I(>240 GeV) <4.5 × 10^-12 cm-2 s^-1 for GRS1915+105, Circinus X-1, and V4641 Sgr, respectively. Conclusions:The γ-ray upper limits obtained using H.E.S.S. are examined in thecontext of previous Cherenkov telescope observations of microquasars.The effect of intrinsic absorption is modelled for each target and foundto have negligible impact on the flux of escaping γ-rays. Whencombined with the X-ray behaviour observed using RXTE, the derivedresults indicate that if detectable VHE γ-ray emission frommicroquasars is commonplace, then it is likely to be highly transient.